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Chapter 1

Organic Resource Management in Smallhold Agriculture

John K. Lekasi

To articulate the principles of organic resource management, it is important to understand the types of materials that are referred to as organic resources. In agriculture, organic resources can simply be described as those organic materials that are used in agriculture as external or recycled inputs to produce crops either for subsistence or for commercial purposes.  This type of farming is particularly characterized by the addition of low-value external inputs into the system. Technologies used to manage organic resources and crop production are directed toward soil and water conservation and soil fertility maintenance. Materials that are commonly used in these farming systems include:

• fresh, dried or composted livestock and poultry manure

• crop residues that are recycled after a crop is harvested

• green manure obtained either on or off the farm

• biomass resulting from short to long-term fallows

• agro-industrial by-products such as coffee husks or sugarcane bagasse

• forest litter, bark or wood shavings

• coarse organic materials applied as surface mulches

The management of these organic resources varies in terms of preparation prior to utilization as inputs. For instance, management is a determinant of choice of application rate, method of application and whether to use them alone or in combination with other inputs. An understanding of the kind of benefit a farmer is likely to accrue from the use of the particular organic resource of choice can help guide on the type of management to invoke at any stage of crop production (Box 1). For example, optimization of biological activity and storage conditions both contribute to the quality of compost, and greatly affects its usefulness as an organic fertilizer. On the other hand, composting results in biomass loss during decomposition. Compost is better applied to soil or used in potting mixtures, while the original resource would better serve as surface mulch.

Organic Resource Utilization in Agriculture

The beneficial role of organic resources in crop production has been recognized for a long time. The capacity of organic resources to provide nutrients, especially nitrogen (N), phosphorus (P) and potassium (K) is one such benefit. Other benefits include an increase in cation exchange capacity (CEC), improved water holding capacity and infiltration rate, and decreased bulk density. This section describes the different roles organic resources play in crop production with special emphasis on composts and manure.

Nutrient release by organic resources

A widely recognized role of organic resources in agriculture is the supply of nutrients. When organic resources are applied as soil amendments, they decompose with a fraction stabilized as soil organic matter (SOM). There are several factors that affect efficient mineralization and subsequent utilization of nutrients by a growing plant. Availability of nutrients is a balance between two opposing processes namely, nutrient mineralization and nutrient immobilization. When organic amendments are applied to the soil, micro-organisms and soil fauna decompose the materials to release nutrients. At the onset of decomposition, the microorganisms require nutrients from the soil in order to metabolize organic materials. In the process, the microorganisms compete with the plants for the same nutrients: the process of nutrient immobilization. A high energy substrate that is low in nutrients will result in net immobilization as it decomposes. Others that are rich in nutrients will result in net mineralization. When the microbial population reaches a maximum and substrate becomes limiting, then nutrient mineralization increases. The net nutrient mineralization is the difference between the nutrient mineralization and nutrient immobilization when the former exceeds the latter. 

Earlier in plant growth, it is advantageous if the organic materials added to the soil mineralize nutrients slowly and the rate of nutrient mineralization increases as the plant growth progresses.  As the plant matures, it is expected that a good soil amendment will have released adequate nutrients for optimum plant growth. Closer synchronization of nutrient mineralization and plant nutrient demand ensures efficient utilization of organic inputs applied to the soil. Organic materials that mineralize too readily subject mineralized nutrients to losses through processes such as leaching and volatilization. On the other hand, organic materials that release nutrients later in the season will not benefit the plant or crop as it will have matured with inadequate availability of nutrients during the critical growing stages.  This example is most applicable to annual crops because perennials require a steady supply of nutrients during seasons with adequate moisture availability.

Organic resources and soil moisture conservation

Availability of adequate soil moisture is a factor that is critical at determining the amount of nutrients that are mineralized and absorbed by the plant.  Application of organic resources maintains moisture levels which are considered to be more favourable for plant growth.  For example, even under drought conditions, fields applied with manure, have been shown to retain moisture for a longer period than fields that have not received manure. 

Excessive soil moisture is not beneficial because it may cause leaching of nutrients beyond the reach of plant roots and an insufficient supply of oxygen to plant roots. Excessive soil moisture also results in anaerobic conditions that cause nutrients to occur in forms that are unavailable to most crops. Mineral soils that contain more soil organic matter tend to be better structured, resulting in improved drainage.

