University of Maryland Extension

Pre-sidedress Nitrate Testing: Crucial for Corn!

Patricia Hoopes

Soil nitrogen levels are very important for crop health and yields, especially when growing corn grain or corn silage. Fertilizer recommendations that are generated for a field that had a previous forage legume crop or manure application assume a significant nitrogen contribution from the previous crop or manure application. However, nitrogen is not a stable compound, and there are many factors that can affect the amount that is actually present when the plant needs it. This can make it tricky to know exactly how much nitrogen is in your soil and thus how much nitrogen fertilizer you need to add.

The pre-sidedress nitrate test (PSNT) takes the guesswork out of nitrogen availability. This test determines how much nitrate nitrogen is present in the soil at a critical time. When the results indicate that the nitrogen level is satisfactory, the grower is saved from any more financial inputs. However, when nitrogen levels are found to be too low, nitrogen can be side-dressed at a rate to ensure an adequate crop.

Do your crop recommendations depend on a nitrogen credit from an organic application or a previous forage legume crop? Are you applying less than 50 pounds of nitrogen per acre in your starter fertilizer? If this scenario describes your corn field, it is highly recommended that you schedule a PSNT study to evaluate the nitrogen levels at this critical time.

Qualifications for PSNT testing:

1) Corn silage or corn grain is being grown.

2) Manure or biosolids have been applied this year or in the past two years, OR a forage legume was grown in the previous year.

3) Less than 50 pounds of commercial fertilizer nitrogen per acre was applied prior to testing.

PSNT is not applicable when:

1) More than 50 pounds of commercial fertilizer nitrogen has been applied to the field.

2) Commercial fertilizer has been the only nutrient source.

3) Irrigation is used.

When should this test be done?

The PSNT must be run when corn is between 6 and 12 inches tall. It is recommended that you let your nutrient management advisor know now that you will want this test done to ensure the advisor’s availability. It is also recommended that growers alert their advisors prior to the 6 inch stage. Corn grows quickly during this critical time, and scheduling can be challenging.

PSNT Services are offered for both UME clients as well as for farmers preparing their own Nutrient Management Plans and for private consultants. Non-clients must provide information on nutrient applications, crop histories, and soil samples.  Forms and directions are available upon request.


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