Improving Cassava Cultivation as an Industrial Raw Material on Acid Soil in Indonesia

Abstract

Cassava is grown nowadays for use in food, feed, and industrial purposes. It is believed that the agro-industrial sector, which uses cassava as a raw material, has more advanced farming technology for improving cassava production. Lampung province in Sumatra Island, Indonesia, is one of the cassava production centers for industrial raw materials, with a planted area of 256,632 ha in 2018. The planting areas are acid soils of Ultisols, Inceptisols, and Oxisols with pH levels ranging from 4.5 to 5.0. Acidic soils have a complicated set of plant growth-limiting constraints. Essential nutrients for plant growth, such as N, P, and K, as well as other cations, are often low due to leaching, nutrients fixed by Fe/Al oxides of clay minerals, and low soil cation exchange capacity. In these acid soils, cassava production ranges from 8 to 15 t ha−1 for traditional farming, 20–24 t ha−1 for semi-developed farming to 25–35 t ha−1 for advanced farming. Meanwhile, with numerous technological advancements, cassava productivity can reach 40–50 t ha−1. Aside from improving varieties, technological updates being pursued include increasing the accuracy of mineral fertilizer dosage, improving planting system technology, bio-fertilizer technology, and in situ organic C enrichment of acid soils.

1. Introduction

Cassava (Manihot esculenta Crantz) is the third staple food after rice and corn in Indonesia, having carbohydrate content 40% higher than rice and 25% higher than corn. Cassava has a starch content of about 24%, which gives it potential as a raw material for bioethanol [1]. It can be grown on marginal land and has a high tolerance to acid soil [2]. These advantages make cassava farming possible with no or low input to the soil.

Cassava cultivation exists throughout Indonesia, such as in large islands like Sumatra, Java, Kalimantan, Sulawesi, and Papua, and small islands like Bali, Sumbawa, and Maluku, etc. Cassava is suitable to be developed in the wet tropical climate that dominates the territory of Indonesia and also grows quite well in the dryer part of Indonesia, like on the island of Nusa Tenggara [3]. Under high temperatures, high light intensity, and heavy rainfall in Indonesia, cassava for industrial raw materials requires a long maintenance time until harvest, about 9 to 10 months [4].

In Indonesia, planting cassava as a food ingredient is rarely expanded by farmers in large areas. Farmers grow cassava as a secondary crop, usually on narrow land (several hundred square meters), and in some areas, the cassava yield sometimes goes into food stocks as the mixing of rice during famine periods like at the end of the dry season or the beginning of the growing season. In places where cassava is grown as an industrial raw material, it is more common to grow it over a large area (> 1 ha). Lampung Province, in the southern part of Sumatra Island, Indonesia, is one of the production center areas. In Lampung, most farmers sell cassava yields to factories that process them for tapioca flour, feed, and bioethanol. At present, there are around 130 units of cassava processing factories in this area, with a demand of about 5 million tons of cassava per year [5]. In 2018, Lampung’s total cassava farming land was around 295,548 ha. Other areas of Indonesia that develop cassava farming extensively are East Java Province, with approximately 157,899 ha, and Central Java Province, with 155,660 ha [6].

The type of soil for cassava farming in the three provinces of cassava production centers is quite different. In the provinces of Central Java and East Java, farmers are growing cassava on Alfisols, and Inceptisols, with slightly acidic to neutral soil pH (pH 5.0–6.5) [4, 7, 8], Meanwhile, in Lampung province, the soil is dominated by Ultisols, Oxisols, and Inceptisols, with soil pH ranging from 4.5 to 5.0 [9, 10].

