Oil Palm-Based Cropping Systems of the Humid Tropics: Addressing Production Sustainability, Resource Efficiency, Food Security and Livelihood Challenges

Abstract

The oil palm (Elaeis guineensis Jacq), is a crop of tremendous food, nutrition and economic importance in the tropics. Weather variability and extremes profoundly impact the establishment, survival and productivity of oil palm on the field. Alleys of palm trees in plantation are cropped with arables during early years following field establishment. Studies were conducted at the Nigerian Institute for Oil Palm Research, Benin City, Nigeria. Oil palm seedlings responded to shading, irrigation and AMF inoculation via enhanced water use efficiency, growth vigor and reduced seedling mortality in the nursery during dry season. Age of oil palm and intercrops of Cassava, Maize and Pepper affected mixture productivity and competitive functions in alleys of 2 to 6 years old oil palm fields. Fertilizers (inorganic/organic) promoted agronomic and physiological efficiencies of N use by alley species. Sole crops had higher N use efficiency compared with the intercrops across the fertilizers.

1. Introduction

The oil palm (Elaeis guineensis Jacq) (Magnoliophyta: Arecaceae), is a perennial monocotyledonous plant which belongs to the family Arecaceae. It is a crop of the humid tropics (rainforest belt). The pulp and nut that provide palm oil and kernel oil, respectively make oil palm a high yielding oil-producing crop [1, 2]. The production of the oil from oil palm fruit pulp and nut, has made the crop second to soybean oil in terms of world vegetable oil production while its demand is expected to increase in future [1, 3, 4]. The oil palm is an evergreen crop that may be used to conserve the environment while palm oil accounts for 21% and 47% of the global oil and fats production and trade [3].

Palm oil is grown in many countries across Africa, South America, and Southeast Asia. The global market for palm oil and other products is dominated by Indonesia and Malaysia which collectively account for 84% of global production and other producers include Thailand, Colombia, Nigeria, Guatemala, and Ecuador. In 2018, the world produced 72 million tonnes of oil palm. Indonesia accounted for 57% of this (41 million tonnes), and Malaysia produced 27% (20 million tonnes [5]. Palm produce contributes more than 15% of the non-oil revenue of Nigeria. The industry provides extensive employment opportunities for Nigerians involved in the production, processing, marketing and distribution of both the main raw materials and the downstream products.

Palm oil is a versatile product which is used in a range of products globally. In the broad sense, it is used as Foods: over two-thirds (68%) is used in foods ranging from margarine to chocolate, pizzas, breads and cooking oils; Industrial applications: 27% is used in industrial applications and consumer products such as soaps, detergents, cosmetics and cleaning agents; and Bioenergy: 5% is used as biofuels for transport, electricity or heat. Palm oil and other palm products have always been used for food/pharmaceuticals/industrial and technological purposes [1, 2]. Palm oil is well suited to various food uses, particularly cooking fats and deep-frying oil, and it appears in bakery products, potato crisps and other snacks, and ice-creams and soap, margarine and cooking fat [1, 3]. Lower quality oils are used for non-edible purposes, such as soaps, resins, candles, glycerol, fatty acids, inks, polishing liquids and cosmetics. From oil palm are produced different basic products: crude palm oil (CPO), neutralized palm oil (NPO), refining bleached and deodorized palm oil (RBD), palm olein, palm stearin, palm kernel oil and palm kernel cake or meal [1, 2]. Due to the recent increase in quality and availability, and developments in technologies: refining, fractional and hydrogenation, palm oil has become highly diversified in its uses. Some of these uses are fatty acid manufacture, oleochemicals in general, additives to animal feed stuffs, potato crisp making, bread and cakes. The new compounds produced from palm oil are known collectively as oleochemicals most of these are molecules with different fatty acids attached to various simple functional groups, such as acids, amines or alcohols [1, 3], and include sulfonated methyl esters, polyols and polyurethanes. Several minor constituents of palm oil can be extracted and used separately, such as carotein, vitamin E and sterols.

The rainforest agroecologies of the humid tropics is characterized by wet and dry season transition and variability in seasonal weather conditions. In this zone, the annual rainfall ranging from 1500 to 2000 mm distributed in a bimodal pattern which results in the rainy and dry season. The dry season is a terminal drought situation characterized by inadequate rainfall, soil moisture deficit, high vapor pressure deficit and temperatures and very clear sky (high intensity of solar radiation [6]. Such unfavorable weather condition has been reported as the cause of the massive seedling and fruiting tree mortality in the dry season [7, 8]). It has been reported that high percentage of oil palm seedlings died on the field during the first and second dry season as a result of soil moisture deficit [7]. Although, fruit trees in plantations (cacao, kola, coffee, citrus species and oil palm) are seldom irrigated, despite the challenges of weather variability and extremity of the tropics. Efforts to develop sustainable production systems for oil palm would involve the evaluation of the value of agronomic practices in the amelioration of extreme growing environmental conditions especially the hydrothermal stresses of the dry season on seedling survival on the field in order to attain optimum seedling population in the field.

Climate Change (temperature and rainfall) scenarios for the deciduous and evergreen rainforest zones of West Africa including Nigeria have been variously constructed using process-based methods that rely on the General Circulation Models (GCM) in conjunction with Simple Climate Models (SCM). The results have indicated projected decline in mean annual rainfall and increases in temperature in year 2020, 2050 and 2080 respectively. The projected climatic changes will exacerbate soil moisture and thermal conditions during the dry season (November to March) and aggravate the vulnerability of crops to such climatic conditions [9]. The changing growing environmental conditions (marginal soils and extreme weather events) impose constraints on cacao growth and productivity. In order to alleviate the constraints imposed by changing growing environmental conditions (marginal soils and extreme weather events) on fruit tree performance. It is imperative to develop climatic-stress adaptive strategies for the fruit tree-based agroforestry systems of the humid tropics in the wake of changing climate/weather conditions (climate change and weather variabilities).

2. The farming Systems of the Humid tropics

Agroforestry involves growing trees in mixtures and arable/food crops and fruit tree species simultaneously on a farm (growing arable crops and fruit tree species together). Alley cropping is an agroforestry technique in which trees are planted in hedgerows, and annuals (arable or fodder) crops are planted in the “alley ways” between the hedge row plants. Alley cropping involves growing short duration trees and shrubs that are compatible with arable or fodder crops. The trees provide other benefits such as reducing erosion, maintaining soil fertility and providing additional income to farmers crop diversity and food security in the early years of tree establishment [10, 11]. The advantages of alley cropping are attributable to improvement of soil quality, increased economic diversity, carbon sequestration, farm yield, resource use efficiencies, and environmental resilience [12, 14].

Intercropping: the simultaneous growing of two or more species in the same field for a significant period of their growth. Such crop combinations has been reported as promising to sustain soil and crop productivity. Intercropping, systems offer crucial ecosystem service that supports food supplies and other livelihood activities. Intercropping practices provide sustainable and stable yields, diversity of flora and fauna and lower risks of crop failure, and implement, sustain and enhance environmental quality, ecosystem services and livelihoods and sustainable landscapes [15, 16]. Intercropping practices are reported to optimize ecological processes including the cycling of nutrients, maintains carbon stocks (sequestration), conservation of soil water, modification of microclimate and reduce soil degradation [11, 15].

