In this issue:

Vegetable Seed Germination and Soil Temperatures

Soil Testing: A Key to Reliable Vegetable Production

National Drought Monitor

Planned Repeal of Rules Governing Migrant Labor Camps

Pests of the Month

Soil Testing:
A Key to Reliable Vegetable Production

Carl J. Rosen, Extension Soil Scientist, Department of Soil, Water, and Climate, University of Minnesota

Soil testing to determine nutrient availability is an extremely valuable diagnostic tool for crop production and should be considered a routine practice for any vegetable operation. Results of a soil test are used primarily as a guide for making fertilizer and lime applications prior to planting. The tests can not only be used to identify nutrient deficient soils, but also to identify soils where additional fertilizer or lime application may not be necessary. Thus, soil testing can take much of the guesswork out of making fertilizer or lime recommendations and lead to more economically and environmentally sound nutrient management.

Effective use of a soil test is dependent on submitting a representative sample and understanding the principles upon which the results and interpretations of a soil test program are based. The following discussion will briefly address topics related to sample collection, sample analysis, and how research-based fertilizer recommendations are made.

Soil Sample Collection: Accurate interpretation of soil test results for making fertilizer recommendations is dependent on collecting a representative sample. Samples not properly collected will result in misleading recommendations. Even the best-equipped laboratory cannot make up for a sample that is poorly taken. The procedure for taking a meaningful soil sample is summarized below.

Each field to be sampled should be divided into uniform areas. Each area should have the same soil texture and color, cropping history, and fertilizer, manure, and lime treatments. One sample should not represent more than 20 acres on a level, uniform field and 5 acres on hilly or rolling land. After scraping off the surface residue, samples should be collected to the 6 to 8 inch depth. For each sample, 15 to 20 subsamples should be collected from randomly selected areas in the field. The soil should be mixed in a clean plastic pail and about 1 pint of the mixture placed in a sample bag. Samples can be sent to the laboratory moist; however, air-drying is recommended if they can not be sent to a laboratory within a few days after sampling.

Soil samples can be collected at any time of the year, although spring and fall sampling are usually the most convenient. If soil tests from a given field are to be compared over the years, it is best that samples be collected at the same time of year. Ideally, fields used for vegetable production should be tested on a yearly basis; although, every two to three years is acceptable for most nutrients except for nitrogen (nitrate), which must tested annually in the spring or fall. On new fields, a soil test is strongly recommended before planting. Consequences of not taking a soil test can be quite costly and in some situations have resulted in crop failure.

Sample Analysis and Interpretation: Fundamental goals of a soil testing program are to: 1) determine whether a particular nutrient is deficient, optimum, or excessive in a soil, 2) assess the need for fertilization, and 3) provide fertilizer recommendations to the producer based, in part, on soil test results. Processes leading up to these goals require extensive background research, both in the laboratory and in the field. Suitable chemical extractants must be identified that will correlate with nutrient uptake or yield. Then, crop response to fertilizer at given soil test levels must be determined. Without adequate research data, soil test results cannot be properly interpreted and may lead to improper fertilizer recommendations.

Specific fertilizer recommendations based on soil tests need to be calibrated for particular regions. Recommendations from one area of the country, in many cases, may not be suitable for other areas due to differences in climate, soil characteristics, and cultural practices. Therefore, the best soil test recommendations are usually the result of local or regional research. In general, calibration of fertilizer recommendations to soil test values is an ongoing process and fine-tuning of recommendations occur as management practices change and more research is conducted. As an example, fertilizer recommendations need to take into account method of fertilizer placement. Banding of P fertilizer as opposed to broadcasting generally increases P use efficiency and therefore lower rates of P fertilizer would be required.

Soil tests are ideally suited to determine organic matter content, available P and K, as well as pH and lime requirements. Soil testing can also be used to determine secondary (Ca, Mg, and S) and micronutrient (B, Zn, and Cu) needs, which can be particularly important for some vegetable crops. Soil tests for iron, manganese, and molybdenum are not considered reliable and needs for these elements are best determined from visual symptoms, soil pH levels, or tissue analysis. The nitrate test to determine residual nitrate in the soil can also be worthwhile is special situations discussed below.

