Sweet corn is an intensively managed crop in Maine, accounting for approximately 30% of the acreage in vegetable production and 300 tons of nitrogen applications per year. There is high potential for excessive soil nitrogen in sweet corn fields, posing a potential environmental threat and adding unnecessary costs to crop production. In 2005, pre-sidedress nitrogen testing (PSNT) was used in commercial sweet corn fields in central and southern Maine to determine if this test could accurately predict plant available nitrogen in the soil prior to sidedress applications, resulting in a reduction in the total amount of nitrogen applied.
Pre-sidedress nitrogen testing was carried out at ten farms just prior to tasseling and the normal time for nitrogen sidedress applications. Very cool, wet soil conditions in 2005 made the accuracy of tests less reliable and more difficult to interpret than anticipated. Growers were reluctant to rely on test results, and sometimes unwilling to wait the 48 hours needed to complete the tests prior to applying a nitrogen sidedress.
At the end of the season, stalk nitrate tests were taken from each field to determine if recommended N applications had provided sufficient fertility for optimum growth, without being excessive. Tests indicated that most farms had higher than optimal N concentrations in the stalks, despite PSNT recommendations. This suggests that available N in the soil was underestimated by the growers regardless of PSNT recommendations. The PSNT itself was not entirely reliable in this study, and grower comfort with it was clearly limited, especially in a difficult growing season such as this. This project demonstrated that growers should identify all sources of plant available N in the soil when determining if sidedress applications are necessary. Further experience with the test may improve its value as a tool for more sustainable sweet corn production in New England.
Pre-sidedress nitrogen testing (PSNT) has been available for silage corn for several years, and recently procedures have been developed for certain vegetable crops, including sweet corn. These soil tests are used to determine if nitrogen levels in the soil are adequate for optimum plant growth through the harvest stage. If the test shows adequate soil nitrogen, then no sidedress need be applied. Nitrogen applications may be reduced by 25% to 50% through the use of PSNT in place of conventional application methods. Despite the availability of these tests, adoption by has been very low. Vegetable growers have not had adequate access to this technology, and feel a high level of risk due to the high value of their crops and the need to maintain high crop quality for their markets. This project sought to introduce pre-sidedress nitrogen testing to Maine sweet corn growers through demonstrations of the technique at a series of “early adopter” farms.
Objective 1. Pre-sidedress nitrogen testing will be evaluated for practicality and reliability for determining plant available nitrogen levels in Maine sweet corn fields
Objective 2. Sweet corn growers will become familiar with pre-sidedress nitrogen testing (PSNT) and its potential for reducing or eliminating nitrogen sidedress applications in some fields.
Performance Target 1. As a result of on-farm demonstrations and post season workshops, 100 farmers will become familiar with PSNT and its potential for reducing N applications in sweet corn fields.
Performance Target 2. As a result of on-farm PSNT demonstrations, ten growers will adopt the use of PSNT to determine the need for N sidedress applications in their sweet corn fields
Ten volunteer farms were selected through the Maine Vegetable & Small Fruit Growers Association as potential “early adopters” and good demonstration sites for the project. Soil samples were collected at each sweet corn field during the whorl (6-8 leaf) stage. Because sweet corn planting is staggered during the spring, several samples were be taken at each farm throughout the late spring and summer. Samples were immediately dried at the Maine Agricultural Experiment Station in Monmouth, and tested for available nitrogen with a “NitraCheck” meter. Results were then e-mailed or faxed directly to the farmers within 30 hours, enabling them to determine whether or not any additional nitrogen, in the form of a sidedress, was needed for optimum plant growth.
When pre-sidedress nitrogen tests indicated that less or more nitrogen is needed than the grower would typically apply, the grower was asked follow the test recommendations on part of the field and follow their routine practice on the remaining part, so that end of season yield and quality comparisons can be made. Following harvest plant samples were taken from fields at each farm. Stalk nitrate tests were used to determine if plant nitrogen tissue content varied due to different soil nitrogen application rates.
Ten sweet corn growers from the southern and coastal regions of the state had at least two fields evaluated with PSNT. Generally an early and mid-season planting was selected for each farm. Farmers were interviewed to get field histories, including any recent manure applications. The fields tested varied greatly in terms of soil type, topography, history, and fertility management. However, all fields were subjected to much greater than average rainfall prior to or following planting, depending on the field. Soil temperatures were also cooler than normal through much of the early part of the growing season. The tests were carried out using the Penn State Quicktest procedure with Merckoquant nitrate strips and a Nitracheck Meter (Hawk Creek Laboratory). Results with the test were initially not satisfactory, with readings much too variable within samples. A new instrument and supply of tests strips appeared to improve the accuracy of the test, although variability still seemed higher than expected. Most of the fields in the trial had not received a manure application, and test results indicated that more than 75% of the fields had nitrate levels below 20 ppm, and should receive sidedress applications of nitrogen ranging from 40 to 120 lbs. per acre (note: recommendations for sidedress applications were not to exceed 60 lbs.). In fact, the recommendations provided for the test appeared to be too conservative, in that recommended supplemental N rates often significantly exceeded what would normally have been applied (e.g. 50 lbs. N). It is likely that very high rainfall and cool soil temperatures had some impact on the test results obtained, resulting in the higher than expected N recommendations.