The essential features of intercropping/agroforestry systems are intensification in the use of space and time of space, light, water and nutrients. Advantages of alley cropping improves soil quality, biodiversity, carbon sequestration, farm yield, resource use efficiencies and environmental resilience (Bedou et al., 2010). Agroforestry involving alley and intercropping are important features of the farming systems of the tropics (references). Research on intercropping has shown that fruit trees can be intercropped successfully with arable crops during the early stages (1 to 5 years) of establishment [10, 17, 18].

Variable availability of growth resources with ages of trees following its establishment exist, thus the variability in the capture and use efficiencies by hedge row crops and alley crops (component species) [19]. When plants are grown together, interspecific competition may occur in relation to the use of growth resources [13] Such biophysical interactions may be positive and negative biophysical interactions exist between trees and the alley crop species. Positive and negative biophysical interactions exists between trees and alley crops in agroforestry/alley cropping systems. Competitive interactions which occur in agroforestry systems based on resource availability for use by the trees and understory plants [20, 21]. In addition, variable complimentality and compatibility between tree crops and the alley crop species in the fruit tree-based intercropping systems is reported. However, proper management of interactions ensures sustainability of the agroforestry systems [20].

Research has shown that tremendous enhancement of growth, development and yield of crops can be obtained through application of fertilizers [11]. Required, is the enhancement of uptake and utilization efficiency by crops without deleterious effect on yield and ecosystem [22]. Nutrient uptake and use efficiencies in crop production is less than 40% worldwide, improvement in the efficiencies of uptake and utilization of nutrients by crops will enhance yields and conserve the environment [23]. Research has shown that nutrient use efficiency indices are affected by cropping systems, fertilizer types and climate [6, 22, 24]. Thus, it is imperative to improve understanding of the use of fertilizers for enhancement of soil fertility, growth and yields of alley crops in the oil palm-based intercropping system of the rainforest zone of Nigeria.

Literature has reported that for most tree crops, the alleys can be intercropped successfully with arable crops during the early stages (1 to 6 years) of establishment. Species combination involving arable crops in oil palm alley and application of fertilizers may enhance competition for resources among intercropped species. Knowledge is however knowledge on the response of oil palm to the presence of cereals, root/tuber and vegetable crops and fertilizers in its alleys, performance of alley crops as sole and intercrops (Cassava, Maize and Pepper) in an oil palm-based agroforestry system, and effects of organic and inorganic fertilizers on soil, alley crops as well as uptake and use efficiencies of fertilizers in an oil palm-based agroforestry system of the rainforest agroecology of Nigeria. There is justification in giving priority to research to improve insights to the performance of cassava, maize and pepper intercrops in the alley of oil palm of different ages (2 to 6 years) as sources of food and income for oil palm farmers during the early stages of oil palm establishment.

3. Materials and methods

3.1 Experimental site and environmental conditions

Series of experiments were conducted to address the themes of the Book Chapter.

An experiment was conducted to examine the effects of shading, watering regimes and mycorrhizal inoculation on the growth and development of oil palm seedlings in the nursery in the dry season. A series of experiments were conducted at the Nursery and Field (Plantations) of the Nigerian Institute for Oil Palm Research (NIFOR), Benin City, Edo State, Nigeria between 2013 and 2018. The Nigerian Institute for Oil palm Research (NIFOR) is located in the rainforest ecological zone of Nigeria between latitude 06^o 33¹N and longitude 05^o 37¹E. Annual rainfall ranges between is 1500 to 2000 mm, average temperature ranges between 28° to 34° centigrade and relative humidity between 54 to 80%.

Age of oil palm and intercrops of Cassava, Maize and Pepper affected mixture productivity and competitive functions in alleys of 2 to 6 years old oil palm fields. Fertilizers (inorganic/organic) promoted agronomic and physiological efficiencies of N use by alley species. The trials were conducted in NIFOR Nursery and Field (Plantations) between 2013 and 2018.

Section A: Studies were conducted to examine the responses of oil palm seedlings to shading, irrigation and AMF inoculation with respect to growth vigor and mortality events in the nursery during dry season. Soil samples were collected from top soil under five-year fallow vegetation and were sieved to remove stones and pebbles. Black polythene bags (1400 cm²) were filled with 6 kg of the soil media and arranged in rows 90 cm both in the open sun and in the shade. The drip lines were placed along the rows after ten days. Two months old oil palm seedlets/plantlets (Tenera) were obtained from NIFOR Pre-nursery and transplanted into the 6 kg pots (black polythene bags:1400 cm²) were filled with top soil obtained from fallow vegetation (secondary forest soil) and arranged in filed plots. Shades measuring 6 x 6 m was constructed using bamboo sticks and palm fronds were used to cover the top and sides. Gravity drip irrigation system was adopted using 200 liter capacity bucket placed on a 1.5 m wooden stand; drippers were installed to apply water (2 liters per palm application). Thermometers each were installed in the shade and in the open sun for the measurement of air temperature weekly and fortnightly intervals.

4. Soil and plant measurements

Data were obtained on growth parameters, pattern of leaf production and senescence, weekly measurement of soil moisture content using a soil moisture sensor while thermometers were installed under shade and in no shade condition to measure soil and air temperatures. At the end of the experiment, 10 plants were gently uprooted and the root washed and shade dried for root measurements. Mycorrhizal Colonization of Roots and Mycorrhizal Spore Count were determined using standard methods [25].

4.1 Irrigation strategy

Oil palm seedlings were drip-irrigated weekly and fortnightly using gravity-drip irrigation system to apply 2 liters of water per plant at each irrigation via point source emitters (2 l/h discharge rate) which were installed on laterals per row of seedlings. Irrigation buckets were suspended on 1.5 m high stakes to provide the required hydraulic heads [26, 27]. There was a two-day pre-irrigation treatment (1.5litres/day) following oil palm seedling transplanting, and thereafter, the weekly and fortnight irrigation treatments were imposed.

Water requirement (WR) was determined using the relation:

$$\mathrm{WR}=\mathrm{A}\mathrm{x}\mathrm{B}\mathrm{x}\mathrm{C}\mathrm{x}\mathrm{D}\mathrm{x}\mathrm{E}$$E1

where: WR = Water requirement (l per day/plant) A = Open Pan evaporation (mm/day) B = Pan factor (1.0, 0.7 and 0.5), C = Spacing of plant (m²), D = Crop factor (Crop coefficient (Kc) for oil palm seedling: initial (0.43) were obtained from Allen et al. [28].

The total water requirement (TWR) was obtained using the relation:

$$\mathrm{TWR}=\mathrm{WR}\mathrm{x}\mathrm{No}.\text{of Plants}$$E2

Maximum allowable deficit (MAD) for oil palm was assumed as 50% of available water storage capacity of the soil (AWC).

Irrigation water requirement is determined using average season wise pan evaporation data for the area. Pan Evaporation (EPan) data used for the experiment were obtained from measurement using a Class-A Pan (121 cm in diameter and 25.5 cm in depth) from the Meteorological Station, Department of Meteorology & Climate Science, Federal University of Technology, Akure, Nigeria located near the plots.