2. Acid soil and constraints for plant growth in Lampung

Mulyani et al. [11] reported that acidic soils develop very widely in Indonesia, especially in wet climates, such as on the island of Sumatra. One of the areas on the island of Sumatra with extensive acidic dryland is the province of Lampung. The acidic dryland in this area reaches approximately 2.87 million ha and is dominated by the orders of Oxisols, Ultisols, and Inceptisols. Oxisols consist of the great group of Hapludox and Kandiudox, Ultisols consist of the great-groups Hapludult and Kanhapludult, while Inceptisols consists of the great-groups Dystrandept, Dystropept, and Eutropept [12]. The classification of acid soils in the Lampung area is following the soil classification as the soil classification of the USDA Taxonomy [13]. The profile of the prime acid soil orders found in the Lampung and their general properties are presented in Tables 1–3.

Table 1.

The Oxisol profile and properties in Lampung Province, Sumatra Island, Indonesia [9].

Table 2.

The Ultisol profile and properties in Lampung Province, Sumatra Island, Indonesia [9].

Table 3.

The Inceptisol profile and properties in Lampung Province, Sumatra Island, Indonesia (private document of Hafif).

The low availability of phosphorus in tropical acid soils is due to P chelation by clay minerals, namely by amorphous and crystalline hydrous oxides of Fe and Al clay minerals, which is very conducive to happening in low pH soil. As shown in Figure 1, the forms of P chelated by clay minerals in acid soils are H₂PO₄⁻ and HPO₄²⁻ [20]. The availability of K is low in acid soils, especially in those that have undergone advanced weathering, such as Ultisols and Oxisols. Rainfall and high temperatures speed up the release of K from rocks and other parent materials into the soil solution. Then, heavy rain keeps washing K out of the soil as low exchangeable K range of 0.03–0.11 meq 100 g⁻¹ in the acid soils of Lampung in Table 4. In the study of cassava cultivation on acid soil, the field experiments were conducted on four great groups of two soil orders, Ultisol and Oxisol. The great group of Oxisol was Hapludox, and the three great groups of Ultisol were Plinthudult, Kandiudult, and Kanhapludult. The great group of Oxisol, Hapludox, is the Oxisol has soil moisture regime udic (soil in a humid climate), with no other identifying horizons. The great groups Plinthudult are sub-order Udult, with plinthite (mixture of clay with other minerals riching iron and humus-poor) found in 150 cm horizon from the soil surface. The Kandiudult is a sub-order Udult of the Ultisols, having a kandic horizon, and the Kanhapudult is the other sub-order Udult having a kandic horizon [13]. The general physicochemical characteristics of the acid soils used for cassava study in the field are presented in Table 4.

Figure 1.

Phosphorus (P) fixation in acidic tropical soil by amorphous and crystalline hydrous oxides of aluminum (AI) clay minerals. Source: Basak and Rakshit [20].

Soil characteristics	Hapludox	Plinthudult	Kandiudult	Kanhapludult
Textures
Sand (%)	39	50	11	17
Silt (%)	13	14	17	48
Clay (%)	48	36	72	35
pH
H2O	4.4	4.6	4.4	4.8
KCl	3.9	4.2	4.1	4.3
Organic C (%)	1.28	1.16	1.51	1.40
Total N (%)	0.09	0.09	0.11	0.11
C/N	14	13	14	13
P2O5 (ppm)	9.3	5.3	23.2	13.0
K2O (HCl 25%) (mg 100 g^−1)	2	4	2	6
Exchangeable Cations (1 N NH4OAc at pH 7)
Ca meq 100 g^−1	1.30	1.75	3.00	2.84
Mg meq 100 g^−1	0.39	1.02	0.81	0.51
K meq 100 g^−1	0.03	0.08	0.03	0.11
Na meq 100 g^−1	0.05	0.09	0.07	0.05
CEC meq 100 g^−1	6.04	5.02	8.56	6.65
BS (%)	29	59	46	45
Al^3+ meq 100 g^−1	1.26	0.47	0.71	0.13
H^+ meq 100 g^−1	0.26	0.21	0.17	0.13
Al saturation (%)	71.1	16.0	18.2	4.3

Table 4.

The physicochemical characteristics of some great-group acid soils at a soil depth of 0–20 cm in Lampung.

Source: Hafif [9], CEC=Cations exchange capacity, BS=Base saturation.

