Take spring nitrate soil samples when corn is 6 inches tall (measuring from the ground surface to the center of the whorl).
Take spring nitrate soil samples when corn is 6 inches tall (measuring from the ground surface to the center of the whorl).

Nitrogen is the most important nutrient for plant growth, development and reproduction, but the minute nitrogen hits soil, it starts transforming. These transformations can greatly impact nitrogen use efficiency in cropping systems.

“Farmers are under a great deal of pressure to reduce the amount of nitrogen they apply to their fields. To make the most of nitrogen, it has to be the right amount, applied at the right time and at the right rate,” explains Dr. Jim Friedericks, Laboratory Manager at AgSource Laboratories in Ellsworth, Iowa. “Late spring nitrate tests can help ensure this season’s crops have adequate nitrogen levels for top production and maximum yields.”

Because so many factors affect nitrogen loss and its availability, estimating crop nitrogen needs can be difficult. As temperatures rise, the rate of nitrogen released from the soil increases. Other factors influencing nitrogen availability include rainfall and tillage. General recommendations for nitrogen, based on Iowa State University research, where the late spring nitrate test was developed, call for 21-25 parts per million (ppm) of nitrate nitrogen for corn following corn or soybeans and for 11-15 ppm of nitrate nitrogen for corn following alfalfa or a recent manure application.

Nitrogen fertilizers come in a variety of forms, which also can influence soil loss and plant availability. 

Regardless of the form used, most nitrogen fertilizers are converted to nitrate in the soil. Excessive rainfall can leach nitrate out of the root zone making it unavailable for plant use. When soils are waterlogged, soil organisms take oxygen from nitrates through a process called denitrification. This type of N loss occurs most often in fine-textured soils. This puts farmers in a tight spot, Friedericks says, because waterlogged soils will result in denitrification but good drainage makes soils more prone to nitrogen loss. Either way, crop potential is lost.

“The weather’s been good for farming in most places this spring,” Friedericks relates. “There’s been ample time for farmers to get in the field, prepare the ground and apply base rates of nitrogen preplant. That’s very appropriate timing because, when soils are cool nitrogen is more stable. Then, using this test, farmers can find out how much more nitrogen they need to apply now in order to carry the crop to maturity. Now, it’s time to think about late spring nitrate tests.”

 Nitrogen is one of the primary nutrients required for plant growth.

Late spring nitrate tests show nitrate concentrations present in soils, making it possible for growers to predict the amount of nitrogen available for the remainder of the season. Sampling is best done in late May to early June when corn is 6-inches to 12-inches tall.  Soil cores should be taken at a depth of one foot with one sample containing 15 to 20 cores, from similar field areas that are no more than 10-acres to 20-acres in size.

The following conditions make late spring nitrogen testing a good investment:

  • Little or no nitrogen was applied right after harvest.
  • Small amounts of nitrogen were applied (< 125 lbs).
  • Nitrogen was applied at sub-optimal conditions (temp<55°F) limiting effectiveness.
  • Small amounts of manure were applied or incorporated after soil temperatures drop below 55°F.

All farmers must be careful when applying nutrients. The best management practice for nitrogen stewardship involves using the right source, the right rate at the right time and right place. Late season nitrate sampling can ensure you have the right amounts of nitrogen for best possible yields. Appropriate sampling strategies for late season nitrate tests varies by location and care should be taken to ensure soil samples are collected in a manner that doesn’t bias results. Specific sampling instructions are detailed online at http://agsource.crinet.com/page305/NitrateAnalysis.

AgSource is a leader in turf, agricultural and environmental laboratory analysis and information management services, with facilities in Iowa, Nebraska, Oregon and Wisconsin. A division of Cooperative Resources International, AgSource Laboratories provides testing services to clients in the United States and across the globe.