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Profitable placement

Agriculture.com Staff 07/06/2010 @ 5:22pm

Many farmers have long been concerned about nutrient stratification in reduced- tillage systems. Last fall, Dave Sovereign, a Cresco, Iowa, farmer, posted his experience, concern, and questions in the Crop Scouting section of Agriculture Online(tm):

"This past growing season some corn showed potash deficiency symptoms," Sovereign related. "Soil tests indicated a high level (of fertility) in the top 4 inches of the soil and very low in the 4-8-inch range. Has anyone else had this type of experience?"

We passed Sovereign's question, and a few of our own, along to Iowa State University agronomist Antonio Mallarino, who conducted a major research project on stratification in the late-1990s.

Mallarino says it's common to see a high potash (K) soil test in the top 4 inches of the soil profile and a very low test level in the 4-8-inch level.

"This is one reason explaining the crop response to deep-band placement we have found in many Iowa no-till and ridge-tilled fields," he says.

Purdue University agronomist Tony Vyn has seen similar patterns throughout Indiana. "Soil exchangeable K concentrations in the top 4 inches are often double those at the 4-8-inch depth when fields are not moldboard plowed," he says.

"But," says Mallarino, "our research in dozens of Iowa fields indicates that the most common cause of spotty K deficiency symptoms is just very high spatial variability of soil K. Spots with low soil K are very common even in fields that average medium or high. And many of these spots are too small to identify (even by dense grid sampling) and too small to be treated."

Vyn says, "High within-field variability in exchangeable K is another reason not to be complacent even with a medium soil-test K level."

Mallarino says it is also very common for the crop to grow out of these potassium-deficiency symptoms.

"Anything that restricts root growth, especially when corn is between 1 and 3 feet high, may result in potassium-deficiency symptoms," Mallarino says. "Many factors can cause this, including diseases and dry topsoil. It is fairly common to see deficiency symptoms fade away after a good summer rain."

Many farmers have long been concerned about nutrient stratification in reduced- tillage systems. Last fall, Dave Sovereign, a Cresco, Iowa, farmer, posted his experience, concern, and questions in the Crop Scouting section of Agriculture Online(tm):

There were some surprises in the Iowa study. For one thing, whether potash was broadcast on the surface or injected had more of an effect on yield than did the placement of phosphorus (P) in stratified soils.

Whether potassium was broadcast or injected had a statistically significant effect on yields at only four sites out of 22 in one phase of the project. At those sites, deep-banding produced higher yields than the other placements. Even when data from all of the sites were combined, deep-banding still increased yields by a statistically significant amount, however.

Because the yield responses to deep-banded K usually were small, the cost effectiveness of this placement will be largely affected by differences in the costs of application and by soil moisture conditions in late spring or summer that cannot be predicted in advance, says Mallarino. "The results suggest that crops respond to deep-band K even two years after the application. Thus, infrequent deep-banding, perhaps every 3 to 4 years, could still increase yields and increase the profitability of this practice," he says.

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