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How legume crops improve soil fertility

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Mr Gerald Ssendaula examines the pods of a groundnut plant in his garden as he explains how to use legumes to boost fertility at his farm. Photo/Michael J Ssali

Soil is a complex ecosystem that has interrelated physical, chemical, and biological properties. You can tell that your soil is fertile if it has plenty of plant and animal activity such as fungi and earthworms. Your continued soil fertility is dependent on the soil management practices on your farm.

Mineral content is one of the vital characteristics of soil fertility. Plants require six essential elements to grow. They include nitrogen, phosphorus, potassium, magnesium, sulphur and calcium.

According to Fanuel Ongua, a soil microbiologist at the National Agricultural Research Laboratories (NARL), it is important that you test your soil not just for NPK but also for microbial activity and micronutrients. Knowing which biology is present in your soil and its function offers great insight on the nutrient cycling capacities of your soil.

“A bountiful harvest is directly proportional to the availability of micro and macro elements required by plants. Tracking mineral content in your soil throughout a season can give you valuable information on how to adjust practices and potentially improve soil and crop health,” he says.

How do you know your garden lacks nitrogen? 

If the plants in your garden are losing their emerald-green color and turning yellow, the reason may be improper watering, viral infections, encroaching weeds, and nutrient deficiencies. Among the deficiencies, the lack of nitrogen usually deprives leaves of their greenness. 

According to Moses Lumu, an agronomist, nitrogen is a constituent of chlorophyll that makes plants’ foliage and stems green. When there is a deficiency, plants suffer from a lack of chlorophyll, which causes the leaves to lose their characteristic green colour.

Besides the chlorophyll, plants with nitrogen insufficiencies have abnormal development of the stems and roots. If the deficiency is severe, the plants may become stunted. 

Plants living in soils that lack nitrogen grow larger root systems that are out of control in an effort to extract as much of this element as possible from the substrate. 

Growing legumes

Soil health is crucial for agricultural productivity, and legumes play a vital role in improving soil fertility. These nutrient-rich crops are not only a cost-effective source of protein for humans and livestock but also enhance soil quality through their beneficial effects on soil biological, chemical, and physical conditions.

High in protein, carbohydrates, dietary fibres, vitamins and minerals, beans and other legumes play a fundamental role in nutritious healthy diets. Both the seeds and leaves are also used as feed for humans and livestock.

For smallholder farmers in Uganda, beans are a cost-effective substitute for animal protein and make up a large proportion of typical diets. Across Europe and the US, they are commonly eaten as tinned beans, chickpeas and lentils, while in sub-Saharan Africa, cowpeas are among the most important legumes.

Pulses, the edible dry seeds of legume plants, are staple foods in the diets of both people and livestock around the world.

In Western Kenya, Rwanda and Burundi, people eat more than 30kg beans a year on average, while many African countries recommend pulses as a meat alternative in dietary guidelines. Pulses can also be stored for extended periods without affecting their nutritional content.

Lumu remarks that growing legumes improves soil quality through their beneficial effects on soil biological, chemical and physical conditions.

He says, “When properly managed, legumes enhance the nitrogen-supplying power of soils, increasing the soil reserves of organic matter. With the ability to fix nitrogen by legumes, crops would need less use of artificial nitrogen fertiliser and soil health would simultaneously improve.”

The magic in the root nodules

Legumes possess a unique ability to fix nitrogen from the air and soil into a form accessible to plants, reducing the need for synthetic fertilizers. This natural process occurs in specialized structures called root nodules, where beneficial bacteria convert gaseous nitrogen into plant nutrients.

According to research by the Enabling Nutrient Symbioses in Agriculture project, high-performing legumes can fix up to 300kg of nitrogen per hectare, equivalent to $1 (Shs3,800) per kg in fertilisers.

In the root nodules, bacteria convert gaseous nitrogen from the air and soil into a form that is accessible to the plant as nutrients. So, legumes need less of the artificial nitrogen fertiliser than cereal and other vegetable crops.

“Legumes, among the earliest domesticated crops, still hold great potential for improvement through breeding and genetic engineering, which can enhance their suitability and sustainability for contemporary agriculture and food systems, despite their long history of cultivation,” Lumu says.

The benefits of more efficient nitrogen fixing in legumes would include greater growth and biomass and higher protein content in the seeds or pulses. This would increase the nutritional value per crop, meaning more high-quality nutrient-rich food could be produced per hectare.

Increased yields would empower small-scale and subsistence farmers to cultivate legumes like soybeans as lucrative cash crops, enhancing rural livelihoods. Moreover, more productive legumes would excel as rotation crops, rejuvenating soil health which is particularly crucial for farmers in sub-saharan Africa who grapple with degraded soil.

Ongua says, “Legumes are considered environmental heroes because they work together with soil microbes to fix nitrogen from the air, enriching the soil with nutrients to allow them to thrive. With such information, it is possible to transform sustainable agriculture, especially in areas where access to synthetic fertiliser is already limited by cost and availability.”

Extending nitrogen fixing to other crops has long been an ambition of crop scientists around the world and as the study of plant biology advances, the pulse of progress is quickening.

By understanding the unique ability of legumes, researchers aim to develop other crops with similar nitrogen-fixing capabilities, transforming sustainable agriculture and improving soil health worldwide.

Other strategies

Leaving soil to fallow does not add nutrients but growing cover crops as pasture, legumes and then grazing your livestock does. Along with nutrients from legumes, manure and compost applications tend to improve soil organic matter and biological activity.

Ongua advises farmers to create a healthy farm ecosystem and stop erosion by wind and running water if they want to increase soil fertility.