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Soil testing may help minimize potassium, phosphorus costs

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Written Tuesday, September 23, 2008  

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, said Purdue University agronomists.

"A soil test is critical to making sure soil pH is good for crop production and managing fertilizer nutrients like potassium and phosphorus," said Jim Camberato, Purdue Extension soil fertility and plant nutrition specialist. "This soil test should analyze potassium and phosphorus 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 per ton, anhydrous ammonia around $1,000 per ton and monoamonnium phosphate and diamonnium phosphate at $1,100 or more.

"Because of these prices, soil testing is critical," Camberato said. "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 said.

"By knowing a field's nutrient levels, you can calculate how long those nutrients can be taken from the soil," he said. "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 potassium and phosphorus, go to http://www.agry.purdue.edu/ext/corn/news/articles.08/PKFert-0915.html .

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 said. 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 potassium and phosphorus, a grower may be able to avoid paying high prices for fertilizer by using the nutrients already in the soil, he added.

Brad Joern, Purdue Extension nutrient management planning specialist, said phosphorus levels will decrease each year on average by about 1 part per million (2 pounds per acre) for every 20 pounds of phosphorus oxide per acre removed by the crop.

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

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

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

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

More information about potassium and phosphorus recommendations for corn silage, wheat grain, wheat straw and alfalfa, as well as corn and soybeans is available at http://www.agry.purdue.edu/ext/corn/news/articles.08/PKFert-0915.html . For more information, contact Camberato at (765) 496-9338, jcambera@purdue.edu or Joern at (765) 494-9767, bjoern@purdue.edu .

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