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Introduction

Before we take a closer look at cover crops, it’s important to understand one of the biggest issues that they help to address. Soil erosion is a major problem in agriculture. An estimated 24 billion tonnes of topsoil are lost because of destructive farming practices every year. Half of all the topsoil on the planet has been lost in the last 150 years. This not only reduces fertility of the land and farmer’s profits, but the eroded soil also runs off and pollutes rivers, lakes and other water systems. Degraded soils are also less able to hold onto rainwater, which can worsen flooding and create cycles of erosion, drought, and desertification. The cost of soil erosion, organic matter decline, salinisation, landslides and contamination is estimated to be €38 billion annually for the EU alone.

Every previous agricultural civilization has collapsed due to land and natural resource mismanagement, and the history of such civilizations is a good reminder that we need to protect our natural resources. The most effective way to control erosion is to maintain a permanent cover on the soil surface.

Cover cropping is hardly new but has found new applications in the modern revival of regenerative agriculture. Some of the earliest known agricultural texts make reference to crop rotations and planting certain species that were not intended for harvesting. An example being Virgil’s volume (70 – 19 BCE) “Georgics,” a tome on all aspects of agriculture, where he recommends alfalfa, clovers, and lupine as cover crops in the off season for increased wheat yields. Since then, and all around the world, there is evidence and current practices of planting that is primarily intended to improve the soil and growing conditions for other crops.

What are “Cover Crops?

The simplest explanation of cover crops is that they are selected plants that are grown in an agricultural field to “cover” the soil in between the seasons when a cash crop is grown there, and are not meant to be harvested. “Covering” the soil however, is only a small part of their benefits, so let’s explore some others.

Advantages of cover cropping

There are many advantages to planting cover crops in agricultural fields, both for the health of the soil and for the farm business so let’s separate them by those two categories.

Advantages for the soil

• Preventing soil erosion: through reduced water runoff and wind damage, cover crops reduce surface runoff and protect soil surface particle detachment by raindrop impact.
• Shading the soil: This prevents evaporation of moisture and drying out of soils. This also has a large impact on reducing temperatures on and below the soil surface.
• Preventing soil surface and subsurface hardpan: Cover crops improve soil water intake by preventing soil surface sealing due to raindrop impact, and consequently, reduce surface runoff. Their roots also help to break up the subsurface “hardpan” or compacted soil caused most commonly by overgrazing, tillage, and machinery driving over it.
• Sequestering carbon out of the atmosphere and into the soil: The chief method of carbon transfer to the soil is via photosynthesis and subsequent storage in plant root tissues, which decompose into soil organic matter. As plant roots penetrate deeper into subsoil layers they deposit short chain carbon through root exudates and long chain carbon as the roots slough off when the plant dies back.
• Building soil aggregate: aggregates are clumps of combined sand, silt, and clay in the soil that range from the micro level (less than 0.25mm in diameter) to the macro level (greater than 0.25mm in diameter) and are formed through physical, chemical and biological activity belowground. These varied shapes allow for healthy soil to have pores spaces for air and water that are essential for healthy plant growth. Cover crops can promote good soil aggregation which leads to the physical protection of soil organic carbon by preventing oxidation and can improve the ability of soils to store carbon and hold on to moisture.
• Adding nitrogen and other nutrients to the soil: Certain cover crop plants are well known for adding nutrients to the soil, either as they grow, or when they die back. Though not all cover crop species fix nitrogen in the soil, legume/rhizobia teams are well known for this and are the most efficient N fixers. Together they pull nitrogen from the atmosphere and store it in nodules around the plant roots which are accessible to other plants. Other cover crops mine minerals from lower subsoil layers making them available at the surface as they die back and decompose. Nearly all of them add some level of nutrients back to the soil if they are allowed to decompose in place at the end of their lifespan.