Managing Cover Crops?

Written by James Warne from Soil First Farming

As we look around at the landscape in the early autumn we can see that some early drilled cover crops are now coming into flower.    Those planted in late July after the first harvests of OSR or Barley will now have been in the ground for 6-8 wks in some parts of the country. Those planted after wholecrop silage or over-winter stubble stewardship options may have had another 4 weeks of good growing conditions.   With the wet and warm summer we have had biomass production will have been large.   The soil biology will have been working overtime mineralising organic compounds in the soil providing the cover crop with nitrogen, phosphorus & sulphur.

While the multiple benefits cover crops provide are undisputed, dealing with them can become also pose multiple problems for the physical activity of drilling and the establishment and early growth of the following cash crop.

Let’s take a quick look a some of the benefits cover crops can bring to the soil and wider environment.   The principle benefit to the farmer is having a living root in the soil.   Do not underestimate the importance of this.    Soil fertility is all about carbon, in simple terms living roots are at worst maintaining the soil carbon stock, and at best increasing the carbon content.   Bare soil is the opposite of this, at best it is maintaining the carbon stock, but most likely the biology will be feeding upon the organic matter it as there is little other food source, releasing carbon as CO2.

Roots are also providing stability, structure and drainage, important functions which would otherwise have been achieved through cultivation, while the biomass above ground provides protection, mulch and insulation.  

While these all sound like a panacea, it’s not so straightforward when it comes to dealing with the cover crop prior to drilling the following cash crop. 

The basic option is to flail off the cover crop then plough/combi drill the following crop.  Simple, straightforward and fulfils our desires of seed into a clean seedbed having buried the trash.   If carbon building, soil fertility and soil biology are your primary aims this option is a failure.   Any gain in soil carbon will be lost by the action of cultivation and the introduction of large amounts of oxygen into the soil, oxidising organic matter and carbon. While the bacteria in the soil may recover from having been turned upside down and buried to 8 inches quickly, the fungi most certainly will not.  Fungi are relatively slow growing and their filaments are very sensitive to be chopped up and disturbed. It can take years for the them to re-inhabit cultivated soil.   And finally the cultivated soil is very susceptible to slumping and erosion by heavy rainfall.

The other option is the drill into the soil unmoved through the cover crop which is every no-tillers desire.  The thought of which can be daunting to a beginner, and the risks can be high.  When do I destroy the cover crop? Do I have a drill that will cope with large amounts of biomass?  Will it look a mess?   Will the pre-emergence herbicide still work. And most importantly, yet underestimated, what will all the decaying biomass contribute to the carbon:nitrogen ratio in the soil.   

Destroy the cover crop too early and risk it becoming a mulch which stops the soil from drying should the weather turns wet.  Allow the drill to get to farm ahead of the sprayer and risk the crop emerging before the cover crop is destroyed.   Flail the biomass too close to drilling and it balls up around the drill.  The decaying cover crop, along with possible chopped straw and residues from the previous crop, will have locked up a large proportion of the soil’s plant available nutrition, slowing the establishment of the cash crop, and in extreme circumstances preventing emergence all together.   The fertilisation of the cover crop needs to be taken seriously to prevent this from happening.   It doesn’t take much cover crop growth to remove most of the available nutrition, right now there is plenty of stripy cover crop & OSR showing exactly where all the volunteers dropped behind the combine has sucked the nutrition from the soil, combined with lots of chaff and possibly chopped straw.

Above: The effect of low plant available nutrition Get it right however and the benefits can be great, simple cheap crop establishment into a friable soil with good aggregate stability and porosity.   The carbon has been captured which improves the functionality and fertility of the soil and can be an ideal medium to drill into.