Your garden may look asleep or almost asleep but under the soil surface beneficial microbes, earthworms, helpful bacteria and fungi are still working on your behalf.
If you do not provide winter food for them, though, their numbers will be reduced by half and your soil in the spring will be half as healthy since harmful nematodes will multiply and chomp on your plants' roots.
George Driever, Oklahoma State University Extension Educator in Pottawatomie County wrote an easy to understand article explaining it all for OK Farm Bureau magazine (link here).
Driever points out the advantage of planting a cool season cover crop -
*Cover crops reduce weed populations by shading them out
*Cover crops store nutrients in their biomass that are later released and help reduce erosion
*Cover crops add organic matter that pull nitrogen from the air, that organic matter helps your soil hold moisture and increases the good microbes.
*The good microbes help release nutrients from the soil to your plants' roots.
There are 3 types of cover crops: 1) grasses, 2) legumes and 3) brassicas
*Brassicas grow fastest and can be killed by a hard freeze, though last night's 24 degrees did not kill my kale, it did burn my Asian mustard plants. Brassicas capture nitrogen and feed it to the soil.
*Legumes make a lot of leaves and stems (my snow peas are thriving, the red beans froze).
Legumes have a symbiotic relationship with Rhizobium, a bacteria that forms nodules on plant roots, converting nitrogen in the air to compounds your plants need.
*Grasses other than rye should already have been planted. Ryegrass is cold tolerant though will freeze and the roots will help the soil. Other cereals include oats, barley, winter wheat.
Among the Ryegrasses, Driever says that Elbon Rye is the best for OK.
Fact Sheet HLA-6436 is at this link where you can get more information on healthy soils, organic fertilizer analysis, cover crops, etc.