A soil test value should be considered as an index of the availability of the nutrient being extracted rather than an absolute quantity of nutrient available. Basically, the soil test value or relative level provides a probability of response to applied fertilizer. As shown in Table 1, the higher the soil test value, the lower the probability of a response to applied fertilizer. Conversely, a low soil test value would have a high probability of response to applied fertilizer.

The use of relative soil test levels is a convenient method of expressing nutrient availability because the extractant used can have a dramatic effect on the absolute value reported. When comparing soil test values from different laboratories, it is important to know the chemical methods used. If the extractants are not the same, then it is likely that the soil values and will be different and may lead to confusion unless the values are categorized into relative levels as described above.

Because of the mobility of nitrate in most soils and the large fraction of nitrogen tied up in organic matter, the nitrate test is not traditionally used determining N needs for vegetable crops in eastern Minnesota. The rate of N to apply in eastern Minnesota is based on yield goal, organic matter level and previous crop. In western Minnesota where rainfall is limited, the nitrate test is useful for determining initial levels of soil nitrate. Research has shown that more accurate N recommendations can be made for many crops in western Minnesota by determining the nitrate-N content in the top two feet of soil. Therefore, use of the nitrate-N test is strongly recommended for the western part of the state except on sandy soils.

Recent research in more humid regions has shown some benefit of using the soil nitrate test especially if manure is the major source of applied nutrients. High levels of residual nitrate will lower or eliminate the need for N fertilizer. Recent studies in New Jersey on sweet corn have shown that sidedress N fertilizer was generally not required to achieve at least 92% of maximum yield when preside dress soil nitrate-N in the top foot was greater than 25 ppm. The test was useful for predicting N sufficient sites, but was of limited use for making N fertilizer rate recommendations. For most vegetable crops, the use of the nitrate test in humid regions still requires further calibration research before specific recommendations can be made.

Fertilizer Recommendations Based on Soil Tests: Once a soil is submitted to a laboratory and the test results are reported, the next step is to determine fertilizer requirements. In most cases, if there is a difference in fertilizer recommendations among laboratories for a given soil sample, the reason is due to differences in soil testing interpretation philosophy. There is more than one approach to making a fertilizer recommendation. The rate of fertilizer recommended can vary depending on which approach is used. Three of the most common approaches to soil test interpretation are: 1) build-up and maintenance approach, 2) sufficiency level approach, and 3) basic cation saturation ratios approach. The build-up and maintenance approach promotes a rapid build-up to a high soil test level, plus annual replacement of an amount the crop is likely to remove regardless of the soil test level. The sufficiency level approach establishes cut-off levels above which no fertilizer is recommended. The saturation approach is used for potassium, calcium, and magnesium and establishes ideal saturation ratios for these ions.

All three of these approaches were developed from university research programs. Therefore, some may be more applicable to certain geographic regions than others. Many laboratories use a combination of these approaches rather solely relying on one. In most cases, fine-tuning of the general recommendation will need to be done regardless of the approach used, but the important point to remember is that the starting point should be research-based. In Minnesota, the sufficiency level approach is generally accepted as being the most reliable. There is very little research to support the use of saturation ratios in Minnesota.

Summary: Soil testing provides a very convenient tool for determining optimum fertilizer and lime needs for profitable vegetable production. Other factors such as fertilizer placement and cropping history may also play a role and should be taken into account when making a recommendation. Soil test interpretations should be based on sound research that preferably incorporates regional fertilizer studies. The calibration of fertilizer recommendations to soil test values is an ongoing process and will be improved or modified as more research is conducted and as management practices change. For more information on soil test interpretation refer to: Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota. University of Minnesota Extension Service, BU-5886-E, 34pp.

Co-Editors:Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@tc.umn.edu
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture, jeanne.ciborowski@state.mn.us
Cindy Tong, Department of Horticulture, University of Minnesota, ctong@extension.umn.edu
Production Editor: Suzanne Wold, Research Specialist, University of Minnesota, woldx018@tc.umn.edu

Disclaimer
Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current directions of the manufacturer

Last Revised April 6, 2000.
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer.©1999 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.