Gardeners often spend a fortune on fertilizers during the season to keep plants healthy and well fed. Yet some of us leave the soil unattended in the winter and early spring only to start the cycle all over again.
Plant cover crops like clover to put nitrogen back in your soil!
A few years back, I became a fan of cover crops to add nitrogen and to keep soils active and healthy during the normally “fallow” season. (Note that I live in an area of California where our growing season is almost year-round. However, there are certain green manures, such as hairy vetch and rye, that are suitable for colder climates, provided they are planted early enough before the winter.) As I was spreading some clover I got to thinking about the cost of cover crops vs fertilizers. The numbers shocked me.
Comparison of fertilizers vs cover crops:
| Fertilizer | Price | Nitrogen | Extra Info | Price/lb N |
|---|---|---|---|---|
| Berseem Clover | $8/lb seed | 125 lb nitrogen/acre | 30 lb/acre | $1.92/lb |
| Banner Fava | $6/lb seed | 150 lb nitrogen/acre | 200 lb/acre | $8.00/lb |
| Common Vetch | $8/lb seed | 110 lb nitrogen/acre | 90 lb/acre | $6,54/lb |
| Blood Meal | $2/lb | 12% | none | $16.67/lb |
Table disclaimer: these figures were taken from a variety of sources that are all subject to some scrutiny. Territorial Seed Company publishes information on cover crops in their catalogs and how much you end up paying for any of the items can vary drastically and how much REAL nitrogen you get from a crop can also vary significantly.
That said, unless your Clover does VERY poorly, you are going to get better results since it is 20 times less expensive with conservative numbers. We purchased Crimson Clover for $1.5 /lb by purchasing 10lbs. Plug in those numbers and we could be paying $0.36/lb of nitrogen.
One important thing to note is that you should choose your cover crops carefully. According to this article in Organic Gardening, some cover crops like rye are allelopathic, meaning that they suppress the growth of neighboring plants. And it’s also important to wait 2-3 wks after turning under your cover crop to make sure that the nitrogen will be available for your veggies.
And now for the punch line: Now I understand the lyrics to “Crimson and Clover”: you are supposed to plant it OVER and OVER! Who woudda known Joan Jett was a gardener! 
All legume seeds need to be inoculated with the appropriate species of rhizobium bacteria to insure that you get the benefits of nitrogen fixation. The bacteria live in the roots of the plants and they are the organisms that capture nitrogen from the air and make it available to the plants.
The questions always is: It there bacteria already there, or do I need to buy the bacteria. This is the argument that I hear going back and forth.
Not necessarily. I’m a restoration ecologist and I have never found a native legume that was not nodulated and apparently fixing Nitrogen. I have also found all exotic naturalized legumes that I’be examined to also fix N. A good indication that N is being fixed is if 1) the roots have nodules and 2) the nodules have are red or pinkish when cut open. I suggest planting with and without inoculant. Keep some physical separation between the plantings. After the plants are established, carefully dig some up and gently wash the soil off the roots. Look for nodules and if they are present, cut them open. If the plants are nodulating without inoculant, then save you money next time and pass on the inoculant.