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Chapter 5

Organic Resources for Integrated Nutrient Management

in Western Kenya

J. Robert Okalebo and Paul L. Woomer

Researchers provide various definitions and explanations for Integrated Nutrient Management (INM).  Some researchers argue that it is a practice whereby both organic and inorganic nutrient resources are applied simultaneously to increase crop yields.  Other scientists argue that in addition to organic and inorganic inputs, other agronomic practices of planting good seed, early planting, weed, pest and disease control, should be the components in the definition of INM.  A third school of thought stresses that nutrient cycling is the centre of INM.  In a review by Franzluebbers et al. (1998), INM attempts to "combine the old and new methods of nutrient management into ecologically sound and economically viable farming systems that utilize available organic and inorganic sources of nutrients in a judicious and efficient way".  By this definition, INM optimizes all aspects of nutrient cycling.

From the biological aspect of soil fertility management, INM seeks tight nutrient cycling with synchrony between nutrient demand of crops and nutrient release within the soil while minimizing loss of nutrients through leaching, runoff, volatilization and immobilization (Figure 1). Janssen (1993) noted that sustainable soil management must involve the judicious application of both mineral and organic nutrient resources within ecologically sound production systems, such as crop rotation, mixed cropping and agroforestry.

In western Kenya, however, mixed cropping, with minimal nutrient inputs is the norm and crop rotation is secondary to continuous maize cropping. Too few farmers recognize the benefit of improved soil fertility through nutrient recycling. Leguminous intercrops and short fallows contribute nitrogen to the soils through biological nitrogen fixation, but this process and root nodulation is not widely recognized as beneficial by farmers.  On the other hand, mineral fertilizers and livestock manure are considered important inputs, but are usually available in too short supply.  The advantages and disadvantages associated with the use of both organic and inorganic nutrient resources are summarized in Table 1.

Types and Qualities of Organic Inputs

Because of their incomplete understanding by farmers, organic inputs and their informed management offer a wide range of opportunities within smallhold farming systems. In farms where manure or inorganic fertilizers are not applied, weeds, crop residues and roots remaining in fields contribute recycled nutrients through decomposition.  Crops acquire nutrients from deeper soil horizons and soil parent materials as well. Therefore, the size of crop yields and the duration when acceptable crop harvests are obtained in absence of nutrient inputs are dependent upon the inherent fertility of soil, including the past management of the farm, local climatic conditions, especially the amounts and patterns of rainfall distribution within seasons, and the nutrient requirements of the crop.  Other factors include the abundance, frequency and types of nitrogen-fixing organisms and the loss of nutrients due to removal, incorporation and grazing of crop residues.  The high nutrient demand by maize removes large quantities of nitrogen (N), potassium (K) and phosphorus (P) from soils while nitrogen-fixing legumes may result in a net nitrogen gain in soils. Organic inputs available at the farm level are often inadequate to supply all nutrient needs (Probert et al., 1992).  This is reflected by the regular importation of farmyard manure from pastoral lands in Kajiado to Kiambu district in Kenya (Lekasi et al., 2001) by farmers growing higher value crops for Nairobi and export markets, despite the abundance of nearby local smallhold dairy enterprises.

Table 1. A comparison between inorganic fertilizers and organic inputs (after Woomer et al., 1999)

The quality of organic nutrient resources has a significant role on the effectiveness of the materials on crop yields.  Organic input quality refers to the nutrient content and the concentration of lignin and polyphenol, two secondary compounds that regulate decomposition and bind with its products.  Large variations in quality occur among sources of manure, including between nearby farms (Table 2).

Before the nutrients in organic inputs can benefit a crop, the  materials must undergo decomposition and nutrient mineralization.  Inputs that are higher in nutrients and lower in lignin and polyphenol, especially those with C:N ratios less than 10, will rapidly decompose and release nutrients into soils. Green manure decomposes more readily than crop residues and woody tissues (Waigwa and Okalebo, 1998; Figure 2).  Variations in decomposition and nutrient release patterns may have either positive or negative effects. In fields of fast-growing annual crops such as maize, there is need for rapid nutrient release to adequately provide nutrients at the early stages of growth, whereas with perennials, a more steady nutrient release is required to provide nutrients over time.

A wide range of organic nutrient sources is available to farmers but different types of organic inputs must be handled and applied in different ways for them to achieve their maximum effect.  The guidelines in Figure 3 integrate both the physical and chemical characteristics of organic inputs that influence their decomposition and nutrient release patterns. For example, the guidelines indicate that when the N content of the organic material is >2.5%, as in leguminous green manure, one is advised to apply this material directly at a recommended rate without additional N input from inorganic fertilizer. Unfortunately, higher quality organic resources are too often in short supply, requiring that farmers apply low quality materials (e.g. N <2.5%, C:N ratio >25) such as crop residues (Table 3).  Reduction in crop yield from these materials after their incorporation into the soil is not uncommon, particularly when added in large quantities (Okalebo et al., 1997). 

