Dave Mengel

How do you maximize yield and minimize fertilizer costs while you’re riding a price rollercoaster?

“Soil testing is still your best tool—you can’t manage P and K without it,” according to Kansas State University Soil Scientist Dave Mengel.

Speaking to farmers at the recent Agronomy Institute in Parsons, Kan., Mengel, who manages K-State soil testing lab, noted that measuring soil nutrients gives producers an excellent indication as whether their ground will reward in investment in phosphorus and potash.

In simple terms, Mengel said, in soil with a phosphorus content of less than 15-20 ppm, or below 100 ppm on potassium, crops will likely respond to a P and K application.

“The bottom-line is easy,” Mengel observed. “If the soil test is low, fertilize.”

The K-State soil scientist also noted that producers may be short-changing the yield potential of their soybean crops.

“We’ve gotten into the habit of fertilizing corn because it responds well and then letting the residual nutrients take care of the following bean crop,” he said. “That may not be adequate. Keep in mind that, if you have tests that are low, you really need to think about fertilizing every crop, every year.”

Explaining that K-State recommendations are based on 95 percent of optimum yield, he said producers get those recommendations figured on both a “feed the crop” and a “buildup” basis.

Feeding the crop means that the suggested rate is intended to provide crop nutrition for a single growing season while the build-up approach allows producers to build the P and K levels in their soil over a given number of years.

In volatile times, Mengel acknowledged, farmers may want to lean toward the single-year approach. Additionally, on rented land the incentive is probably stronger to provide just what the crop needs.

Building soil nutrient levels, however, gives producers the flexibility to reduce rates, or not fertilize, in years when fertilizer prices spike because they already have adequate P and K levels for that particular crop, he pointed out.

Mengel also suggested that when phosphorus tests are low—less than 10 ppm—producers should consider using a phosphorus-containing starter, especially when planting in cool soils.

Regarding the type of phosphorus, Mengel said that liquids are not better than dry formulations and low-salt types are no better than the standard.

Asked about the use of Avail, a coating for dry phosphorus or an additive for liquid P which is designed to increase availability, Mengel said test results have been erratic. While the response has been excellent at times, it has been poor in other cases. Suggesting that research has not yet pegged the factors which make the use of Avail beneficial, Mengel invited producers to “take a look, but take care.”

The K-State agronomist also told farmers that there are alternatives to soil testing. Optical sensing systems such as “GreenSeeker” have been very successful in determining the amount of nutrients available to a crop, he said.

Citing K-State economic analysis revealing lost profitability due to inaccurate fertilizer application, Extension Crops and Soils Specialist Doug Shoup discussed good soil sampling techniques.

For soil pH, phosphorus and potassium measurement, Shoup suggested sampling at a 6-inch depth. In homogeneous fields where yields are consistent, a random approach to gathering samples works fine, he said, but he urged farmers to keep unique areas in mind for separate sampling.

Regarding the timing of sampling, Shoup recommended late winter or early spring for corn and sorghum and prior to planting in the fall for wheat.

Sampling at the same time of year, on a three-five year basis, is also important as is waiting for 30 days after a lime application, he said.

Balancing time and level of accuracy, Shoup said 10-20 cores should take for a homogeneous field or area.

Regarding nitrogen, the agronomist explained that K-State’s recommendation takes into consideration N required for yield goals, balanced by organic matter content, previous crop and other credits. Following a soybean crop, for example, the rule of thumb for the credit is one pound of nitrogen for one bushel of beans harvested.

For organic matter credit in the fertilization of summer crops, the formula is 20 pounds of N credited for every percent of organic matter. The credit for winter crops is somewhat less.

Noting that corn and wheat are more dependent on sulphur than soybeans are, Shoup said that he is seeing some need for additional sulphur.

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