The actual evapotranspiration (ETc) of oil palm seedlings the irrigation regimes was calculated with the water balance equation (Eq. (1)) [26].

$$\mathrm{ET}+\mathrm{I}+\mathrm{P}+\mathrm{\Delta S}–\mathrm{Dp}-\mathrm{Rf}$$E3

where, ET, is actual crop evapotranspiration (mm); I, the amount of irrigation water applied (mm); P the precipitation (mm); ΔSW, changes in the soil water content (mm); Dp, the deep percolation (mm); Rf, amount of runoff (mm). Since the amount of irrigation water was controlled, deep percolation and run off were assumed to be negligible.

Soil water measurements were taken throughout the growing season using the gravimetric method.

The volume of water required per plant (irrigation requirement, I_R) was estimated as:

$${\mathrm{I}}_{\mathrm{R}}={\mathrm{ET}}_{\text{peak}}\ast \text{area}/\text{crop}/{\mathrm{E}}_{\mathrm{n}}$$E4

where En is emitter uniformity for drip irrigation system (0.94) and area per crop.

Peak evapotranspiration (ET_peak) rate for the crop under drip irrigation treatment was estimated as:

$${\mathrm{ET}}_{\text{peak}}=\mathrm{ETo}\ast \mathrm{P}/85$$E5

where ET_peak is peak evapotranspiration rate for the month or period, ETo is the reference evapotranspiration, for the month/period (e.g. 5.1 mm/day), P is the proportion of total land area covered by the crop leaf area (cm) which is assumed 80% (after [26]).

Crop evapotranspiration (ETa) was also calculated using data obtained from FUTA in the formula of Doorenbos and Pruitt [29] and [28] in the form:

$$\mathit{ETa}=\mathit{\text{KcETo}}$$E6

where ETo is potential evapotranspiration and Kc is the crop coefficient.

Weather variables at site of experiment during crop growth cycle (soil and air temperatures, vapor pressure deficit (vpd), solar radiation, wind speed will be monitored from Meteorological Observatory, 500 m from site of experiment). Data collected were subjected to analysis of variance (ANOVA) while significant treatment means were separated using the Duncan Multiple Range Test (DMRT).

Section B: Experiments were conducted between 2016 and 2017 cropping seasons to examine the effects of age of oil palm in plantation on the growth, competitive interactions and mixture productivity of cassava, maize and pepper in oil palm-based strip intercropping system in the rainforest of Nigeria. The studies were conducted on 2, 3, 4 and 5 years old oil palm fields for oil palm field which were established in the fields during 2014, 2015, 2016 and 2017 using Tenera (hybrid).

The experiment was 3 x 3 factorial combination of ages (2, 3, 4 and 5 years) of oil palm plantations and 3 species of arable crops arranged in a split-plot design interrow spaces (alley) between oil palm plans constitute the main plot and arable crops species as sub-plot treatment. Treatments were replicated 3 times. The planting space for oil palms was 9 x 9 m triangular with eight (8) stands per field plot. The plots were spaced by 1 m between plots and replicates (inter row) and 1 m at the borders. Strip intercropping system was adopted in the experiments. Spacing cassava, maize and pepper was 1 x1m given a plant population of 141 plants per plot.

4.2 Data collection

Oil palm data collections include number of leaves, canopy extent and number of fresh fruit bunch (FFB). These data were collected quarterly. The canopy extent was recorded in meters and canopy spread calculated using the multiplication values of the palm measured in two dimensions of ‘North–South spread’ and ‘East–West spread’ and canopy volume of oil palm trees. Oil palm canopy volume and ground coverage by Tripathy et al. [30] and canopy spread (Cs) was estimates as:

$$\mathrm{Cs}=\mathrm{Ns}\mathrm{x}\mathrm{EW}$$E7

where: Cs = canopy spread, NS = North–South, EW = East–West.

Oil palm yield traits were sampled which include the number of fresh fruit bunches (FF) and weight of fresh fruit bunch (FBB) and total bunch yield per treatment plots. The growth and yield data were collected from ten tagged plants randomly selected from the experimental plots. Data collected include number of leaves, canopy extent and number of fresh fruit bunch (FFB). Palm yield traits: yield per palm, number of fresh fruit bunches (FF) and weight of fresh fruit bunch/palm and total palm yield/experimental plot. At harvesting, cassava tuber weight, maize seed weight and pepper fruit weight for each individual plant was according to estimate – the intra- land variability. Biomass yield, dry above ground biomass at harvest and final yield were determined from the sampled plants.

4.3 Indicators of crop mixture productivity and competitive interaction

Different measures or indices of productivity have been developed to determine the productivity of crops in crop mixtures. These indices include relative yield, relative yield total and land equivalent ratio, aggressivity. Relative yield is the biomass or yield of a species in mixture or intercropping expressed as a ratio of its yield in monoculture [31]. Relative yield total (RYT) is the sum of the relative yields of the species in mixture expressed as a ratio of its yield in monoculture [31]. Land equivalent ration (LER) is an indication of biological efficiency of intercropping in use of environmental resources compared to the sole crop [32]. The percentage land saved from intercropping was estimated using the formular described by Willey [14]. Aggressivity is a measure of competitive relationships between two crops in mixed cropping [21] and an important competition function to determine the competitive ability of a crop when grown in association with another crop [33]. This index compares the yields between intercropping and monoculture, as well as their respective land occupancy [34, 35].

Section C: Studies were conducted to examine the effects of fertilizer (NPK compound fertilizer, poultry manure and pelletized organic fertilizer) and age (3, 4 and 6 years) of oil palm on nutrient uptake and use efficiencies of strip intercropped cassava, maize and pepper in an oil palm-based intercropping system. Cassava, maize and pepper were strip-intercropped in the alleys of 3, 4 and 6 years oil palm fields. The fertilizers (NPK, Ferti plus and poultry manure) were respectively applied at the rate of 67.5, 168.75 and 337.5 g/plant) as determined by the soil test.

The indices of N uptake and agronomic and physiological efficiencies of N use were calculated using the procedures described in the literature [36, 37, 38]. Nutrient uptake refers to the ability of crop to extract or absorb nutrients from the soil. The uptake of nitrogen was calculated as the product of the measured N concentrations in shoot biomass and reproductive structures (fruit/seed/tuber) the weight of the biomass (shoot and reproductive structures) [37, 38]. The proportion of total plant N partitioned to the shoot is called the N harvest index (NHI). It is also defined as the percentage of grain N uptake to total plant N uptake [37]. Nutrient Use Efficiency (NUE): This is a term used to indicate the ratio between the amount of fertilizer N removed from the field by the crop and the amount of fertilizer applied. NUE is expressed in several ways as the efficiency of conversion of nutrient taken up by the plant into crop biomass. This ratio describes the efficiency of N fertilizer utilization in crop production.

Agronomic Efficiency (AE) is calculated as the unit of yield increase per unit nutrient applied. It reflects the direction of production impact of applied fertilizer and also relates to economic return. The calculation of AE requires knowledge of yield without nutrient input, so is only known when research plots with zero nutrient is been implemented on the farm [11]. AE is expressed as the efficiency of conversion of nutrient taken up by the plant into crop biomass.

Physiological Efficiency of N use (PE), is defined as the yield increase in relation to the increase in crop uptake of the nutrient in above ground parts of the plant (Dobermann [36]. Similar to AE, it needs a plot without application of the nutrient of interest to be implemented, and requires measurement of nutrient concentrations in crop biomass (shoot and reproductive structures).

