One the greatest challenges to food security in Malawi is the quality of the soil. Many soils have been depleted of their nutrients. Three major nutrients that plants take in through their roots are nitrogen (N), phosphorus (P), and potassium (K). Inorganic fertilizers supply these three nutrients and are identified by their NPK ratio (e.g. 23:21:0 or 46:0:0) which tells you the percent mass of each nutrient per 100g of fertilizer. Urea fertilizer is 46:0:0 which is 46% N, 0% P, and 0% K, by mass.
The NPK ratio allows a farmer to apply exactly the amount of each nutrient he wants. But things get complicated from here. For example, Calcium Ammonium Nitrate is dissolves and evaporates quickly when it is in contact with open air. If not buried in the soil, the nutrients will not be available to the plants. Other fertilizers take a long time to become available in the soil: anywhere from a few hours to weeks. Organic compost can take even longer to make its nutrients available.
Some plants are heavy feeders when it comes to different nutrients. The one that most farmers watch out for is nitrogen. Corn, Sunflower, Wheat, Tobacco, and many other widely cultivated plants eat a lot of nitrogen from their soils. They typically require large quantities of fertilizer to maximize their yields. One solution to this problem is to use legumes in the crop rotation to resupply the soil with nitrogen.
Legumes are plants that have the capability to use atmospheric nitrogen (i.e. the nitrogen that we inhale and exhale a thousand times a day) for their growth. Legumes are an excellent way to reduce the amount of inorganic fertilizers applied to heavy feeders. A field previously planted with soya beans will not require as much nitrogen fertilizers as a field that did not have a legume on it recently. Common legumes in sub-Saharan Africa are soya, beans, peas, ground nuts and acacia.
What is often overlooked about legumes is that they must be infected with the bacteria rhizobia before they can begin fixing nitrogen from the air. Before the plant is infected with rhizobia it utilizes soil nitrogen just as non-legumes would. A farmer who is trying to increase the nitrogen content of his soil by planting legumes must understand this. If his soil does not host rhizobia, then his legumes will never be able to fix nitrogen. The solution to this is first apply rhizobium inoculant to the seeds before planting. This will establish rhisobia in the soil. After the bacteria is established the crops can be managed to preserve the rhizobia population. This will increase the yield of legumes over time and also the nitrogen content of the soil.
However, some common practices can reduce the rhizobia population in the soil. Plowing the field will expose rhizobia to sunlight which can kill the bacteria. Low pH (acidic conditions) will also kill rhizobia. Low pH is a consequence of using inorganic fertilizers and raw animal manure in the fields. A farmer can manage his pH by applying lime before planting.
pH also affects plant growth. Most plants crops grown in Malawi grow best under mildly acidic conditions with pH around 6. If the pH falls bellow 5, significant crop losses can occur. Many farmers do not realize that their soil is too acidic and don't understand why their fertilizers are not effective. Again, pH can be effectively managed with lime (or sulfur, if the conditions are too alkaline).
Crop rotation is known as an effective way to improve soil nutrition. Many Malawian farmers understand that heavy nitrogen feeders such as maize should be rotated with legumes. But what is often overlooked is role crop rotation plays in disease and pest control. A farmer might decide to alternate growing soya and maize on his plot. This provides him with good nitrogen levels for his maize and sustained rhizobium population, but over time his yields will diminish. That is because the soil becomes laden with bacteria, viruses, and nematodes that target soya and maize. If millet and ground nuts could be introduced, they might provide bumper yields with no signs of disease. This is another benefit to crop diversity.
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good understanding of soil science is key to increasing crop yields. Many farmers actually damage their soil and their crops' growing conditions because they do not understand the soil. This is one area where extension workers can easily improve the community's annual yields without incurring large subsidy costs. Lime and rhisobia inoculants are cheaper than fertilizer. Good soil management will reduce the over all demand for fertilizer there by reducing its cost on the market, further dropping the cost to the farmer and the consumers.
