Farming Magazine - June, 2009

GROWING

The Pre-Sidedress Nitrate Test

A simple test you should consider
By Vern Grubinger

Want to save some money, protect water quality and grow better crops? There’s an easy way to do this that costs just a few dollars and doesn’t take much time. It’s called the PSNT, or pre-sidedress nitrate test.

We all know that adequate nitrogen is needed for crops to produce their best yields and quality. Less well-known is the fact that too much nitrogen can actually reduce the yield of crops like pumpkins, winter squash, peppers and tomatoes due to an excess of vegetative growth. This may also promote certain diseases, especially those that require leaf wetness and thrive when there’s poor air movement in the crop canopy.

So, why not just apply exactly the right amount of nitrogen fertilizer for your crops?

N is the problem

Nitrogen is a tricky nutrient for growers to manage because it’s always changing. Sometimes it’s in the proteins of organic matter, and sometimes it gets broken down into simpler available forms like ammonium and nitrate. From there, it may leach out of soils or be taken up by plants or microbes, where it becomes part of organic matter again.

To complicate things further, the total amount of nitrogen in the soil is also changing. Besides being lost by leaching, nitrogen can also evaporate, or volatilize, especially when soils are wet. On the other hand, nitrogen gets added to soil by nodulated legume roots that can fix it from the air, or by additions of compost, manure or fertilizer. Nitrogen can also build up if soil organic matter levels are increased by the addition of lots of organic residues over time.

Mineralization matters

The eventual release of available nitrogen from decaying plants, compost, manure or soil organic matter is called mineralization. Think of this process as soil microbes munching up those materials and spitting out any extra nitrogen they don’t need to build their bodies. The microbes involved are especially active when the soil is warm and moist, and rather sleepy if it’s cold or dry, so environmental conditions have a big impact on the rate of mineralization.

What the PSNT does is measure how much available nitrogen is in the soil at the time when long-season crops are just about to take off. That number is used that to predict how much more nitrogen will be mineralized and thus made available to the crop while it’s growing. This information allows you to fine-tune your nitrogen fertilizer applications to make sure the crop gets what it needs without putting on extra N, which would be a waste of money and a potential threat to water quality. By using this test, many growers find that soil organic matter and previous applications of compost, legume cover crops and manure can meet some or all of their crop’s nitrogen needs.

Photo by Tim McCabe, Courtesy of USDA-NRCS.
Soil samples for the PSNT test are best taken when corn is 6 to 12 inches high. The sample should go down 1 foot to assure accurate results. Sampling within the row is not recommended if any nitrogen has been applied as a starter fertilizer. In that case it is important to stay away from the rows to avoid picking up any fertilizer N in the sample.

Some history

The PSNT, also called the June Nitrate Test, was originally developed for field corn in the mid-80s by Dr. Fred Magdoff at the University of Vermont. The focus of the test at first was on dairy farms where manure is frequently applied and legumes like alfalfa and clover are often included in the cropping system. the goal was to be able to account for the available nitrogen that would be provided to the corn from those sources.

It is now well-established that if the nitrate-N level in the soil is above 25 ppm when field corn is 6 to 12 inches tall, adding more N from fertilizer will not increase yield. Not surprisingly, it turns out that the same is true for sweet corn: 25 ppm is the threshold for shutting off any additional N fertilization. As PSNT results get lower, fertilizer recommendations go higher.

It’s not just corn anymore

Additional research has been done with other crops over the past decade, in particular, long-season vegetables that are typically side-dressed, such as tomatoes, peppers, fall cabbage, butternut squash and pumpkins. Al-though the PSNT works to predict the fertilizer needs of these crops, too, a slightly higher threshold is usually used to determine whether or not to apply additional nitrogen. That’s because corn has a deeper and more extensive root system than most vegetables and is better able to extract N from the soil. Thus, vegetables with shallower root systems require a higher level of N in their rootzones to meet their needs.

