By STANSLOUS NGOSA
Soil fertility is declining in many parts of sub-Saharan Africa and Zambia has not been spared.
There are various reasons for the decline in soil fertility, chief among them, climate change and it is believed that ever changing climatic conditions are bound to cause more flooding, drought and unpredictable weather patterns.
Closely associated to this are some of the common traditional farming practices, which include burning of crop residues that leave the soil bare and unprotected from the sun and wind.
Other causes are excessive or insufficient use of fertilizer and improper crop rotation.
Thankfully, there are many traditional and modern practices which can help boost soil fertility, and assist farmers in making their farms more flexible and resistant to the changing climate.
Rather than burn crop residues and other organic matter, farmers can use methods such as micro-dosing of fertilizer, planting nitrogen-fixing crops and trees, making good use of compost manure, and taking steps to prevent soil erosion through wind and water.
A good soil fertility strategy for farmers is to make best use of all organic sources of plant nutrients.
It is also wise to use ways that conserve soil and water, and, when possible, to make careful use of small amounts of expensive inorganic fertilizers.
Organic sources of nutrients include legumes, green manures and cover crops, composts, and animal manures.
The best practises for soil fertility vary depending on the area of farming.
Zambia has four agro-ecological regions that comprise different soil types. The main soils are loamy-sand or sand Alfisols, interspersed with clay.
Upland soils are of low inherent fertility; most are moderately leached sand veldts, loams and clays.
Their texture and structure and physical properties vary. Soils in most dambo areas are characterised by a dark colour in the top 30 centimetres of the profile.
The Food Agriculture Organisation (FAO) says in many cases, silt loams underlain by silt clay loams or clay soils developing in deeper layers.
Clay appears to have elevated to lower horizons into water tables. Black colour is chiefly on account of their high organic matter content.
Dambo soils have supported crop production on a sustainable basis for decades without having to apply external inorganic fertiliser on account of their organic matter and high nutrient pool.
There is currently an increasing trend to dambo cultivation due to their wetness and high fertility status.
The variation in fertility of both upland and lowland dambo soils has to be viewed and approached cautiously with different land management practises for production to be sustainable.
Bulima Organic Cooperative Limited vice-chairperson Mike Ng’uni says when the nutrients, which lead to soil fertility are removed and not replaced, or when the conditions that support soil fertility are not maintained, soil becomes degraded.
“This leads to poor yields,” Mr Ng’uni says.
He says soils are often low in nutrients if added to the combined effects of shorter fallow times and continuous cropping. Slash-and-burn agriculture leads to grave soil degradation or depletion.
Mr Ng’uni says soil degradation occurs through too much tilling of the soil, which damages soil structure.
“Also overusing inputs such as synthetic fertilisers and herbicides can leave residues and build-ups that hinder the work of micro-organisms,” Mr N’guni says.
He says a build-up of salt in the soil, often associated with irrigation, deplete fertility and limit crop yields.
Mr Ng’uni says the soil left bare after burning residues or harvesting crops is vulnerable to erosion from wind and rain.
He says the top soil washed away, especially in flood-prone areas or blown away contains most important nutrients needed for an effective agriculture production.
The Farm Radio International says physical characteristics of the soil determine how much water a soil will hold.
Soils have many small pores, which hold water until it can be taken up by plant roots, but in order to do this, the soil must be adequately aerated. The air is stored in larger pores in the soil.
Soils, which have been compressed by the constant weight of equipment or livestock will not have a good structure and often contain little water or air, and therefore have poor fertility.
Soil chemistry affects the availability of plant nutrients in the soil.
The pH level of the soil, together with its exposure to air, affects the form in which nutrients are found in the soil.
A measure called Cation Exchange Capacity (CEC) indicates the amount and type of clay in soils and organic matter content.
A good CEC level indicates that a soil can hold nutrients in a form that is readily available for uptake by plant roots.
In general, soils, which contain higher amounts of clay rather than sand, and a high percentage of organic matter, have better CEC levels.
The biological life in the soil – soil-dwelling macro-organisms and micro-organisms – breaks down crop residues into organic matter. A healthy biological life in the soil also limits many plant diseases and soil-dwelling pests, which damage crops.
The level of organic matter in the soil is critical to soil fertility.
Organic matter levels affect soil structure, CEC, the amount of water that the soil can hold, and the level of nutrients available for plant growth.
A system called integrated plant nutrient management recommends that farmers use the best soil fertility principle- maximise the use of organic sources of fertiliser and minimise the loss of soil nutrients by using soil and water conservation practises.
The other one is the careful use of inorganic fertilisers according to need.
Legumes add nitrogen to the soil, mostly through falling leaves but also when their roots and nitrogen-rich root nodules decompose underground.
The organic matter produced by legumes is rich in nitrogen, decomposes quickly and is a good source of nitrogen for other plants.
Herbaceous non-woody green manure legumes often called cover crops and fast-growing leguminous trees are excellent ways to improve soil fertility.
Cover crops also provide a dense soil cover that can prevent soil erosion, evaporation of water, and suppress weeds.
But legumes need good conditions to grow and farmers may have to improve the soil first, so that legumes can contribute to soil fertility.
The most common problem is a shortage of phosphorus. In highly acid soils, liming or adding animal manure can raise the pH and increase the availability of phosphorus.
In most soils, however, the only option is to add phosphorus. As phosphorus fertiliser not normally affordable for African farmers, a good strategy is to use rock phosphates. A farmer should be careful to obtain sources of rock phosphate that are effective at providing nutrients for their crops.
Scientists analyse the quality of crop residues in a laboratory. They use equipment to conduct tests, which measure the amount of nitrogen, carbon, lignin and other substances in the residues.
But farmers can also conduct tests. For example, nitrogen levels in leaves and other material can be estimated simply on the basis of colour.
Dark green leaves are higher in nitrogen, and make good fertiliser. Yellow leaves are lower in nitrogen and do not, by themselves, make good fertiliser.
Estelle Munsitala, whose 15-hectare farm is located in a flood-prone area of Mpongwe says conservation farming and the planting of Faidherbia trees is the answer to improving soil fertility.
She says after using Faidherbia trees, soil loss had decreased tremendously, while soil fertility and soil structure had strongly improved.
Mrs Munsitala says practising the right methods of farming like crop rotation also helps to improve the soil.
Soil erosion as the result of various factors, especially climate change is evident in Zambia and if left unmitigated and farmers left uninformed, it will continue affecting agriculture yields.
Reprinted with permission from The Times of Zambia