Nutrient Management Spotlight - The Late Spring Nitrate Test

Don't feel neglected, the latest Manure Scoop has arrived. Hope everyone is having a successful planting season; may the leaves be green and your soils fertile (from that wonderful manure).

As you might have saw the new IMMAG newsletter just came out (you can find the complete newsletter at http://www.agronext.iastate.edu/immag/updates/may2015update.html). So I'm going to borrow one of the articles I wrote for the newsletter and post it here too. This one is on using the late spring nitrate test.

This year as part of the Manure Applicator Certification program, we asked you what you were doing as part of the Nutrient Reduction Strategy to reduce nutrient loses from your farm. One of the more popular answers was switching to split fertilizer application. Some potential benefits of split fertilizer application may include reduced opportunity for nitrate loss through leaching and denitrification, the potential to use less fertilizer, having less investment in the field if you are forced to replant to soybean after weather related losses or planting delays, or even slightly delaying to get additional information about this year’s markets and growing conditions. However, there can be concerns about the costs of making a second fertilizer application trip across the field, or even if the weather will permit this fertilizer application.

Often times when we switch to split application our general plan is be to apply 50-60% of the nitrogen recommendation in the fall or early spring, and then to sidedress the remaining 40-50% into the growing crop. An alternative approach is to determine our sidedress amount using the late spring nitrate test (LSNT). The late spring nitrate test is a nitrate only soil test where soil samples are taken to a depth of 12 inches when the corn plant is 6-12 inches tall. This test is supposed to inform us about available nitrogen concentrations in the soil just as our corn growth, and nitrogen need, is about to take off. In using the results you’ll want to break your field up into different management zone, parts of the field that have similar management histories and soil types (a management zone probably shouldn’t be any bigger than 10 acres). Within each management zone 16 to 24 soil cores should be collected. As these samples are collected you need to make sure that any banded fertilizer or manure isn’t biasing your results; sampling in a pattern relative to the corn (or banded fertilizer) row can help eliminate the effect of the banded application. For example, go to the first sampling location in your management zone and pull the first soil sample in the row, then move to your next sampling spot and pull the soil sample one-eighth the distance between rows, go to your next sampling location and pull the sample one-fourth the distance between rows, and continue this pattern.

Although this may seem a little complicated, the real difficulty starts in interpreting the results. Iowa State research says corn needs 25 ppm of nitrate-nitrogen in the top 12 inches of soil to produce maximum yield; however, the interpretation of the results vary with cropping system, manure history, and even weather conditions prior to and after sampling. Selecting the “critical” soil nitrate concentration (the one you are trying to achieve) is one of the more difficult parts of using the late-spring nitrate test to make management decisions.

Table 1. Nitrogen fertilizer recommendations for corn on manures soils.

	Recommended N rate
Soil Test	Excess rainfall	Normal rainfall
ppm NO3-N	lb N/acre	lb N/acre
0-10	90	90
11-15	0	60
16-20	0	0-30
> 20	0	0

In fields that have received manure, a “critical” soil nitrate-nitrogen concentration of 15-20 ppm nitrate-nitrogen is recommended. You’ll note that this is lower than non-manured fields; this is because the manure application provided more organic nitrogen that will be mineralized throughout the growing season and become plant available, but isn’t detected by this test. Based on your sample results you can then calculate the amount of nitrogen that would be recommended to sidedress. The formula for calculating nitrogen application is if your soil test was greater than 20 ppm then 0, otherwise (20 ppm - soil test nitrate) * 8 = lbs of N/acre to apply. Alternatively, table 1 provides a way to select a sidedress nitrogen application rate. In this table excess rainfall would be May precipitation that exceeded 5 inches, normal rainfall should be used for other cases.

As with any new fertility management program, first-time users are encouraged to experiment with the test in small areas before using it to guide fertilization on all their fields. As with most recommendations this test is intended to maximize profits when used across many years and sites, not to give the “perfect” rate in a specific year.

For more information related to using the Late Spring Nitrate Test please see, ISU PM1714-Nitrogen Fertilizer Recommendations for Corn in Iowa. More information on best management practices for reducing nutrient loss from agriculture can be found in SP435 - Reducing Nutrient Loss: Science Shows What Works.