MAINTAINING SOIL FERTILITY

In previous sections of our treatment of sustainable agriculture, we have discussed trophic issues and two broad approaches to decreasing soil losses; the CRP and conservation tillage systems. This section focuses on methods for preventing depletion of soil nutrients, in part by maintaining the soil's inherent fertility, and thus minimizing the need for inorganic fertilizer supplements. While it is likely that many farmers will never be completely free of such supplements, much can be done to minimize their use. Treatment below basically involves a list of possible techniques:

Conserving soil does this, of course. For example, use of conservation till in the US corn belt can reduce losses of phosphorus by 34 - 91 %.

Crop rotation can also decrease the need for inorganic supplements -- and reap other benefits as well. I hope that those of you who garden in the future keep the benefits of rotating your "crops" in mind! For example, when soil depleters, such as grains, are rotated with soil builders, such as legumes nutrients can be added and soil structure can also be improved. Rotation also can have beneficial effects on pest reduction, and mulches (as in cover crops during winter) can decrease weeds and increase retention of soil. The effectiveness of rotation at boosting soil fertility depends on the legume, how long it is there, whether part of it is harvested before it is plowed in, etc. However, a well-managed legume rotation can add 150#/acre of nitrogen --about half the inorganic nitrogen applied to corn in the Midwestern US!

Economically viable use of crop rotation may require financial incentives or rewards if it involves losing grain productivity every other year on the land. Such incentives could be derived if alfalfa and other forage crops were eligible for crop price support subsidies (something that would benefit those attempting to mix livestock and crop production in an integrated, sustainable system as well). The Leopold Center (spring '06) of Iowa State University recently reported on how various rotation systems compared in terms of soil quality ratings and profits. They found that rotations of at least 5 years duration that included at least 3 years of forage crops between corn and soy plantings yielded better soil quality ratings and more profit than did continuous corn plantings. For the comparison with a 2-year corn/soy sequence, they found that soil quality ratings were better for the rotations involving forage crops, but that rotations involving forages were not more profitable. Clearly, if we want to build soil and enhance profits, we need to find a way to make forage rotations economical.

An alternative to conventional year-to-year rotation, when removing the land from crop production every other year or so isn't possible, is to plant legumes between crop rows in late summer, and to plow them in the next spring. (This only works well in relatively mild climates such as we have here in the Willamette valley of course.)

Plants planted like this (during the non-crop growing season) are often referred to as "catch crops." These prevent leaching of nitrogen and other nutrients during the time of year when no crop is on the land -- i.e. during fall, winter, and spring rains here in the Willamette Valley.

Monitor more closely the nutrient content of soils, and adding only what is needed. This can often reduce inputs of inorganic fertilizers greatly. The nutrient content of soils often varies greatly from field to field on one farm, and different amounts are left in the soil at the end of the season, depending on what was planted there that year. Some farmers use GPS (Global Positioning System) devices coupled with soil testing to monitor and guide the administration of fertilizers to each part of the field very precisely, rather than just giving all parts of the fields the maximum to be sure that all receive enough.

In addition to savings in fertilizer purchase costs when fertilizer application rates are diminished, the American Carbon Registry will credit farmers for decreasing emissions of the greenhouse gase, nitrous oxide, associated with nitrogen fertilizers. Basically, the registry pays farmers to decrease their nitrogen fertilizer inputs, using one of the methods mentioned just above or below this paragraph or others. Farmers can earn "carbon credits" that could be sold on the market, once the demand for such credits is there. [Frontiers in Ecology and the Environment 9/12].

Band fertilizer in the rows, rather than broadcasting it over the entire field. Modern equipment makes this possible, and the fertilizer is then largely available to the crop rather than to plants all over the field, so this approach not only decreases the amounts of fertilizer that must be applied, but also has weed control benefits.

Apply fertilizers only at certain seasons. This would involve applying fertilizer only when the crop is there (or nearly there, as when it has just been planted or just before that) and able to take it up. Farmers often apply fertilizers in the fall instead, and most then simply leaches out from the soil. (It isn't that farmers are unintelligent; they sometimes feel constrained apply fertilizers at inefficient times because those are when they have free time and equipment!) However, increasing understanding of the consequences of excessive fertilizer use (both on-site and off-site) have helped many farmers to adjust the timing of their fertilizer applications, with a consequent decrease in amounts that must be applied.

Utilize manure more effectively. (See discussion of manure management under trophic issues.)

Utilize crop residues more effectively: Complete on-site recycling of corn and wheat residues in the US could replace 1/3 of the N, 1/3-1/5 of the P, and 100% of the K now applied to those fields in inorganic form. In fact, the 1985 and 1990 Farm Bills linked eligibility for federal farm Ppogram benefits to a Crop Residue Management Action Plan, designed to increase water conservation and decrease soil erosion. Nationwide, about 70% of straw and stover as of 1999 was left on fields after harvest in the US, and their direct recycling into the fields was the leading method of residue disposal in most US farming regions. However, this means that about 30% of residues are used in other ways, either by being burned (as here in the Willamette Valley with a portion of grass straw residues), baled for animal feed or other ways. (Increased interest in using such material to produce biofuel-based ethanol will likely further reduce return of nutrients from these materials to soils...) If these materials were left on site, needs for inorganic supplements could decrease. (Of course, nutrients from fresh organic material like this aren't available immediately to plants, requiring microbial action for their release. Since this material has a high ratio of carbon to nitrogen (C:N), and decomposers need N to do their work, N availabiity may, in fact, be temporarily reduced when the material is first being worked on. You may have experienced this problem in your own garden, if you've added sawdust or straw to enhance oganic matter in your soil, then noticing that your plants look somewhat yellow or don't grow as well? The N shortage is temporary, of course, and can be alleviated by paying attention to the provision of N-rich materials as well.)

There is also increased interest in producing "biochar" from crop residues or other agricultural wastes. Biochar is similar to charcoal except it is produced by heating biomass in a low/no oxygen environment so that it doesn't combust. Pre-Columbian Amazonians are believed to have covered burning biomass with soil to produce biochar, and are believed to have used it to enhance soils. Biochar enhances soil abiolity to retain nutrients, water, and chemicals; may decrease emissions of the greenhouse gas, N2O, nitrous oxide, because the biochar retains nitrogen; and may also reduce emissions of CH4, methane, from soils, these emissions otherwise being associated with decomposition of organic material under conditions lacking oxygen. The suggestion is that farmers could produce biochar from crop residue, perhaps as part of also creating biofuels from the residue. Centralized pyrolysis plants could produce liquid fuels, gases, and biochar; farmers could also operate their own low-tech kilns to make it, or trucks equipped with pyrolyzers could travel from farm to farm.

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This page is maintained by Patricia Muir at Oregon State University. Page last updated Nov. 25, 2012.

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