The Apparent Recovery Efficiency (ARE) is the ratio of nutrient uptake to nutrient applied, it is also defined as the difference in nutrient uptake in above ground parts of the plant between the fertilizer treated and untreated crop relative to quantity of nutrient applied. Nutrient utilization efficiency (NUE) is calculated as the product of physiological and recovery efficiency. It is calculated based on the method described by Dobermann [36]. Internal Utilization Efficiency (IE) is defined as the yield in relation to total nutrient uptake. It varies with genotype, environment and management. It is an indication of the efficiency of internal nutrient conversion, which may be affected by other stresses (deficiencies of other nutrients, drought stress, heat stress, mineral toxicities, pest etc.) [36]. The total factor productivity (TFP) relates an index of output to a composite index of all inputs while Partial Factor Productivity measure relates output to a single input. Partial Factor Productivity (PFP) is a simple production efficiency expression, calculated in units of crop yield per unit of nutrient applied [36].

Data analysis: Data collected were subjected to analysis of variance (ANOVA) and significant treatment means were separated for 5% (P < 0.05) probability level.

5. Results and discussion

Section A: Response of growth and development of oil palm seedlings to shading, irrigation regimes and mycorrhizal inoculation in the dry season in the nursery.

Treatments were shading and no-shading (open sun), 7- and 14- day irrigation intervals and mycorrhizal inoculation or non-inoculation. Across irrigation treatments, un-shaded oil palm seedlings had lower biomass weights (leaf, frond and shoot) while the treatments significantly affected plant height and frond length from 2 to 20 weeks after planting (Table 1). The seedlings irrigated fortnightly produced longer roots compared with the unshaded and weekly irrigated (Table 2). Shading and weekly irrigation significantly enhanced soil moisture contents and seedling water use efficiency compared with the unshaded. Shaded seedlings under weekly irrigation consumed more water compared with fortnight irrigation (Table 3). Shading and weekly irrigated seedlings combined with mycorrhizal innoculation were more vigorous compared with the unshaded and non-inoculated (Table 4). Mycorrhizal inoculation enhanced oil palm seedling growth while weekly irrigation produced more vigorous seedlings. Shade conserved soil moisture whie unshaded had lower soil moisture contents across period of observation (Figure 1). The unshaded oil palm seedlings had significantly higher water use compared with the shaded for both weekly and fortnight irrigation (Figure 2). Shade combined with AMF inoculation enhanced vigor of growth across the irrigation treatments, and reduced mortality of oil palm seedlings in the nursery in the dry season (Figure 3).

Treatment	Root length (cm)	Root weight (g)	Shoot weight (g)	Leaf weight (g)	Frond weight (g)	Number of roots
Shaded	39.06b	10.30a	94.17a	40.76a	35.42a	10.15a
Unshaded	45.10a	11.46b	57.83b	23.41b	20.35b	11.14a
LSD (0.05)	6.70	1.10	18.43	5.54	6.80	1.10

Table 1.

Effect of shading on growth parameters of oil palm seedlings.

Treatment	Root length (cm)	Root weight (g)	Shoot weight (g)	Leaf weight (g)	Frond weight (g)	Number of roots
Weekly	44.81b	12.61b	91.28a	39.15a	35.10a	11.37a
Fortnightly	56.72a	16.64a	65.56b	32.54b	28.40b	12.24a
LSD (0.05)	3.25	2.08	10.31	4.98	4.44	1.63

Table 2.

Effect of irrigation on growth parameters of oil palm seedlings.

Shade	Irrigation	Plant height (cm)	Number of fronds	Frond length (cm)	Root length (cm)	Root weight (g)	Shoot weight (g)	Leaf weight (g)	Frond weight (g)	Number of roots
Shaded	Weekly	62.2	9.38	51	43	13.15	109.3	47.57	39.1	11.62
	Fortnightly	63.1	8.88	54.5	42.7	10.51	94.3	40.55	37.75	12.62
Unshaded	Weekly	43.3	9.38	34.3	40.8	12.07	61.3	25.89	22.13	12.12
	Fortnightly	40.3	9.38	33.4	55	12.61	64.8	26.13	22.65	12.25
LSD		6.42	ns	6.97	8.66	2.13	17.41	6.80	7.23	ns

Table 3.

Interaction of shade and irrigation on growth variables of oil palm seedling.

Treatment	Root length (cm)	Root weight (g)	Shoot weight (g)	Leaf weight (g)	Frond weight (g)	Number of root
Inoculated	42.35b	13.42a	89.04a	37.24a	32.73a	12.81a
Non-inoculated	48.42a	10.76a	75.79a	33.82b	28.08b	11.50a
LSD (0.05)	5.35	ns	ns	3.80	3.98	ns

Table 4.

Effect of mycorrhizal inoculation on growth parameters of oil palm seedlings.

Means in a column with the same letter (s) are not significantly different by DMRT (P = 0.05).

Figure 1.

Effect of shading on soil moisture contents.

Figure 2.

Effect of irrigation on soil moisture contents.

Figure 3.

Effects of irrigation and shading on water use of oil pam seedlings.

Section B1: Effects of age of oil palm on the growth and yields of alley crop species.

The performance of strip intercropped mixtures of Cassava, Maize and Pepper sown in the alleys of 2 to 6 years old oil palm fields were investigated. In the alleys of oil palm ages 2 to 6 years, sole crops of cassava, maize and pepper out-yielded the respective intercrops (Figures 4–6). Among oil palm ages, yields of the intercrops were higher and similar for the 3 and 4 years and lower for 6 years old fields. The ages of oil palm affected fruit yield and yield components of pepper (Figure 4). The weight of fruits of pepper produced was significantly influenced by ages of oil palm. The weight of pepper fruits harvested for 2 years old oil palm was higher followed by 3 and 5 old oil palm. The weight of fruits of pepper obtained for 5 years old oil palm reduced significantly. Seed yield of pepper followed the same trend for number of fruits and fruits yield. The number and weight of fruits harvested decreased as the age of oil palm increased. There were significant differences in the yield components of cassava in oil palm alleys.. Age of oil palm fields also affected number of tubers, tuber yield and harvest index (HI), significant differences were found between values of these variables for 3, 4 and 6 years oil palms (Figure 5). Maize cob and seed weight and number of seeds/cob were significantly different among the ages of oil palm to which the maize was alley cropped. The weight of cob was high for 2 and 3 years old oil palm. There was reduction in value for 5 years old oil palm. The seed weight of maize alley in oil palm of 2 years old recoded high compared to other ages of oil palm. The highest seed yield were obtained for 2 and 3 years oil palm while seed yield of maize sown in the alley of 5 years old oil palm had low yield. While, the shoot biomass yield, cob weight, number of seed, seed weight, seed yield, canopy spread, air and soil temperature were significantly affected by the ages of oil palm (Figure 6). Ages of oil palm influenced yield and its components of the alley crop mixtures. The differences in yield of the different ages (2, 3 and 5) can be attributed to low vigorous growth as a result of low nutrient content of the soil which led to poor vegetative growth. Contrary to the results obtained from cassava and maize, growth and fruit yields of pepper differed. Height and leaf development were not significantly different among the ages of oil palm fields despite the canopy differentials among palm ages. Although, root and shoot biomass and pepper yields of pepper were better for young palms (2 and 3 years) the differences were not significant for root and shoot biomass and pepper yields of pepper. The effect of age of palm was not significant for harvest index (HI), values were close for the different ages of oil palm.

