By Beatrice Nakibuuka
Healthy soil is the backbone of productive agriculture, supporting plant growth, sustaining biodiversity, and ensuring long-term food security. Soil comprises physical, chemical, and biological components that interact to maintain fertility. However, soil health is not static; it requires continuous management and replenishment.
Role of soil health in agriculture
One key indicator of soil health is biological activity, including the presence of earthworms, fungi, and beneficial microbes. These organisms play a crucial role in nutrient cycling, organic matter decomposition, and soil structure improvement. Farmers who adopt sustainable soil management practices can enhance productivity, lower input costs, and ensure long-term agricultural sustainability.
Soil fertility is heavily influenced by its mineral content. Plants require six essential nutrients: nitrogen, phosphorus, potassium, magnesium, sulphur, and calcium. A balanced supply of these nutrients is critical for optimal crop yields and resilience against pests, diseases, and environmental stressors.
Fanuel Ongua, a soil microbiologist at the National Agricultural Research Laboratories (NARL), advises that soil testing should go beyond nitrogen, phosphorus, and potassium (NPK) levels to include microbial activity and micronutrient availability.
“Understanding the biological composition of your soil provides valuable insights into nutrient cycling and overall soil health,” Ongua explains, adding: “A bountiful harvest is directly linked to the presence of both macro and micronutrients essential for plant growth.”
Identifying nitrogen deficiency in soil
Plants often exhibit visible symptoms when soil nutrients are inadequate. If crops lose their lush green colour and turn yellow, several factors could be responsible, including improper watering, viral infections, weed competition, or nutrient deficiencies. Among these, nitrogen deficiency is one of the most common and can severely impact plant growth.
Moses Lumu, an agronomist, explains that nitrogen is a key component of chlorophyll, the green pigment responsible for photosynthesis. When plants lack nitrogen, their leaves turn yellow or pale, indicating distress.
“In addition to chlorophyll deficiency, nitrogen-deficient plants exhibit stunted growth, weak stems, and poorly developed roots,” Lumu says.
In nitrogen-deficient soils, plants develop extensive root systems in an attempt to extract more nitrogen, often at the expense of healthy foliage and crop yield.
Legumes as natural solution for soil fertility
Legumes play an essential role in improving soil fertility. These nutrient-rich crops not only provide an affordable source of protein for humans and livestock but also enhance soil quality by improving its biological, chemical, and physical properties.
Legumes such as beans, peas, lentils, and cowpeas are rich in protein, carbohydrates, dietary fibre, vitamins, and minerals. In sub-Saharan Africa, beans and cowpeas are staple foods. In Western Kenya, Rwanda, and Burundi, for example, annual bean consumption exceeds 30 kilogrammes per person. Pulses, the dried edible seeds of legumes, are widely consumed globally and can be stored for long periods without losing nutritional value.
According to Lumu, cultivating legumes significantly improves soil quality.
“Legumes increase nitrogen levels in the soil, enhancing organic matter content. This reduces the need for synthetic nitrogen fertilisers, which improves soil health and lowers farming costs,” he explains.
Magic of root nodules
Legumes possess a unique ability to fix atmospheric nitrogen into the soil through a symbiotic relationship with bacteria known as rhizobia. This process occurs in specialised root nodules, where rhizobia convert atmospheric nitrogen into a form that plants can absorb and utilise.
Research from the Enabling Nutrient Symbioses in Agriculture project shows that high-performing legumes can fix up to 300 kilogrammes of nitrogen per hectare, equivalent to purchasing nitrogen fertiliser at approximately $1 per kilogramme (around UGX 3,800). By reducing reliance on synthetic fertilisers, legumes offer a cost-effective and environmentally friendly solution for soil fertility management.
Role of legumes in crop rotation, soil conservation
Integrating legumes into crop rotation systems is an effective way to maintain soil fertility. By alternating legumes with cereals or vegetables, farmers can naturally replenish soil nitrogen while disrupting pest and disease cycles. This practice improves soil structure, reduces erosion, and supports long-term agricultural sustainability.
Ongua describes legumes as “environmental heroes” due to their positive impact on soil ecosystems.
Crop scientists are actively researching ways to transfer nitrogen-fixing abilities to non-legume crops.
Other soil improvement strategies
While legumes are a natural way to enhance soil fertility, additional strategies such as organic matter application, mulching, and composting can further improve soil health.
Leaving land to fallow alone is not always sufficient for replenishing nutrients. However, planting cover crops like legumes and incorporating livestock grazing can significantly boost soil organic matter and microbial activity. The application of manure and compost also enhances soil fertility, increasing moisture retention and supporting beneficial soil organisms.
Ongua encourages farmers to adopt sustainable soil management practices to build a healthier farming ecosystem.
“Preventing soil erosion, maintaining adequate ground cover, and using organic soil amendments are crucial for increasing soil fertility and ensuring long-term agricultural productivity,” he advises.