Table 2.  The nutrient content of different cattle manures from some farms in Kenya (after Probert et al., 1992).

Table 3. Effects of crop residue and nitrogen fertilizer additions on the grain yield of maize grown on a Ferralsol near Eldoret, Kenya.

WS = wheat straw, SYT = soybean trash applied at 2 t/ha; N = nitrogen applied as urea at 80 kg N/ha; WS contained 0.67% N, 0.09% P, 8.63% lignin and 1.11% polyphenolics; SYT contained 1.07% N, 0.20% P, 9.31% lignin and 1.17% polyphenolics.

The negative effect of low quality organics is explained in terms of nutrient immobilization because the microorganisms active in organic matter decomposition (e.g. bacteria, fungi) also obtain their nutrient requirements from the decomposing organic materials.  Microbes in effect have “priority access” to applied resources because of their size, abundance, distribution and metabolism.

Case Studies on the Use of Organic Inputs

Large amounts of maize stover and wheat straw are produced in the high agricultural potential areas of Uasin Gishu and Trans Nzoia districts in Kenya (Lwayo et al., 2000).  In the sugarcane growing areas, similar quality residues are also available in large quantities.  Disposal of these materials may in some cases even pose problems to land managers.  These  materials are often  left in   the field, fed to cattle or used as cooking fuel.  Frequently, residues remain in the field to dry and then burned to facilitate tillage operations.  Burning recycles some nutrients, but loses most carbon, nitrogen and sulfur to the atmosphere.  Yet alternative, more environmentally-friendly methods of utilization are available. 

The large scale farmers with suitable machinery may chop the materials soon after harvests and incorporate them into the seedbed.  This system facilitates the decomposition and nutrient release from materials including the improvement of soil physical characteristics, such as soil structure and infiltration.  Decomposition and residual effects are enhanced by incorporating a small amount of mineral N with the residues and then plough the mixtures into the seedbed, preferably before the successive crop is planted.  With regard to incorporation of N into residues, a study in Uasin Gishu district investigated maize response to the addition of wheat straw and soybean trash.  These two residues have contrasting qualities and are also common within the district.  Treatments where mineral N was incorporated with these residues significantly out-performed those without N incorporation (Table 3).  Conservation tillage strategies retain chopped crop residues as surface mulch, and then direct seed into them, a management approach that relies upon specialized equipment and chemical control of weeds later in the growing season.

PREP-PAC is a product intended to correct the symptomatic low fertility patches common in croplands of western Kenya.  It is based upon the principles of integrated nutrient management (Figure 4).

Table 4. Maize grain yield at 3 sites in western Kenya (after Obura et al., 2001)

Low soil fertility patches result from nutrient removal  and their formation is  enhanced by continuous  cropping of land without the addition of external nutrient sources (Woomer et al., 1997).  A diagnostic survey in Vihiga, Busia and Bungoma districts showed that low soil fertility patches occupy between 10% and 30% of farm land and that the soil test parameters (pH, C, N and available P) were lower in these patches compared to the similar parameters obtained in soils within close areas where crops grew vigorously (Nekesa et al., 1999).  PREP-PAC consists of 2 kg Minjingu phosphate rock, 200 g of urea, 120 g of food legume seed, rhizobial inoculant, gum arabic sticker and lime pelleting material.  Instructions are provided in English, Kiswahili and local languages.  The product costs KSh 41 (US $0.55) to assemble.

PREP-PAC was tested on smallhold maize-legume intercrops in several low fertility soils of western Kenya.  Treatments were selected to determine the response of maize and N-fixing soybean intercrops to individual PREP-PAC components and their interactions (Table 4). The yields varied with the treatments and sites, and ranged from 1.1 to 4.8 t ha^-1.  Although the main PREP-PAC components (PR, urea and Biofix) applied individually or separately increased maize yields, the PR sole application gave the highest yield increases across the 3 sites, particularly in Kakamega with red soil of high clay content.  On the average, PR combinations with Biofix and urea resulted in significant maize yield increases, but the complete pack (PR + urea + Biofix) gave the highest yield increase of 205% above control in Siaya.  These yields resulted in 2.6 and 3.7 benefit to cost ratios in Bungoma and Siaya, respectively, good returns by any standard.  Fuller discussion on soybean yields is given in Obura et al. (2001). 

The practice of planting improved relay fallows with maize is promoted by the World Agroforestry Centre and other research organizations in western Kenya. The fallows continue to grow for several months following crop harvests and then the leaves and twigs are incorporated into the soil and stems are recovered for use as poles and cooking fuel.  Poor establishment of these fallows occurs on the least fertile soils, a situation that may be corrected through the strategic application of mineral fertilizers. 

