Why include legumes in your crop rotation for soil health and fertility benefits

Sustaining soil health and fertility is critical for maintaining long-term profitability in arable farming systems.

Incorporating legumes such as red clover, peas, beans, and vetches into crop rotations offers a proven strategy to improve soil fertility in both the short and long term.

These crops are renowned for their nitrogen-fixing benefits, providing a valuable nitrogen supply for following crops.

This blog explains how nitrogen fixation works, and how farmers can benefit from incorporating legumes into their crop rotations to achieve better soil health and fertility.

The science behind legumes’ nitrogen fixation

Legumes, such as clover, peas, beans, and vetches, have a unique biological advantage: their ability to form symbiotic relationships with Rhizobium bacteria in soils.

This process is called biological nitrogen fixation.

How nitrogen fixation works:

1. Initiating the symbiosis:

• When legume seeds germinate, they release chemical signals called flavonoids into the soil.
• Rhizobium bacteria present in the soil detect the flavonoid signals and respond by producing Nod factors, which stimulate the formation of root nodules on the legume.

2. Formation of root nodules:

• The bacteria invade the plant root hairs and establish themselves within specialised structures called nodules. These nodules act as biological factories where nitrogen fixation occurs.
• Inside these nodules, the bacteria convert atmospheric nitrogen gas (N₂), which plants cannot use, into ammonium (NH₄⁺), a form readily available for plant uptake.

3. Nitrogenase enzyme activity:

• The conversion of N₂ into NH₄⁺ in the nodules is facilitated by the enzyme nitrogenase, which requires energy and anaerobic conditions to function effectively.
• The plant provides this energy in the form of carbohydrates, a byproduct of photosynthesis, to sustain the Rhizobium bacteria.

4. Nitrogen availability for the plant:

• The fixed nitrogen is directly absorbed by the legume for its growth and development.
• Surplus nitrogen is often stored in the root nodules and released into the soil after the plant residues decompose, enriching the nitrogen pool for subsequent crops in the rotation.

Soil health and fertility benefits of nitrogen fixation

Legumes offer more than just nitrogen fixation; they play a pivotal role in improving soil health by boosting microbial activity and increasing soil organic matter.

These benefits have long-term positive effects on soil fertility and structure, supporting optimal crop yields.

Boosting soil microbial activity

Microorganisms, such as bacteria, fungi, and actinomycetes, are crucial for soil fertility. They decompose organic matter, recycle nutrients, and improve soil structure.

Legumes enhance microbial activity in several ways:

• Root exudates: Legumes release carbohydrates, amino acids, and organic acids in to soil through their roots, serving as an energy source for beneficial soil microbes and stimulating their growth and activity.
• Enhanced diversity: Legumes create a diverse microbial community by creating microhabitats in their root zones. This diversity improves nutrient cycling efficiency and helps suppress soil-borne pathogens.
• Improved enzymatic activity: Increased microbial populations lead to higher soil enzyme activity, such as nitrogenase (involved in nitrogen fixation) and phosphatases (which release phosphate from organic matter).

Increasing soil organic matter

Legume crops leave behind a significant amount of beneficial plant residues after harvest, including:

• Root biomass: Deep-rooted legumes like clover or lucerne (alfalfa) contribute root material that breaks down slowly, adding carbon to the soil.
• Above-ground biomass: Stems, leaves, and pods left after harvest decompose rapidly, contributing to the soil’s organic matter pool.

There are several benefits of increased organic matter, including:

• Carbon sequestration: Organic matter binds carbon in the soil, helping mitigate greenhouse gas emissions.
• Improved water retention: Organic matter increases the soil’s capacity to retain water, which is critical during dry periods.
• Enhanced soil structure: Organic matter binds soil particles into aggregates, improving porosity, aeration, and root penetration.

Long term soil health benefits

• Nutrient reservoir: Legume residues act as a reservoir of nutrients, not just nitrogen but also phosphorus, potassium, and micronutrients that are slowly released from the soil and made available to crops.
• Reduced soil erosion: Improved soil structure, thanks to increased organic manner content, reduces erosion risks during heavy rainfall.
• Resilience to climate stress: Soils rich in organic matter are better buffered against climate extremes, ensuring crop stability even in adverse conditions.

Pairing nitrogen-fixing legumes with fertiliser application for optimal results

Incorporating legumes or other cover crops that fix nitrogen naturally can complement the use of nitrogen-based fertilisers, such as OCI Nutramon.

The nitrogen fixed by legumes provides a steady and natural supply to the soil, while targeted applications of synthetic fertilisers can address the specific needs of subsequent high-demand crops like wheat or maize – ensuring the crop has enough nitrogen during critical growth stages.

Learn more about OCI Nutramon and its high nitrogen efficiency.