3. Plantation systems, fertilizer, and cassava production

Generally, there are two systems in cassava plantations in Lampung Province, namely monoculture and intercropping. The study by Manihuruk et al. [21] found the factors influencing the farmers in choosing a plantation system were land area, the distance of the farming area from the processing factory, and the source of income. Farmers having small areas (< 1 ha) prefer the intercropping system because it generates more revenue than the monoculture system. Farmers with farming areas near processing plants prefer the monoculture system because the cost of production transport to the factory is relatively low. Meanwhile, farmers with other sources of income choose an intercropping system because the earnings from the monoculture cassava are lower than those from intercropping, especially if they have small areas (≤ 0.5 ha).

The planting distance of cassava in a monoculture system widely used by farmers is 1 m x 1 m or 1 m x 0.8 m. While in the intercropping system, the distance of cassava between double rows was 160 cm, and in double rows, it was 80 cm. In an intercropping system, the first step is to grow seasonal crops such as soybeans, peanuts, or corn on land. Then the cassava planting is carried out after the seasonal crops are 15–30 days old.

Based on cassava farming patterns such as planting system, inputs to the land, seedling kind, and land area, Hafif [9] reported that there were three types of cassava farming in Lampung; traditional, semi-developed, and advanced. Farmers in the traditional type typically work on 1 ha or less of cassava land, using a monoculture or intercropping system, cultivating the land with family labor and livestock, using random seeds (derived from previously planted cassava), and using very little mineral fertilizer. The input to the land is only manure at a modest rate of around 1–1.5 t ha⁻¹, and sometimes a little urea is added (±100 kg ha⁻¹). Another habit is often harvesting too-young cassava due to pressing economic needs. When converted to hectares areas, traditional farmer’s land produced cassava of around 8–15 t ha⁻¹ (Table 5). Most of them consider cassava products as additional/side income.

Variables	Farmers of
	Traditional	Semi-developed	Advanced
Cultivated area	< 1 ha	0.5–2 ha	2–5 ha
Cassava seedling	random seedlings	random seedlings- superior varieties	superior varieties
Organic fertilizer	1–1.5 t ha^−1	1–4 t ha^−1	2.5–4 t ha^−1
Chemical fertilizers	Urea 100 kg ha^−1	Compound fertilizer, NPK (15:15:15): 50–200 kg ha^−1	Compound fertilizer, NPK (15:15:15): 300–500 kg ha^−1
Soil Cultivation	family labor and or own livestock	Livestock or tractor services	Full tractor services
Cassava Production	8–15 t ha^−1	20–24 t ha^−1	25–35 t ha^−1

Table 5.

Comparison of traditional, semi-developed, and advanced cassava farming characteristics in Lampung Province, Indonesia.

Source: Hafif [9].

Farmers in the semi-developed farming category prefer a monoculture system and have used complete chemical and organic fertilizers. They mostly used compound fertilizers (NPK 15:15:15%) with doses varying between 50 and 200 kg ha⁻¹ and supported by manure ranging from 1 to 4 t ha⁻¹ in a cassava planting area of 0.5–2 ha. For soil cultivation, the farmers used livestock or tractor services (hand tractor) and still relied on random seeds. Cassava production from semi-developed farming for monoculture patterns ranges from 20 to 24 t ha⁻¹ (Table 5).

In advanced cassava farming, the farmers have considered the efficiency and effectiveness of a farm. In cultivating the land, farmers have fully used the services of a tractor to cultivate the land in a shorter amount of time, about 1–2 hours per ha or 7 ha per day, whereas it would take five days per ha to do the same work with livestock. Most cassava seeds are derived from outside the land, based on recommendations from extension workers or factories. The area of cassava planting ranges from 2 to 5 ha. Advanced farmers preferred the monoculture system and used fully compound fertilizers (NPK 15:15:15%), sometimes adding urea and SP-36. The amount of compound fertilizer applied is high, between 300 and 500 kg ha⁻¹, and manure as much as 2.5–4 t ha⁻¹. Advanced farmers harvest cassava until it is 9–10 months old, with cassava production ranging from 25 to 35 t ha⁻¹ (Table 5).

