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Corn+Soybean Digest

Give P-K A Quick Kick

Despite the tight farm economy, it can pay to rapidly build up phosphorus (P) and potassium (K) when soil levels are low to medium. That's according to an Ohio State University economic study.

This finding is true whether an entire field tests low or if there are low-testing “islands” within a field. These “islands” come to light when soil testing is done in grids of about 2.5 acres using site-specific technology.

The rapid buildup concept differs from traditional thinking where the buildup of soil fertility was done on a gradual basis.

“Financial analysis indicates that if it's profitable to build soil P and K levels, it is most profitable to do it as quickly as possible,” explains Jess Lowenberg-DeBoer, a Purdue University ag economist. “A buildup program of one to two years may be most appropriate under today's economic and risk management constraints. A rapid buildup reduces the risk of lost profits.”

Independent crop consultant Don Brucker, Melvin, IL, has found that the rapid buildup of P, K and lime pays off in the real world as well as in university studies. He says this is especially true on high exchange-capacity soils.

“The faster the buildup on low-testing soils, the faster the yield response,” Brucker says. “We have one newer client who divided an 80-acre field — half corn and half soybeans — in two. He's taken a 120' swath down the middle of each and applied the traditional 200 lbs/acre of 18-46-0 and 200 lbs/acre of 0-0-60 every other year. On the rest of the field he's followed our program of fertilizing by exchange capacity, which gave him a rapid buildup to optimum levels of P, K and lime.”

Corn yields after two years are 4.3 bu/acre higher and soybeans after three years are 2.1 bu/acre higher in areas where the grower followed Brucker's recommendations.

Brucker samples soils by management zones ranging from 2 to 15 acres and averaging 7-8. He varies fertilizer and lime rates by zone.

“After the first year or two, when the exchange capacity reaches optimum, it's less costly to maintain that level than to use the traditional application amounts,” Brucker reports. “Our average cost to maintain P, K, micronutrients and lime is only $10-12/acre.”

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