Organic matter management (OMM) is a way of maintaining and/or increasing soil fertility. Sufficient amounts of organic matter (OM) improve storage and provision of nutrients, contribute to a porous soil structure that increases infiltration of water into the soil and that retains the moisture containing soluble minerals. It also improves aeration of the soil. Two relevant processes influence the amount of OM in soil: the addition of OM (roots, surface residue, manure, etc.) and the loss of OM through harvest, decomposition (by microbes, enhanced through tillage, etc.) or erosion. Both the production and accumulation of OM as well as the decomposition are accelerated in the warm and humid conditions of tropical climates.
Significance of OMM in relation to soil fertility, water management and climate risk adaptation
OMM as a measure to enhance soil fertility is an integral part of devising farming- and cropping systems. The design principle aims at maintaining a cycle of organic matter between soil, plants and animals, creating equilibrium between in- and outputs at the field and farm level.
In tropical smallholder agriculture many useful practices of OMM can be found, but they are often not sufficiently adapted to respond to current changes like shrinking farm sizes etc. The lack of OM results in soil exhaustion and declining productivity, which in turn worsens the water-holding capacity of soils. The key to OMM is sufficient biomass production and a nutrient balance on field and farm-level.
By increasing soil fertility OMM contributes to food security and higher economic returns, which is mainly a result of mobilising local farm resources. In most cases mineral fertilisers show a better response level when combined with organic fertilisers (saving on costly external inputs). OMM provides smallholders with the most important locally available means of maintaining and generating soil fertility.
The significance of OM for agriculture water management lies not only in improved soil properties like infiltration-rate and water-holding capacities but also in a number of OMM practices which increase water harvest potentials (hedgerow cropping, mulching etc) and reduce unproductive water losses (evaporation, etc.).
OMM provides a number of effective responses to the current challenge of adapting farming practices to the extremes caused by climate change (rainstorms, flooding, drought). Humus-rich soils are less vulnerable to erosive rains and drought. Farms designed with the aim of creating maximum biomass production and storage (agroforestry, multiple cropping) add to the protection of soils and create carbon-rich agro-ecosystems (carbon sinks of living and dead organic matter). In irrigation, agriculture watershed management is important in order to reduce flooding or drought. Hillsides managed with the principles of OMM can contribute to this.
Impact on soil parameters
The properties of tropical soils are greatly variable and the production and decomposition of OM highly dependent on climate. These two factors have an influence on how far soil parameters are affected through OMM. The main effects are as follows:
- OM affects the cation exchange capacity (CEC) of a soil. In very sandy or highly- weathered and acid soils humus provides almost the entire CEC.
- OM provides a slow-flowing source of plant nutrients that are contained in the plant tissue and are released through its decomposition (mineralisation).
- OM contributes to improvement of soil structure (enhancement of soil life, increasing aggregate size and with it the macro-pore-space), reducing erodibility of soils.
- Through its impact on soil structure higher amounts of rainwater can infiltrate into the humus-rich soils (infiltration-rate)
- The retention capacity of water (field capacity) and the available water capacity usable by plants are influenced by the OM content of the soil.
Optimal C-contents of soils are generally higher in humid climates than dry ones (6% - 1,5% C ). Continuous cultivation without restitution of OM results in rapid decline of fertility of most tropical soils.
Farm-sources of organic matter
The first source to be found are the residues left by crops and weeds on the field. They can stay on the field or be removed and transformed into compost or animal manure. In most smallholder farms the amount of crop residues produced on the farm is not sufficient for maintaining soil fertility. When this is the case, additional biomass can be produced in various ways:
- Cultivating green manure and/or cover crops as part of regular crop rotations
- Agroforestry and hedgerow cropping practices
- Production on specific plots may be destined to grow biomass for another field (for example for mulching of coffee plantations)
- Fields with fodder crops are also a source of organic matter (via the animal stomach). Grazing outside the farmed area may benefit soils inside the farm when animals are kept inside for part of the day or night (dung left during stabling, kraaling)
There are manifold practices and ways to return the produced biomass back to the field depending on the farming system and site conditions. A general rule for most tropical smallholdings is as follows: produce the maximum amount of organic matter which can be recycled to the soil to preserve soil fertility and increase the productivity of the farm.
Plant residues for no-tillage systems and for incorporation into the soil
Residues of crops and weeds (leaves, stems, fruits) can be used for incorporation into the soil or for mulching. The effect will depend largely on the amount of biomass left after harvest (above ground biomass, but also roots, microorganisms). The amount is determined by the type of cropping system, by the harvest-index of crop varieties, by yield and fertility status of the soil. In case, for example, of a rich maize harvest, the residues might be sufficient to provide enough mulch cover of the soil to protect it until the moment the subsequent crop has covered the soil. The new crop can be directly sown into the crop residue layer (shredded / chopped or not) after having treated the weeds (often with the help of herbicides). Specific tools and machines are available for this purpose. The technique is widely used in intensive, mechanised farming and has the advantage of not having to till the soil. The soil is continuously protected against erosive rains as well as from the accelerated breakdown of OM that goes along with soil tillage. (For further benefits see mulching.)
