09/20/2024
Hey garden/farmer folks who are always as asking me soil tests here is some solid info from my colleagues at UMASS Amherst.
Fall Soil Testing
Although soil samples can be taken any time of year, many growers prefer to take samples in the fall. This allows time to apply lime if soil pH needs to be adjusted, plant a cover crop to recover any leftover nutrients, make a nutrient management plan, and order materials well in advance of spring planting. Furthermore, the UMass Soil Testing Lab and most similar labs tend to have higher demand in the spring and turn-around times are faster in the fall and winter.
Soil Sampling Tips
It is best to take soil samples at the same time of year for the most consistent and reliable results. Avoid sampling when the soil is very wet or soon after a lime or fertilizer application. If a field is uniform, a single composite sample is sufficient. A composite sample consists of 10 to 20 sub-samples taken from around the field that are then mixed together. To obtain sub-samples, you can use a spade to take thin slices of soil representing the top 6 to 8” of soil. A soil probe is faster and more convenient to use than a spade and is worth the small investment if you have many fields to sample. Remove any thatch or other organic material such as manure from the surface before taking your sample, as this will result in inaccurate soil organic matter levels. Put the slices or cores into a clean container and thoroughly mix. Take about one cup of the mixture, spread it out on paper to dry overnight at room temperature, and put it in a zip lock bag. Samples do not need to be fully dry to submit, but drying them overnight will speed up the process at the lab. Label each sample on the outside of the bag, and fill out the sample submission form thoroughly—see links below. The instructions below are correct for the UMass Lab; make sure you follow specific sampling, packaging, and labeling instructions for the lab you use.
Many fields are not uniform, for many reasons, including uneven topography, wet and dry areas, different soil types, and areas with varying previous crop and fertilizing practices. For example, there might be a place in a field where a tractor accidentally dumped an excess amount of lime. In such cases, the field should be subdivided and composite samples collected and tested for each section. Alternatively, known problem areas can be avoided entirely.
Soils should be tested for nutrient levels and organic matter content every two or three years, unless a specific soil management process needs to be evaluated sooner. Organic matter testing is not included in routine UMass Soil Lab tests, so be sure to check the box on the sample submission form if you want organic matter results. A routine soil test at the UMass Soil Lab costs $20; organic matter is an additional $6.
Submitting Soil Samples
It’s important to request the appropriate test for the type of material you are submitting: field soil vs. soilless greenhouse media vs. high tunnel soil. Different materials are tested with different procedures that mimic how nutrients will become available to plants in the relevant field or greenhouse setting. If you have any questions about what type of test to request, you can contact the UMass Soil Lab at [email protected](link sends e-mail) or the UMass Vegetable Program at [email protected](link sends e-mail) or (413) 577-3976.
Ordering information and forms for the UMass Soil Lab are available here. For Routine Soil Tests for field soil, commercial growers should use the Commercial Vegetables and Fruits order form. Nutrient recommendations for commercial tests will be given in pounds per acre. Home gardeners should use the Home Grounds and Gardening order form, which will provide recommendations in pounds per 100 square feet. If you want to receive crop-specific nutrient recommendations with your test results, be sure to indicate the crops that will be grown on the submission form.
The UMass Soil Lab is currently offering routine soil tests, pre-sidedress soil nitrate tests (PSNTs), soil texture analysis, and soilless greenhouse media tests. Manure or compost analysis and plant tissue analysis is available through the University of Maine Soil Lab(link is external).
Types of Fall Soil Tests
The most common tests to request in the fall are routine soil tests and nitrate tests.