Soil moisture conservation can also be achieved by use of organic materials as surface mulches. Mulched organic materials later decompose and become additional sources of plant nutrients. Carbon dioxide is generated as a byproduct of decomposition.  This gas provides carbon during the process of photosynthesis and crop productivity is favored by increasing its concentration within the plant canopy. Organic mulches also create favourable conditions for soil macrofauna that serve as soil engineers by their channeling and burrowing activities.

Mulches suppress weed emergence and reduce the cost of weeding (Lekasi et al., 1999) as well as improving soil physical conditions. The beneficial effect of mulch in soil moisture conservation and other effects with respect to crop growth are demonstrated in the example of mulching cabbage with banana residues (Table 1), a practice that improves yield and promotes soil biological activity

Table 1. Application of banana mulch in conjunction with other management practices on cabbage yield

¹Banana residue mulch was applied at 15 t ha^-1 while plastic mulch was to conserve moisture.  ²Cabbage yield LSD_0.05 = 19.7. There was no significant difference on cabbage yield between the inorganic fertilizer and mulch treatments.

There are different sources of organic materials that farmers can use in soil management. The choice of organic materials depends upon the availability in the farm and their alternative uses. In the case of soil and water conservation, any material that can be used as mulch is most suitable regardless of its nutrient content. Good mulch should cover the soil adequately to minimize runoff and erosion. This will allow water infiltration and at the same time reduce weed infestation. Some organic materials used as mulch may be long or short lasting depending on the age and texture at the time they are applied. Other factors, such as termite infestation, can have negative effects on the utilization of organic materials in many locations. Organic materials used primarily for soil nutrient replenishments need to be considered more critically since the availability of the nutrients is controlled by many factors. In general, the amount and ease at which nitrogen is released from the organic materials is generally used as a measure of suitability of that material as a soil amendment.

Organic Resource Management and the Environment

Organic resource management should be practiced in such a manner that the environment is not harmed. This section gives a summary of attributes that are related to environmental issues with regard to management and utilization of animal wastes, with emphasis on intensive farming systems.  This type of production system often produces concentrated animal wastes. Livestock producers should particularly recognize the threat their operations pose to surface and ground water. Prevention of air and water pollution by animal wastes requires proven methods of source reduction, storage, preservation, distribution and utilization of plant nutrients in the animal wastes (Waggoner et al., 1995). The main sources of environmental pollutants are nutrients originating from animal excreta (feaces and urine) and compound derivatives after undergoing composting during storage. Gaseous loss of N from animal waste through volatilization and denitrification are potential sources of environmental pollution and greenhouse gas emissions.

Leached nutrients are transported in runoff and ground seepage and later deposited in groundwater and streams. The extent to which plant nutrients in animal waste are readily leached also depends on the nature of the compounds carrying the nutrients before they dissolve in the transportation medium. A study reported by Pakrou and Dillon (1995) has shown urinary-N leaching to 15 cm of up to 40% for irrigated pasture and up to 24% for non-irrigated pastures within one day of application. The remainder of the remaining urinary-N was converted from urea to ammonium within the same day. The fate of such ammonium depends on the soil conditions and is greatly susceptible to leaching losses if soil moisture is high, or to gaseous loss as ammonia if the soil pH is basic (>7.0).

These studies have demonstrated that livestock systems have the potential to contaminate the environment if animal wastes are not handled in a manner that would minimize N volatilization and denitrification and also reduce leaching of other nutrients. On a global scale, agriculture emits large amounts of greenhouse gases and leaches nutrients into surface and ground waters.  Care must be exercised to restrict these negative environmental impacts when organic resources are being processed for use as plant nutrients.  For example, water hyacinth (Eichhornia crassipes) invasion of Lake Victoria has been attributed to leaching of nutrients from the surrounding urban and rural areas into the lake, thereby encouraging proliferation of this water weed to uncontrollable levels (Woomer et al., 1998), yet techniques are available to process water hyacinth into useful products.

Organic Resources

The roles of organic resources in improved and sustained crop production are physical, chemical and biological in nature.

Physical functions:- Organic matter (OM) binds soil particles into aggregates, giving rise to good soil structure and associated soil porosity, important in relation to root proliferation, gas exchange and water retention and movement.  Crop residues or tree prunings left on the surface of the soil will reduce soil loss through erosion and subsequent humification of these materials.  Beneficial effects of surface OM include reductions in soil temperatures, splash, crusting and compaction as well as soil moisture storage.