4. Cassava varieties as industrial raw matter

Cassava of UJ-5, UJ-3, Malang 4, Malang 6, Ardira 2, Ardira 4, and Litbang UK 2 were among the new high-yielding varieties introduced by the Indonesian Ministry of Agriculture in Lampung since 2000 [4]. One of the varieties that are becoming a favorite and being developed by many farmers in Lampung is UJ-5. The Indonesian Legumes and Tuber Crops Research Institute was the inventor of the superior cassava varieties, especially for industrial raw materials.

The UJ-5 variety could meet the requirements as a fuel-grade ethanol (FGE) raw material, due to having the following properties; 1) high starch content, 2) high yield potential, 3) resistance to biotic and abiotic stresses, and 4) flexibility in farming and harvesting time [22]. According to the Indonesian Legumes and Tuber Research Institute, the UJ-5 could produce tubers in the range of 25–38 t ha⁻¹, had starch content of 20–30% fresh weight (FW), and HCN content >100 ppm (slightly bitter taste) and harvest age of 9–10 months. UJ-5 was relatively resistant to cassava bacterial blight (CBB) and had high dry matter (% DM), starch content from dry matter (% DM), sugar content (% FW), and amylose (%) of which was 43.9, 27.5, 39.1, and 22.4, respectively, and the conversion of fresh tubers to bioethanol was 4.5 kg liter⁻¹ (Table 4) [22, 23]. Other beneficial properties of the UJ-5 are; 1) leaves do not fall quickly, 2) it can grow on low and high pH soils, 3) it can grow in high populations, and 4) they can develop in an intercropping system [24]. Table 6 shows in more detail the benefits of the UJ-5 compared to several other types of industrial raw materials and bioethanol.

Cassava clones	Water content	Dry weight	Starch	Sugar	Amy lose	Tubers for 1 L bioethanol	Yield
	(%)	(%)	(%)	(%)	(%)	Kg	t ha^−1
UJ-3	58.8 d	40.8 b	25.7 c	36.7 b	21.3 bc	4.9	25
UJ-5	54.8 e	43.9 a	27.5 a	39.1 a	22.4 a	4.5	30
Adira 4	64.5 a	37.7 c	22.6 f	31.0 d	18.8 e	4.7	17
Malang 6	60.3 c	40.4 b	23.5 e	34.1 c	20.8 c	5.0	15.6
MLG3011	59.7 c	40.7 b	24.3 d	33.3 c	21.0 c	4.3	27
CMM99023–4	64.5 a	37.6 c	21.9 g	30.3 d	20.1 d	5.1	25.3
CMM99008–3	58.4 d	40.2 b	26.9 b	37.0 b	21.7 b	4.2	22.3
OMM9908–4	61.7 b	40.1 b	24.5 d	34.1 c	19.8 d	4.5	31.7
LSD	0.8	1.1	0.46	0.91	0.53

Table 6.

Comparison of some new high-yielding varieties and the properties of cassava as industrial raw materials and bioethanol in Lampung, Indonesia.

Column means followed by the unequal letter are significantly different at an LSD of 0.05. Source: Ginting et al. [22].

5. Cassava yield quality

The low available nutrient content in acid soils, especially P and K, was the cause of low cassava quality due to low starch content and high cyanogenic glucosides [25]. Another cause of low starch content was harvesting cassava before maturity (at 6–7 months old), which was common among farmers in traditional and semi-advanced farming. On average, the starch content of the cassava grown on the acidic soil of Lampung is around 18–22% (manufacturer’s information), although the potential starch content of the UJ-5 variety can reach 30%. However, cassava yield factories may accept these starch levels and limit cassava purchases to only those with a starch content is at least 18% [9].

