A-Z of fixing nitrogen in the soil

It is of great importance that farmers employ agricultural practices that minimise the loss of soil and its contents, which include water, nutrients and micro-organisms.

These components, when maintained at required levels, guarantee better income for farmers.

Farmers must thus strive to build up soil moisture and nutrient levels, using methods and practices which are less destructive to all physical, biological and chemical status of the soil.

Two practices can help farmers achieve this, that is, green manuring and cover cropping. Green manuring is the growth of crops capable of fixing nitrogen, which are then incorporated into the soil to improve it.

On the other hand, cover cropping involves planting certain species of crops, mainly broad–leafed together with the main crop - intercropping. The aim of cover cropping is to maximise on the ground cover to prevent direct sunlight from hitting the soil to curb moisture loss.

Merits of green manuring

It increases organic matter content in the soil

During incorporation of the plants at flowering stage into the soil, the main aim is to trigger decomposition. These materials are broken down by fungi, bacteria, insects, earthworms, and burrowing animals that are found in the soil, to release nutrients that are needed by the main crop.

The remaining materials form humus and organic matter that are of great influence on the soil physical and chemical properties. Physically, soils with high organic matter are less compact, hence root penetration is easy, as well as water and air circulation.

Increased biological activity

Micro-organisms in the soil feed on remnants of dead plants and animals. The materials help in creating a conducive environment in the soil as they decompose.

Reduces soil erosion by improving structure

Aggregation of individual soil particles leads to formation of structures that are well-packed and are relatively resistant to agents of erosion such as water, wind and anthropogenic (human) activities. During decomposition, the materials produce liquid substances known as leachates which help to bind soil particles.

Improve nutrient availability to plants

This is the main goal of green manuring. One plants legumes which have the ability to fix nitrogen in the soil. The nitrogen and others are released into the soil in their available forms and taken by the main crop improving yields.

 Weed suppression

Green manuring helps in suppressing weeds as the ground is covered. This also increases the competition with the nutrients and since crops cover significant areas, their competing ability is favoured. Some cover crops and green manure species also produce substances that are detrimental to the survival of the weeds (allelopathic).

Increases water infiltration

Infiltration of water into the soil significantly reduces if the ground surface is bare since water runs off when it rains. Planting cover crops helps reduce forces of rain drops and encourages water infiltration.

Best crops to grow in green manuring

Leguminous crops are preferred since they have the ability to fix nitrogen from the atmosphere into the soil through the help of rhizobium. With proper management, legumes can enhance the nitrogen–supplying power of the soil after fixation. N content is always high during flowering of the crop.

Some of the crops that have been scientifically proven to fix a reasonable amount of nitrogen are tithonia, alfalfa, lablab, canola, crotalaria and pigeon pea.

Tithonia (Tithonia diversifolia) has the following nutrients - 3.5 per cent nitrogen (N), 0.37 per cent phosphorus (P) and 4.1 per cent potassium (K) on dry matter basis. Analysis of alfalfa for nitrogen indicates that it contains between 2.5 per cent to 3.0 per cent in the above ground dry matter. Dolichos lablab, commonly known as Dolichos bean (Lablab purpureus) contains approximately 3.12 per cent nitrogen.

Canola (Brassica napus) can grow in wide range of soil types due to its N fixation ability making it require less fertiliser application. Out of its above ground dry matter, canola contains about 3.26 per cent nitrogen. In comparison to total dry matter, Crotalaria (Crotalaria spectabilis) contain 3.07 per cent of nitrogen. 

However, of all the crops, pigeon pea (Cajanus cajan) supplies the highest amount of N when used as green manure, as it contains approximately 3.94 per cent of nitrogen. Some of crops that can be used for green manuring and cover cropping are common bean (Phaseolus vulgaris), cowpeas (Vigna unguiqulata) and mung bean (Vigna radiata).

You can minimise the use of chemical fertilisers and other chemical farm inputs if you embrace green manure and cover cropping.

The writer is based at the Department of Crops, Horticulture and Soils, Egerton University.