Photo by Vern Grubinger.
The time to sample soil for the PSNT test in winter squash is a week or so before the vines begin to run, so there is still time to get the results back from the lab and side-dress N fertilizer if needed.

Taking the sample

To get good PSNT results you need to send a good sample to the soil test lab for analysis. For each sample, collect 15 to 20 cores or slices of soil to a depth of 12 inches, and mix together thoroughly to form a composite sample. (You may want to purchase a soil corer so you can take samples quickly; they are available from farm supply companies for under $50.) Avoid sampling fertilizer bands in the rows or areas that may have received extra N. The sampled area should be consistent for past crop, soil types and manure applications. If fields have significant differences sample them separately.

Preparing the sample

About 1 cup of the composite sample should be dried to stabilize the nitrate. Because microbial activity can rapidly change the concentration of nitrate in soil samples, it’s important to start drying the sample right away. Either air-dry it by spreading out in a thin layer on a sheet of plastic overnight (using a fan will reduce drying time) or by placing on a cookie sheet and heating in an oven at 200 degrees Fahrenheit until dry. Samples can also be dried in a microwave by spreading the cupful of soil thinly on a plate and microwaving at full power for five to eight minutes, depending on the moisture content of the soil.

When drying, don’t place damp soil samples on absorbent material because it can absorb some of the nitrate. If the soil samples cannot be dried right away, keep them cool at less than 50 degrees Fahrenheit to slow down microbial activity. Storing warm, moist samples in plastic bags is the worst as this will promote biological activity and changes in soil nitrate levels.

You can skip the drying step if you can deliver the samples to the soil testing lab within a day, or if the lab you are working with provides you with special cloth mailer bags.

Almost all land-grant university soil test labs in the Northeast offer the PSNT or June nitrate testing service for under $10 per sample.

Timing the sampling

The soil should be sampled about a week to 10 days before you expect to side-dress, as this will to allow time to collect the sample, have it analyzed and receive the results. Typically, this is when sweet corn is 6 to 12 inches tall, or just before pumpkin vines start to run. Samples taken too early will not be as accurate because soil releases nitrate continually in the spring. Soil test labs typically have a fast turnaround time for these tests because they understand that samples taken just a week or so before side-dress applications of fertilizer will allow the most accurate assessment of plant-available nitrogen. You might be able to sample even closer to the date you want to side-dress, but check with your lab about turn-around time and ability to e-mail the test results.

Using the test results

Your soil test lab will likely provide you with nitrogen fertilizerrecommendations along with your results. Most labs have recommendations for field corn and sweet corn, but not all have developed them for other vegetables. Keep in mind that the best way to use the PSNT test is to submit samples for several years and keep records of the results, as well as your fertilizer applications, growing conditions and crop performance. That way you’ll gain the ability to interpret the results yourself.

An excellent fact sheet is “Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops” by Joseph R. Heckman, Ph.D., specialist in soil fertility for Rutgers Cooperative Extension(http://njaes.rutgers.edu/pubs/publication.asp?pid=E285). The publication includes the table shown at left.

Note that in years with unusually dry spring weather, soil nitrate concentrations typically will be higher than normal; in years with unusually wet spring weather, soil nitrate concentrations typically will be much lower than normal. The only way to know what constitutes the normal range of soil nitrate concentrations for your soils and N management is to test your fields for a few years in a row and maintain records of the results.

PSNT test NO3-N (ppm) Interpretation
Less than 20 Very likely N deficient, side-dress N is recommended.
20 to 24 May be sufficient for some crops. A low rate of side-dress N may be applied to ensure that N is sufficient.
25 to 30 Sufficient N is available for most crops. Side-dress N is usually not recommended.
Greater than 30 Sidedress N is not recommended.
Greater than 50 Excessive. Indicates excessive application of manure, compost or other sources of N.

The author is vegetable and berry Specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at vernon.grubinger@uvm.edu.