As the late summer and autumn vegetables are harvested large areas of soil remain exposed and begging to be covered.
Cover crops or green manures are subsequently sown at this time of the year by the astute organic gardener, in the knowledge of the inherent advantages they bestow on the soil.
However, many gardeners are unaware of the different roles that a green manure preforms compared to a cover crop.
Essentially a cover crop protects the most critical component of any garden, the top soil, from the harsh environment, be it wind, heavy rains, frost, scorching sun etc, thus preventing erosion and degradation of the organic matter (humus) within the soil. It ensures the biological processes continue to occur by maintaining the microbial activity in the soil. They also protect the habitats of the earthworms and other essential soil invertebrates.
A green manure can have a broader role, often including protecting the soil as a cover crop, but more importantly collecting the nutrients which could be lost by leaching or oxidation and recycling them back to the soil. These plants can then be either dug in, prior to becoming too stringy, or harvested (some can be cut 2 or 3 times) and used as a mulch or the basis of your compost.
By careful selection of the types of green manure used, the nutrients and trace elements can be drawn from a range of soil depths and incorporated with the nitrogen extracted from the air.
The legumes, such as board beans, grey peas, lupins or clovers provide the advantage of little nitrogen factories (rhizobia bacteria nodules) busily manufacturing Nitrogen from the air, along with deep foraging root systems. Cereal grasses such as ryecorn, oats, barley and millet exploit the shallower depths of soil and combine well with a legume.
Rape and mustard are commonly used as green manures, and are known to collect potash. Mustard is also known to rid soils of wireworms. It must also be remembered that rape and mustard are from the brassica family and should never be followed by crops of other brassicas, such as broccoli, cabbage and cauliflower.
Finally make the most of the warmth still in the soil and plant your green manure or cover crop before the cold winter weather sets in.