Soil tests may not show accurate nutrient availability

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Growers conducting soil tests this fall may be surprised at unexpectedly low potassium (K) levels. It’s likely, however, that fields tested in drought-stricken areas have plenty of nutrients waiting to move back into the soil, according to DuPont Pioneer agronomy experts.

“The K is actually quite safe in the plant residues, so growers shouldn’t be alarmed if soil test levels of this nutrient are lower than expected. Just be aware that more nutrients will be released into the soil with precipitation,” said Andy Heggenstaller, DuPont Pioneer agronomy research manager.

With little rain on most fields in 2012, K, absorbed by corn plants during the growing season, has not yet been released back into the soil from deteriorating corn stalks. In a drought year, K also can become fixed between clay layers until water moves through the soil again. Expect that K test levels will increase to more normal values if you can wait to sample following a significant fall rain event.

“Some growers may think it’s a better idea to wait and sample soil in the spring because it will give them a more reliable nutrient reading,” says Heggenstaller. “But I would caution against spring sampling unless this is your normal practices, because you would end up comparing apples and oranges and couldn’t rely on previous soil tests as a basis.”

Even though K levels from this year’s soil test will likely be lower than actual amounts, farmers can rely on crop removal rates and previous years’ soil test results as a guide to estimate next year’s K needs. To determine crop removal this year, multiply the field’s harvested bushels by an estimated 0.3 pounds of K removed per bushel of corn or 1.5 pounds of K removed per bushel of soybean. The calculated amount is a good estimate of how much K was consumed by the crop during the growing season and thus the minimum amount that should be replaced for the next crop if historical soil test levels were in the optimum range.     

“Depending on management style, this method of calculating K needs might not be the best long-term approach for managing K fertility, but in drought years it is often the best option for growers because of the variability in soil sampling,” says Heggenstaller. “I still recommend that producers test soils this year because it will prove useful when determining future fertilizer needs.”

Other key nutrients, including phosphorous (P), should not show as much variability in soil tests as a result of drought. Phosphorus does not get fixed in clay soils to the extent that K does and is not nearly as abundant in crop residues as K. In the case of nitrogen (N), most producers applied enough to achieve a high-yielding crop in 2012, but ended up receiving little precipitation and lower than anticipated yields. In these fields, it is very likely that extra N is present in the soil. Depending on rainfall between now and next spring, some of this N may be available for next year’s crop.  

Lack of water also may cause a drop in soil pH from previous years. Without precipitation, lime applied to help balance pH in spring 2012 cannot infiltrate the soil and take full effect. Additionally, dry soil conditions are often associated with increased salt concentration at the soil surface, which can also result in lower than expected pH test values.

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Robert Miller    
Colorado  |  October, 31, 2012 at 09:50 AM

A couple of points missed. Soil test K values are actually going to be higher this year (relative to 2011 and 2010) across much of the corn belt, as yields are lower and that topsoil dried out early last spring, thus there was less K removal than in past years. Fall reports form labs in IA and Illinois bare this out. Growers that had yields over 180 bu/ac (north central corn region) are likely to experience somewhat lower K levels. As to soil pH, yes the pH values maybe lower this fall, and all though it maybe the result of the failure of lime to react, it more the effect of the drought. With lack of rain soluble salts accumulate in the surface of the soil. The increase in salt affects the Lab pH instrument thus biasing the pH down. Its been noted in dry years soil pH my drop 0.5 units. The drop in pH is an artifact of the measurement. Last point. To the many growers across the corn belt who are bailing and removing corn stover, they should consider the value of nutrients that are being removed, and recognize that there will be increased fertilizer needs on these fields to meet productivity in the coming years. R Miller PhD.

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