Figure 4.

Effect of age of oil palm field on cassava tuber yields.

Figure 5.

Effect of age of oil palm on maize seed yields.

Figure 6.

Effect of age of oil palm on pepper fruit yields.

Section B2: Effects of age of oil palm in plantation on competitive interactions and mixture productivity of alley intercrops of cassava, maize and pepper.

The effects of strip intercropped species in the alleys of 2 to 6 years old oil palm fields on competitive interactions and mixture productivity were investigated. Compared with their sole crops, the intercrops had yield advantages and higher land-use efficiencies (LER >1: Figures 7 and 9). The land equivalent ratio (LER) expresses the magnitude/extent of yield advantage of crop mixtures over sole crops, thus represents the land required for sole crop to produce the total yield by the component crops in intercropping. The intercrop mixtures outyielded the sole crops of the component crops (cassava,maize and pepper). The magnitudes of the LER confirms larger percentage land saved by crop mixtures over the respective sole crops. This observation supported reported advantage of intercropping in terms of productivity per unit land area, use of growth resources, in addition to greater percent land saving. Age of oil palm fields significantly affected LER, lowest LER was obtained for 6 years old palm field and values were close for 3 and 4 years old fields (Figure 7). Relative yield (RY) is defined as the sum of relative yields of the species in mixture expressed as a ratio of its yield in monoculture (Figure 8). The values of RY of the intercrops infer that the system is advantageous and complementary being able to utilize growth resources efficiently and were able to stimulate the growth of one another when grown in the alley of oil palm (2 to 6 years after establishment). The relative yields of component crops in the mixture over were lower compare with their counterpart sole crops. The competitive functions were computed in the form of aggressivity (Agg) while the relative species competition was therefore evaluated as competitive ratio (CR) which is a measure of the times by which the component crops are more competitive than the other. Aggressivity values were high (> 0.25) across intercropped compared to sole crops (Figure 9). The ages of oil palm affected competition for growth resources (space and soil nutrients) and the productivity of crop mixtures. The intensity of aggressiveness was almost similar for 3 year oil old palm field, differences were found for 4 and 6 years old. Strip intercropping of cassava, maize and pepper displayed yield advantages with high relative yield total (RYT) and land equivalence (LER), and low aggressivity in the alleys of 2 to 6 years old oil palm. These observations denote complementarity and low competitive interactions among intercrops, and the significant advantage of intercropping in terms of productivity per unit land area and greater percentage land saved under intercropping (Tables 5–7).

Figure 7.

Effect of age of oil palm on LER of maize.

Figure 8.

Effect of age of palm on LER of cassava.

Figure 9.

Effect of age of palm and fertilizers on LER of pepper.

Plant variables	2 years	3 years	5 years	Means	LSD (0.05)
Number of leaves	148.2	157.1	218.3	174.3	6.49
Leaf area (cm2)	18.9	23.2	23	21.7	1.53
plant height (cm)	65.3	63.2	62.9	63.8	1.21
Root biomass (g)	13.3	13.2	11.8	12.8	0.97
Shoot biomass (g)	74.2	73.5	70.5	72.7	1.46
Number of fruits/stand	66.8	65.8	63.4	65.4	1.38
Fruit yield/stand(g)	413	39.1	36.6	39.3	1.61
Fruit weight (g)	411.6	390.9	364.3	388.9	5.12
Harvest index	4.7	4.4	4.2	4.5	0.54

Table 5.

Effects of age of oil palm on growth and yield of pepper.

Plant variables	2 years	3 years	5 years	Means	LSD (0.05)
Number of leaves	9.3	9.6	7.9	8.9	1.01
Leaf area (cm2)	294.4	307.4	297.3	210	2.74
Plant height (cm)	92.6	91	86.4	90	1.88
Root biomass (g)	6.7	6.7	6.2	6.5	0.52
Shoot biomass (g)	56.7	57.2	54.9	56.2	1.15
Cob weights (g)	143.3	141.2	119.1	134.5	3.84
Seed yield (g)	500.5	510	444.4	484.9	6.25
Number of seeds/cob	87.3	90.1	76.6	84.7	2.80
Harvest index	7.9	7.1	7.0	7.6	0.73

Table 6.

Effects of age of oil palm on growth and yield of maize.

Plant variables	2 years	3 years	5 years	Means	LSD (0.05)
Number of leaves	40.3	40	38.3	39.6	1.09
Leaf area (cm2)	43.8	31.6	34.5	36.7	2.63
plant height (cm)	79.3	75.6	62.9	63.8	1.61
Shoot biomass (g)	57.2	47.8	40.4	48.5	2.53
Number of tubers/stand	4.8	4.1	3.4	4	0.51
Tuber yield/stand (kg)	5.0	4.45	4.02	4.16	0.61
Harvest index	4.7	4.4	4.2	4.5	0.54

Table 7.

Effects of age of oil palm on growth and yield of cassava.

Various studies have established that cropping the alleys of oil palm with arable crops is successful for at least the first 5 years of field establishment of oil palm [2, 35]. For oil palm of 2 to 6 years old, palm leaves and canopy extent had fully developed to completely close and cover soil surface and canopy overlap over the alleys and consequent reduction in space and solar radiation interception and transmission. This is in line with [2, 35] who reported that 1, 2 and 3 years of cropping after planting produced no subsequent deleterious effects up to 16 years after planting. A similar trial conducted on forest land near Benin in Nigeria including cropping for 2 years concluded that alley cropping remained possible in the early years of oil palm before complete canopy closure [2]. This early experiment evaluated maize, yams and cassava, and shade-resistant cocoyam as the only crop up until 12 years. For the first 2 or 3 years on the cropped plots, good yields from crops and growth from palms were obtained. Results were also obtained for cocoyam (Xanthosoma sagittifolium) in a more recent experiment near Benin City, in the fifth and sixth years after planting [2, 35]. The relative yields of component crops in the mixture were low compared with their counterpart sole crops. The strip intercrops of cassava, maize and pepper in the alleys of 2, 3 and 4 years old were characterized by relative yield (RY) values less than one (< 1). This may be attributed to inter-specific competition among oil palm of 2, 3, 4, 5 and 6 years old and the intercrop species [2, 35]. Relative yield greater than one (1) indicates that intercropping system has high competitive advantage and complementary in the utilization of resources efficiently [13]. In general, relative yield total (RYT) was best for cassava among the intercrops and lowest for maize. Apart from the yield advantage, RYT is often used to express economic feasibility of intercropping system. The low competition between intercrops implies that one component stimulated the growth of the other [13]. This is however, contrary to other reports that RYT less than one (<1) showed inter-specific competition among species. The envisage advantage of growing crops in mixture for enhanced use efficiency of growth resources and input (fertilizer) attract farmers to intercropping [10, 13, 39]. The greater yield advantage of crop mixtures was established from the land equivalent ratio (LER), land equivalence and percentage land saved (PLS)variables of competitive interaction of crop mixtures confirmed high yield advantage of cassava, maize and pepper intercrops in alleys of oil palm of 2–6 years, and also indicated high productivity of the intercropping system. This observation is consistent with those of Miller and Pallardy [40] and Agele et al. [10] on intercrop species. These results confirmed that cassava, maize and pepper can grow together in mixture without adverse effect on each other but offer yield advantage when intercropped with oil palm alley of 2 to 6 years old. Similar findings were reported by Malay et al. [13] on maize-legume intercropping systems and Agele et al. [20] on cashew-based intercrop of Sesame and bambara groundnut in the southern guinea savanna of Nigeria. Aggressivity of the intercrops was about 0.01 for sole crops of cassava, maize and pepper intercropped into 6 years old oil palm implying equal competition. However, cassava, maize and pepper had both negative and positive symbol greater than one (1). This is consistent with the report of Koohi et al. [41] who stated that crowing coefficient greater than one (>1) had yield advantage, RCC of equal to one (1) and above one (>1) has yield advantage while less than one (< 1) is disadvantageous (Figures 10 and 11).

