USING THE  PSNT TEST TO MANAGE N FERTILIZATION OF VEGETABLE CROPS
By Vern Grubinger, Vegetable and Berry Specialist, University of Vermont Extension

Want to save some money, protect water quality, and grow better crops? There’s a pretty 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? If only it was that simple…

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 changing, too. 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 up 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. The significance of that information is it allows you to fine-tune your nitrogen fertilizer applications to make sure the crop gets what it needs without putting on extra N that would be a waste of money and a potential threat to water quality. By using this test, many growers will find that soil organic matter and previous applications of compost, legume cover crops and manure can met some or all of their crop’s nitrogen needs.

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, so 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 six to twelve 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 any more. Additional research has been done with other crops over the past decade, in particular, long-season vegetables that are typically sidedressed, such as tomatoes, peppers, fall cabbage, butternut squash and pumpkins. Although 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 root zones to meet their needs.

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 one 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 F 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 5 to 8 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, keeping them cool, less than 50 degrees F to slow down microbial activity. Storing warm, moist samples in plastic bags is the worst – 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 for this test.

Timing the sampling. The soil should be sampled about a week to 10 days before you expect to sidedress 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 will 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 very fast turnaround time for these tests because they understand that PSNT samples taken just a week or so before sidedress 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 sidedress but check with your lab about turn-around time and ability to e-mail you the test results.

Where to send the sample. Almost all land-grant university soil test labs in the Northeast offer the PSNT, or June nitrate test, for about $10 per sample. The University of Vermont Agricultural and Environmental Testing Lab offers the test for $8. Samples should be sent to: Room 262, 63 Carrigan Drive, University of Vermont, Burlington, VT  05405-1737.

Using the test results. Your soil test lab will likely provide you with nitrogen fertilizer recommendations along with your test 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 for more information 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. That publication includes this table:

PSNT test results: ppm NO3-N                                Interpretation

Less than 20                     Very likely N deficient, sidedress N is recommended.

20 to 24                            May be sufficient for some crops.
                                        A low rate of sidedress N may be applied to ensure that N is sufficient.

25 to 30                            Sufficient N is available for most crops. Sidedress 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.

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.
revised 2-9-12

RETURN TO VERMONT VEGETABLE AND BERRY PAGE