A routine soil test includes phosphorous, potassium, calcium, magnesium, and sulfur, as well as micronutrients. At UMass, the routine test automatically includes pH, but does not include nitrate or organic matter; these additional parameters can be requested as add-ons to the routine test. If the crop is indicated on the submission form, routine soil test results will inform how much of what type of fertilizer or soil amendment you should add to a field for that crop the following season, based on that crop’s nutrient needs and the soil test results. Haven’t prepared your crop rotation plans yet? No worries. You may ask for recommendations for up to 3 different crops without extra charge. The Home Grounds and Gardening form gives you the option to request general vegetable nutrient recommendations instead of crop by crop—this can also be useful for small-scale commercial growers if you don’t have the capacity to tailor your fertilization to each crop.
Nitrate levels are evaluated separately from the routine soil test. These tests are called pre-sidedress soil nitrate tests or PSNTs, although they can be used for reasons other than informing the need for sidedressing. A mid-season nitrate test, will tell you if sidedressing with nitrogen (N) is necessary. A fall nitrate test will tell you if you have excess N in the soil at the end of the season. This will inform whether to increase or decrease N fertilizer applications next season and whether you should plant a fall cover crop to scavenge and store the excess N for next spring. Soil nitrate content above 20 ppm in the fall indicates that too much N fertilizer was applied that season and an N-scavenging fall cover crop should be planted. Nitrate is very ephemeral in soils and is easily lost to leaching and to the air, so nitrate left in the soil at the end of the season will not be there next spring. If you have high soil nitrate content in the fall, planting a fall cover crop will scavenge this N. When the cover crop is tilled in the following spring, the N will then be available for your next cash crop. At the UMass Soil Lab, use the Pre-Sidedress Soil Nitrate Test form to submit a nitrate test soil sample, or check the box for nitrate on the routine soil test submission form; it is only an additional $8.
Interpreting Results and Choosing Amendments
Fall soil test results are primarily used to calculate fertilizer and other amendment needs for the following spring. We can provide fertilizer calculation support for commercial growers—contact us at [email protected](link sends e-mail) or (413) 577-3976. Or, see our Calculating Fertilizer Applications article for step-by-step instructions. The UMass Soil Lab also provides a fact sheet on Interpreting Your Soil Test Results, with an explanation of all the information that appears on your results. In addition to fertilizers, there are several other common soil amendments that can be considered, based on fall soil test results. Below are explanations of some amendments:
Lime
Most New England soils are naturally acidic (pH of 4.5 to 5.5) and need to be limed periodically to keep the pH in the range of 6.0 to 7.0, which is ideal for most vegetable crops and beneficial microbes. Heavier soils or soils with high organic matter will buffer pH change better than sandy soils, and soil lab lime recommendations will take this into account. Lime can be applied any time, but it will change pH slowly, so fall applications are sometimes preferred to allow several months to raise the pH. Split applications (half in the fall and half in the spring) may also be effective. Powdered lime and pelleted lime (which is lime that is powdered then re-formed into pellets for ease of application) will react and raise the pH faster than regular lime, but are also much more expensive. Use dolomitic lime if your soil has low magnesium and calcitic lime if not. See Soil Acidity, pH, and Liming(link is external) in the New England Vegetable Management Guide for more information on liming.
Compost
Compost is often applied in order to increase soil organic matter but also contain soluble nutrients that are available for crop use. Composts usually primarily add phosphorous to soils. Composts do also usually contain N, but only about 10% of the total N content in compost is available to the crop each year. If not applied to actively growing crops, phosphorous from composts can be lost to the environment.
Nutrient levels in composts can vary widely—e.g. phosphorous levels can vary from 0.1-3%—so analysis is important for determining rates of application. A compost analysis should be completed to measure nutrient availability and to determine if the product is finished before applying to the field. Unfinished compost applied to the field may pose microbial food safety risks or cause crop ammonium toxicity as it continues to decompose. Ammonium content below 100mg/kg and a carbon:nitrogen ratio of 20:1 indicates a finished compost. Higher amounts of ammonium indicate active decomposition, or unfinished compost. The C:N ratio is reduced as microbes break down carbon content in the pile and convert it to CO2. Compost analysis is available through the UMaine Soil Lab(link is external).
