Open access peer-reviewed chapter
Abstract
Organic fertilizers with low C:N ratios can be applied to supply both macro and micronutrients to the soil. Aside nutrient supply, they can improve soil structure, texture, water holding capacity and nutrient holding capacity. The mechanisms that may interplay to allow organic fertilizers to affect the soil and crop yields may include improved nutrient synchrony, general improvement in fertility and/or priming effects. The rate, timing and method of organic fertilizer application must be considered to reduce N and P losses during organic fertilizer application. To meet the nutrient requirement of crops, organic fertilizers must be applied in large quantities, so it is more prudent to apply locally available resources. In a case study where sole organic fertilizer, sole inorganic fertilizer and their combinations were applied under rain-fed conditions, it was observed that manure had the potential to hold nutrients longer. This is a positive finding for drought prone areas.
Keywords
- climate smart agriculture
- manure
- water holding capacity
- nutrient holding capacity
1. Introduction
Poor soil fertility is the major biophysical factor affecting crop production in the world [1]. It is a major threat to food security considering the ever-increasing growth rate in human population which is projected to reach about 10 billion by 2050 [2]. In times of old, forests and marginal lands were converted to farmlands to meet the food demands of the growing population. This practice caused the extinction and endangerment of many plant and animal species; hence it is frowned upon by many stakeholders. As such, today, it would not be prudent to encroach land reserves and other marginal lands for agricultural purposes. It is therefore imperative that we improve soil fertility and health of the available land, to increase food production and to ensure the world’s food security under the current and projected climate change.
Previously, the use of mineral fertilizer was thought of as the most appropriate remedy to soil fertility problems due to its rapid nutrient release [3]. However, mineral fertilizer lacks the ability to improve the soil’s physical properties causing fertility improvement by fertilizers alone to be unsustainable. Over-reliance on mineral fertilizer without due diligence to the organics may lead to increased soil erosion, surface and groundwater contamination, increased greenhouse gas emission and reduced biodiversity [4]. In addition, mineral fertilizers are expensive, and many farmers may not have the purchasing power to acquire it [5]. As a result, the attention of various stakeholders has been drawn to use of organic resources [6]. The application of organic fertilizers presents a more sustainable method of food production. There is unending literature reporting the efficiency and effectiveness of organic nutrient sources in maintaining soil quality (physical, biological and chemical properties), improving crop yields and sustaining productivity [6, 7, 8]. The benefits of applying organic fertilizers to the soil are elaborated in this chapter.
2. Types of organic fertilizers
Organic nutrient sources are specifically derived from plant and animal origins [9]. They include plant residues, animal wastes and biofertilizers. In this era where climate change and the COVID-19 pandemic has impacted agricultural production and the financial capabilities of all workforces including farmers, farmers could use organic fertilizers available to them for soil fertility purposes because they are cheaper and more environmentally friendly when they are locally available [10].
Organic fertilizers include poultry manure, cattle manure, green manure (often legumes), field crop residues, composts, bone meal, household waste, blood meal, slurry, cocoa pod husks, palm kernel cake, among others. Biofertilizers are products containing single micro-organisms or combinations of them which when applied help fix atmospheric N, solubilize nutrients, mobilize nutrients, or secrete growth promoting substances to aid crop growth. These products do not supply nutrients themselves but enhance the activities of soil microbes to make more nutrients available to crops. They are categorized into N-fixing biofertilizers, phosphorus solubilizing biofertilizers, composting accelerators and plant growth promoting rhizobacteria [9]. Most of the plant and animal residues are often by-products and nuisance to the environment. Using them as nutrient sources would help reduce waste and greenhouse gas emission.
