Vegetable and Ornamental Horticulture : Using Nitrogen Wisely

USING NITROGEN WISELY

Steve Reiners
Associate Professor in Horticultural Sciences at Cornell University

Growers are well aware by now that nitrogen fertilizer prices will probably be twice that of a year ago. In addition, supplies may be more limited than the past couple of seasons. The reason of course is the high price of natural gas. Natural gas is used to produce most of the synthetic nitrogen that farmers use. Prices for gas neared $10 per 1000 cubic feet in January, up from $2.50 in 1999. Once prices rise above $3.00 it becomes more economical to produce the nitrogen fertilizer where gas prices are cheaper and ship the fertilizer here. As a result, US nitrogen production has just about shut down and we will be depending on imports this season.

Of course there are things that growers can do to conserve N, lower costs, and still keep the crop quality that their markets require. Before planting, growers need to take credit for N that may already be in their soils. Darryl Warncke, Soil Fertility Specialist with Michigan State University, offers these suggestions. They include;

ß Estimate the N contribution from a winter cover crop. A cereal crop like rye or wheat may recycle 20 to 40 pounds of N per acre while a legume crop may provide 40 to 80 pounds. Some of this N will be tied up early in the season as soil microbes use the available N to break down the cover crop. Cover crops that are especially high in carbon (more woody) will be slower to release their N.

ß Estimate the N contribution from soil organic matter. Muck soils will release 60 to 100 lbs of N per acre over a season. For mineral soils, assume about 5 to 10 pounds of N for every 1% organic matter. For example, a soil with 2.5% organic matter will release about 12 to 24 pounds of N.

ß Manure can provide 4 to 12 pounds of available N per dry ton depending on composition.

When estimating N credits from last year, be aware that the heavy rains and winter snowpack has resulted in N leaching from soils, especially on gravelly or sandy soils and soils that contained no fall cover crops.

Use N more effectively on your farm by matching application timing with the period that vegetables will need the most N. For most crops, only about 25 pounds of N/acre is needed the first 4 to 5 weeks after seeding or transplanting. Simply broadcasting and plowing down the full N dose prior to planting is the most inefficient way to use N. Although it will more than meet the needs of the early plant growth, most will leach away, and not be available when the plants really need it. Try applying only 25 to 40 pounds of N as starter fertilizer. Follow this us with sidedressing later in the year based on a pre-sidedress nitrogen test (PSNT).

Note: Contact CCE Washington to get a PSNT kit

Over the last decade, Cornell and many institutions around the country have been researching the use of the PSNT for vegetable crops. The PSNT allows growers to test their soil 4 to 6 weeks after planting to determine the amount of nitrate that is present. This is normally the time that most vegetable crops will respond to a sidedressing of N. Based on trials conducted in the northeast and mid-west, it's estimated that about 25% of all crops will not require any additional N - the level in the soil is already adequate. Another 50% may require only about half of what is traditionally applied. Only about 25% of all crops will need a full N sidedressing.

To perform a PSNT, take a soil core12 inches deep in 15 to 20 locations in a representative field. For vegetables, its best if these samples do not represent more than a10 acre field. Check with your local CCE office to see if they can perform the test at their office or recommend a lab where the sample should be sent. Dry the soil ASAP. To dry, spread the soil thinly and allow to air dry, or place in a conventional oven at 200F, or in a microwave on full power for 5 to 8 minutes. Use the dry soil to run the test. The number is given as PPM of nitrate. Based on most of the PSNT trials I have seen around the country, if the nitrate level is below 30, use a full amount for a sidedressing. If its 30 - 39, you may want to try using half the amount. Above 40, you can probably skip the application entirely. This estimate is somewhat conservative. Like any tool, the PSNT should be used in combination with the grower's knowledge of soil type, cropping history, and local weather conditions (overly dry or wet). Using the PSNT along with other techniques will allow you to reduce N use, save some money, and maintain yields and quality.