On-farm studies were performed in western Kenya to determine the agronomic and economic viability of intercropping improved fallows of Crotalaria grahamiana and Tephrosia vogelii with maize and beans in the same season.  These studies indicated that the application of 30 kg N and 20 kg P ha^-1 as urea and Minjingu phosphate rock, respectively, greatly enhances the growth of both maize or beans due to greater biomass accumulation and incorporation of the improved fallow.  Incorporating the fallow with MPR increased the levels of nitrate in soils, resulting in better yields of maize over several consecutive cropping seasons (Ndungu, 2002).  Fallow biomass incorporated into soils at 2 t ha^-1 in conjunction with 20 kg P ha^-1 as Minjingu phosphate rock was an economically viable option for improved fallow technology, assuming that land availability does not restricting its adoption by farmers.

Conclusion

This chapter discussed the concept of Integrated Nutrient Management and the important role of organic resources within that strategy.  Several examples were provided where organic materials by themselves were unable to guarantee crop performance and sustain soil fertility but, the combination of organic and mineral inputs has resulted in greatly improved crop yields under smallhold farming conditions in western Kenya.  This improvement is due, in part, to manipulating soil biological processes in favor of better supply and timing of nutrient availability.  While the utilization of nutrient-rich organic materials is direct and straightforward, fuller use of low quality organic inputs, such as maize stover or straw, requires combination with other inputs in a manner that requires understanding by land managers.

References

Franzluebbers, K., Lloyd, R. and Juo, A.S.R. (Eds.). 1998. Integrated Nutrient Management for Sustained Crop Production in Sub-Saharan Africa: A Review.  TropSoils/TAMU Technical Bulletin 98-03, Texas A&M University, USA. 50 pp.

Giller, K. 2000.  Translating science into action for agricultural development in the tropics: An example from decomposition studies. Applied Soil Ecology 14:1-3.

Janssen, B.H. 1993.  Integrated nutrient management:  The Use of Organic and Mineral Fertilizers, pp. 89-105.  In: H. van Reuler and W. H. Prins (Eds.).  The Role of Plant Nutrients for Sustainable Crop Production in Sub-Saharan Africa, Ponsen and Looijen, Wageningen, The Netherlands.

Lekasi, J.K., Tanner, J.C., Kimani, S.K. and Harris, P.J. C. 2001.  Managing manure to sustain smallholder livelihoods in the East African Highlands.  HDRA Publications, Coventry, UK. 32 pp.

Lwayo, M.K., Okalebo J.R. Muasya, R.M. and Mongare, P.O. (2000).  A diagnostic survey on the utilisation of phosphate fortified wheat straw compost for increasing cereal production in Uasin Gishu district, Kenya.  African Crop Science Conference Proceedings 4:619-622.

Nekesa, P., Maritim, H.K., Okalebo, J.R. and Woomer, P.L. 1999.  Economic analysis of maize-bean production using a soil fertility replenishment product (PREP-PAC) in western Kenya.  African Crop Science Journal 7:585 - 590.

Ndungu, K.W. 2002.  Effects of Minjingu Phosphate Rock on Maize Yield and Changes in Soil Phosphorus under Improved Fallows in Western Kenya.  M.Ph. Thesis, Moi University, Eldoret, Kenya.

Obura, P.A., Okalebo, J.R. Othieno, C.O. and Woomer, P.L. 2001.  Effects of PREP-PAC product on  maize-soybean intercrop in the acid soils of western Kenya.  African Crop Science Conference Proceedings 5:889-896.

Okalebo, J.R., Simpson, J.R., Okwach, G.E., Probert, M.E. and McCown, R.L. 1997.  Conservation of soil fertility under intensive maize cropping in semi-arid eastern Kenya.  Proceedings of the African Crop Science Conference Proceedings 3:429-437.

Palm, C.A., Gahengeo, C.N., Delve, R.J., Cadish, G. and Giller, K.E. 2001. Organic inputs for soil fertility management in tropical agroecosystems: application of an organic resource database.  Agriculture, Ecosystems and Environment 83: 27-42.

Probert, M.E., Okalebo, J.R., Simpson, J.R. and Jones, R.K. 1992. The role of boma manure for improving soil fertility.  In:  M. E. Probert (Ed.).  A Search for Strategies for Sustainable Dryland Cropping in Semi-arid Eastern Kenya.  ACIAR Proceedings 41:63-70.

Waigwa, M.W. and Okalebo, J.R. 1998.  Decomposition and nutrient release from residue/prunings of Acacia mearnsii, Melitia dura and wheat straw in a ferralsol of Uasin Gishu, Kenya.  Proceedings of the 16^th Conference of the Soil Science Society of East Africa, Tanga, Tanzania, pp. 166-174.

Woomer, P.L., Okalebo, J.R. and Sanchez, P.A. 1997.  Phosphorus replenishment in western Kenya:  from field experiments to an operational strategy.  African Crop Science Conference Proceedings 3:559-570.

Woomer, P.L., Karanja, N.K. and Okalebo, J.R. 1999. Opportunities for improving integrated nutrient management by smallhold farmers in the Central Highlands of Kenya.  African Crop Science Journal 7:441-464.

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