The degradation of soil over the past 40 years has been due to the depletion of organic matter content, resulting in decreased crop yields. To produce two tonnes of maize per acre, an uptake of Nitrogen from the soil is, for example, 48kg.
A soil rich in organic matter (four percent) is sufficiently supplied with Nitrogen to produce high yields and makes sense financially. But organic matter does even more than that. It enriches the soil with a multitude of micro-organisms, which not only enable soil fertility, but also help to fend off pests and diseases.
One of the limiting factors, especially on red soils, is phosphorus. Red soils are red because they are rich in iron and in Kenya, also in aluminium. Phosphorus, in its usual form of PO3, is highly attracted to Ferrum (Fe3+) and forms insoluble Fe-Phosphates.
In this form, phosphorus is no longer available to crops. This phenomenon is called phosphorus-fixation. Even phosphorus added to such soils in fertiliser form will be bound up.
The only solution in such soils is to increase the amount of phosphorus in the biomass. If phosphorus is taken up by microorganisms, it is protected from forming compounds with Fe and Al and is released in plant-available forms in a steady process of recycling it.
Tithonia, a plant valuable because of its ability to take up phosphorus from the soil, is an invaluable component of any compost, as it increases the amount of plant-available phosphorus.
Reconstructing your soil cheaply
The loss of organic matter can be attributed to high decomposition rates, dry spells, heavy rains, erosion, leaching and harmful fertilisation.
However, an underutilised, cheap and easily available source of organic matter rich in nutrients is available to any smallholder in Kenya. Farmers can be taught how to use their cow manure and turn harvest residues and weeds into compost.
Compost plays an important role in the reconstruction of soils. It is an ideal source to sustain a great number of microorganisms, which form the crucial living component of biomass.
It decomposes any organic materials into a plant-available form of manure, but it also has a negative electric charge. As such, it is able to attract positively charged cations such as Ca2+, Mg2+, K+, NH4+ (Ammonium), and acidic H+.
This increases the so-called cation-exchange capacity of the soil, which enables nutrients to be taken up by plants and stabilizes the pH.
Compost also plays an important role in the physical structure of a soil. A soil rich in compost has a better water-retention capacity. It can sustain a crop for two weeks longer without rain than soil low in organic matter.
How many times does a crop die because the rains are two weeks late? Soils high in compost are also better protected against erosion and have abetter tilth.
This means ease of tilling the soil and better growing conditions for the seedling. It also supports the development of stronger roots, which are necessary for strong, healthy plants.
How to make good compost
Compost is organic matter, mainly plant materials that have decayed. Compost manure supplies soil nutrients, including both macro and micronutrients. Inorganic fertilisers often provide a few nutrients per product, and, therefore, for more sustainable production, compost works well.
While plant materials and other organic waste can decompose naturally and get incorporated into the soil, composting hastens the process. It also helps to build soil structure through the addition of humus, enhances moisture absorption and retention, and reduces soil erosion.
What do you need to do composting?
Composting uses all available organic materials.
Brown materials, including dried plant residues, dried grass, leaves, maize stover, etc., green plant materials such as kitchen waste and green plants, and animal manure.
Select a site that is not exposed to wind, rain/runoff- preferably under a shade. It should be shaded to ensure the compost material is not exposed to direct sunlight.
Ensure the site is close to where the compost manure will ultimately be used to reduce the hassle of having to carry it there.
There are two main ways of composting
– heap and pit. Pit composting is suita ble in areas with less rainfall/water as it helps to conserve moisture. The heap method is suitable in areas with significant rainfall as it prevents logging/excess water entering the compost, which would affect the process.
Establishing a compost heap/pit
A heap is done on the ground surface while a pit is dug 2-3ft in the ground. If the pit is dug deeper, the materials placed deep in the ground will might not decompose properly as some microorganisms would not survive under those circumstances. The rest of the processes are similar for both methods.
Select the method that will suit your prevailing/current weather.
The compost site should always be 1-1.5m wide and can be as long as needed based on available material for composting. This width enables you to easily turn the material as you can reach the entire width of the pit/heap.
The compost pit can be smeared with fresh cow dung to reduce the leaching of nutrients. The heap on the other end is supported by small poles and droppers around it on both corners to ensure that it is stable.
Making the compost layers
The foundation layer: This is what sits at the bottom of the heap or pit of thecompost. It includes larger plant materials (dry) such as dried maize stalks, grass, and dried leaves. Cut the materials into small pieces and spread them until they are about 25cm high from the base. Sprinkle some water on this layer and ensure the materials are MOIST but not WET.
The first layer: This includes green plant materials such as green grass cuttings, kitchen waste and other green planting materials.
It is recommended to include shrubs such as tithonia (Mexican sunflower), glyricidia, leucaena, sesbania, crotalaria and lantana to increase the Nitrogen content. Apply this up to about 30cm.
The second layer: Includes animal manure.
Also add slightly decomposed manure so as to introduce microorganisms to the compost to hasten this. The animal manure may be mixed with some soil. Apply this up to 20cm only to allow for good air penetration.
Cover layer: Add a cover layer with either banana leaves or other broad leaves or polythene cover to prevent exposure to rain, direct sunlight and animal disturbance.
Check the site every two weeks for temperature and moisture.
This is done by inserting a stick in the centre of the heap. Feel the stick with your fingers to get the temperature and moisture. If the composting is going on well the stick will be moist and warm.
If the compost is not moist, sprinkle some water evenly as you turn it. Add enough water to make the material moist – not wet. High temperatures can be corrected by suspending the cover for a while to allow the hot air to escape. Low temperatures mean slow composting and can be corrected by adding covering and allowing sunlight to reach the heap so that it can be heated up.
Turn the compost after three weeks and add some water to make it moist (only when necessary). Your manure should be properly composed within six weeks. You will be able to tell from the sweet smell it produces and also the uniformity of the final material
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