Figure 10.

Effect of age of palm on relative yields of species in the intercropping system.

Figure 11.

Effect of age of palm on aggressivity of the intercrop species.

Section B3: Effects of alley intercropping on the growth and yield of oil palm.

Oil palm alleys of ages 2, 3, 4 and 6 years were planted with cassava, maize and pepper in intercropping system. By this time, the oil palm plants had not completely closed canopy but expected to create competition in both above-ground and below-ground for alley crops. The alley intercrop species affected growth attributes of oil palm (Table 8). The growth and development of oil palm especially, canopy formation during the early years of establishment (1 to 6 years) appeared to favor sowing arable crops (such as cassava, maize and pepper) in its alleys. The alley crop species (maize, cassava and pepper) exerted no significant detrimental effects on the measured characters of oil palm (including number of leaves, canopy extent, number of fresh fruits bunch (FFB) and weigh of fresh fruits bunch (FFB) (Table 8). Oil palm canopy spread increased as the age of the oil palm increased, canopy extent was largest for 5 and 6 years old palm trees compared with the 2, 3 and 4 years. The number of FFB in young palms were more and small in size and lower in weight while the number of FFB in older palms were few and bigger in size. The number and weight of fresh fruit bunch (FFB) were different between the 4 years old and 5 and 6 years old palm trees. The number of fruits per palm ranged from 10 to 16, 8–16, and 10–15 and yields of FFB were on the average, 8.98, 9.28 and 31.93 kg.m² for the respective 4, 5 and 6 years old oil palm trees (Table 8). The number and weights of fruits were in line with NIFOR record on oil palm production [2]. The yields of FFB from 4 to 6 years old palm trees were close to NIFOR standard. The tested plant species grown in mixtures had enhanced productivity over their respective sole crops possibly by exploiting species complementarities for resource capture. The complementarity could have resulted in part from competition avoidance responses for resource capture and use for growth of the individual species [10, 13]. Individual species responses in the mixture (e.g. shade avoidance) and hence differences in resource acquisition in time and space improved the performance of the intercrop community as a whole [23, 35].

Plant variables	2 years	3 years	5 years	Means	LSD (0.05)
Number of leaves	16.22	17.9	35	23.1	1.13
Canopy extent (m)	1.41	1.51	2.35	1.76	0.25
Number of fresh fruit bunches/tree			9.71	9.71
Weight of fresh fruit bunches/tree (kg)			98.68	98.68

Table 8.

Effects of alley intercropping (cassava, maize and pepper) on the growth and yield of oil palm.

Section C1: Effects of fertilizers and age of oil palm on efficiencies of N uptake and use for biomass and yield production by alley crop species.

The fertilizers exerted significant effects on nitrogen uptake for yield production of cassava, maize and pepper strip intercropped in the alleys of 3, 4 and 6 years oil palm plot (Table 9). Compared with the unmanure (PuO), application of fertilizers enhanced nitrogen contents, N uptake and N yields. NPK compound fertilizer enhanced the contents, uptake and yields of N compared with other fertilizer treatments while poultry manure had varied effects on nitrogen contents, N uptake and N yields of the intercrops. The effects of ferti plus organo mineral fertilizer on N uptake and N yields of the intercrops was similar to that of poultry manure (Table 9). in addition, uptake and yields of N were higher for the respective sole crops of cassava, maize and pepper compared with the strip intercrops. While N content was highest for fruits of pepper, highest N uptake and N yields were found for cassava leaves and tubers for which in addition, shoot biomass was heaviest. The interaction of intercrop and fertilizer types were significant for most of the indicators of nutrient uptake measured (Table 9). The biomass yields were heaviest for cassava, therefore, its N uptake and N yields were higher compared with maize and pepper under application of poultry manure (Table 9). Fertilizer application using NPK, fertiplus and poultry manure enhanced the nutrient uptake in cassava planted in in the alleys 3, 4 and 6 years old oil palm. However, NPK produced slight increases in N uptake of cassava under 4 and 6 years old oil palm compared with other fertilizers (NPK, Ferti Plus and poultry manure). The age of oil palm fields influenced the effects of fertilizers on the uptake of nitrogen for biomass production in cassava. Nitrogen uptake for tuber production in cassava was better under 3 years old field by NPK and 4 years old by poultry manure. The fertilizers and age of oil palm affected nutrient uptake in the leaf and seed of maize. NPK and Ferti plus significantly enhanced leaf and seed nutrient uptake of maize compared to poultry manure across the ages of oil palm. The least values for N uptake were however recorded for poultry manure and highest for 3 years old oil palm. Among fertilizer treatments, least decline in effect of age of oil palm was found for 6 years old. NPK and Poultry manure had no differences in values for 4 years old palm hence enhanced nutrient uptake in leaf production. 6 years old oil palm had the least value. Close values were found fertilizer effects on leaf and seed nutrient uptake and for 3 and 4 years old oil palm which recorded the greatest effects (Table 9). Ferti plus and NPK enhanced the nutrient uptake in pepper leaves and fruits transplanted into 3 years old palm. NPK fertilizer influenced nutrient uptake compared to poultry manure for 3 and 4 years oil palm. Lowest fertilizer efficiency was found for 6 years old oil palm for both leaf and fruits of pepper. Ferti plus enhanced N nutrient uptake in leaf and fruits of pepper compared with other fertilizers (Table 9).

Intercrop species		N contents	Biomass	PuN	PuO	PuN (sole)	Intercrop mixture; Yield (+N)	Crop mixture Yield (unfertilized)	Sole crop Yield (N)	Sole crop; Yield (unfertilized)	NHI; Mixture	NHI; sole crop
Cassava	NPK	1.81	1212	1775.61	1324.35	1844.3	37.8	31.2	42	35.7	0.047884	0.058013
Maize		1.68	125	164.64	113.68	178.1	11.3	10.4	14.2	11.4	0.148673	0.161538
Pepper		2.03	129	170.52	126.84	205.0	12.3	11.2	15.3	12.3	0.165041	0.18125
Cassava	Poultry manure	1.49	1108	1183.06	897.22	1247.2	33.7	27.3	37.6	35.7	0.044214	0.054579
Maize		1.38	109	111.78	87.48	187.0	10.4	8.3	13.4	11.4	0.132692	0.166265
Pepper		1.54	97	113.96	88.06	173.4	11.7	11.2	14.6	12.3	0.131624	0.1375
Cassava	Ferti Plus	1.53	1194	1320.39	957.93	1844.4	31.3	26.8	37.4	35.7	0.048882	0.05709
Maize		1.37	118	119.19	87.87	154.5	9.3	7.4	11.7	11.4	0.147312	0.185135
Pepper		1.73	123	134.94	102.96	172.4	10.8	9.3	14.1	12.3	0.160185	0.186022
	LSD	0.37	35.98	20.57	17.73	21.9	2.74	2.47	2.84	2.33	0.005	0.004
Fertilizers (Fz)		Significant	*	*	*	*	*	*	*	*	*	*
Crop types; (Ct)		Significant	*	*	*	*	*	*	*	*	*	*
Fz*Ct		Significant	*	*							*	*

Table 9.