3. Benefits of organic fertilizers to the soil
3.1 Balanced nutrient supply
Organic fertilizers supply all essential crop nutrients (N, P, K, S, Ca, Mg, B, Cl, Cu, Fe, Mn, Mo, Ni and Zn) in balanced forms, including micronutrients. This is often not the case for any one inorganic fertilizer. Since all these nutrients make up the biomass of organic residues, they are released during the decomposition process into the soil. The downside to applying organic fertilizers alone is that they contain very minimal amounts of these nutrients and as such must be applied in bulky quantities to meet crop nutrient demands [11]. Also, the fact that only a fraction of the nutrients in organic fertilizers can be released per season must be factored in when applying organic fertilizers. On the average, as a rule of thumb, only about 50% of nutrients in organic fertilizers are mineralized in the first season of application [12]. Usually, the focal nutrient used to calculate the amount of organic fertilizer to apply is its nitrogen (N) concentration. For example, 30% decomposed cattle manure (DCM) contains about 2% N [13]. Assuming a farmer grows maize, which requires about 90 kg/ha N, that means:
Therefore, to supply 90 kg N = (90 × 100)/2 = 4500 kg DCM.
Since the applied DCM will only supply half the amount of N required in a season, the amount must be doubled to make
To supply same amount of N through mineral fertilizer, a farmer would only need about 200 kg Urea, however, in organic applications, other nutrients are concurrently being applied. Since a large amount of DCM would supply the required N and other nutrients, it must be available to the farmer. Hence advocates of organic fertilizers must emphasize on ways to raise such large amounts of materials for application locally if sole organic production is desired.
3.2 Improving nutrient holding capacity
Aside the balanced nutrient supply, organic fertilizers add organic matter to the soil if a long-term application is practiced. Organic matter improves the nutrient holding capacity of the soil because it contains organic acids that increase the H+ ions and surface charge of the soil, causing the soil’s cation exchange capacity to increase [15]. Thus, the soil’s ability to hold more cations (nutrients) at exchange sites is increased and hence the nutrient holding capacity of the soil is also improved. Organic matter also improves the buffer capacity of the soil and increases the soil’s ability to resist a change in pH, which in turn affects nutrient loss or gain to the soil [16]. Organic fertilizers increase microbial activity in the soil, causing increased nutrient mineralization rates for the benefit of crops. They stimulate the activities of aerobic and anaerobic bacteria [17] and arbuscular mycorrhizae fungi that form networks of root extension for extensive nutrient availability to crops. Upon the lysis and decomposition of soil microbes, nutrients retained in their biomass are made available in the soil and to crops.
3.3 Improving water holding capacity
Soil structure, texture, bulk density, and organic matter content are the controls on soil water holding capacity; therefore, any management practice that improves these soil properties, in turn, improves water holding capacity (WHC) of the soil. Soil moisture content is largely dependent on the specific surface area of the soil and the thickness of films of water surrounding the pores [18]. The addition of organic matter through organic fertilizer application improves soil aggregation and increases the surface area of the soil, presenting the soil with more room for soil particles to be surrounded by films of water. As a result, the soil can hold more water against the pull of gravity which drains water from the soil.
While soil organic matter binds soil particles, it also stimulates the activity of soil microfauna whose movement create micro and macropores in the soil, creating extra room for water infiltration [19]. Thus, soil water holding capacity can be improved by the addition of organic fertilizers. In the wake of climate change, where unexpected droughts may be imminent, improving the water holding capacities of the soil with the application of organic fertilizer is the way to go. Also, the physical presence of organic materials on the soil serves as mulch that reduces evaporation and retains moisture in the soil. It also reduces the speed of runoff water and allows rain or irrigation water to infiltrate the soil at favorable speed, thereby reducing erosion, soil and nutrient loss [19].
3.4 Improving soil texture and structure
The soil binding properties of organic matter and improvement in soil aggregation helps to improve soil structure [20]. The addition of organic matter also improves soil texture and aeration. Soils with improved structure and texture allow easy air, water, and root movement to support healthy crop growth.
