By Dr. James Mutegi
Legumes are the most important household crops, especially in the drier areas. On average, farmers harvest less than a tonne per hectare while the potential for most legume producing regions could go up to 3.5 tonnes.
Legumes require nitrogen, phosphorus, potassium, magnesium, sulphur and non-acidic soils.
On average, a hectare of most common legumes will take up about 150kg of nitrogen, 20kg of phosphorus, 14kg of magnesium and 1 kg of sulfur to produce an average of 2.7 tonnes of grain per hectare.
The actual amount of nutrients that should be supplied externally through fertilizers is the difference between those already contained in the soil and the crop requirement.
Most legumes can make their own nitrogen from the atmosphere and leave excess nitrogen in the soil that benefits subsequent crops.
This is free nitrogen that a farmer doesn’t have to buy. Where such plants are unable to make nitrogen, inoculum could be applied on the seeds to aid nitrogen fixation. Farmers are, therefore, advised to rotate legumes with other crops like cereals or root crops, instead of continuously planting crops of the same family.
At a market value of between Sh50,000 and Sh80,000 per tonne, the unit value of grain legumes is up to three times higher than that of maize and other common cereals.
The Alliance for a Green Revolution in Africa-funded Kenya Soil Health Consortium Programme has identified the main causes of low maize yields as Striga infestation, acidic soils, and declining soil fertility.
SITE-SPECIFIC RECOMMENDATIONS
Striga is a parasitic weed that siphons water and nutrients from growing cereals, often reducing crop yields by over 20 percent. Striga can be controlled by rotating cereals with legumes.
Striga is a pest of low fertile soils and its proliferation decreases as soil fertility improves. Rough estimates suggest that for every tonne of maize, the crop takes up approximately 13kg of nitrogen, 3kg of phosphorus, 4.2kg of potassium, 1.2kg of magnesium and 1kg of sulfur from the soil. The crop also takes up micronutrients.
Yet the current fertilizer recommendations in Kenya are based only on nitrogen and phosphorus. Recent work by the International Plant Nutrition Institute (IPNI) and other actors in central and western Kenya show a need for developing site-specific recommendations that also include elements like potassium, magnesium, sulfur, and zinc.
Since most of the fertilizers in the Kenyan market, such as DAP, NPK, CAN and UREA, only supply nitrogen, phosphorus, and potassium, there is a need for developing fertilizer blends that supply more of the required nutrients.
Most of the agricultural lands in central, western and Rift Valley are also acidic. Estimates show that over 30 percent of crop yield could be lost even with appropriate fertilizer application due to acidic soils. The most common solution for acidic soils is application agricultural lime.
The best way of supplying nutrients is by applying a combination of both commercial fertilizers and organic products like crop residues and manure. In addition to cutting the fertilizer bill, organic resources build the capacity of soils to hold crucial nutrients and manage among other roles.
To some extent, farmers can diagnose the limiting nutrients on their own by viewing the crop growth characteristics. For example, a maize crop that is deficient in nitrogen will have pale green or yellow leaves starting from the older leaves, those with phosphorus deficiency will exhibit purplish leaves and those with potassium deficiency have dry leaves along the edges.
These visual symptoms are good indicators of nutrient deficiency. Exhaustive determination should be done in labs.
Dr. Mutegi is a soil scientist and farming systems analyst at IPNI Sub-Saharan Africa Program, and Project Manager, Soil Health Consortia for Eastern and Southern Africa.