Part 2: Growing Green Manures with Limited Water
Our most limiting factor in the Northern Great Plains is water. Irrigation is largely limited to those areas near rivers because most water pumped out of the ground is so laden with salt that it will ruin the soil after only a few years of application. The reason that dry land farmers normally grow only one cash crop every two years is to allow water to build up in the soil for the cash crop year.
In years past, summer fallow was the most common way to conserve water the year before a cash crop was planted. This was done by cultivating the fields three or four times during the summer to keep the weeds from growing, using the moisture and going to seed. One problem with this program was that it led to an increase in water and wind erosion and by the end of the summer there was little left on top of the ground to protect the soil. The highest risks for erosion came in the late winter, especially if the fields had not been seeded to winter wheat and were completely bare. Also, with nothing on the soil, snow catch during the winter was reduced.
This problem was reduced by dividing large fields into long narrow strips which usually ran cross ways to the prevailing winds. In recent years, chemical fallow has become popular. With this system Roundup® is applied to the field to kill weeds during the fallow season. This leaves the stubble standing from the previous year to catch snow and protect the soil from erosion. But continuous use of Roundup® has resulted in the appearance of super weeds which are resistant to Roundup®, as well as a more serious problem where we are finding glyphosate, the active ingredient of Roundup®, and its breakdown products building up in the soil, in the surface water and even in the rain. More questions are being asked about connections between these residual chemical traces and a host of chronic diseases. And, of course, there is the high cost of chemical agriculture in general, which is a problem especially with today’s low non-organic grain prices. After the successful results of the “Stop Acid Rain” campaign from years ago, I think it is time for a new campaign: “Stop Roundup Rain.”
The good news is all these difficulties brought on by chemical agriculture can be avoided by using green manures to build the soil. For example, legumes, thanks to the bacteria which live in nodules on the plant roots, take the nitrogen from the atmosphere and put it or fix it into a form of nitrogen that the plant can use—hence nitrogen fixation. Over the years, we have found on our farm that rotating our legumes is as important as rotating our cash crops in breaking up cycles of disease, pests and weeds. We use the following soil building crops in our rotation: alfalfa, yellow blossom sweet clover, peas and lentils. We also plant buckwheat though it is not a legume. (I will include a chart at the end of the final part of Chapter 3 with a comparison of these soil building crops.) We plant one of these soil-building crops every other year in our rotation. Even though buckwheat is not a legume, it has the advantage of smothering weeds and solubilizing phosphate. It also has a long taproot, which opens up the soil and may explain a significant increase in stored soil moisture in the year following a buckwheat green-fallow crop, compared to other green-fallow crops.
Some worry that the water required in growing legumes will reduce the yield of the cash crop the following year. However, a research project on our farm in the early 90’s demonstrated that an early June termination of a crop of yellow blossom sweet clover used no more water than the conventional summer fallow.
Green manure crops, except buckwheat, are normally terminated by early to mid-June, before the normal rains falling that month are finished. In areas of very low rain fall, the green manure crops might be better planted just after harvest—the same day as the grain is cut, if possible—and then it can grow through the fall when normal rains arrive. However, they would also be competing against weeds during this time. But no termination would be necessary, as the annuals would be killed naturally by the cold of the winter while the biennials, like yellow blossom sweet clover, or short perennials, like alfalfa, would continue to grow the next year.