Matured compost applications are usually made in the spring; however, testing may happen in the fall in order to estimate plant available nutrients for next year’s crop and help determine future compost application rates.
Manure
Manure is an excellent source of nutrients, however, as manure ages and decays, considerable nutrient loss occurs from leaching, surface runoff, or volatilization of ammonia into the atmosphere. Manure may also contain pathogens such as E. coli and Salmonella. If raw manure is applied to vegetable fields, especially for crops that are likely to be consumed raw (e.g. carrots, but not potatoes), this food safety risk should be minimized by maximizing the interval between manure application and crop harvest. Certified organic producers must follow the National Organic Program Standards (NOP) 90/120 day rule that says that you need to wait 120 days after raw manure application to harvest vegetables where the harvesteable portion of the crop is likely to touch the soil (e.g. carrots, melons, lettuce), or 90 days for vegetables where the harvestable portion of the crop is not likely to touch the soil (e.g. tomatoes, peppers). For farms covered by the Food Safety Modernization Act, there is no specified application interval required but this NOP rule is a good rule-of-thumb to follow per the FDA. Ideally, manure should be applied in the fall or to a non-food rotation crop. Fall-applied manure should be incorporated immediately and a winter cover crop should be planted to protect N from leaching. Make manure applications in cold weather to reduce volatilization, but not to frozen ground, as this increases surface runoff potential. In no-till systems, research has shown that surface-applying manure to a growing cover crop will reduce nutrient losses compared to surface applications to bare ground. In order to make accurate nutrient applications to best fit your crop needs, a manure analysis should be conducted—manure analysis is available through the UMaine Soil Lab(link is external). Be sure to submit your samples in a tightly sealed container or the postal service will be very unhappy with you!
Cover Crops
Cover crops planted in the fall, (historically before September 15, although that date is moving later and later every year), are an excellent way to capture and store nutrients for your crops in the following spring. All cover crops will scavenge leftover N, but different cover crop types add or immobilize different amounts of plant-available nitrogen (PAN) to the soil when incorporated in the spring. See our Late Season Cover Crops article for details in addition to what is below.
Legume cover crops provide up to 100 lb PAN/A when incorporated, because they fix N from the air. To maximize PAN contribution from legumes, kill the cover crop at bud stage in the spring.
Cereal cover crops immobilize up to 50 lb PAN/A because the high carbon content leads to a spike in soil microbe activity, which ties up the N in the soil. To minimize PAN immobilization from cereals, kill the cover crop during the early stem elongation (jointing) growth stage.
Legume/cereal cover crop mixtures vary widely in how much PAN they add to the soil, depending on legume content. When the dry matter from a cover crop mix is 75% from cereals + 25% from legumes, PAN contributions is usually near zero when incorporated. Higher legume content means higher PAN contribution and higher cereal content means more N immobilization.
Micronutrients
Routine soil tests are calibrated for macronutrient content levels. Because micronutrient levels in soil are comparatively very low, these levels are not accurately measured by routine soil tests. Plants require very low levels of most micronutrients and it is generally uncommon to have micronutrient deficiencies in New England soils. Additionally, the “recommended” ranges listed for micronutrients are not optimal ranges, as they are for macronutrients, but instead are the typical ranges found among samples submitted to the lab. The best way to diagnose and address micronutrient deficiencies in vegetable crops is to scout for nutrient deficiency symptoms and submit tissue samples during the growing season. For recommendations on specific micronutrients needed for crop growth, such as boron, see the Micronutrient(link is external) section of the New England Vegetable Management Guide.
Most nutrient applications should be made in spring, when a growing crop is best able to use the applied nutrients and avoid leaching, runoff, or volatilization.
Need further assistance interpreting your soil test results? Contact the UMass Soil Lab at [email protected](link sends e-mail) or our new Soil Health Extension Educator, Arthur Siller, at [email protected](link sends e-mail) or (413) 545-9747.
--UMass Vegetable Program