The low quality of cassava causes the price of this commodity to fluctuate. From 2011 to 2016, the average cassava price decreased by around 2.38% per year [6]. The decline in prices caused some cassava farmers to switch farming to other commodities resulting in a reduction in the cassava planting area of 10.8% per year in Lampung [5]. However, with improvements in cultivation technology and growing superior varieties, cassava productivity was indicated to increase. In 2018, the cassava productivity of Lampung Province was about 26.04 t ha⁻¹, which was better than the average national productivity of 24.39 t ha⁻¹, and the selling price of cassava at the farmer level has continued to improve [6].

6. Technology improvement of cassava cultivation on acid soil

6.1 Mineral fertilizers

The study by Wargiono [26] on the acid soil in Lampung found that cassava in an intercropping system with rainfed rice gave the best result with the application of 90 kg N, 50 kg P₂O₅, and 90 kg K₂O per ha. The results of Ernawati’s research [27] on Kanhapludult acid soil found the application of a mixture of urea, SP36, and KCl fertilizer in a ratio of 2:1:1 or the equivalent of a mix of 90 kg N: 36 kg P₂O₅: 60 kg K₂O with an application dose starting at 40 g plant⁻¹, 80 g plant⁻¹, 120 g plant⁻¹, and 160 g plant⁻¹ or the equivalent of 400 kg ha⁻¹, 800 kg ha⁻¹, 1200 kg ha⁻¹ and 1600 kg ha⁻¹ if the cassava population was 10,000 ha⁻¹, the yield of cassava was not significantly different, namely 54 kg plant⁻¹ or 54 t ha⁻¹. That means the lowest dose of mixed fertilizers, 400 kg ha⁻¹, was sufficient for cassava planted on a ha of acid soil.

KCl application of as much as 300 kg ha⁻¹ on acid soil in Lampung increased the weight of cassava tubers by an average of 1.98 kg plant⁻¹ compared to an average of 1.45 kg plant⁻¹ by application of 200 kg KCl ha⁻¹ [28]. Meanwhile, the study of Hafif [9] found that the application of straw compost (2 t ha⁻¹), each enriched with 50 kg KCl, 100 kg KCl and 200 kg KCl, to acid soil in Lampung, significantly increased the weight of tubers of cassava from 7.35 kg plant⁻¹ (without enrichment) to 7.97, 8.26 and 8.42 kg plant⁻¹ (Table 4), respectively, and the same treatments increased tuber starch content from 30.1% to 30.9%, 32.3%, and 33% (FW), and decreased total cyanogen content by 13.8%, 26.4%, and 28% from 246 ppm (Table 7).

The treatments	Stem diameter (cm)	Tuber weight (kg plant^−1)	Tuber number plant^−1	Starch content (%FW)	Total cyanogen (ppm)
K0	2.39	7.35 b	22.7	30.1 d	246 a
K50	2.52	7.97 a	25.8	30.9 c	212 b
K100	2.55	8.26 a	25.1	32.3 b	181 c
K200	2.57	8.42 a	25.2	33.0 a	177 c
LSD		0.97		2.11	26.9

Table 7.

The application effect of straw compost (2 t ha⁻¹) enriched by 50, 100 dan 200 kg KCl on stem diameter, tuber weight, tuber number, starch, and the total cyanogen of cassava in acid soil in Lampung.

The column means followed by the unequal letter are significantly different at an LSD of 0.05. Source: Hafif [9].

To increase cassava production on acid soils, it is necessary to solve some problems such as Al toxicity [29], low P and K availability [2], and aggregate instability due to low soil organic matter content [30]. Although cassava is a tolerant plant for marginal lands, without fertilizer application, the yield of cassava was far from the target. Even soil fertility under cassava plants will rapidly decline due to the high nutrient uptake of cassava [31]. Howeler [32] reported the plantation of cassava for eight years consecutively without fertilization, which caused cassava production to decrease from 22 tons ha⁻¹ to 13 tons ha⁻¹. Therefore, to get a high yield of cassava on marginal land, one should apply sufficient NPK and organic matter [25, 30]. Among the macronutrients, the K mineral is the one that plays a principal role in increasing the quantity and quality of cassava in acid soils [4, 24].

