April 21 2010 08:23 AM

The time is now to test your manure for nitrogen; the fertilizer savings could surprise you.

The author is a nutrient management educator for Michigan State University Extension. For more information, visit www.animalagteam.msu.edu.

How much nitrogen is manure supplying my corn?" It's a common question from farmers who are frustrated when I answer, "It depends."

But contrary to what they think, I'm not trying to be elusive. Numerous factors have to be taken into account before we can come up with an answer. The same manure applied at the same rate might vary from 25 to 125 pounds of nitrogen per acre due to different timings, application methods, and weather conditions. Soil nitrate testing is the quickest - and most trusted - route to the answer.

The financial risks of guessing too high or too low are enormous this spring. If you think you don't have the staffing to test, consider this: the money you save from a good, accurate nitrate test could pay most of one employee's salary for a full year or buy new tires for your manure tank or at least offset the milk check.

Whether you call it a late-season nitrate test (LSNT) or a pre-sidedress nitrate test (PSNT), the goal is all the same: to gauge the amount of plant-available nitrogen in soil after the temperatures have warmed up enough to convert ammonium and organic nitrogen to the nitrate form. This nitrate is the primary form of nitrogen taken up by plants and what the soil test measures. The soil tests need to be taken later than what is convenient, when the corn is 6 to 12 inches tall. They also need to be taken deeper and handled differently than regular samples.

It might not be convenient

The timing of this test is frustrating for dairy producers - it coincides with first-cutting hay and herbicide spraying. Many dairy farmers choose to put down their nitrogen for corn just prior to or at planting to free up time for other necessities.

The soil test will be most accurate at measuring the nitrogen from manure applications if synthetic nitrogen has not been applied. Minimal amounts in the starter have little impact, but taking the test after full nitrogen fertilizer rates have been applied on a manured field is counterproductive. Though pre-sidedressing nitrogen may seem inconvenient, it allows you to adjust any deficit with a sidedress application. Or, better yet, find out that the manure application is sufficient.

The nitrate soil testing is just one piece of the system that helps you to fine-tune nitrogen management. Following it consistently on your farm will show a pattern for success.

Start with good manure samples, and make the best estimate of plant-available nitrogen from these samples. Timing is crucial - you will get several different answers if you apply the same manure at different times. You'll get one estimate of plant-available nitrogen for surface-applied manure in the summer, another one for injected manure in the fall, and yet another estimate for manure applied in the spring.

In stratified manure systems, such as those with sand bedding, you should pull several manure samples from each storage area - one from the top, watery area; one after agitation; and a third from the sand that is scooped out of the bottom. Then, you can send each of these samples, frozen, to a laboratory for testing. Each of these samples will result in different application recommendations. Different goals will also alter application rates. Is the goal to apply a full nitrogen rate to a cornfield? When manure rates are limited by phosphorus standards, how much additional fertilizer nitrogen is still needed for corn yield potentials? Teaming the soil and manure tests with a stalk nitrate test at corn black layer stage and monitoring yields can help you further refine your nitrogen decisions.

The wild card

Even when manure is tested, rates are estimated, and calibration confirmed there is still one remaining wild card: WEATHER.

Nitrogen in manure can be lost just as quickly as it can be retained. The many variables surrounding nitrogen losses and retention are exactly the reason you need the nitrogen test to quantify all these differences and put accurate recommendations to the individual situations so you can manage nitrogen profitably.

I don't recommend that any producer blindly follow nitrate soil tests the first time they take them. The soil nitrate tests should be part of your overall system - and may bring other components to light. If manure has been randomly applied, the nitrate soil tests will show random results. When a soil nitrate test comes back differently than expected, backing up and looking for the reason may prompt closer attention to manure sampling, calibration, spreading, communication, and record keeping.

To gain confidence in nitrate soil testing sample from a variety of potential nitrogen credit situations. You will be amazed at how logical they are. Take a sample from a field that has not had any manure, one from a field that has not had a manure application in two years, and one from a field that has received manure every year, and note the progression of additional nitrogen.

Consider past crops: corn following alfalfa, beans and corn will show declining N values. Sort out fall-, winter-, and spring-applied manure situations. Note what was surface-applied versus fields that were incorporated. Records of weather conditions on surface-applied manure and how soon it was incorporated will probably correlate to the amount of nitrogen found. During hot weather, delaying incorporation from hours to days will quickly show up in reduced plant-available nitrogen. It would not be uncommon for these different variables to create a range of plant-available nitrogen rates from 25 pounds per acre to 125 pounds per acre from the same rate of manure applied at different times, methods, and weather conditions.

Interpret and monitor
I've made a point to visit with producers about their soil nitrate test results, and usually we come to a very logical reason for the differences they find. They also find consistency on their farm from year to year. Spot checking these different situations annually will help quantify the impact that weather and soil conditions produce in any one season.

Nitrogen may be lost by leaching, volatilization, or denitrified, but how much is always the question. Determining if and how much nitrogen has been lost can be determined in time for sidedress corrections and with confidence in purchasing expensive inputs. Available nitrogen really does depend on numerous site-specific factors, but the soil nitrate testing can bring logic to a dynamic system.

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