Spectrum Agronomic Library

Knowledge is key to using your analytic results to their fullest. The Spectrum Agronomic Library provides you with useful information that will help you to better understand the complex science of agronomy. Our agronomists will be continually adding original and reprinted articles, so check the library regularly for new information.

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Building Up and Drawing Down Soil P and K Levels

It is common to hear people discuss fertilizer recommendations in terms of crop removal, plus or minus soil test buildup as if they were somehow disconnected from each other. In fact, they are two aspects of the same subject. Most of us fall into the habit of thinking that we are fertilizing plants. Except for foliar fertilizer or tree trunk injection, we do not fertilize plants… we fertilize soil. Because of this, soil chemistry will determine how much of the applied nutrients the plants will be able to take up. If a soil is low in phosphorus (P) or potassium (K), it will tie-up or “fix” much of the applied fertilizer P and K (P2O5 and K2O) into forms that are not available to the plants. This nutrient fixation is simply another way of saying that the soil is trying to build itself up in these nutrients… whether that is your intention or not. The soil is a reservoir for the nutrients that have been applied or generated by other means over the years. The nutrients that a plant gets in any one season are likely to be ones that have been in the soil for many years. Therefore you can think of fertilization as putting nutrients into one side of a reservoir while the plants are taking them out of another end. What happens inside of this nutrient reservoir is soil chemistry and microbiology. These processes, along with weather, determine how much access the plants have to the nutrients within the reservoir.

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Artificial Soil Media

Soybean Cyst Nematodes

Soybean cyst nematodes (Heterodera glycines) have been an increasing problem for soybean producers over the past decade or more. H. glycines is the only cyst nematode known to attack soybeans in the U.S. Most of the major soybean producing areas of the U.S. now recognize that SCN is present in at least part of the area. SCN is and obligate parasite of higher plants, more than 1100 species of plants have been found to serve as hosts. Fortunately, most are weed and crop plants not commonly found in soybean fields or cropping rotations. Populations of the nematode increase rapidly under continuous soybeans, but decline drastically during the first year under non-host crops.

Severe infestations of SCN can have a devastating effect on yields. Yield losses can range from slight to as much as 90% depending upon the degree of infestation, soil fertility, cultivar susceptibility, environmental conditions, and race of the nematode. Root systems of heavily infected plants are drastically reduced, necrotic and practically devoid of Rhizobium nodules. In Iowa, susceptible varieties yielded about 40% less in infested fields than in non-infested fields. In Ohio on a fertile, dark colored soil, varieties resistant to SCN yielded over 50 bu/ac, whereas those susceptible to SCN yielded from 24-39 bu/ac (a yield loss of from 52% to 22%). Another study using aldicarb to control SCN found that high SCN populations may reduce yields by 30 bu/ac.

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The Nutrient Uptake Process

Calculating Salt Index

Dr. John J. Mortvedt

Salt content is one of the most critical characteristics of fertilizers that should be considered when fertilizers are applied, especially with seed-row or “in furrow” placement.

Summary: Salt index (SI) of a fertilizer is a measure of the salt concentration that fertilizer induces in the soil solution. SI does not predict the exact amount of a fertilizer material or formulation that could produce crop injury on a particular soil, but it does allow comparisons of fluid formulations regarding their potential salt effects. As we all know, placement of some formulations in or near the seed may decrease seed germination or result in seedling injury.

Fluid fertilizers containing potassium phosphate as the source of K have lower SI values than those containing KCI. When applied near the seed, fertilizers with lower SI values generally cause fewer problems in seed germination or seedling injury. SI of any fluid formulation can be calculated using the SI values of the most common fertilizer sources. Dealers or growers then can select those formulations with lower SI values that best fit their needs.

Banding of nutrients has received much attention over the years. Usually, the fertilizer is placed at a depth greater than that of the seed to allow root interception of the fertilizer band as roots grow outward and downward in the soil.

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library/start.txt · Last modified: 2010/03/31 11:41 by wayland