The alternative to the no-tillage method is to incorporate the residues into the upper layer of soil after harvest or before sowing for the following growing season. In mechanised agriculture bulky plant residues like maize straw are shredded and then integrated into the soil by shallow ploughing or with a rotary tiller. The crop type impacts on the organic matter status of the soil. When incorporating material like straw with wide C/N-ratios an application of small amounts of N-fertilizers is needed to avoid the fixation of nitrogen in the microorganisms (N- block). Crop residues are mostly integrated with other fertilisers like manure or mineral. (For further effects see green manure.)
Limiting factors: Smallholder farmers are often eager to remove plant residues from the field as they are valued for various purposes, like animal fodder or for bedding, thatching of roofs etc.
Compost (to be written)
Definition:
Principles of composting
Techniques of compost production
Application of compost
Effects on soils and crops
Potential and limitations
Animal manure
Animal manure is made from animal excrement like droppings and urine with the addition of bedding material and fodder residue in different combinations. The excrement can also be used in liquid state (slurry) for fertilising. There are many practices to produce and use manure. They are determined by the type of livestock, the way the animals are kept (from collection of droppings of grazing animals to permanent stabling), the process of producing the manure and the sources of fodder and bedding material. In certain farming systems it is possible to use animal manure as the sole source for maintaining soil fertility.
The effect of manure on soil fertility and plant growth is highly variable and determined by several factors:
(i) the quality of manure,
(ii) the available quantity and
(iii) the application and spreading techniques
Quality: The type and proportions of raw material from which the manure is made and the practise of preparing the manure determine its content of nutrients and organic matter. The nutrient content of solid and liquid excrement varies considerably according to the type of animal. During storage the fresh manure undergoes a rotting process, which narrows the C/N-ratio, builds up humic substances, mineralises OM. The process of collecting and storing the manure can result in a higher concentration of nutrients in the manure, and determines the level of loss (leaching through rains, loss of OM through mineralisation). Animal manures usually possess a higher concentration of nutrients than compost from plants.
Available quantity: This largely depends on the amount of fodder available and the livestock number that can be sustained from it on the farm. An extensive keeping system of grazing and collection of droppings may only produce a tenth of what can be produced from the same number of animals kept permanently in a stable with sufficient bedding material. The highest quantities are obtained in deep-litter stabling where the animals stand on their manure (needing high quantities of bedding) thus providing good storage conditions and protection of manure against rains.
Application and spreading techniques (valid also for composting): Quantities and intervals of application depend on the crop rotation. Manure should be given to those crops in a rotation which show the highest response. Subsequent crops will feed on the carry-over effect from the previous season. In arable crops one often finds 15 – 30 t/ha of manure every third year, whereas in intensive horticulture manure is applied every growing season in much higher quantities. Manure should be spread evenly over the field (no clumps) and get a shallow working-in. There are also practices to apply manure to single plants for a localised impact. This helps to save manure while getting a good return.
The systematic integration of animal husbandry into agriculture through fodder production, stabling and efficient storage and application techniques provides a good opportunity to intensify production in smallholder agriculture with the help of larger quantities of an effective organic fertiliser. At the same time one has to take into account that the amount of work related to the transport of manure and fodder production is a limiting factor.
Green manure / cover crops
Green manure/cover crops (GMCCs) are plants that are grown in order to provide soil cover and to improve the physical, chemical, and biological characteristics of soil. GMCCs may be sown independently or in association with crops. In general, green manure/cover crops are used to pursue the following objectives:
Provide soil cover for No-Tillage (reducing unproductive evaporation of water from soil and lowering soil temperature)
Provide soil cover for no-tillage (reducing unproductive evaporation of water from soil and lowering soil temperature)
Increase water infiltration and reduce run-off and protect from splash and run-off erosion of soils from erosion
Reduce weed infestation
Add biomass to soil (and nitrogen when using leguminous plants) and feed soil life
Improve soil structure
Promote biological soil preparation
Reduce pest and disease infestation
Substitute other fertilisers
Mulching
Mulching consists in complete or partial soil-coverage mainly using layers of plant material, such as crop residues (see above). (Mulch with plastic sheets is not considered here.) Mulching is usually applied to intensive crop production, in gardens and orchards and with commercial crops such as coffee, pineapples and bananas, as it is labour-intensive when not mechanised.
