Cover Crop and Green Manure


Above: Red clover growing amongst corn stubble, in early June. The clover was undersown into the corn crop the previous year (below).
The photo at right illustrates bountiful growth of red clover in the spring in a field where there had been a corn crop the previous year. The clover was seeded into the corn after the last cultivation for weeds, just before the canopy closed in early July (see photo below) The corn was harvested for silage in mid-September. Then, the increased light available to the clover allowed it to become well established (with a large root system) by the time cold weather set in. It overwintered, and in early April began to grow vigorously. It is shown here in early June, just before it was to be cultivated into the soil.

In the absence of clover, the ground would have been mostly bare, exposing it to erosion, and there could be a large loss of nutrients by leaching. The clover protected the soil and took up nutrients that might otherwise be lost - in this way, it functioned as a "cover crop". In addition, as a legume it added nitrogen to the system by nitrogen fixation, fixing probably 100 kg N per hectare. That nitrogen, and nitrogen taken uo from the soil are contained in the clover biomass. By turning it in at the flowering stage, when it is still succulent, about half of the contained nitrogen will be mineralized and made available to the subsequent crop over the ensuing 4 months; in this way, it functions as a "green manure".