You may have heard of cover crops for nitrogen fixation to help replenish your garden soil. But what does it really mean and is it something you should incorporate into your garden?
Nitrogen is a plant macronutrient – meaning that along with phosphorous and potassium, it is required in greater quantities than all other nutrients. In general, nitrogen is considered the “greening” element. It is primarily responsible for plant growth, but also utilized in many aspects of function and development. Due to the scope and scale of its usage, nitrogen is easily depleted from soils.
Besides cover crops, alternative methods to replenish nitrogen include adding compost, manures or fertilizers. Certain plants, mainly those in the legume family (fabaceae), are able to utilize atmospheric nitrogen and with the aid of soil bacteria (primarily rhizobium) turn that free nitrogen into a usable form: ammonia.
Fall is a great time to grow cover crops because the most efficient of them prefer cool temperatures. Additionally, a summer vegetable garden has pulled a lot of nitrogen out of the soil, so the time is ripe to replenish. Cover crops can also be planted between mature plants and fruit trees. The most common fall cover crops are clover (trifolium), fava beans (broad beans, horse beans), lupines, peas and vetch (vicia). The goal is to plant dense to ensure abundant root mass.
▪ Example: For fava beans, the target is one plant per square foot.
▪ Direct sow the seeds of your desired plants starting in late September into November.
▪ Most of these plants will reach maturity in late spring, just before summer vegetable gardening starts up.
As stated earlier, legumes have a symbiotic relationship with rhizobium and other bacteria, which is required for the nitrogen fixation to occur. Most soils have this rhizobium, however it is generally a good idea to buy the inoculate. This is found at various online sites.
Simply coat the seeds with the inoculate and plant. You will know if you have rhizobia only after you pull out a plant and see root nodules. These nodules form a few weeks after the bacteria have invaded the roots. The bacteria are reliant on sugars from the host plant, so if it is stressed from water, temperature or other factors, the bacteria will not be as abundant. The nodules in most annual cover crops will be pinkish-red. Midseason if you pull a plant out and don’t see nodules or none that are pink, you will need to inoculate when you plant the following season’s cover crop.
To achieve maximum nitrogen deposition, it is critical to turn under the plant material or cut it and let it decay on top of the soil. Keep in mind, the nitrogen is in the plant and not in the surrounding soil or nodules. Because the amount of nitrogen is highest when plants are flowering and before they set seed, this means bad news for fava bean/pea lovers. A good idea if you want to grow fava beans/peas for both consumption and a cover crop is to allocate some for you and some for your soil.
These same tactics apply to soybeans and green beans, which are two nitrogen fixing summer cover crops. However, you’ll often see green beans omitted as a cover crop, as they are not nearly as efficient a nitrogen fixer relative to other legumes.