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NOTES By Richard Glenister |
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April 27, 2005: One of my major concerns about the transition to organic is my ability to maintain soil fertility. Conventional grain farmers routinely apply nitrogen fertilizer as a hedge to maximize yield, even on fields of high fertility. Organic grain farmers don't have the luxury of inexpensive, readily available nitrogen fertilizers. Instead, they must carefully manage their soils, patiently building and maintaining soil fertility with legume fallow crops, crop rotations, manures and composts. Here in upstate New York, organic farmers often transition former hay fields since they most easily meet the certification requirements. In these situations, the initial soil nitrogen status may be unknown. As fields move through the rotation, moreover, the decrease or increase in soil nitrogen depends on (among other factors) the weather over the past few years, each year’s crop yield and the success of the legume fallow crop. Unlike with other fertility elements such as phosphorous and potassium, many factors can cause soil nitrogen loss. Even plowing and cultivating stimulates nitrification, denitrification and subsequent nitrogen loss. Historically, conventional grain farmers have estimated nitrogen fertility by using the pre-plant soil nitrate test. Unfortunately, nitrate is typically the smallest and most variable fraction of the total supply. The soil nitrate level has been shown to vary radically with soil temperature, moisture and numerous other factors. It is also the form of soil nitrogen most susceptible to leaching and loss. About 20 years ago, in an effort to eliminate some of the uncertainty, a pre-sidedress nitrate test was developed for conventional corn production. This test had the advantage that application of nitrogen fertilizer could be delayed until soil nitrate values were higher, so that nitrogen could be applied only where needed. Unfortunately, the variability of soil nitrate was still a factor, and unless test results showed very high levels the usual practice was to apply nitrogen anyway as a hedge. Obviously this is not practical for an organic farmer. A new way to measure soil nitrogen Soil scientists have long known that the total soil nitrogen content of even highly depleted soils was many times higher than the nitrogen available to the crop. The total soil nitrogen of an acre-furrow of soil is typically 2,000 to 4,000 pounds per acre, far more than the 200 pounds per acre required by a corn crop. At any given moment, soil nitrate can account for only 10 to 30 pounds of soil nitrogen per acre. It is apparent, then, that there is some component of total soil nitrogen that acts as a reservoir for the growing crop. The other obvious form of nitrogen is ammonium. Unfortunately, the ammonium form is even more volatile and harder to measure than nitrate. Soil scientists speculate that this more readily available fraction of soil nitrogen probably consists of the plant material from previous crops and soil microbes both living and dead. In the 1990s, two soil scientists from the University of Illinois, Richard Mulvaney and Saeed Khan, were trying to explain why many soils that were judged low in available soil nitrogen by soil analysis and cropping history gave no increased yield when nitrogen fertilizer was applied. Their goal was to avoid the unnecessary over-fertilization that causes water pollution. They showed evidence that there are two soil nitrogen fractions: first, a readily available fraction consisting of sugar-like compounds loosely defined as 'amino-sugars;' second, a less available fraction consisting of amino acids and protein-derived compounds called the 'amino acid' fraction. Both of these fractions contained 200 to 400 pounds per acre of the nitrogen needed by a growing crop. When they compared the yield response of a wide variety of soils, they found that the amino-sugar fraction correlated well with the resulting yield response. They observed that if the soil amino-sugar content was over 245 parts per million (ppm), there was no yield increase to applied nitrogen. And in addition, if the soil amino-sugar content was less than this 245 ppm level, the yield response was inversely related to the 'amino-sugar' content. The lower the amino-sugar level, the more dramatic the yield response. They subsequently went on to develop a fairly simple practical test, which they called the “alkali-labile soil derived” nitrogen test, to measure this amino-sugar fraction. Since it is performed in a wide-mouth Mason jar, we refer to it as the “Mason Jar” soil nitrogen test. Unfortunately, this information is of little value to conventional farmers since they either rely heavily on applied fertilizer or, in the case of large livestock and dairy farms, they have an excess of manure which they must dispose of in any event. However, for the organic grain farmer this new test offers a way to monitor the success of his or her farming practices and can remove some uncertainly from decision making. The test is conducted on air-dried soil samples collected in late winter or early spring, leaving plenty of time to make decisions before the busy spring season. Since the components being measured are relatively stable, no complicated storage and handling conditions are necessary. The only precaution is that if there has been a recent application of manure or ammonium fertilizer, the test must be modified to correct for this with a pre-treatment step. Trying it for yourself In order to discover how this new test might be used, we assembled the necessary materials and equipment and began testing soils from our own farm. We collect at least 30 core samples to a depth of 10-12 inches from each field. Samples are spread out on a large cookie sheet and allowed to air dry in a warm place (80-90° F) until completely dry. Each sample is then pulverized and blended before being sub-sampled for the test. We have obtained values ranging from 200 to 350 ppm depending on the field and the stage in the soil fertility building process. For the most part, the test values follow trends anticipated by the cropping history and yield potential. Two situations gave somewhat surprising results, however. The first was a lower than expected value following one year of clover fallow. It may be that one year of fallow is not enough to accumulate and replenish this soil nitrogen fraction. The second was an old grass hay meadow from which the hay had been harvested for over 15 years without any added fertilizer. Despite a very low phosphorous level and low hay yield, the soil had test values of about 350 ppm. In a third case, a field tested following three years of alfalfa-timothy
sod and one corn crop gave a Mason jar test value of greater than
250 ppm. This suggests that a second corn crop might be feasible
in the crop rotation for this field. As test results accumulate
in the future, we should gain a more complete understanding of their
significance. Richard Glenister has been raising cattle on a small farm in central New York for many years, and is in the process of transitioning to certified organic mixed grain and livestock production. He wishes to acknowledge the generous assistance of Dr. Mulvaney with this project.
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