Effects of fertilizers and intercrops on N content and uptake, crop yields and nitrogen harvest index (NHI).

Section C2: Effects of fertilizers and age of oil palm on agronomic and physiological efficiencies of N use by alley crop species.

In general, the fertilizers following their application to the strip intercrops in the alleys of 3, 5 and 6 years old palm influenced most of the indicators of nutrients use efficiencies (Table 10). Agronomy Efficiency (AE) differed significantly. Apparent Recovery Efficiency (RE), Apparent recovery Efficiency by difference(RE%), Physiological Efficiency(PE), Utilization Efficiency(UE), Internal utilization Efficiency(IE) and partial factor productivity (PFP) were not significantly different among the intercrops under the fertilizers. Fertilizer treatments enhanced most of measured variables of nutrient use efficiencies compared to unfertilized plots (control) (Tables 10 and 11).The ages of oil palm significantly affected most of the measured variables among the intercrops except the N removed at harvest. However, apparent recovery of N differed among intercrops and ages of oil palm plots. The values of the measured parameters were highest for 5 and 6 years oil palms across the intercrops (Table 10). For cassava, poultry manure enhanced the utilization efficiencies while ferti plus promoted the efficiencies of N recovery from the applied fertilizers and physiological efficiency of its use. NPK enhanced both recovery and utilization efficiencies of nutrients, partial factor productivity, N removed at crop harvest and the competitive ability for uptake and use of nutrients from the applied fertilizers (Table 10). In maize, NPK enhanced recovery efficiency from applied fertilizers, partial factor productivity, N removed at harvest and competitive ability for nutrients (Table 10). Ferti plus enhanced physiological, utilization and conversion efficiencies of nutrients while poultry manure improved physiological and utilization efficiencies of nutrients. In pepper, ferti plus and poultry manure enhanced the utilization efficiency of nutrients while NPK significantly enhanced N removed at harvest. The fertilizers affected almost all variables measured as indicators of agronomic and physiological efficiencies of nutrient use (Table 10). The effects of fertilizer was significant on utilization and recovery efficiencies of N and competitive ability for nutrients, agronomic and physiological efficiency of N use and apparent recovery of N (Table 10). Poultry manure and ferti plus fertilizer enhanced agronomic efficiency, apparent recovery of nutrients from above ground biomass, internal utilization efficiency as well as N conversion efficiency while NPK enhanced partial factor productivity and NHI for both sole and intercrop mixtures (Table 10). The summary of uptake, recovery, agronomic and physiological efficiencies of N use, N removed at harvest and conversion efficiencies is presented in Table 11. Maize and pepper were efficient in terms of ANR, agronomic and physiological efficiencies of N use and its conversion efficiencies while cassava was outstanding for N uptake and its recovery at harvest. Among the fertilizers tested, poultry manure and Ferti plus were outstanding with respect to ANR, agronomic and physiological efficiencies of NUE and N conversion efficiencies while NPK out-performed other fertilizers for NHI and N recovery at harvest. The interactions of intercrop species and fertilizer was significant for most of the variables of agronomic and physiological efficiencies of N use evaluated (Table 11). The fertilizers (NPK compound fertilizer, ferti plus and poultry manure exerted significant effects on nitrogen uptake for biomass and yield production. While NPK enhanced N uptake and apparent recovery of N in above ground biomass, ferti plus and poultry manure promoted most other indicators of agronomic and physiological N use efficiencies. The efficiencies of uptake and use of N were higher for NPK and ferti plus compared with poultry manure across the ages of oil palm fields. The values of the measured indicators of N uptake and use efficiencies were highest for 4 and 6 years compared with the 3 years old fields across the intercrop species. The uptake and yields of N were higher for the respective sole crops of cassava, maize and pepper compared with the intercrops across the fertilizers compared with the control.

Intercrop species	Fertilizers	Agronomic efficiency of fertilizers	Physiological efficiency of fertilizers	N recovery efficiency	Apparent recovery of N	Utilization efficiency of N	Internal utilization efficiency	Partial factor productivity	N conversion efficiency	N removed at harvest	Relative interaction intensity	Competitive ability for nutrients
Cassava	NPK	247.61905	0.0146257	1.504200	15	0.2193857	0.0212885	0.12600	0.5524862	1771.1955	0.809406	0.2907
Maize		276.10619	0.0176609	0.169866	1.7	0.0300235	0.0686346	0.03767	0.5952381	164.26107	0.784000	0.2626
Pepper		273.17073	0.0251832	0.145600	1.4	0.0352564	0.0721323	0.04100	0.4926108	170.0972	0.651162	0.2635
Cassava	Poultry manure	4050.4451	0.0223901	1.084666	11	0.2462916	0.0284855	0.00674	0.6711409	1182.8806	0.716606	0.3032
Maize		3990.3846	0.0864198	0.004860	5	0.4320988	0.0930399	0.00208	0.7246377	111.7625	0.743119	0.2561
Pepper		4786.3248	0.019305	0.005180	6	0.1158301	0.1026676	0.00234	0.6493506	113.9423	0.762886	0.2644
Cassava	Ferti Plus	381.81818	0.0124152	0.906150	9	0.1117365	0.0237051	0.07825	0.6535948	1017.110	0.722780	0.2745
Maize		389.74359	0.0606641	0.078300	8	0.4853129	0.0780267	0.02325	0.7299270	118.9703	0.737288	0.2475
Pepper		348.93617	0.0469043	0.079950	9	0.4221388	0.0800356	0.02700	0.5780347	134.6826	0.634146	0.2423
LSD (0.05)		1991.04	0.025	0.566	4.36	0.176	0.031	0.041	0.079	62.68	0.057	0.019
Fertilizers (Fz)		*	*	*	*	*	*			*	*	*
Crop types (Ct)		*	*	*	*	*	*			*	*	*
Fz*Ct			*					*	*			*

Table 10.

Effects of fertilizers and intercrops on indices of N uptake and use efficiencies.

	N Uptake	ANR (%)	Agron; NUE (kg/kg)	Physiol.; NUE (kg/kg)	N yield	NHI	N removed @ harvest	N conversion efficiency
Cassava	513.65	7.02	2011.03	0.0175	21.22	0.10	691.3	0.62
Maize	337.01	8.01	2021.14	0.0211	17.14	0.11	477.7	0.65
Pepper	335.48	7.06	2031.52	0.0233	18.51	0.12	445.6	0.64
LSD	11.51	0.41	19.43	0.001	7.02	0.002	15.31	0.02
Zero	617.5	4.71	248.33	0.012	13.56	0.09	342.4	0.44
NPK	521.62	6.03	365.53	0.019	20.47	0.12	702.5	0.55
Poultry manure	357.59	7.33	427.92	0.027	17.03	0.11	470.3	0.68
Ferti Plus	382.93	8.68	373.49	0.026	18.66	0.12	524.7	0.65
LSD	40.63	0.22	28.34	0.002	0.45	0.003	23.6	0.02
Crop Type; (CT)	Significant	Significant	Significant	Significant	Significant		Significant	Significant
Fertilizers; (FZs)	*	*	*	*	*	*	*	*
CT x FZs	*	*	*	*	*		*	*

Table 11.

