What is a Cover Crop

Diversity is really important when you consider adopting and integrating cover crops into your rotation. The first question you should ask yourself is what do I want to achieve from using cover crops? Do you want to help alleviate compaction, increase soil organic matter levels, suppress weeds, improve soil structure, improve your soils ability to infiltrate water… the list can go on.

I would answer that question by having a multi-benefit approach. Do not settle on one reason to want to have cover crops. Please do not get into the habit of having a single species mix. Be as adventurous as you can; why not have 5, 8 or 12 species?

What are some of the basic aspects to cover cropping?
They are sown after harvest, usually in late September or early October, and stay in the ground until late-January and early-February. The earlier you can sow your cover crop, the better it will perform for you. Like any plant, including your cash crop, they require good soil temperature and enough sunlight to grow. The later you can leave your cover crop in the ground, the greater the benefits will be.

Species and mixes

Choose a mixed cover crop, which could include species like phacelia, oats, chicory, or clover (see table below). What are some of the benefits cover crops can provide to your farm?

  • Fixing atmospheric nitrogen
  • Soil structure and compaction
  • Nutrient scavenging
  • Attracting more pollinators and insects
  • Soil organic matter and improving biology
SpeciesBenefits
PhaceliaThis is a late flowering plant and will provide habitat for pollinators and beneficial insects over winter.
Clover (white/red)Clover species are atmospheric nitrogen fixing plants. They will support goals to reduce fertiliser needs. They are also hardy.
ChicoryThis is a good plant to help bust through compaction or pans. These plants are also suitable, like most species in this table, for livestock.
BuckwheatThis plant is a good scavenger of nutrients which help reduce leaching through the soil profile. This will help maintain soil fertility.
Birdsfoot TrefoilThis is another nitrogen fixing plant. It is also a good flowering plant for pollinators.
RyeGrows quickly, providing good soil cover to reduce erosion. It is also a vigorous rooting plant and will provide good organic material.
Fodder RaddishThis plant is also good at breaking up compaction and soil pans.
OatsAnother cereal crop which grows proficiently and provides good rooting to help build soil structure.
VetchThis is another nitrogen fixing crop. It also has good medicinal properties for livestock.
RyegrassRye grass provides good early growth, helping to protect the soil from erosion.
MustardThis is a good crop at growing quickly and mopping up left over nutrients from harvest.

A key thing to think about is the carbon to nitrogen (C:N) ratio of your cover crop mix. There is always more carbon than nitrogen in organic matter. A ratio of 20:1 means that there is 20g of carbon to every 1g of nitrogen. The lower the C:N ratio, the quick the substrate (e.g. straw, leaf, plant roots etc.) will be broken down into organic matter, and the quicker nitrogen will be released into the soil for plant (i.e. cash crop) uptake. A ratio of more than >35 means that microbial activity is immobilised and the substrate takes much longer to break down. Research has shown that soil microbes are functioning at there best with a ratio of 24:1.

For example, grasses (at vegetative stage) have a C:N ratio of 15:1 and legumes (clover, vetch and peas) have a C:N ratio of 10:1. Well rotted manure or compost has a C:N ratio of 20:1, providing a good balance between mineralisation and immobilisation. If you have a very low C:N ratio, your soil biology will release nitrogen too quickly and it will be lost to the atmosphere. Once you start to move beyond the 34:1 ratio, you will start to lock up nutrients in your soil.

What are some of the draw backs to cover crops?
First of all, there is an associated cost that comes with buying seed and sowing it. However, I firmly believe that you get more out than you have to pay in. To save money, you can buy straight seeds and mix them yourself. Cover crops do not have to be perfect; far from it. I do not believe there is a magic ratio of species.
Secondly, you might not see the benefits overnight. You need to manage your expectations in season one. I believe you will start to see benefits straight away, but they might not match the very high expectations people place on cover crops. Keep persisting and do not give up. You can make cover crops work for you, no matter what your situation is.
Thirdly, there can be issues with certain crops setting seed before destruction and causing weed problems – this is only really an issue with mustard. This plant in particular grows quickly and can easily go to seed if you are not careful. In my opinion, mustard is one of the species I would consider leaving out of your mix; unless you are looking for a cheap filler.

Destruction methods

Many in the farming sector believe that glyphosate is the only answer to destroying cover crops effectively. In reality, there are several steps you can take before turning to this herbicide. Think of it as a last resort.

I have seen farmers use a combination of grazing and crimp rolling. Sheep or cattle go on in December and grazing through to January. Once the first frost come in January, the crimp roller is attached to the tractor and the cover crops is mercilessly rolled into submission. I think this combination can be very effective when timed right. Consider using Holistic Grazing (i.e. mob-grazing); a tight-knit herd or flock can cause considerable damage. A side benefit is that you are presenting an opportunity to bring livestock onto your farm (if you want to). This helps to deposit manure (for fertility) and a second income stream from the same piece of land. You can either use your livestock, or rent the land to a neighbour – I believe they call this a “win-win”. Many cover crop species are susceptible to frost and when crimp rolling, you can break the stalk and expose the plant. This helps to speed up the degradation of the plant.

If needed, these methods can then be followed by a minimal glypshosate application to finish the cover crop off. I would like to add one last point. Once you have established a Regenerative Agriculture system and have started to build up soil health, you will find that soil biology (especially earthworms) can do a very good job at breaking down trash left on the surface and dragging it into the soil structure.

To me, cover crops are an absolute no-brainer no matter what your situation is. They will require some tinkering as you find the species mixes that work for you. Be persistent and don’t be afraid to try making your own seed mixes.

References:

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