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Corn+Soybean Digest

Soil Testing May Help Minimize Potassium, Phosphorus Costs

With fertilizer prices the highest they've ever been, the most important thing row-crop growers can do is take a soil sample and have it analyzed, say Purdue University agronomists Jim Camberato and Brad Joern.

"A soil test is critical to making sure soil pH is good for crop production and managing fertilizer nutrients like potassium (K) and phosphorus (P)," says Camberato, Purdue Extension soil fertility and plant nutrition specialist. "This soil test should analyze K and P levels, which will help a grower determine whether or not those nutrients need to be added."

Purdue agricultural economists expect to see prices for potash at or more than $900/ton, anhydrous ammonia around $1,000/ton and monoamonnium phosphate and diamonnium phosphate at $1,100 or more.

"Because of these prices, soil testing is critical," Camberato says. "You can't just look at your soils and know the nutrient level. The only way to know what your nutrient levels are is to take a soil sample and have it analyzed by a laboratory."

If the results show high levels of potassium and phosphorus, the grower then has the opportunity to withdraw those nutrients this year and delay the purchase and application of fertilizer to another year, Camberato says.

"By knowing a field's nutrient levels, you can calculate how long those nutrients can be taken from the soil," he says. "It's kind of like banking; you put nutrients in and after a while you have enough built up that you can take some out."

(To learn more about banking in K and P, go to Dealing With High-Priced P and K Fertilizer.

If a particular field is at or below the critical level, or the level when there is still a good chance of getting a yield response to nutrient additions, a producer should not skip fertilizer application, Camberato says. But if a field is above the maintenance limit or the level where there is very little chance of getting a yield response from additional K and P, a grower may be able to avoid paying high prices for fertilizer by using the nutrients already in the soil, he adds.

Brad Joern, Purdue Extension nutrient management planning specialist, says P levels will decrease each year on average by about 1 ppm (2 lbs./acre) for every 20 lbs. of P oxide/acre removed by the crop.

"If soil test results display levels greater than 45-50 ppm, this means you don't need to apply fertilizer for a pretty long time," Joern says. "Once P levels reach 40 ppm for corn and soybeans and 50 ppm for wheat and alfalfa, we don't recommend applying."

The same thing can be done with K, but growers need to realize that K fertilizer levels will change much more quickly than P levels, he says.

Joern says it would not be unusual to expect K levels to change as much as 10-15 ppm in a given year, and even quicker in sandier soils and a little slower in heavier textured soils. Once K levels reach 138, 150, 175 and 200 ppm for soils with a cation exchange capacity (CEC) of 5, 10, 20 and 30 meq/100g, respectively, we don't recommend applying, he says. The CEC is the amount of positive ions like K, calcium and magnesium that a soil can hold.

"The bottom line is fields with high K and P soil test levels can go without fertilizer additions for at least a couple of years to offset high fertilizer prices," Joern says. "When soil test K and P levels fall below the maintenance limit, regular fertilizer additions are recommended to help maintain optimum yield."

More information about K and P recommendations for corn silage, wheat grain, wheat straw and alfalfa, as well as corn and soybeans is available at Dealing With High-Priced P and K Fertilizer. For more information, contact Camberato (765-496-9338, [email protected]) or Joern (765-494-9767, [email protected]).

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