Summary of N uptake, agronomic and physiological efficiencies of N use.

*

ANR (Apparent Recovery of N in above ground biomass); ANUE (Agronomic N use efficiency); PNUE (Physiological N use efficiency).

The age of oil palm fields and fertilizers influenced nutrient uptake and use efficiencies of the intercrop species (leaf, tuber of cassava, fruits of pepper and seeds of maize) in the alleys of 3, 4 and 6 years old oil palm trees and thus, the resultant yield improvement compared with unmanure treatments [42]. The intercrop species differed in shoot morphological and physiological attributes, and rooting patterns of rooting, biomass and nutrient accumulation and partitioning. These attributes have implications for nutrient uptake, use efficiencies and yield production among the alley crop species [22]. Crop species and fertilizer type affected the uptake and accumulation oof nutrients to the vegetative and reproductive structures [20, 23]. Information about differences among species and varieties with respect to N use efficiencies have been used to develop cultivars adapted to low fertilizer input management systems. The sole crops of the intercrop species under NPK treatment recorded the highest nutrient contents in their leaves which indicates that physiologically, nutrients uptake may depend of the degree of competition (below ground) for resources [20, 23]. The fertilizer NPK, contains high N content which is released rapidly into soil solution and promoted its availability, this must have enhanced its uptake by the plant and utilization for biomass production (improved plant growth) compared with organic fertilizers (poultry manure for example). Fertilizers enhance nutrient availability and bring about decreases in competition placed on nutrient resources by intercrop species (crop mixtures) [20]. Literature reports has indicated that differences in soil nutrient status are a major source of variation in uptake and use efficiencies and of crop yields [20, 36]. These reports also attributed crop yield enhancement by fertilizers to improvements in the efficiency of uptake and use of nutrient resources for both sole and intercropping systems. Agele et al. [6] also attributed high yield performance of sole and intercrop combinations of crop species to improvements in efficiency of nutrient utilization. Our study showed that application of poultry manure, ferti plus and NPK fertilizers to alley crops in oil palm soil affected nutrient uptake leaf and tuber/seed nutrient contents compared with the control (un-manure) treatments. The fertilizers enhanced soil nutrients (the sandy loam soil of experimental site appeared to have low fertility status especially N) while improvement in other soil (chemical, physical and biological) properties would have promoted biomass accumulation and yield production by the alley crop species. It is reported that nutrient availability depends on nutrient concentration in the soil and environment and release pattern in synchrony with the crop needs [22, 23]. However, highest N uptake values were obtained for the un-manure. Nitrogen harvest index was higher under ferti plus and poultry manure compared with NPK, this result was in line to the conclusion of Agele et al. [6, 22] that the crop yields and nutrient availability were higher in plots for which farmyard manure was applied, and to longer time availability. Manure decompose slowly and release their constituent nutrients slowly, may be to meet time dynamics of nutrient demand by growing crops [6, 43].

Highest nitrogen harvest index values for seed and leaf of intercrop species were obtained from the un-manure treated plants. The superiority of these may be attributed to more vigorous nutrient exploitation advantage [23]. Oil palm also take up nutrient from the soil for its growth, especially from sandy loam soil of experimental sites, nutrient recycling in palms is slow. However, oil palm provides nutritional elements like phosphorus [44], and nutrient P is reported to decrease species competition placed on nutrient resources [11]. For arable crops grown in the alley of oil palm (1 to 6 years old), supplementary input of nutrients especially nitrogen from fertilizers (organic and inorganic) is needed to meet the nutrient requirements of alley crops.

6. Summary and Conclusions

Based on the measured growth parameters of oil palm seedlings, shading and weekly irrigation enhanced seedling vigor compared with fortnight and open sun (unshaded) treatments. These treatments enhanced vigor, growth and reduce mortality of oil palm seedlings in the nursery in the dry season. Mycorrhizal inoculation of oil palm seedlings, shade and weekly irrigation are recommended. The findings from this study acclaimed the relevance of dry season irrigation and shade to supplement soil moisture and reduced temperature for oil palm seedling growth and development. The drip irrigation-shade strategy adopted ameliorated dry season terminal drought (hydrothermal stresses) in cacao. This is a veritable tool to scale up growth, survival, establishment and flower/pod production. The results of this study will contribute to the development of sustainable cacao production practices and development of shade and irrigation management guidelines for small holder farmers.

Oil palm age significantly affected most of the measured growth variables and indicators of competitive interactions and crop mixture productivity, and agronomic and physiological efficiencies of N use by alley intercrop species. The study established that intercrop mixtures of cassava, maize and pepper in the alleys of oil palm of 2 to 6 years old exhibited some levels of compatibility and complementarity and confirmed low competitive interactions but high growth resource use efficiencies. The results of competitive functions and crop mixture productivity indicate the yield advantage of crop mixtures over sole cropping and hence the overall biological advantage of intercropping of cassava, maize and pepper in the oil palm-based intercropping system. Strip intercropping of cassava, maize and pepper in the alleys of 2 to 6 years old oil palm in the rainforest zone had no detrimental effects on the growth and yield of oil palm. Crop performance results from the behavior of the individual plants interacting through competitiveness (vigor of growth of individual species in the mixture) and complementary which drive resource capture and utilization. The study improved understanding of compatibility and complementarity of growth resource use (space, light, nutrients and possibly water) of oil palm with some selected arable crops in oil palm-based intercropping system of the rainforest of Nigeria. Improved insight to unravel the primary drivers and dynamics of competitive and complementary growth responses of crop mixtures in cropping systems. Such knowledge is relevant to the promotion and adoption of crop mixtures (intercropping systems) for sustainable increases in crop yields at acceptable input levels. Over their respective sole crops, the strip intercrops of cassava, maize and pepper exhibited high values of land equivalence, percentage land saved (PLS) and low aggressivity in the alleys of oil palm which indicates greater yield advantage of crop mixtures over sole cropping. These observations denote the advantage of intercropping in terms of productivity per unit land area, in addition to greater percentage land saved.

The fertilizers (organic and inorganic) enhanced nutrient uptake and use efficiencies in the respective leaf, tuber, seed and fruits of cassava, maize and pepper in the alleys of 3, 4 and 6 years oil palm fields. The indicators of uptake and use efficiencies of N differed among the alley crop species and fertilizer types across the ages of oil palm plants in plantation. While NPK promoted the uptake and apparent recovery of N in above ground biomass, the organic fertilizers enhanced other indicators of agronomic and physiological N use efficiencies, and the efficiencies of uptake and use of N were higher for NPK and ferti plus compared with poultry manure. Uptake and use efficiencies of N were higher for the respective sole crops of cassava, maize and pepper compared with the intercrops across the fertilizers. N content was highest for fruits of pepper while N uptake and yields were highest for cassava tubers and seeds of maize. For arable crops grown in the alley of oil palm (1 to 6 years old), supplementary input of nutrients especially nitrogen from fertilizers (organic and inorganic) is needed to meet the nutrient requirements of alley crops.