4. Mechanisms underlying organic fertilizer effects on soil
Many research works have observed extra crop yields with organic fertilizer application compared to when its nutrient equivalents are applied through mineral fertilizer [21, 22, 23]. Various mechanisms have been proposed to explain this added crop yields from organic fertilizer application. Some of which include
Under the improved nutrient synchrony mechanism proposed by Vanlauwe et al. [23], when organic fertilizers are applied, they supply microbes with energy from the carbon they contain, to drive decomposition processes. This leads to temporal immobilization of soil N [24, 25] to build their body tissues. The immobilized N is made available at a later stage of plant growth when the microbes have decomposed the organic material to make nutrients available and/or some microbes have lysed and released their nutrients to the plant when it needs nutrients most. In effect, the peak of nutrient supply coincides with highest crop nutrient demand point when crops have matured, so that the nutrients are efficiently utilized, and little is lost to the environment. Kapkiyai et al. [26] reported that a combination of organic and mineral nutrient sources has been shown to result into synergy and improved synchronization of nutrient release and nutrient demand and uptake by plants leading to higher yields.
The general fertility improvement mechanism [23] is based on the theory that organic matter, aside its addition of nutrients to the soil, improves other physical properties of the soil that helps to perpetuate the nutrient addition effect in real time. Some of these benefits include the improvement of soil structure, water and nutrient holding capacities as discussed above. It also adds micronutrients which is usually not the focus of inorganic fertilizer application.
Priming effect is another mechanism proposed by Kuzyakov et al. [27], in which organic fertilizers affect additional crop yields. Priming refers to strong short - term changes in the turnover of soil nutrients caused by the addition of easily decomposable organic materials. Changes may be positive or negative depending on whether nutrients are rapidly mineralized or immobilized. Under this mechanism, a sum of nutrients available in the soil after harvest and nutrients in crops from the field are higher than a sum of the initial soil nutrients and nutrients in the organic materials. Thus, the additional unaccountable nutrient is the result of organic fertilizer precursing a more rapid mineralization rate and dissolution of previously unavailable/fixed nutrients into solution. This is made effective by the improvement in microbial population, diversity and activity affected by the organic material addition.
These mechanisms, though proposed by different authors, all point to the fact that organic fertilizers are beneficial to the soil and consequently, crops.
Despite the benefits of organic fertilizers to the soil, organic resources application is limited by the large amounts required to meet nutrient demand [28]. Hence locally available organic resources must be used to overcome this limitation. In areas where animal production is common, feedlot manure is the most available organic fertilizer resource. Crop residue retention and cash crop- cover crops rotation is an option to increase on-farm residue production. One other option that has proven to be effective is an integrated nutrient management approach where organic and inorganic fertilizers are applied in right quantities [29]. This approach helps to harness the mechanisms underlying the effects of organic fertilizer application on crops, resulting in synergy in terms of crop yields.
5. Role of organic fertilizer in climate smart agriculture
In times when climate change is imminent and its effect on agriculture tends to endanger food security, it is paramount that farmers and other stakeholders use strategies and resources that adapt farming systems to the changing climate. Climate change is mainly driven by natural and anthropogenic activities that pump greenhouse gases (examples CO2, CH4, N2O) into the atmosphere [30, 31]. It may lead to extreme droughts or extreme floods, which may have devastating impacts on food production and agriculture. In this light, organic fertilizers are a great resort due to their replenishing effects on soil physical and chemical properties. Aside the benefits of organic fertilizers discussed above which may adapt the soil to drought conditions, soils should be well drained and loose in flood prone areas in wait of climate change. In compact and poorly drained soils, the addition of organic fertilizers would improve soil particle aggregation and structure to give the soil more room to infiltrate water without settling on the top for too long to cause floods. The addition of organic matter reduces the inventory of greenhouse gasses contributed to climate change by agriculture. This is achieved by the sequestration of carbon into the soil from organic fertilizers applied. The carbon would have been lost to the atmosphere as CO2 or CH4 if it had not been incorporated into the soil [32]. As a result, the application of organic fertilizers to the soil helps to reduce greenhouse gas emission leading to global warming and a consequent climate change and helps adapt the soil to the current and future changes in the climate.