Mulch materials: crop residues and weeds (available in situ, less transport, but often not in sufficient quantity, except for plants with high ratio of crop residues like bananas). Nearby sources from hedgerows, field trees or plants from erosion control strips can be used without too much labour input for transport. Special mulch production plots for high-value crops like coffee are used when the crop provides high income. The properties of mulch material have to be considered: physical properties like large and hard material have to be chopped before applying, densely-layering material like fresh grass with more narrow C/N-ratio decompose more rapidly than bulky, loosely layering aerated materials. This means that the aim of physical protection of the soil and that of plant nutrition might be in competition.
Amount and intervals: There are only general rules. Mulching is dependent on local climate, crops and properties of mulch material. The thickness of layer affects the ground cover and with it the protection of the soil. But to allow rainfall to reach the soil the layer should not be too thick. . Light rains might be absorbed by too thick a mulching layer. Material with speedy decay requires regular replacement or thicker layers if permanent cover is intended (permanent crops like banana plantation). Layers of 5 to 15 cm are usual. Green material in thin layers under humid and warm conditions may decay within two months. But there might be no need to replace the layer as crop stands form an effective soil cover several weeks after sowing. Attention has to be paid to weed propagation through seed or fresh cuttings when applying mulch material from outside sources.
Timing: Mulching should be applied to annual crops before or shortly after sowing / planting, depending on the technique. In the case of permanent crops it should be in place before first intensive rains start.
Spatial arrangement: Total surface cover is often used, but this needs the highest amount of mulch material. The alternative is mulch strips covering the area on both sides of seed-rows or every second row (not possible with traditional broadcasted sowing). It is also possible to mulch single but tall plants when they are widely spaced, reducing the amount of mulch needed per field.
Specific benefits: As with cover-crops, there are the particular benefits of protecting the soil against run-off loss of water and top-soil, reducing splash erosion, maximizing water infiltration through improved soil structure and physical barriers to surface run-off, reducing evaporation losses of water from soil, lower soil temperatures, control of weeds, and increase of OM content in soil. The effects on crop yield also depend on the speed of mineralisation of the mulch (Müller-Sämann, 1986).
Agroforestry and hedgerow cropping
Definition: Agroforestry integrates trees into agriculture for the purpose of delivering products (fruits, forage, wood, etc.) and services like windbreaks, shading for improved microclimate and less evaporation, but also in terms of supplying important quantities of organic matter through their root biomass, shedding and harvesting of leaf biomass. Agroforestry systems can be designed in such a way as to maximise benefits for soil organic matter, for controlling run-off and splash erosion and for improved infiltration of water.
Practices: Agroforestry methods were already widely practiced in traditional agriculture. Modern techniques aim at optimising agroforestry through the adequate choice and spatial arrangements of trees and management of the tree component in order to reduce competition with the field crops under the trees. A widely applied technique consists in hedgerow-cropping (alley-cropping), where hedges are massively pruned before the cropping season and leaf biomass integrated into the soil or left as mulch.
Specific benefits are the sheltering of agriculture crops, the enlargement of the root and above-ground borders of the cropping systems. Deep-rooting trees might tap nutrients and water in deeper strata of the soil than the usual crops. There is a high-production potential of biomass for all purposes of organic matter use: mulching, incorporation into the soil, composting, for animal feed and for manure production, but also for fruits. When not adequately managing the tree-component competition with crop yields for light, water, nutrients will be higher.
References
For basic knowledge regarding organic matter management:
http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html
Publication of FAO / GIZ, 2011: Green manure/cover crops and crop rotation in Conservation Agriculture on small farms. While written in relation to a development project in Paraguay, the publication also provides basic knowledge regarding no-tillage, the use of green manures, and the practice of crop rotation on small farms.
http://www.fao.org/fileadmin/user_upload/agp/icm12.pdf
Literature
W. E. H. Blum: Bodenkunde in Stichworten, 2007, ISBN 3-443-03117-x
Dupriez H. and P. de Leener: Ways of Water. 1992. ISBN 2-87105-011-2
Dupriez H. and P. de Leener: African Gardens and Orchards. 1989 ISBN 0-333-49076-2
Kotschi, J. et al.: Standortgerechte Landwirtschaft zur Entwicklung kleinbäuerlicher Betriebe in den Tropen und Subtropen, GTZ 1984, ISBN 3-88085-264-2
Kotschi, J. et al.: Standortgerechte Landwirtschaft in Ruanda. Zehn Jahre Forschung und Entwicklung in Nyabisindu, GTZ, 1991, ISBN 3-88085-464-5
Müller-Sämann, Karl M.: Bodenfruchtbarkeit und standortgerechte Landwirtschaft. Maßnahmen und Methoden im Tropischen Pflanzenbau, GTZ 1986, ISBN 3-88085-317-7
Pietrowicz, P. et al: Agriculture Ecologique au Rwanda. GTZ 1998, ISBN 3-8236-1294-8