Chemical functions:- Continuous organic inputs to soils enhance plant nutritional status, particularly inrelation to direct supply of nitrogen (N), phosphorus (P), sulphur (S) and potassium (K).  There is evidence that organic N and S are readily mineralized to inorganic forms more readily than the organic P.  Incorporation of organic N and S protects these elements from leaching.   The slow

release of N, S and P through mineralization is synchronized with plant requirements, to a certain extent, offering the prospect of developing management practices for improving soil fertility and nutrient supply through timed application and resulting decomposition patterns.  Organic inputs enhance cation exchange capacity (CEC) particularly in sandy soils.  Organic inputs reduce aluminum (Al) toxicity and P-fixation in strongly acid soils with oxide mineralogy.

Biological functions:- SOM stimulates the activities of macrofauna and microorganisms in soil which in turn contribute to nutrient release.  Earthworms influence physical and biological conditions of the soil, which interact to effect nutrient supply to plants.  Litter and SOM are the main food for earthworms.  Organic inputs stimulate soil microbial biomass which may in turn immobilize nutrients, either temporarily or longer depending on the nutrient concentration of the inputs.  The decomposition process is catalyzed by the soil microorganisms and fauna (termites, mites and collembola), and the microflora (bacteria, fungi), which together, constitute the soil biomass.  In the tropics, SOM decomposes rapidly due to higher temperatures and favourable moisture conditions but decomposition is slower in drier or cooler environments.

Conclusion

A profound transition is underway in East Africa as smallhold farmers move from subsistence, cereal-based farming to mixed-enterprise, market-oriented systems (Figure 1).  In subsistence systems, relatively few resources are available and their use is straightforward, with crop residues from maize-legume intercrops fed to relatively few livestock and the obtainable manure applied to home gardens.  This trend leads to nutrient depletion in crop outfields, particularly when land is no longer available for natural fallow.  As their systems diversify, a wider range of organic resources become available to land managers and more possible uses emerge.  Those resources that serve as feed for confined animals are generally used for that purpose.  Livestock manure is more fully recovered, and generally composted and applied to cash crops, but is also available to fodder and field crops.  Income generated through cash cropping allows for purchased inputs, particularly feed and fertilizer. Low fertility patches or fields are corrected through the use of specialized technologies including strategically combined fertilizers and short-tem improved fallows.  Field and fodder crops are more frequently rotated.  Composting makes better use of assorted organic resources, and allows for improved nutrient contents of otherwise lower quality materials.  Orchard and other tree enterprises are initiated that not only generate revenue, but also biomass that is used elsewhere on the farm.  Household enterprises may also extend beyond agriculture, particularly into cottage industries.  This sort of diversified organic resource management leads to better lives and contributes to rural transformation.

References

Kirchmann, H. 1985. Losses, plant uptake and utilization of manure nitrogen during a production cycle. Acta Agriculturae Scandinavica (Suppl. 24), 77 pp.

Lekasi, J.K., Bekunda, M.A, Woomer, P.L and Tenywa, J.S. 1999. Decomposition of crop residues in banana-based cropping systems of Uganda. Biological Agriculture and Horticulture 17:1-10.

Mugwira, L.M. and Murwira, H.K. 1997. Use of cattle manure to improve soil fertility in Zimbabwe: past, current and future research needs. Soil Fertility Network for Maize-Based Cropping Systems in Malawi and Zimbabwe. Working Paper No. 2. Soil Fertility Network, Harare.

Pakrou, N. and Dillon, P. 1995. Preferential flow, nitrogen transformation and N-15 balance under urine-affected areas of irrigated and non-irrigated clover based pasture. Journal of Contaminant Hydrology 20:329-347.

Waggonner, D.K., Nipp, T.L., Harris, B.L., Waggoner, D.B. and Weber, G.M. 1995. Protection of water quality: A multi-component challenge to livestock producers. Journal of Sustainable Agriculture 6:157-176.

Woomer P.L., Bekunda, M.A., Karanja N.K., Moorehouse T. and Okalebo J.R. 1998. Agricultural resource management by smallhold farmers in East Africa. Natural Resources 34(4):22-33

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