Farmers who cut back on high-cost fertilizer inputs in 2009, and then had record yields, are wondering how much fertilizer is actually needed. For 2010, let fall soil tests be your guide, says Stewart Seeds agronomist Paul Brautigam.

Adequate levels of nutrients like phosphorus and potassium can go a long way in boosting yield potential, especially if next season brings more "normal" weather conditions.

"We can’t control the weather, but we can control, to a greater extent, our soil fertility," Brautigam says. "We were fortunate that timely rainfall, and very little plant stress this year, helped overcome average soil fertility. We may not be as lucky in 2010 if we have any drought or excess heat.

"Soil testing and fall fertilization would be good preparation for what Mother Nature may throw at us."

You can sample through December for spring fertilizer applications, in most instances, as well as sample in March to April for fall applications. University specialists note both timeframes generally have the lowest amount of testing variability.

They also suggest avoiding late winter sampling of heavy-textured soils, since freezing and thawing can release potassium from clay and give unusually high test levels. More in-depth advice can be found from < a href="">Purdue University.

"If you’ve been able to apply fertilizer already this fall, you’re in good shape. If the late harvest has not allowed a window, there is still time," Brautigam says. "Test soils soon and commit to needed fertilizer right away. Apply as the weather allows, even if you have to wait until spring."

GPS and variable-rate application are two of the best tools for use with soil testing. Brautigam suggests using one of three systems to soil test: grid sampling of 2.5- to 10-acre grids (the larger grids for more uniform fields); sampling by soil type; or using a modified grid (a combination of grid, yield zones and soil type) for fields with little or no soil test history.

"The key to all three systems is the GPS coordinates," he says. "Using the coordinates allows consistent testing in the future and variable-rate application for maximum return on investment of fertilizer dollars. Farmers will find an accurate test history over a decade or more is priceless."

Once you have test results, Brautigam recommends taking action as soon as possible. Lime application to achieve proper soil pH should be the first step, he says. Correct pH allows nutrient release already stockpiled in the soil profile.

Farmers should also address critical soil test findings. For example, corn and soybeans need 30 pounds per acre of phosphorus for a critical level and 200 to 300 pounds per acre of potassium for a critical reading, depending on cation exchange capacity (CEC).

"Phosphorous and potash prices have fallen significantly from a year ago, to below the 5-year average," he says. "For nitrogen, demand overseas has stabilized and is on the rise. Since natural gas prices are relatively low and inventories are normal, it may be a good time to buy.

"We are advising customers through not to run the risk of 'hidden hunger,' or starving crops. Make the time and investment to replenish the soil bank for the 2010 growing season. Even higher yields are possible when you focus on strong fertility balanced with optimum pH."