Improving soil organic matter

• Growing green manure, mostly legumes, for the amount of biomass they build. Before fl owering they are cut and worked into the soil.
• Intercropping cover crops such as velvet bean, Tithonia, lablab and others as living mulch. The cover crop is regularly slashed, when it competes too much with the main crop.
• Mulching with especially hard-to-compost or woody materials, like dry crop residues or green manure crops, which have been grown to maturity, also can ontribute to a slow increase of soil organic matter over time.
• Trees and shrubs for agroforestry can be grown in the fi elds with crops where they are regularly pruned and the branches are used as mulch. They may also be planted on the edges of a field or on fallow plots.
• Residues from harvested crops in the form of husks, leaves, roots, peelings, branches and twigs should be returned to the fi elds either as compost, as mulching materials, or for incorporation into the soil.
• Depending on the fi nancial situation of the farm, additional materials from agro-processing like wood shavings, or coffee or rice husks, or from food industry like seed cakes can be purchased.
• Integration of livestock helps to quickly improve soil organic matter, when livestock excreta and bedding are properly recycled.

The amount and the quality of organic matter supplied to the soil influence the content of organic matter in the soil. A regular supply of organic matter provides the best conditions for balanced plant nutrition. Estimates say that in humid tropical climates 8.5 tons, in sub-humid climate 4 tons, and in semi-arid 2 tons of biomass are needed per hectare and year to maintain soil carbon levels of 2, 1 and 0.5 % respectively.
Burning organic residues and standing dead biomass (such as crops left on a field) is a crime to the environment! All the benefits that may be derived from incorporating organic matter are lost and, if the plant material is burned, the atmosphere is polluted. The ashes contain nutrients that are directly available to the plants; however, large amounts of carbon, nitrogen and sulphur are released as gas and are lost. The nutrients in the ash are also easily washed out with the first rain. The burning also harms benefi cial insects and soil organisms.

The majority of the farmers by far do not exploit the potentials of on-farm production of organic soil inputs. Realizing the potentials of farm-own resources can be essential for long-term sustainability of the farm, as it helps to reduce the cost of buying organic materials like manure or mineral fertilizers. Instead of buying farm inputs the farmers may use part of the savings for buying seeds for green manures and feed plants or for purchasing own livestock.
Due to limited landholdings and lack of livestock, some of the farmers may be unable to produce adequate quantities of green manure and compost. These farmers will depend on outside sources of organic materials to maintain the fertility of their soils.

Green manures can be grown for more than one season in the following ways:

• They can be planted in a bush-fallow system to restore poor soil. They are broadcast and left to grow uninterrupted for as long as required. The longer the period, the greater the benefit on soil fertility is.
• They can be grown on new land before it is prepared for use, especially to help control diffi cult perennial weeds like couch grass and spear grass.
• They can also be grown to produce large amounts of green plant material, which are then cut and carried to other fi elds, fed to livestock or used for composting. An example of such a perennial species is alfalfa (Medicago sativa).

Agroforestry involves growing trees or shrubs together with crops. The trees or shrubs act as long-term green manures, where the leaves are spread on the field and dug into the top soil layer. For example, every sixth row, pigeon pea trees are intercropped with traditional grain crops. At the end of the season the crop residues are gathered under the pigeon pea plants and left for decomposition together with pigeon pea leaves. After six months the mixture is spread on the surface and incorporated into the soil.
Regular pruning of agroforestry trees, before or as soon as they flower, increases the amount of green materials obtained and reduces competition with the main crop.

The composting process should be conducted in a place that is easy to access for easy transport of materials to the composting site and close to the fi elds where the compost is to be used after production, and next to a water source. A well-drained and levelled ground minimizes the risk of sieving out of nutrients by run-off rainwater. Natural shade such as a tree or a built shade reduces evaporation. An adjustable structure may allow its removal during rain.
There should be an appropriate distance from short term crops such as vegetables to avoid the risk of contamination, especially if animal waste is used.

