In-Season Tools Manage N Closer To Margin

In-Season Tools Manage N Closer To Margin

Pre-sidedress nitrate test (PSNT) offers corn growers confidence.

Figure 4. Recommended sampling positions for PSNT testing of irrigated fields.

Current nitrogen (N) fertilizer recommendations in Colorado are based on soil samples taken in the fall or in the early spring. However, most N uptake by corn occurs in midsummer from the 8-leaf growth stage to pollination. Mineralization of N from manure or other organic matter, and nitrate leaching, can significantly change soil N status before this time. The pre-sidedress nitrate test (PSNT) measures these potential changes. By complementing preplant soil testing with PSNT, growers can better predict yield response from N fertilizer, saving unnecessary fertilizer costs.

The PSNT is based on nitrate concentration in the top 12 inches of soil when corn is 6 to 12 inches tall (V6 growth stage). Under typical Colorado conditions, CSU researchers found the critical PSNT level is 15-ppm nitrate-N (NO3-N) in the top foot of soil at this growth stage. If the PSNT level is lower than 15 ppm NO3-N, sidedress N should be applied. If the PSNT level is higher than 15 ppm NO3-N, the probability of a yield response to additional N is very low (see Table 6). Although the PSNT was originally calibrated for non-manured fields in Colorado, the 15 ppm NO3-N should also be sufficient for fields with recent manure applications or legume crops. The test is most useful for predicting whether or not soil N is sufficient - not for making an N rate prediction. You must assess yield potential as well as soil nitrate levels to determine how much additional N is needed if the PSNT is below 15 ppm.

Proper soil sampling may be the most critical step in the PSNT procedure. To sample a field, take a minimum of 15 to 20 random soil core samples from a uniform soil area or 40-acre field. On surface irrigated fields, we recommend collecting equal numbers of soil samples from the furrow and shoulder of the bed and sampling depth of 12 inches (see Figure 4). Get the soil sample to a testing lab right away and tell the lab you are evaluating the sample for PSNT and need your results quickly. Using the PSNT will give you more confidence to evaluate your sidedress decision, and you may see savings in both fertilizer and sleep.

Table 6. The yield response of corn to sidedress N application of 60 lbs/acre when PSNT was above or below the critical NO3-N concentration at V6.

(Sampling Depth: 0 - 12”)	Number of Observations	Yield response from sidedress N	Prediction accuracy

		# of Sites	%
Below critical level (15 ppm)	35	19	54**
Above critical level (15 ppm)	21	0	100
Total	56		71

Based on equal sampling intensity from both furrow and shoulder positions *16 sites did not respond to additional N

by Reagan Waskom
Extension Specialist
Water Resources
CSU Water Center

Interested in Trying the PSNT?

With N fertilizer costs high and water supply projections low, this is a good year to consider using an in-season N assessment for determining side dress needs for corn. However, many folks may be nervous about trying a newer practice without some technical support. Growers or county faculty that would like to try using the PSNT should give Troy Bauder (970-491-4923) a call. He is willing to provide advice and field support to increase the use of this practice among Colorado corn producers.