Cover Crops...Our Allies in Nutrient Management

by Richard de Wilde
Every year we are intrigued by cover crops and find ourselves wondering why more farmers don’t utilize them.  Late summer and fall is an important time of year when we start to wrap things up for the growing season, making our final passes through the fields and putting them to bed for the winter.  We remove the mulch and irrigation lines, take down tomato stakes and chop any remaining plant material (such as broccoli stalks) in the field.  Starting in mid-late summer, as soon as a crop is finished, we start this process with the goal of getting a cover crop planted as soon as possible.  We’ve been planting cover crops since August, so many fields are already covered with a lush blanket of green growth.  Cover crops are a very important part of our production system and are important for maintaining the health of our soil as well as investing in future crops we’ll take off the land.

Richard kneeling in a cover crop planting

Cover crops are an excellent example of how it pays to work in alignment with nature.  While we plant most of our cover crops in the fall, they could be planted at other times of the year in certain scenarios.  We choose cereal grains, grasses and legumes as our plants.  It’s important to understand why we plant them and what purpose they serve.  First, cover crops will out-compete any fall weeds that might germinate in a field….and we hate weeds!  There are actually some weeds that germinate and start their growth cycle in the fall. Once they are established, we have to deal with them in the spring when they start to bloom.   The more weeds we can prevent from getting established in the fall, the better it will be in the spring.   Cover crops also help hold soil in place.  Winter winds and moisture can carry precious topsoil away if there isn’t something to hold onto it.  We try to get cover crops established as soon as possible so we can maximize their growth potential and form a strong root structure to hold the soil in place and prevent erosion.

Field planted with a cover crop mix of annual rye grass,
oats, crimson clover, Japanese millet and Austrian winter peas

Another important reason for planting cover crops is to build soil health and nutrition while building a system for holding nutrients.  Cover crop plants can both synthesize and extract nutrients from their environment and then act like a sponge to take these nutrients up and hold onto them.  Through photosynthesis they are able to take carbon from the air and use it to build nutrients in the plant and soil system.  Some scientists studying climate change have theorized that if all farmers used cover crop systems, we could mitigate the problem of excess carbon and the effects of climate change.  Many nutrients in the soil are water-soluble and can be lost when they wash away with melting snow and moisture over the winter and in the spring.  If you have a plant in the soil, it will take up the nutrients and utilize or hold onto them.

This year we’ve chosen to diversify our cover crop plant mixes.  We have two different mixes.  The first mix is a combination of four different plants that have the ability to overwinter.  This means they will start to grow again in the spring time.  We plant this mix in fields that we do not plan to plant early crops in.  This mix includes hairy vetch and mammoth red clover which are both legumes.  The other two components are annual rye grass and cereal rye.  Each component of the mix has a specific purpose.  The legumes are important because they have the ability to take nitrogen from the air and fix it in the soil.  Annual rye grass is a fast-growing, aggressive plant that can out-compete weeds.  While it’s part of the overwinter mix for this purpose, it’s actually one component that will not come back in the spring.  Cereal rye is important because it takes up the nutrients, including the nitrogen synthesized by the legumes, and acts like the sponge to hold onto them.  They release them into the soil as needed, or at the end of their life cycle when we cut the cover crop and work it back into the soil.

Austrian winter peas, rye and clover in our cover crop mix.

Our second mix contains five components that will winter-kill.  While this means that the plants will die when we get temperatures of 10°F or less, these amazing plants can and will continue to grow (slowly) up until this point.  This is yet another reason that cover crops are so amazing!  We use this mix on fields that we know we’ll need to get into early in the spring to plant some of our early crops such as parsnips, salad greens, early cabbage, kohlrabi, peas, etc.  This mix also contains the annual rye grass for its fast-growing abilities.  The nitrogen-fixing legumes in this mix include winter peas and crimson clover.  The sponges in this mix include two cereal grains, oats & Japanese millet.  While creating these mixes has added a level of complexity to the process, it also has added a higher level of diversity to our cover crop system which in turn will create a wider diversity of microbes in the soil.

Our standard operating procedure when we finish harvesting a crop is to immediately follow with the chopper to break down any remaining plant material, then do a light disking.  Next, we spread compost and then the cover crop seeds are planted.  This happens fast and the whole process can be completed in 24-36 hours!  This is very time-sensitive and every day matters because you really want to maximize the growth of the cover crop while the fall days are still warm.  Of course we need moisture in the soil to germinate the seeds, so sometimes we dance with the weather and try to time the seeding right before or after a rain.

Using cover crops is a very efficient way to hold and add nutrients to the soil.  Once the crop is planted, everything happens in place.  There is no additional need to haul or spread additional fertilizer…the plant does all the work for us!  Management, teamwork and timeliness are key components to making this all come together.