Presidedress Soil Nitrate Testing for Sweet Corn
1. Aggressively advocate for the adoption of PSNT to determine N requirements for sweet corn.
2. Conduct an agronomic and economic evaluation of the recently establishment 30 ppm critical concentration for the PSNT in sweet corn.
3. Conduct an agronomic and economic evaluation of the UV 200 preplant soil nitrate test as an indicator of available soil N and as a predicator of N requirements for sweet corn.
Based on the 83 test sites in three states over two years, the critical concentration PSNT was found to be 25 ppm. This is less than the 30 ppm originally postulated.
At demonstration sites on commercial farms in Connecticut, 70 percent of the field samples used less nitrogen in 1995 and 75 percent in 1996. Comparable figures for New Hampshire are 60 percent and 70 percent, respectively. New Jersey reported 61 percent of the sites sampled in 1995 required reduced nitrogen and projects a 30 percent overall reduction in nitrogen use on sweet corn as a result of adoption of the PSNT.
The PSNT will be available as a routine soil test option in Connecticut, New Hampshire and New Jersey as well as other northeastern states.
All three participants observed significant reductions in nitrogen use on PSNT-sampled sweet corn fields. Reductions ranged from 0 to 160 lb/acre in research calibration plots. On commercial farms, 75 percent of Connecticut fields and 70 percent of New Hampshire fields required no nitrogen sidedressing. This would result in savings of $18-20 per acre. New Jersey reports anticipated reduction in nitrogen fertilizer of 30 percent valued at about $14-15 per acre.
Our research confirms that the PSNT critical concentration is less than 30 ppm or about 25 ppm. One of the PSNT experiments had a PSNT of 25 ppm and there was no response to sidedress N. Using the PSNT at this site would have saved about 125 lbs. N/acre or an equivalent of $37.50 per acre. Using the PSNT recommendations for sweet corn (as published in Table 1 of Rutgers Cooperative Extension Fact Sheet 760) for each of the twelve PSNT experiments would have resulted in an average sidedress N recommendation of 95 lbs. N/acre. This is about 30 to 65 lbs. less than the standard recommendation of applying 125-160 lbs. N/acre without the PSNT. Results suggest that using the PSNT to make sidedress N recommendations for sweet corn may reduce N fertilizer usage by about 30 percent. This savings is similar to that which has been found for using the PSNT on field corn.
In 1995 and 1996, at least 12 commercial sweet corn growers in New Jersey used the PSNT on part of their crop acreage. The PSNT was used on a total of 46 acres of sweet corn. We compared the PSNT recommendation to the growers' usual practices. The results showed that in 61 percent of the fields, the PSNT recommendations lowered the N fertilizer use from the farmers' usual practice, in 6 percent of the fields there was no change from the usual practice, and in 33 percent of the fields the PSNT recommended more N than the farmers had intended to apply. Overall use of the PSNT on these 46 acres reduced N fertilizer recommendations by an average of 31 lbs. N/acre. This amounts to a potential net savings of $9.30 per acre.
The PSNT is probably most useful to growers that produce sweet corn on manured land or have fine-textured soils with relatively high organic matter content. In these situations, the PSNT will likely measure significant levels of NO3-N and reduce sidedress N application rates accordingly.
The PSNT also can be used where modest amounts of preplant nitrogen fertilizer have been applied. Growers in Connecticut and New Hampshire have successfully used the test by applying 60 to 100 pounds of preplant nitrogen and then sidedressing the amount of nitrogen recommended by use of the PSNT. Frequently the test recommendation is for little or no fertilizer nitrogen.