If you didn’t fertilize last fall, soil sampling can be one of the best fertilizer management tools this spring. “If you don’t soil sample every year and you didn’t sample last fall, this is the year to do it, says Gary Hergert, University of Nebraska soil and nutrient management specialist.
“Farmers can control input costs; they can’t control the weather. But if they soil-sample, they will know what is out there and that will pay off. It’s not too late to sample this spring.”
Early spring sampling will also reflect any changes in nitrogen (N) from winter precipitation. And a look at drought history tells a tale.
“There are parallels between the droughts of 1934 and 2012,” says Drew Lerner, president of World Weather, Kansas City, Mo. “Last year joins the four previous major droughts in 118 years of data collection: 1906, 1934, 1936 and 1988.”
The resulting low yields in many areas may mean a higher than normal nutrient carryover, especially N. That’s why soil sampling should pay off.
The Midwest P and K management system is based on soil testing, response-based fertilizer application for low-testing soils or removal-based fertilizer application to maintain desirable soil-test P and K values.
“On most soils in the Corn Belt, P and K bond to soil minerals and don’t move very far from point of application,” says T. Scott Murrell, the Indiana-based north central director, International Plant Nutrition Institute (IPNI). “While there may be some loss from surface runoff, in most situations, P and K not taken up by the crop carry over for use in future years.
“Lower yields caused by the drought mean less P and K were removed with grain harvest. However, if corn intended for grain harvest was instead cut for silage, and that silage contained grain, P and K removal will be greater than planned,” he says.
Now is not the time to guess about P and K if you don’t have recent test results as the starting point for estimates, says Hergert. “If you don’t know what’s out there, don’t guess, soil-test.”
“When you take soil samples, it gives you a chance to evaluate where you are with P and K,” says Kyle Freeman, manager, new product development at the Mosaic Company in Plymouth, Minn. “Then you can adjust your nutrient management in the spring if needed. Based on fall soil tests, we’ve seen lower yields don’t necessarily mean you’ll see a huge jump in soil-test P.
“With or without soil sampling, plan for success by using best practices,” Freeman urges. “If you are planning on a 225-bushel corn crop, make sure P and K needs are taken care of.”
“If you have soil sample results from last fall, compare them with your previous test,” says John Sawyer, Iowa State University Extension soil fertility specialist. Sampling with dry late-summer and fall soil conditions may have affected results. “Do they look unusual? If so, you may want to resample in the spring or use previous trends in soil test results. If your normal sampling frequency was to not sample last fall, and your crop yields were not drastically affected by dry conditions last summer, don’t worry; apply what has been normal for recent years. If your crop yields were significantly affected by the dry conditions, then you can sample in the spring and adjust application rates in the spring or fall, if needed.”
If fall or spring soil test results look different than expected, Sawyer adds, “remember that reduced yields will have less nutrient removal. If corn was cut for silage instead of for intended grain then, especially for K, there will be more removal with the silage than for grain only because there is a lot of K in the plant vegetation. We’d like to maintain an agronomic optimum soil test level for the long term and best match inputs to removals.”
Murrell concludes, “If soil tests don’t change as expected based on past applications, drought-induced changes in soil chemical reactions are likely a significant part of the explanation.”