Ideally composting plant material is a mixture of 50 % different fresh green material and 50 % of dry material. The rate of coarse material should not exceed 10 %. If too much fresh material is used, aeration of the heap will be poor. As a result the heap will start to smell and nitrogen will be lost. If too much dry material is used, bacteria lack food and the composting process will not start. Larger quantities of dry material are thus best left in the fi eld to protect the soil surface from drying out and being washed away.
As most crop residues are low in nitrogen, sources high in nitrogen such as leaves of green legume plants or prunings from leguminous trees or legume stalks may be used to insure suffi cient nitrogen for the composting process. Also materials from shrubs such as tithonia, gliricidia, leucaena, sesbania, crotalaria and lantana leaves are good materials to use.
Whenever possible, plant materials should be composted together with animal manure. Addition of animal manure accelerates the composting process and results in compost with higher fertilizer value. Dung can be dissolved in or mixed with water and poured over the compost heap when preparing the compost. Urine and slurry, both rich in nitrogen, can encourage decomposition of dry material when poured over it.
Ashes can be spread in thin layers between the other materials. Too much ash, however, can result in gaseous nitrogen losses. Some earth or old compost can be used as well. Earth will adsorb escaping nitrogen well.
Where soils have the tendency to fi x phosphate, ground rock phosphate is best added to the compost, as it will be more readily available to the plants than if it is added to the soil directly.
Lime can be added in small quantities, but is in general not necessary for the composting process.
Tools needed for composting include a hand hoe, machete (panga), stick pegs, spade or forked hoe, watering can, wheelbarrow, sharp stick or compost thermometer (to monitor the temperature changes in the compost heap). For watering, a watering can or a sprayer should be used rather than a bucket, as this allows the material to better soak up the water.
Materials that should not be used for composting include materials from diseased or pest infested plants or plants that have been sprayed with pesticides or herbicides, materials with hard prickles or thorns, which may hurt the persons handling the compost. Persistent perennial weeds should not be composted either. Instead they should be destroyed by spreading them out in the sun to dry, or even burning. The dried material or ashes can then be added to the compost heap. Non-organic materials such as metal or plastic, rubber, leather and textile materials cannot be composted.

There are different methods for making compost based on different approaches and origins. They include the Indore and the Bangalore method, which were developed in India, the heating process/block method, the Chinese high temperature stack, the pit, trench, basket or the Boma composting. Each of these methods has advantages and disadvantages.
In the Bangalore method, the composting materials are mixed with urine, slurry or dung. The heap, once set up, is plastered with a layer of mud and is not turned. Due to the mud layer, the composting process becomes semi-anaerobic after a few weeks. The method is simple to use and needs little labour and water. It has fewer nutrient losses than the Indore-method, but may not destroy all diseases and needs more time to reach maturity.
In the Indore method the heap is turned twice. It is more labour intensive and needs more water than the Bangalore method, but has a shorter composting period. The materials go through an intensive heating phase.
In dry climates composting is mainly practiced in pits to keep the compost humid and save on water and labour for maintaining ideal conditions.
Vermi-composting uses specially introduced earthworms for decomposition.It is a good technique for recycling food waste and crop residues from vegetable gardens in the proximity of the house. The composting period is longer as compared to other methods and varies between six and twelve weeks.
In this manual only the heap/pit method and vermi-composting are described in further details.

1. Collect adequate quantities of the materials needed.
2. Measure out an area 1.5 meters wide and of any convenient length. The width should enable to work with the compost without having to walk on it.
3. Dig out a shallow pit of the planned size of the compost heap. The more arid the climate, the deeper the pit is usually dug. Compost pits should, however, not be deeper than 50 cm to ensure aeration. If no pit is dug in a humid climate, loosen the ground where the compost heap will be, as the materials need close contact with the loose soil at the bottom. The topsoil obtained when digging the trench should be carefully put to one side beside the trench so that it can be used in the compost.
4. Woody materials should be chopped into pieces 5 to 10 cm in length or spread on a road or used as livestock bedding before composting to be bruised and increase its surface for better decomposition. Wet plant material such as sea-weed or fresh grass should be wilted before mixing it with other material. Straw should be pre-soaked in water, if possible. Ideally dry material is thoroughly mixed with urine and animal dung.
5. Lay down the bottom layer of rough vegetation such as maize stalks or hedge cuttings. This layer should be about 30 cm thick. Such materials allow for air circulation into the heap.
6. Then add a layer of mixed green material and animal manure (if available) followed by a layer of dry material. Then mix both layers and water well. The better the different materials are mixed, the better the composting process. Plant material infected with viruses should be placed in the center of the compost, and should be covered fast to avoid that the viruses are propagated to healthy plants by sucking insects.
7. Repeat the process to build a heap to a height of 1 to 1.5 meters. Make sure to water each new layer well to create humid conditions. As for composting, aerated conditions are needed, the compost heap should not be stamped. A well-made heap has almost vertical sides and a fl at top. If you have a lot of materials, it is good to make several heaps of about 2 meters in length.
8. To complete the pile, ideally cover it with 10 cm of topsoil to prevent gases from escaping from the compost pile. Lastly, cover the whole pile with dryvegetation or banana leaves to prevent loss of moisture through evaporation.
9. Take a long, sharp, pointed stick and drive it into the pile at an angle. The stick helps to check the condition of the pile from time to time. If the stick is pulled out and is warm after two to three days, this indicates that decomposition has started. If the stick is white, this is an indication that the heap is dry inside. The heap should be turned and watered well.
10. Do not grow cover plants such as pumpkin on the compost heap itself, as this dries it out. Plant them next to it.

