By Anne Verhallen, Soil Management Specialist (Hort), OMAF and MRA
From ONvegetables in The Grower, December 2013
There has been a lot of interest this past year in cover crops, particularly as a source of livestock feed or pasture. However this tends to raise some questions about what you are sacrificing from the soil improvements value of the cover crop by removing most of the top growth.
It’s somewhat telling that Dan Towery, a well- known cover crop researcher and promoter in the United States said at the Cover Crop meeting in Altoona Iowa, “below ground cover crop growth may be more important than above ground”. Well in my opinion, there is no “may” about it. While the above ground growth – the leaves and stems are critical for erosion protection, the roots are probably more important for all the benefits that we see from cover crops.
Cover crop roots help to build soil structure. The roots themselves release exudates – complex carbohydrates – sugars and other compounds. These materials act like glue – binding soil particles together. The rhizosphere or the root zone is the hotbed of microbial activity. The sugars feed and support the soil life, which in turn further builds soil structure.
Now lets’ take a closer look at those roots. Not surprisingly, cover crop roots are really quite different from the cover crop top growth. We often talk about the carbon to nitrogen ratios of the cover crop top growth in regards to whether the cover crop will tie up nitrogen or easily cycle nitrogen. Generally carbon to nitrogen ratios that are around 25:1 basically between 20 and 30 to 1 or below will cycle nitrogen relatively well. We see these ratios with young growing grasses like wheat and rye, but as the grass matures it lays down more carbon and more longer lasting materials like lignin, the carbon to nitrogen ratio expands to 60 to 1. Breaking down cereal straw will tie up nitrogen.
Back to the roots though – the carbon to nitrogen ratios on cover crops are wider than the top growth. Makes sense – the roots are usually a storage organ for the plant – so more complex carbon molecules are laid down. The carbon to nitrogen ratio of roots does not change with depth in the soil. It varies more with the crop and the crop growth stage. Generally, if the cover crops are immature and lush in their growth, the carbon to nitrogen ratio will be well within the 25:1 and nutrients will be readily cycled. However as the cover crop matures, this changes and the differences between species become more apparent. For example brassicas like oilseed radish and canola and grasses like wheat or rye can be as high as 40 to 45 to 1. In contrast legumes like peas are below 20 to 1.
So what does that really mean? It means that the brassica and grass roots do not break down as readily and it helps to explain field observations. Why do pea roots seem to disappear quickly? Why don’t we give a nitrogen credit to pea cover crops? The pea cover crop breaks down quickly and releases the nitrogen too early in the season to mesh well with a corn crop. This also supports the concept of mixing cover crop species to create a better overall cover for the field – use the grass and brassica but add in a legume to help feed the system.
And the bottom line for cover crops? If you have a need for feed or an opportunity to sell your cover crop for feed or pasturing – it is not a total loss. The cover crop roots will still be there and they are more important than the top growth for building and maintaining soil structure.