6. Qualities of a good organic fertilizer
Since organic materials are diverse in type and nutrient composition, it is difficult to give a general recommendation of an organic material. The lignin, polyphenol and nitrogen contents of organic material are important controls on its nutrient mineralization, once applied. It is important to evaluate the carbon to nitrogen (C:N) ratio of an organic material to determine if application of the material will lead to N mineralization or immobilization. A C:N ratio of 25 would enhance decomposition and mineralization by soil microbes while a C:N ratio above that would enhance N immobilization [14]. Hence the lower the C:N ratio, the more rapidly nutrients will be made available to the soil. Organic materials high in lignin (>15%) and polyphenol (>5%) contents usually have high C:N ratios and are resistant to microbial decomposition; hence will decompose slowly. If the N content of the material is 2.5% or more, it would likely decompose and mineralize faster [33].
7. How to apply organic fertilizers to harness all the benefit
The effectiveness of an organic material as a fertilizer is also dependent on how it is applied. Surface application of organic fertilizer enhances the loss of N through ammonia volatilization or loss of N and P through runoff and erosion. Judicious methods by which organic materials may be applied to reduce wastage and nutrient losses include band spreading, trailing hose method, burying method, rapid soil incorporation, and the addition of nitrogen inhibitors [34].
Band spreading is the application of the organic material(s) in narrow bands usually a few centimeters away from the crops. This reduces the surface area of the material to the atmosphere so that ammonia volatilization is reduced. To reduce the rate of denitrification as well, band spreading should be done during cool weather with no excessive soil moisture and at right rates. The crop canopies also serve as a physical barrier that further reduces the rate of ammonia volatilization from band spreading applications.
Slurries or liquefied organic fertilizers could be applied in these narrow bands through trailing hoses which hang down from a boom and run along or just above the surface of the soil.
Organic amendments could also be buried at about 5-30 cm depth depending on the crop establishment. Deeper depth burying can be practiced before crops are grown while shallower depth is suited for already established crop fields. This method greatly reduced N loss through ammonia volatilization and the loss of material through erosion.
Manure could be rapidly incorporated into the soil during soil tillage (before planting) or with hand implements to reduce N and P losses in volatilization and runoff.
Under conditions with high denitrification potential, nitrification inhibitors could be added to organic fertilizers to delay the rate at which ammonium is converted to nitrates, which is a suitable substrate that precursors the denitrification process. It is important to apply organic fertilizers at cool times of the day and at the right rates to reduce nutrient losses.
8. Case study (Research)
The sole application of organic fertilizers has proved to be a slow means of nutrient supply to the soil. Hence the combined use of organic and inorganic nutrient sources has been proposed [29]. Such applications harness the benefits of synergistic interaction between the organic and inorganic nutrient sources. The main objective of this research was to increase maize yield with the application of organic manure or a combination of it with mineral fertilizer. To arrive at this objective, the yield of maize following varying rates of combined manure and mineral fertilizer applications were estimated at harvest, synergistic benefits of combined applications were quantified and the effect on soil nutrient stocks were analyzed.
