Use a PSNT to measure nutrients from manure

The Pre-Sidedress Soil Nitrate Test (PSNT) is a valuable tool for optimizing nitrogen fertilizer applications on fields that have received manure.

Manure applications on fields can help decrease commercial fertilizer use. Nitrogen (N) is one of the most valuable nutrients contained in manure. This manure N comes in different forms; some N may be available to the plant right away but the rest of it undergoes a process known as mineralization. This mineralization process will continue to make N from manure available to the plant for several years. So when soils have had manure applied, how do you know how much nitrogen is available for the plant when it needs it?

A Pre-Sidedress Soil Nitrate Test (PSNT) is a valuable tool that measures the N that is readily available to the plant. It is especially useful in corn fields to decide how much additional N to apply when the plant needs are high—typically when the corn measure 6- to 12-inches tall from the soil surface to the top of the leaf canopy. A critical level of soil nitrate measured by PSNT has been established for determining whether or not a field needs additional side-dress nitrogen for optimal yields.

PSNT testing takes some planning to put into practice. Typically in lower Michigan, the corn will be at the proper 6- to 12-inche height sometime after Memorial Day. That tends to be a very busy time for farmers, so planning which fields to sample and who is going to be responsible for this time-sensitive test is critical. In addition, a composite PSNT soil sample should contain 15 to 20 soil cores per field, so it does take effort to make the information more accurate. Ideally samples are taken 12-inches deep but if that’s not possible be sure to record your sample depth and be consistent. Mapping out fields, making sure equipment will be available and that personnel are assigned and trained to do the sampling is a good job to do before the weather gets nice!

Samples can be taken to Michigan State University or other labs and the cost is minimal. Once you have tests back, interpretation of the results can help determine whether side-dress N is needed and at what rates. Keeping track of this information over the years is especially helpful in determining if your manure spreading plans are on-track. With the rising cost of commercial N, PSNT testing can be a good use of your time and effort both in terms of profitability and environmental stewardship.

For more information, contact Roberta Osborne. Use those rainy days to make your farm more profitable by planning a PSNT testing program for your farm!

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