A trend has recently cropped up in the agricultural industry, and this one’s a keeper. Cover crops have overtaken the practices of heavy irrigation and chemical fertilizers to revitalize soil after the seven months between harvest and planting each year.
According to the 2017 agricultural census, the percent of U.S. cropland planted with cover crops increased by 50 percent between 2012 and 2017, from just over 10 million acres to more than 15 million.
Planting cover crops is an accessible agricultural strategy for improving soil health and water retention, nutrient cycling, and preventing weeds and nitrogen pollution in water and erosion. Some science even shows that cover crops can improve the ability of soils to store carbon.
Here we look at some of these benefits in a little more detail:
Soil Health and Fertility
Cover crops, particularly those with nitrogen-fixing abilities such as legumes and radishes create a continual loop of nutrient and nitrogen cycling that rivals compost and manure. Nitrogen is produced in the nodules of the legume roots which contain Rhizobium bacteria. These bacteria take nitrogen from the air and convert it to a form plants can use. Legumes also help with their ability to convert the unavailable form of phosphorus to an available form through releasing some organic acids through their roots. This is important to note since phosphorus is one of the most limiting nutrients for plant growth.
Improved Soil Texture
Compacted soil does not allow necessary air circulation and water infiltration into a plant’s root zone. Oxygen is a necessary component in many plant processes including respiration and nutrient movement from the soil into the roots. Unlike animals, plants do not have an active transport system for oxygen, instead it moves slowly within a plant through diffusion. In the absence of oxygen, root growth stops and over time the roots begin to die. Plant roots have trouble moving through compacted soils, weakening their interactions with symbiotic microbes in the soil that bring back nutrients and waters. Selecting and planting crops based on the characteristics and benefits of their roots improves soil texture, which is as important as its fertility. Plants like daikon radish have roots that break up compacted soil, while other plants like annual rye have fibrous roots that hold together loose soils.
Half of the topsoil on the planet has been lost in the last 150 years. Beyond losing fertile land, the effects of soil erosion lead to increased sedimentation in streams and rivers, clogging these waterways and causing declines in fish and other species. Soil erosion also affects water absorption rates, which can worsen flooding. Cover crops are a great tool to minimize soil movement off of the field. Maintaining active roots holds the soil in place during rain, while the above ground growth shields soil from drying out and being carried off by wind. They also help to manage absorption of rainfall, reduce runoff, and maintain pore space (avoiding compaction).
Cover crops have two ways to help with water infiltration, getting water into the soil, above ground and below ground. Below ground, the shallow roots of the cover crop give water droplets an easy path to follow since the root is already penetrating the soil. The green foliage above ground allows water droplets to slow down and dissipate their force before entering the soil profile. Cover crops also contribute to a reduction in evaporation, preserving moisture during drought periods.
Prevention of Weeds
Cover crops, which are typically grown densely, fill the space weeds would otherwise invade, and block sunlight from weed seeds, so that they don’t have a fighting chance.
As cover crops, otherwise known as “green manure,” are on the rise in U.S. agriculture, the state of Maryland takes the cake: Politico reported that Maryland farmers planted cover crops on more than 43 percent of the commodity corn, soybeans, and cotton acres in 2017, and numbers reported by the state department of agriculture since then have been upwards of 50 percent. Other Northeast states report more than 30 percent of cropland in cover crops, but many Midwest states come in at under five percent, while only about four percent of the nation’s total cropland is planted with cover crops. The next census won’t be done until 2022, but the practice is spreading. Whether farms are large or small, that's less fertilizer used, which is a good thing for the environment.