1. About every third day, depending on the weather conditions and if it has rained or not, the heap must be watered.
2. If all goes well, the heap should be turned after 3 weeks, after the temperature of the pile has fallen. Compost heaps are usually turned 2 to 3 times in their early stages. When turning the compost heap, make sure the outside material comes inside. Thus, when turning the heap, fi rst take the material from the top and the outside to make the new heap. This procedure ensures that all parts of the compost go through a proper heating phase. Do not add new material during turning.
3. After 3 to 6 weeks the heap should be turned again. By now the compost should have a fresh earth smell and no grass, leaves or animal droppings should be visible. Some woody branches or stalks may still be present, as they take a longer time to rot.
4. In 3 to 6 weeks after the second turning, the compost should be ready for use. Mature compost turns blackish-brown in colour and has a pleasant smell. If the planting season is still far off, leave the pile covered where it is. The pile should always be kept moist and covered with dry material. If the heap becomes too wet it should be opened up and mixed with dry organic matter or allowed to dry in the sun before rebuilding.The decomposition process is quickened by adding large amounts of fresh animal manure and by turning the heap more frequently.

In an African context, there is no such thing as too much compost. Usually the amount that can be produced by a smallholder farmer is rather small. So it is, therefore, important that compost be applied where the cultivated plants can use it and where it directly contributes to better plant nutrition and water retention. Efficiency of the compost can be improved through targeted application to the root zone of the crop plants through banding or placing into planting holes rather than spreading it over the entire fi eld. Labour requirements may be higher with targeted application, it may however result in higher productivity and help to reduce the size of land required to produce the required quantities of food.
In planted crops, compost is best applied into the planting holes and mixed with topsoil. Compost should be applied first to the crops with high nutrient demands such as tomatoes. In sown crops, compost is best brought out in the sowing rows prior to sowing and worked into the topsoil. In perennial tree crops, compost application is most effi cient when applied along the drip line of the trees (and not at the foot of the trunk). Good quality compost is ideal for seedbeds also. Compost should not be ploughed deeply into the soil. Compost can also be hoed into the topsoil as a top dressing.
Effect of nitrogen from compost is generally little and extends over a long period. Availability of nitrogen and other nutrients from compost can be increased, when the compost is supplemented by nitrogen rich liquid manure or fresh animal manure after its application before crops.
Compost that has not fully decomposed can be used for mulching between crops or around tree crops. It will continue to mature on the ground and organisms in the soil will draw it into the soil where it will decompose further. When using compost as mulch, it should be covered with a thin layer of straw or dry grass or leaves. This will avoid loss of nutrients due to direct exposure to sunlight and heat. Young composts are best applied to the soil together with nitrogen rich plant material like green manures, or they are applied before sowing a green manure crop.
The compost can be used immediately or stored for later use. Ripe compost for storage should be kept in the shade and covered with 10 cm of top soil to keep it humid and prevent loss of nutrients.

Build a brick and mortar enclosure with a concrete bottom, one or two chambers and proper water outlets. Convenient chamber size is 2 m x 1 m x 0.75 m. However, the size of the chambers should be determined according to the volume of the composting material. Alternatively, a sizeable plastic or metal container or wooden boxes with a secure and removable lid to keep out predators and with ventilation holes on the side walls and holes on the bottom to release excess moisture from the container, but small enough to keep out fl ies if possible. The ‘four tank’ or ‘four chamber’ method of chamber construction is also commonly used because it facilitates easy and continuous movement of earthworms from one chamber with fully composted matter to a fresh chamber. Whatever container is used or built, it should be placed in a dark and damp place.

• A layer of good moist loamy soil (vermin bed) is placed at the bottom, about 15 to 20 cm thick above a thin layer (5 cm) of broken bricks and coarse sand.
• Earthworms are introduced (about 150) into the loamy soil, which the worms will inhabit as their home.
• Then, a small quantity of fresh cattle dung is placed over the vermin bed.
• The compost pile is then layered to about 5 cm with dry leaves or preferably chopped hay/straw or agricultural waste biomass such as vegetable peels, leftover food, dead leaves and plants. Egg shells can also be broken into small pieces and added to the pile.
• For the next 30 days, materials are continuously added to the pit until it is full and is kept moist by watering it whenever necessary. Meat or fish scraps, greasy foods, dairy products or bones should not be added into the pile, as these will attract ants and rodents. The pile should be covered with porous material to keep off predators.

The compost should be ready within 60 to 90 days. The material will be moderately loose and not as heavy and with a dark brown colour.
In the two or four pit system, watering should be stopped in the first chamber so that worms will automatically move to another chamber where the required environment for the worms are maintained in a cyclical manner and harvesting can be done continuously in cycles.
To remove some of the compost, let the top of the heap dry out by discontinuing the watering for two to three days so that the worms move down to the cool base of the heap. Compost can then be removed and taken back to a fresh pile.