6.2 Improvement of the plantation system

Intercropping and monoculture systems are two options for cropping systems developed and used by Lampung cassava farmers. Research by Asnawi and Arief [33] found that cassava productivity could increase if the monoculture cropping system was changed to a monoculture with a double-row system. The monoculture system with a double row was different from the monoculture system of farmers, especially in terms of spacing, population per hectare, and fertilization rate. In the monoculture system of farmers, the spacing varies, namely 60 x 70 cm, 70 x 80 cm, or 80 x 80 cm. The total population of cassava in this monoculture system ranges from 15,000 to 20,000 plants per ha⁻¹. Fertilization is usually only 75–100 kg of urea plus a little SP-36 (50 kg ha⁻¹) and manure of about 1 t ha⁻¹.

The monoculture system with double rows changed the spacing to 160 x 80 x 80 cm so that the cassava plant population per hectare is only around 11,200 plants. This system recommended a fertilizer dose of 100 kg Urea +150 kg NPK + 100 kg KCl and manure to be 5 t ha⁻¹. Cassava yields can reach 50–60 t ha⁻¹ with this system [33]. In addition, the system with double rows can join the intercropping system by planting annual crops in the 160 cm space between the double rows. This method is even considered more profitable. The first step in the intercropping system with double rows was to grow seasonal crops such as soybeans, corn, peanuts, and green beans on the space between double rows (160 cm), then plant cassava when the crop was two weeks to a month old. The performance of the intercropping system with double rows and corn as an intercropping crop is shown in Figure 2.

Figure 2.

Design of the intercropping double-rows system (a) and performance of the intercropping double-rows system in the field (B). Source: Robet and Arief [33].

6.3 Bio-fertilizer of mycorrhizae (arbuscular mycorrhiza)

According to Howeler [34], another ingredient that also had the potential to improve the growth, yield, and yield quality of cassava is Arbuscular mycorrhizae (AM). Cassava can grow well on acid soils with low P contents because it has a very efficient symbiosis with AM, which occurs naturally. Cassava is most dependent on AM. At low concentrations of P in acidic soils, the growth and branching of AM hyphae will increase. The AM performed a symbiosis with the cassava roots, as illustrated in Figure 3 [9].

Figure 3.

Symbiosis mutualism between arbuscular mycorrhiza (AM) and cassava roots. Source: Hafif [9].

One way of enriching soil mycorrhiza is through the application of biofertilizers. When AM and plant roots form a symbiotic mutualism, the plant roots will supply exudate to AM, and vice versa, AM will help deliver nutrients and water to the roots. AM hyphae will extend the root system of plants up to 100 times and help plants absorb more nutrients and water, especially in soil with less available nutrients like P and microelements like Zn, Mo, and Cu. AM also increases plant tolerance to drought, high temperatures, infections from fungal diseases, and even high soil acidity. Good plant growth with the help of AM is easier to see in the crops planted in acid soils with a high level of weathering, low base cations and P, and high Al content [35].

According to the findings of Hafif [9], the use of AM bio-fertilizer combined with zeolite as a carrier was able to enhance cassava yield from 7.1 kg plant⁻¹ to 8.8 kg plant⁻¹, and the amount of starch produced increased from 29.5% to 32.1% (FW) or 75.1% to 76.7% (DW) (Table 8).

Treatment	Tuber weight (kg plant^−1)	Tuber number plant^−1	Starch content (% FW)	Total cyanogen (ppm)
M0	7.1 b	22.9	29.4 b	210
M1	8.8 a	25.5	32.1 a	198
LSD	0.85		2.13

Table 8.

The effect of AM bio-fertilizer on quantity and quality of cassava yield on acid soils.

The column means followed by the unequal letter are significantly different at an LSD of 0.05. Notes: FW = fresh weight, M0 = no mycorrhiza, M1 = with mycorrhiza.

Source: Hafif [9].

