Using Soil Nitrogen Tests to Verify
Nitrogen Credits to Farmers

Julie Hahm
University of Wisconsin - Green Bay

Introduction

Nutrient runoff from farms is one of the main sources of non-point pollution. This runoff can contaminate both ground and surface water resources which leads to a disruption in the nutrient balance for aquatic organisms. Any nutrients not used by the crop can contribute to the runoff. In this project, nitrogen use by corn was studied. Depending on soil type, corn uses between 120 and 160 lbs of nitrogen per acre. Any nitrogen applied in excess of this recommendation will not produce a substantial increase in yield, but only be waste and can end up in either surface or groundwater.

Overapplication is due to a lack of education. Most farmers are unaware of the amount of nitrogen already present in their soil from previous manure applications, legume plowdowns or fertilizer carryover. During an average growing season, a farmer may apply manure to his field, use nitrogen as a herbicide carrier and add starter fertilizer. Depending upon how the corn looks the farmer will sidedress more nitrogen on a field at a given amount. In most cases, this results in an over application of nitrogen. When the nitrogen released into the soil by the break down of the previous legume crop and the nitrogen released by the manure is credited, some farmers will find that the application of side dress fertilizer is uncalled for or only a small fraction of it was necessary.

First year plow down of alfalfa will leave between 90 and 190 lbs of N, depending on soil type and alfalfa management. Application of manure to a field adds 3 to 4 lbs of nitrogen per ton of manure (3 lbs if the manure is not plowed under in 3 days). The nitrogen from these two sources will decrease the amount of nitrogen needed in a side-dress fertilizer.

The first step in convincing farmers about this is to add only the amount of N required and compare it to the nitrogen rate the farmer would usually apply. Once the farmer believes that they don't need the excess nitrogen, teach them how to credit manure application and legume plowdown correctly.

Methods and Materials

The area that we worked in was the Embarrass River watershed, 292 square miles, known for it's bass fishing and canoeing. A subwatershed of the Fox-Wolf basin, it is listed by the Wisconsin Department of Natural Resources as a high priority for selection for a priority watershed project due to sedimentation and soil erosion concerns. We selected the Embarrass for this special project because of the positive response from farmers to smaller scale educational efforts. Funding was provided by Outagamie Soil Improvement Association and Waupaca Water Quality Committee.

At the beginning of the summer of 1997, we chose land owners at random from a plat map of the Watershed. A total of 120 farmers were selected between the three counties-Outagamie, Shawano, and Waupaca. We compared each chosen land owner with aerial photos of the area to ensure that it was an active farm. We decided to offer a manure spreader calibration and 2-3 Pre-Sidedress Nitrogen Tests (PSNT) to each farmer. The PSNT is a 1 foot soil test that examines the nitrate in the soil. A letter describing the project, its goals and the free services was sent to each land owner. Additionally, any farmer expressing interest outside those chosen at random was also included. We followed up by calling each of the farmers to determine their interest in the project; if interested, we went out to the farm. Once on the farm, we performed a manure spreader calibration, took a manure sample, conducted a survey on nutrient management practices, and recorded field history on fields enrolled in the PSNT program.

The manure spreader calibration was performed by weighing each axle of the spreader and tractor when full and empty. The manure load was calculated and a sample taken and analyzed by a lab to determine nutrient content. We did this to show the farmer how much manure he was applying to his field per load. The lab analysis showed how much nitrogen, phosphorus, potassium and sulfur he was applying to his field with every application.

To gain a better understanding of farmer's management practices, we asked each participant to fill out a survey. Responses were kept confidential. The survey concerned practices such as manure application, fertilizer application and manure and legume crediting.

Farmers chose up to four corn fields for the PSNT test. The PSNT test reveals the amount of nitrogen present in the field. This translates into a nitrogen application rate. It reveals how much nitrogen the legume plowdown and manure application contributed to the soil. For each of these fields, the manure application, starter fertilizer application, and previous crop was recorded.

Toward the end of June, we returned to take the soil samples when the corn was between 6"- 12" high. We took 8-12 samples from a 20 acre field; then took a subsample for our lab analysis. We sent the sample to a soil lab to be analyzed for nitrogen. In two days, we received the results from the lab. Then, we interpreted the results for the farmer and recommended a nitrogen application rate. All of these services were provided free of charge. The farmers were free to apply any nitrogen rate, but were encouraged to do a side by side comparison of the PSNT rate with their normal rate.

The results of the Embarrass Special Project include 81 PSNT tests and 70 surveys. Farmer participation was encouraging, 60% of farmers contacted cooperated or took part in the project. This shows the majority of farmers are concerned with nutrient management practices. The results of the PSNT tests opened a lot of eyes. Of the fields tested:

By showing almost 60% of the farmers that there was some nitrogen already available in the soil, we hope to have farmers questioning their traditional practices. Based on follow up conversations with several of the farmers, we have documented at least 10,000 lbs of nitrogen not applied as a result of the project.

The surveys showed the practices of the farmers in the area to be in need of improvement. The average total nitrogen application (all sources) was 198 lbs per acre compared to a crop need range between 120 and 160 lbs/acre. 47% of farmers are applying more than 200 lbs of nitrogen to the acre. Starter fertilizer rates are also significantly higher than necessary. The average producer is spending $10.21 more per corn acre per year in fertilizer than necessary.

Crediting of legume plowdowns and manure applications are another aspect that farmers are improperly calculating. 69% of farmers could save money by using manure crediting. Only 18% take any credit on their manure--of these, just 3% credit properly. Legume crediting could save money for 31% of farmers. Of the 87% who do take credit for legume nitrogen, only 13% credit properly. This information will be used in future farmer education programs in the watershed.

To prove that legume credits are effective, we encouraged the farmers to install test plots. One farmer agreed to a replicated plot and hosted a field day in August. The recommended rate was 120 lbs if the soybean credit was taken. Strips of 80, 120, and 160 were applied. Yield checks were done and the results showed no difference between applying 120 and 160 lbs.

The excess nitrogen did not increase the yield enough to economically warrant its application. The surveys showed that 55% of farmers are overapplying N; of these, 10% were applying more than twice as much as needed.

This study was a successful attempt to educate farmers about the validity of legume and manure crediting and nutrient management. Farmers are applying too much nitrogen in this area, but they are open to decrease application rates. Using the PSNT as a tool to prove to farmers that nitrogen from manure and legumes is in the soil is a vital step in getting them to reduce nitrogen applications. Coupled with one-on-one education, including manure spreader calibration, farmers can take steps to decrease costs and clean up the water in the surrounding area. This was the first step in helping farmers better manage their nutrients. In the future, more farmers need to be reached and all need to be taught how to properly credit for manure and legume crops.

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