Montana State offers reference sheets to calculate fertilizer rates from soil tests

BOZEMAN - At the closing of each growing season, growers start thinking about the next year, and soil testing is an important part of that planning. The Montana State University Extension Soil Fertility Program recently released two online reference sheets to help growers interpret soil test reports and calculate fertilizer rates.

"At first glance, soil test reports can seem daunting," said Clain Jones, Extension soil fertility specialist and professor in MSU's Department of Land Resources and Environmental Sciences. "They list the soil properties analyzed, a numeric test value in the form of a number or bar graph, and often have an interpretation of that value. Looking for properties with values that are interpreted as 'high' or 'low' can help correctly focus attention on specific nutrients or issues such as salts."

Extension's "Soil Test Interpretation" reference sheet can help growers determine how soil test levels impact plant production and whether levels are low or sufficient by MSU standards. The soil nutrient level that is sufficient for crop growth is often called the critical level. Adding a nutrient that is already at or above the critical level will likely not increase yield, whereas adding a nutrient when levels are low has a good chance of improving yield, Jones said.

Some laboratories also provide fertilizer rate recommendations in soil test reports. If there is no recommended rate or the lab does not use MSU guidelines, then the soil test values are used to calculate fertilizer rates. The process is outlined in Extension's "Fertilizer Rate Calculations" reference sheet.

Fertilizer rates for phosphorus, potassium and micronutrients such as zinc are based on soil test values that estimate plant availability, Jones said.

"Determining fertilizer rates for these nutrients is relatively straightforward," he said. "We provide tables that specify the amount of these nutrients that should be added based on the crop and the soil test result."

Jones said nitrogen fertilizer rate calculations can be more complicated. Plant-available nitrogen can be gained or lost relatively quickly during the growing season. Therefore, fertilizer nitrogen is not only based on crop need and soil nitrate from the lab report, but it is also adjusted for potential nitrate gains and losses specific to a given field or garden. These adjustments are based on several characteristics, including the level of soil organic matter, the prior crop, when the soil sample was taken and the depth of the soil sample.

"We provide estimates for each of these adjustments based on Montana field research," Jones said.

Proper fertilization starts with knowing what nutrients are available in the soil. Extension's online series, The Soil Scoop, offers additional reference information on soil testing, including how to get a good sample, what information to supply to the testing laboratory and which soil tests to request. The series' reference sheets also provide short summaries on other soil fertility topics.

To best understand soil test interpretation and calculating fertilizer rates, visit MSU MontGuides "Interpretation of Soil Test Reports for Agriculture" and "Developing Fertilizer Recommendations for Agriculture," in addition to MSU Extension bulletins such as guides for gardeners.

For additional information and reference sheets, visit Extension's publications and presentations site, soil fertility site or contact Jones at 406-994-6076 or [email protected].