How cover crops sync with nature to replenish soil without chemical fertilisers

‍Editor's note: Even before its current status as a nutrient-rich superfood, ragi has been a crucial chapter in the history of Indian agriculture. Finger millet, as it is commonly known, has been a true friend of the farmer and consumer thanks to its climate resilience and ability to miraculously grow in unfavourable conditions. As we look towards an uncertain, possibly food-insecure future, the importance of ragi as a reliable crop cannot be understated. In this series, the Good Food Movement explains why the millet deserves space on our farms and dinner plates. Alongside an ongoing video documentation of what it takes to grow ragi, this series will delve into the related concerns of intercropping, cover crops and how ragi fares compared to other grains. 

In the mid-1960s, the world used just over 46 million metric tonnes of chemical fertilisers—a volume that reflected the growing pressure to feed a globally booming population. By 2022, that number had quadrupled to nearly 188 million tonnes. Today, nitrogen-based fertilisers dominate, making up roughly 58% of all fertiliser use, while phosphates account for 23% and potash for about 18%. This dramatic rise isn’t just a story of technological progress; it’s a window into how modern agriculture has reshaped soils, economies and ecologies over the past six decades.

Farmers apply chemical fertilisers to enrich the soil with essential nutrients—notably the big three: nitrogen, phosphorus and potassium. Chemical fertilisers inject these into the land instantly, resulting in boosted yields. Given that they are heavily subsidised by the government, these fertilisers seem like a  cost-effective and profitable way of efficiently managing farms. Often, this results in overapplication, and fertilisers end up doing more harm than good—they change soil composition and compromise microbial life and soil health. The chemicals then leach into groundwater, contaminating drinking water. 

As any organic farming guide will tell you, there is a better way to replenish soil nutrients, one that works with nature: cover crops. Cover crops are a class of crops grown before or in between the main crop(s). They get their name because they ‘cover’ the land outside of the main cropping season. This act of covering the land with some plant, any plant, is necessary to preserve the soil’s nutrients. Cover crops keep the soil aerated, maintain (and even increase) microbial populations, reduce soil temperature, fix atmospheric nitrogen, and increase soil organic carbon, all of which are essential to soil health. 

Replacing fertilisers

When the sun beats down on exposed soil, it extracts valuable moisture from the soil, and kills crucial soil microbes due to the heat. The cover crop, then, becomes a protective layer. Alongside protecting soil and lowering its temperature, cover crops actively contribute to soil health. 

Legumes are a popular choice for cover crops since they fix atmospheric nitrogen, converting it into a form which plants can use. Legumes form a symbiotic relationship with certain bacteria, most notably Rhizobium spp., which stimulate nodule formation on the plant’s roots, to create an entryway into the root cells. Here, the plant provides the bacteria with carbohydrates and the bacteria converts atmospheric nitrogen into ammonia, which the plant can then use for nutrient synthesis. Upon harvesting, the whole crop is  mixed back into the soil using an agricultural implement called a rotavator. As the stem and roots decompose, they release that nitrogen into the soil, making it available for the main harvest crop which will be grown next. 

When cover crops are crushed by a rotavator to form mulch, this liquid, which is toxic to most pests and pathogens, is released. Since these crops replace gaseous pesticides (or fumigants), they are termed biofumigants.

Sometimes, a main crop may get infected by a pest or disease. Cover crops offer an opportunity to break this cycle. Potent compounds like isothiocyanates which are synthetically produced for use in commercially-sold pesticides, also occur as natural compounds in certain plants. When sprayed indiscriminately, these compounds can impact biodiversity – killing crucial pollinators, contaminating soil and water, and unintentionally entering our body through food, contributing to antimicrobial resistance. Cover crops are a natural way to introduce these compounds to the soil. 

For instance, plants like mustard, which belong to the Brassica genus, have defence mechanisms that allow them to produce these compounds on their own. The plant stores two chemicals, glucosinolate and myrosinase, which can interact to form isothiocyanate in separate compartments. If the plant gets injured, the compartments are damaged, and isothiocyanates are formed. When cover crops are crushed by a rotavator to form mulch, this liquid, which is toxic to most pests and pathogens, is released. Since these crops replace gaseous pesticides (or fumigants), they are termed biofumigants.

Cover crops also host many bugs, beetles, and bees that keep pest populations in check, while also serving as pollinators. For example, aphids are sap-sucking pests that damage the plant and slow its growth. The ladybird beetle, hosted by mustard plants, is capable of feeding on 50 to 200 aphids in a day—acting as swift, natural pest-controllers.

Cover crops also host many bugs, beetles, and bees that keep pest populations in check, while also serving as pollinators

Also read: Why Akkadi Salu, an ancient practice of intercropping ragi, deserves a comeback

Increasing soil organic matter

Ultimately, cover crops aim to enrich the soil. All the aforementioned benefits are tied together by one metric that serves as the best measure of soil quality: soil organic matter. Soil organic matter refers to all the living and non-living organic components of soil—from bacteria and earthworms to leaves that trees shed. It plays a role in every physical, chemical, and biological soil property—from water retention and aeration to storing nutrients and supporting microbial life. 

Soil organic matter also enhances carbon sequestration, or the conversion of atmospheric carbon dioxide into a form that can be stored in the soil. On an average, almost 58% of the mass of soil organic matter is estimated to be carbon. This means healthy soil is not just important to grow good food, it directly contributes to the fight against climate change. So, how does one measure how much soil organic matter is being returned to the soil by cover crops?   

Also read: Decoding ragi’s cropping conditions: How farmers study soil, rains and temperature

Measuring biomass

One simple way of measuring biomass is to create a 1 m² PVC frame, using waste pipes and some elbow pipes. Then, throw this frame randomly, and harvest the 1 m² area. Aim to throw the frame in a Z-shaped route to ensure that samples are collected from different parts of the field. Weigh each sample. Since 1 acre is approximately 4,000 square metres, the average of 5-10 such samples can be multiplied by 4,000 to estimate the productivity per acre of cover crops. 

While soil organic matter is often conflated with soil organic carbon, it is important to remember that they are distinct terms. The carbon that is added back to the soil is important both for soil health and for carbon sequestration. However, cover crops add back more than just carbon to the soil—nutrients like nitrogen, phosphorus and sulphur make their way back too. And most importantly, soil organic matter enhances the soil’s capacity to be a storehouse for water, minerals, and carbon.  If re-adopted into farming practices, cover crops have indispensable value to add to agricultural output, soil and water health and, consequently, to the well-being of our planet. 

‍

{{quiz}}