8.1 Methodology
A field experiment was conducted at the plantation section of the Kwame Nkrumah University of Science and Technology under rain-fed conditions. Nine treatments (three levels of mineral fertilizer at 0, 50 and 100% of the 90-60-70 kg/ha NPK recommended rate (RR) by three levels of manure at 0, 50, 100% of 5 t/ha RR) were applied on the field in a factorial fashion arranged in Randomized Complete Block Design (RCBD) with three replications. The land was slashed and burned and later plowed and harrowed to a fine tilt. Plot layouts were done with lines and pegs with each plot measuring 3 m by 2 m. There were 2 m alleys between replications and 1 m alleys between plots. Initial soil and manure sampling and analyses were done to characterize them. Randomized manure treatments units were allocated to their designated plots. The Akposoe maize variety developed by the Crops Research Institute of Ghana was planted 2 weeks after manure allocation. Weeding was done manually when necessary. Mineral fertilizer application was done 2 weeks after planting (WAP). The fertilizers were applied as urea, triple superphosphate and murate of potash. The urea was split applied in the first fertilizer application (2 WAP). The other half of the urea was applied 6 WAP. The manure was spread in the plots and raked in to about 5 cm depth. The fertilizer was applied by the band placement method, about 5 cm away from the maize plants. A final soil analysis was done after harvest to determine soil nitrogen (N), phosphorus (P) and potassium (K) levels. Data was subjected to analysis of variance (ANOVA) with the GENSTAT statistical package and significant means were separated with least significant difference at 5%.
Note: Rains were quite erratic at the start of the experiment until an unexpected shortage during the reproductive stage of maize growth. Though unfortunate, this was a good situation to determine if manure applied to the soil would help maintain more soil moisture and consequently impact maize yield.
8.2 Results and discussion
The lack of rains crippled any effect of the manure alone or its combinations with mineral fertilizer to create differences in the yield of maize. Limited soil moisture has been reported to constraint maize yield [35], because all the processes involved in nutrient movement to roots, uptake by roots and translocation through the transpiration stream use water [36].
After harvest, soil and statistical analysis showed that plots receiving 50 and 100% rates of manure had a significant 20% more total soil N than the control and mineral fertilizer rates. It is possible that due to the rapid nutrient release mechanism of mineral fertilizer, most of its nutrients was released during the early stages of the maize growth, subject to rapid loss from the soil system. The C:N ratio of the manure was 23.08, which is an indication that N was being mineralized [35] into the soil system over a long period, even after the shortage of rains. A combined use of the full rate of manure and full rate of mineral fertilizer also had 20% more total soil N than each individual nutrient source. It is evident that combining organic and inorganic inputs creates a balance between increasing N availability for plant uptake over sole organic application and decreasing N availability for potential system losses compared to fertilizer alone [37].
The rather erratic rains at the beginning of the experiment might have caused soil P and K to leach beyond root zone, hence the lack of differences between the effects of sole manure and mineral fertilizer applications or their combinations at the end of the experiment.
8.3 Conclusion
Overall, it was concluded that organic manure had the potential to hold nutrients in the soil longer than inorganic fertilizers. In the advent of climate change, it could be a very useful tool especially in areas was droughts are expected.
References
- 1.
Tandzi NL, Mutengwa SC. Factors affecting yield of crops. Agronomy SP—Ch. 2 UR—DO— 10.5772/intechopen.90672 SN—978-1-83881-223-2 PB—IntechOpen CY—Rijeka Y2-2021-07-02 ER—. 2019. DOI: 10.5772/intechopen.90672 - 2.
Bruinsma J. The resource outlook to 2050. By how much do land, water and crop yields need to increase by 2050? In: FAO Expert Meeting on How to Feed the World in 2050. Rome, Italy: FAO; 2009. p. 33 - 3.
International Food Policy Research Institute (IFPRI). Sustainable Options for Ending Hunger and Poverty. Green Revolution: Blessing or Curse. Washington DC, USA; 2002 - 4.
Tilman D, Cassman KG, Matson PA, Naylor R, Polasky S. Agricultural sustainability and intensive production practices. Nature. 2002; 418 :671-677 - 5.
Tetteh FM, Issaka RN, Ennin S, Buri MM. Soil Fertility Initiative, Fertilizer Update and Reccommendation Trials. Ghana: Soil Research Institute; 2008. p. 33 - 6.
Suge JK, Omunyin ME, Omami EN. Effect of organic and inorganic sources of fertilizer on growth, yield and fruit quality of eggplant ( Solanum melongena L). Archives of Applied Science Research. 2011;3 :470-479 - 7.