Several growers expressed dissatisfaction with the testing procedure and results. Although recommendations were usually sent to the grower within 48 hours of the sampling, there was a sense of urgency on the part of the growers, perhaps due to labor and time restrictions, and several applied sidedress N prior to receiving a test result. Further, some of the test results did not appear to make sense to the growers based on their knowledge of the field and its history. Thus they were not comfortable following the recommendations. Given that the results often recommended more N than the grower would have typically applied, which was the opposite of what we had hoped to demonstrate, it was difficult to put much confidence the recommendations, and instead we worked to resolve what we perceived to be problems with the testing procedure and recommendations. By season’s end we felt that we had resolved most of the issues that hampered the testing. However, it also became clear that this technique may only have limited usefulness under the present growing conditions for sweet corn in Maine. Most of the fields were regularly rotated with other crops, relatively few received manure applications, and none had regularly been rotated with legumes. Therefore, unlike silage corn grown in the state, the potential savings in N applications from both farm input and environmental risk perspectives was relatively low. However, there certainly were some exceptions to this, as there are a few large, wholesale sweet corn growers who do regularly apply manure to their fields and typically don’t rotate crops. It is likely that these growers could see benefits from adopting PSNT in most years, and, as part of this project we have worked with a private crop advisor to serve some of these growers in the future.
In this study, pre-sidedress nitrogen testing using the Penn State Quicktest procedure with Merckoquant nitrate strips and a Nitracheck Meter indicated that most sweet corn fields in Maine did not generally hold large reserves of plant available nitrogen from sources such as previously applied manures or cover crops, as has been shown to frequently occur in silage corn fields.
According to the PSNT results, most sweet corn fields did require supplemental applications of nitrogen during the growing season, although the particular season of this study was notable for excessive rainfall early in the season, which likely impacted preplant soil nitrogen applications and any plant available nitrogen reserves.
As a tool, the PSNT presented some difficulties in regard to its use in sweet corn fields. We found the testing unit gave variable results, and that resampling and recalibration were often required to get repeatable results. This was problematic, in that growers were usually anxious to apply a sidedress and unwilling to wait for test results due to the rapid growth rate of the corn, and growers’ time and labor limitations.
While most sweet corn fields in the study did require nitrogen sidedress applications, and thus saw little direct benefit from PSNT in this season, two large-acreage growers who typically use pre-plant manure applications found that reserve soil nitrogen was adequate for growth according to the PSNT and did not apply additional nitrogen. In individual cases like these, i.e. large acreages, little or no crop rotation, regular manure applications, PSNT may provide significant benefits. Growers in these or similar situations are encouraged to use PSNT in the future through a crop advisor who offers the service.
Education & Outreach Activities and Participation Summary
An introduction to PSNT and its potential for sweet corn was held at the Maine Vegetable & Fruit School in March, 2005 with approximately 65 growers attending. During the growing season, a training and demonstration of PSNT was held at a sweet corn field at the Agricultural Experiment Station in Monmouth with 35 growers attending. Approximately 220 growers from throughout New England attended a session on Soil Health at the New England Vegetable & Fruit Conference in Manchester, NH where the PSNT and this trial on sweet corn were reviewed as part of a presentation on Nutrient Accumulation. Results of the project were also shared with over 80 growers at the annual meeting of the Maine Vegetable & Small Fruit Growers Association in January 2006. The project summary was also reviewed at the Maine Vegetable & Fruit School in March, 2006 with 72 growers attending.
We have continued to work with several sweet corn growers on PSNT, in cooperation with a private crop advisor who offers the test. These larger growers have developed confidence in the test and have seen economic benefits from its use. The crop advisor (AgMatters, Vassalboro) is now expanding the use of the test for other vegetable crops with his customers.
The pre-sidedress nitrogen tests used in this study indicated that most of the sweet corn field soils evaluated did not have adequate reserve plant available nitrogen to permit optimum plant growth throughout the season. Therefore, sidedress applications were often recommended at rates similar to what growers typically would have applied regardless of the test. As a result, most growers saw no economic benefit from using the test in this season. However, the exceptionally high rainfall early in the season may have impacted normal soil nitrogen levels, and some fields may have shown less need for supplemental nitrogen under better conditions. Further, two large acreage fields did reduce nitrogen applications as a result of the tests, saving an estimated $42 per acre in fertilizer costs (or roughly $500 on one farm), and reducing potential nitrogen leaching in those fields. The cost of the test at this time would be approximately the same as the savings potential from one acre of a sidedress application (~$50), and thus would probably be a worthwhile investment for growers with large sweet corn acreage. Some smaller growers expressed the opinion that putting on a nitrogen sidedress was relatively inexpensive “insurance” for their crop and more reliable than a test.
It is perhaps still too early to determine whether or not sweet corn growers will adopt PSNT as a tool for determining rates for sidedress applications of nitrogen. The results of this trial did not suggest that potential savings from using the test in sweet corn fields, especially smaller acreages that are regularly rotated and do not receive regular manure applications, would be significant. In addition, for most farmers the test would require either a consultant or state university to run the tests and interpret results for them, which adds costs and time to a crop that requires a tight schedule and is often not highly profitable. As of yet, with the exception of a few larger acreage growers working with a private crop advisor, we have not seen significant adoption of this test by sweet corn growers. However, as larger growers become more familiar with the test and see the potential economic and environmental benefits of using it, it is likely that smaller growers will also become interested in PSNT and may see it allow reductions of nitrogen applications in some fields.
Areas needing additional study
In order to better determine the potential impact of PSNT on nitrogen applications to sweet corn fields, a study using fewer specific fields with different histories over several years could provide the data needed to predict benefits of using test recommendations and determine in which situations the test is most likely to have an impact.
People who carry out the tests need to be thoroughly trained by experienced technicians, as the test can often produce confusing data, especially if the technician is not well acquainted with the history of the field being tested.
Research toward a quicker, reliable plant available nitrogen test that could be carried out in the field with a short turn around time, could generate considerably more interest from high-value retail vegetable growers.