6.4 In situ enrichment of soil organic C with root exudates of Brachiaria

The research on degraded soils in Madagascar showed that Brachiaria grass, as the source of nutritious feed for livestock in the tropic, planted as an intercrop between cassava, had a good effect on cassava production, namely being able to increase cassava yield from 4 to 13 t ha⁻¹ to 11–30 t ha⁻¹ or an average increase of 240% [36]. The beneficial effects of Brachiaria root exudates include their ability to improve soil aggregates, nutrient cycles, and organic carbon levels [36, 37, 38].

A study conducted by Hafif [9] demonstrated that the roots of signal grass (Brachiaria decumbens) released low molecular weight organic acids into the rhizosphere. These acids included citric, malic, and oxalic. When compared to results obtained without the presence of root exudate from Brachiaria grass, the organic acids secreted by Brachiaria roots were able to chelate aluminum with a significantly higher organic aluminum content. Research on acid soil in Lampung found the root exudates of Brachiaria could reduce the amount of exchangeable aluminum by up to 33%. A decrease in exchangeable Al by root exudates will increase P mobilization in acid soil by inhibiting P fixation by the Al oxide-hydroxide adsorption complex [39]. Planting Brachiaria grass as an intercrop between cassava on acid soils in Lampung (Figure 4) increased yield and cassava starch. Brachiaria grass increased cassava tuber weight from 7.1 kg plant⁻¹ to 8.2 kg plant⁻¹ and starch content from 29.2% to 31.0% (FW) and reduced total cyanogen from 214 ppm to 195 ppm (Table 9).

Figure 4.

Brachiaria grass performance as an intercrop between cassava in field lab (A) and farmer’s field (B) on acid soil in Lampung, Indonesia. Source: Hafif [9].

Treatments	Tuber weight (kg plant^−1)	Tuber number plant^−1	Starch content (%) (FW)	Total cyanogen (ppm)
BD0	7.1 b	22.9	29.2	214 a
BD1	8.2 a	24.7	31.0	195 b
LSD	0.85			19.0

Table 9.

The effect of Brachiaria root exudates on the yield quantity and quality of acid soil Lampung Indonesia.

The column means followed by the unequal letter are significantly different at an LSD of 0.05. Source: Hafif [9].

7. Conclusion

Cassava in a tropical climate like Indonesia is one of the principal food sources, especially for marginalized people in rural areas. However, in certain areas, such as Lampung Province, cassava, which initially received little attention from farmers, has instead developed into one of the leading commodities. This positive development started in 2005, along with the rapid development of cassava processing factories in this region [5].

Farmers in Lampung are not discouraged from growing cassava because of the acidic soil. Cassava planting on acid soil with little external input could produce around 8–15 t ha⁻¹. However, if the next planting still has low input, then cassava production will decrease because cassava absorbs soil nutrients very highly. Based on that experience and supported by intensive counseling from the factory officer and agricultural extension from the local and central governments, the way farmers cultivate cassava is improving. In semi-developed and advanced cassava farming, cassava can produce 20 to 35 t ha⁻¹.

In Lampung, the average productivity of cassava is still around 17.53 t ha⁻¹ [5]. This productivity is far from optimal because the experimental results can reach 40–50 t ha⁻¹. The productivity of cassava on acid soils can increase if farmers improve or adopt cultivation technologies such as planting superior varieties, increasing the doses of mineral fertilizers and organic fertilizers, and improving cropping systems. In the future, it is necessary to encourage the use of biological fertilizers of mycorrhiza, organic C enrichment, and increased mobilization of soil nutrients in situ by planting intercrops that produce root exudates like Brachiaria among cassava plants.

On the other hand, the slow absorption of cassava cultivation technology in Lampung was due to several factors. One of the most influential is the unstable and relatively low selling price of cassava at the farmer level. Low prices make it difficult for farmers to survive in cassava farming. As a result, from 2011 to 2016, the cassava planting areas in Lampung decreased by 10.8% per year because farmers switched their farming to other commodities. However, since 2018, the price of cassava has continued to improve, and this is the hope that farmers will get excited again about growing cassava in Lampung [6].