Negassa W, Negisho K, Friesen DK, Ransom J, Yadessa A. Determination of optimum farmyard manure and N, P fertilizers for maize on farmers’ fields. In: Seventh Eastern and Southern Africa Regional Maize Conference, 11th–15th February 2001. 2001. pp. 387-393 - 8.
Vanlauwe B, Diels J, Aihou K, Iwuafor ENO, Lyasse O, Sanginga N, et al. Direct interactions between N fertilizer and organic matter: Evidence from trials with 15 N-labelled fertilizer. In: Vanlauwe B, Diels J, Sanginga N, Merckx R, editors. Integrated Plant Nutrient Management in Sub-Saharan Africa. Wallingford: CAB International; 2002. pp. 173-184 - 9.
FAO. Plant nutrition for food security. A guide to integrated nutrient management. In: Roy RN, Finck A, Blair GJ, Tandon HLS. 2006. ISBN: 92-5-105490-8. Available from: https://www.fao.org/3/a0443e/a0443e.pdf - 10.
Adejobi KB, Fameye AO, Akenbi OSO, Adeosun SA, Nduka AB, Adeniyi DO. Potentials of cocoa pod husk ash as fertilizer and liming materials on nutrient uptake and growth performance of cocoa. Research Journal of Agriculture and Environmental Management. 2013; 2 (9):243-251 - 11.
Timsina J. Can organic sources of nutrients increase crop yields to meet global food demand? Review paper. Agronomy. 2018; 8 :214 - 12.
Eghball B. Nitrogen mineralization from field-applied beef cattle feedlot manure or compost. Soil Science Society of America Journal. 2000; 64 :2024-2030. DOI: 10.2136/sssaj 2000.64620 24x - 13.
BARC (Bangladesh Agricultural Research Council). Fertilizer Recommendation Guide 2012. Soils Pub. No. 45. Farmgate, Dhaka: Bangladesh Agricultural Research Council; 2012 - 14.
Brust GE. Management Strategies for Organic Vegetable Fertility. Safety and Practice for Organic Food2019. pp. 397-408. DOI: 10.1016/b978-0-12-812060-6.00009-x - 15.
Havlin JL, Tisdale SL, Nelson WL, Beaton JD. Soil Fertility and Fertilizers. An Introduction to Nutrient Management. 8th ed. Upper Saddle River, New Jersey: Pearson Educational, Inc.; 2014. p. 98 ISBN 978-81-203-4868-4 - 16.
Buresh RJ, Dobermann A. Organic materials and rice. In: Annual Rice Forum 2009: Revisiting the Organic Fertilizer Issue in Rice. Vol. 2010. College, Laguna, Phiilippines: Asia Rice Foundation; 2010. pp. 17-33 - 17.
Mamaril CP, Castillo MB, Sebastian LS. Facts and Myths about Organic Fertilizers. Muñoz, Nueva Ecija, Philippines: Philippine Rice Research Institute (PhilRice); 2019 - 18.
Vengadaramana A, Thairiyanathan JJP. Effect of organic fertilizers on the water holding capacity of soil in different terrains of Jaffna peninsula in Sri Lanka. The Journal of Natural Product and Plant Resources. 2012; 2 (4):500-503 - 19.
FAO. The importance of soil organic matter. Key to drought-resistant soil and sustained food production. FAO Soils Bulletin 80. 2005 - 20.
Funderburg, E. What Does Organic Matter Do in Soil?. 2001. Available from: https://www.noble.org/news/publications/ag-news-and-views/2001/august/what-does- organic-matter-do-in-soil/ - 21.
Giller KE. Targetting management of organic resources and mineral fertilizes: Can we match scientists fantasies with farmers’ realities? In: Vanlauwe B, Sanginga N, Diels J, Merckx R, editors. Balanced Nutrient Management Systems for the Moist Savannah and Humid Forest Zones of Africa. Wallingford, U.K.: CAB International; 2002. pp. 155-177 - 22.
Palm CA, Myers RJK, Nandwa SM. Organic-inorganic nutrient interaction in soil fertility replenishment. In: Buresh RJ, Sanchez PA, Calhoun F, editors. Replenishing Soil Fertility in Africa. Madison, Wisconsin: Soil Science society of America; 1997. pp. 193-218 - 23.
Vanlauwe B, Wendt J, Diels J. Combined application of organic matter and fertilizer. In: Tian G, Ishida F, Keating JDH, editors. Sustaining Soil Fertility in West Africa SSSA Special Publication no. 58. Vol. 9 (Jan–Jun 2006). Madison, USA: Soil Science Society of America; 2001. pp. 247, 12-280, 18 - 24.
Myers MG, McGarity JW. The urease activity in profiles of five great soil groups from northern New South Wales. Plant and Soil. 1968; 28 (1):25-37 - 25.
Palm CA, Gachengo CN, Delve RJ, Cadisch G, Giller KE. Organic inputs for soil fertility management: Some rules and tools. Agriculture Ecosystems and Environment. 2001; 83 :27-42 - 26.
Kapkiyai JJ, Karanja NK, Woomer PL, Qureshi JN. Soil organic carbon fractions in a long-term experiment and the potential for their use as a diagnostic assays in highland farming’s of Central Kenya. African Crop Science Journal. 1998; 6 :19-28 - 27.
Kuzyakov Y, Friedelb JK, Stahra K. Review of mechanisms and quantification of priming effects. Journal of Soil Biology and Biochemistry. Institute of Soil Science and Land Evaluation, University of Hohenheim, D-70593 Stuttgart, Germany. Institute of Organic Farming, University of Agricultural Sciences, A-1180 Vienna, Austria; 2000; 32 :1485-1498 - 28.
Vanlauwe B, Giller KE. Popular myths around soil fertility management in sub-Saharan Africa. Agriculture, Ecosystems and Environment. 2006; 116 (1-2):34-46. DOI: 10.1016/j.agee.2006.03.016 - 29.
Brempong MB, Opoku A, Ewusi-Mensah N, Abaidoo RC. Evaluating added benefits from combined cattle manure and fertilizer application in am maize cropping system. Journal of Environmental Science and Engineering. 2017; B6 :34-40 - 30.
Denchak M. Global Climate Change: What You Need to Know. 2017. Available from: https://www.nrdc.org/stories/global-climate-change-what-you-need-know - 31.
Nwankwoala HNL. Causes of climate and environmental changes: The need for environmental-friendly education policy in Nigeria. Journal of Education and Practice. 2015; 6 :30 - 32.
Slave C, Man C. The contribution of human activities to climate changes. In: Agrarian Economy and Rural Development—Realities and Perspectives for Romania. 3rd Edition of the International Symposium, October 2012, Bucharest. Bucharest: The Research Institute for Agricultural Economy and Rural Development (ICEADR); 2012. pp. 292-295 - 33.
Haile W, Tesfamaraiam AA. Potential of local plants as a source of NPK on small holder fields in Southern Ethiopia. Technical Report. ISBN: 978-9988-633-73-82013. pp. 8-10 - 34.
Misselbrook T, Bittman S, Cordovil CMdS, Rees B, Sylvester-Bradley R, Olesen J, Vallejo A. Field Application of Organic and Inorganic Fertilizers and Manure. Draft Section for a Guidance Document. 2019 - 35.
Bationo A, Waswa B, Okeyo MJ, Maina F, Kihara J. Innovations as key to the gree revolution in Africa. Exploring the Scientific facts. 2007; 1 :123-133 - 36.
Olsen SR, Kemper WD. Movement of nutrients to plant roots. Advances in Agronomy. 1968; 20 :91-149 - 37.
Gentile R, Vanlauwe B, Chivenge P. Interactive effects from combining fertilizer and organic residue inputs on nitrogen transformations. Journal of Soil Biology and Biochemistry. 2008; 40 :2375-2384