If a nutrient deficiency is suspected in your field(s) or you’re planning your fertility for next season, a soil test is a good place to start. Keep in mind, although soil tests are key in the investigation they shouldn’t be the only tool used for diagnosis. Scouting for crop symptoms, comparative soil and tissue testing and examining previous crop yields and rotations are all pert of the diagnostic and planning process. Thoroughly reading and correctly interpreting a soil test result is a critical step in making sound economic decisions on crop nutrition.
How to Soil Sample
The best time to test is as close to seeding as possible. Many years, an intense spring workload means fall testing is the only time that works. Early fall testing is possible; in fact, it can begin right after harvest if that’s what works best. But testing should be delayed until later in the fall, or even spring, in the following situations:
- Pulse crop stubble – Because potentially mineralizable N levels are higher in pulse residue (a low carbon to nitrogen ratio), rain and warm weather might encourage fall microbial activity and subsequent N mineralization. This means if testing occurs right after harvest, there’s still time for microorganisms to convert organic N to plant available NO3-N before freeze up, and an early fall soil test won’t account for this added N.
- Freeze-thaw or wetting-drying cycles – Under extreme weather conditions, continual freezing thawing or wetting-drying cycles encourage microbial N mineralization from OM, resulting in a surge of NO3-N. An early fall test won’t pick up the added N.
- High initial N or OM levels – Soils with relatively high N or OM levels have more potentially mineralizable N. Early fall testing in fields with the above conditions could underestimate available N. But if growers recognize, understand, and accept this risk, testing can be executed any time in the fall.
There are four basic methods to field testing:
Benchmark – a predetermined area of a field, usually 1/4 ac, chosen to be representative of the entire field. Generally this location is referenced by GPS.
Random – sampling occurs throughout the whole field but only areas considered average or representative are included. Knolls, depressions, or other atypical areas of the field (such as old farm yards or saline areas) should be avoided.
Grid – a detailed sampling method that superimposes a grid pattern onto the field (grids are usually 5-10 acres). Composite samples consisting of 8 – 10 cores are collected around each grid intersection.
Landscape zones – separates the field according to topography – knoll, midslope, depression – and samples each landscape position separately. Most useful when the crop nutrition program is also based on landscape zones.
To match sampling efficiency with statistical accuracy, extract 15 to 20 cores and mix into one composite sample. The number of cores is irrespective of field size (or zone). So whether it’s a 1 acre vegetable field or a 320 wheat field, 15 to 20 cores should be taken. What constitutes a field? Any area that has been farmed as a single unit– same crop nutrition program, same crop production inputs - in the last few years. The ideal depths for a sample are 0-6, 6-12, and 12-24”. These depths allow accurate analyses of pH, OM, P, and K in the shallow depth and includes the deeper depths for more mobile nutrients such as NO3-N and SO4-S. If a truck mount probe is used and can’t separate depths, a 0-12” core is acceptable. But, if OM or pH analysis is required for herbicide label guidelines or for soil health information, a 0-6” sample is critical. Some agronomists use a truck mount sampler for the 12” sample and a hand probe or auger for the 6” sample.
The Chance of Error
The soil testing process generally consists of four steps:
- Soil Sampling
- Sample extraction and analysis
- Correlation and calibration of results
- Crop nutrient recommendation philosophy
It’s generally the first step – soil sampling – that presents the biggest potential for errors in the soil testing process. Incorrect sampling, mixing and packaging can lead to sample contamination – all common sources of error. Following correct soil sampling procedures is essential for maintaining sample integrity and generating accurate results.
Completing the sample submission form as thoroughly as possible also helps improve your recommendation. For example:
- The legal land location is required because it places a field into a specific soil climatic zone, which is required to predict soil moisture, precipitation and mineralization.
- Stubble management determines if more nitrogen should be recommended (if the straw is spread and immobilization is predominant) or if less nitrogen is required (because the straw is baled and mineralization is favoured).
pH is a measure of acidity or alkalinity and is based on a scale from 1 to 14, where 1 is acidic (also called sour) and 14 is alkaline (or basic). Prairie soils generally fall between 6.5 and 8. pH should always be measured on a 0-6” sample.
pH is important for cropping guidelines after applying a residual herbicide and for accurate nutrient recommendations. For example, because phosphate easily binds with calcium and magnesium in soils with a higher pH, the recommendation for phosphate in these soils will be adjusted.
Another possible implication of extreme pH is impaired crop growth. For example, while blueberries grow well in low pH soils alfalfa prefers a higher (or at least neutral) pH.
E.C. (electrical conductivity) is a measure of soluble salts, reported as mS/cm (millisiemens per centimeter). Labs will often use a 1:2 soil to water paste to determine soluble salt content, but the actual salinity rating is based on a calculated saturated extract. A rating less than 2 indicates the soil is non-saline, 2 to 4 is slightly saline, 4 to 6 is moderately saline, and 6 to 8 is severely saline. Once E.C. climbs above 4 yield of most crops is threatened.
Few labs routinely determine texture. If assessed, texture is usually determined by a hand-texturing procedure. When specially requested, labs can do a more detailed analysis called a P.S.A. (particle size analysis).
Despite seldom testing, texture is an important parameter in western Canadian soils because it reflects the capacity to hold moisture and retain positively charged nutrients. For example, clay or heave soils are better able to retain moisture and nutrients; so from a practical standpoint, anhydrous could be banded closer to the surface in clay than sand. Or, more seedrow placed fertilizer is allowed on clay soils compared to sandy.
Labs extract and measure nitrate-nitrogen (NO3-N), available phosphorus (P), potassium (K) and sulfate-sulphur (SO4-S). Nitrogen recommendations are based on the amount of nitrate measured in 24” (or estimated id a 24” depth isn’t submitted). Labs implement different methods to extract available phosphorus and potassium, but all lab recommendations are ultimately based on the concentration of these nutrients in the top 6”; which makes sense because these nutrients are relatively immobile in the soil. Sulphur recommendations are based on the amount of sulphate in the top 12 or 24”, depending on the lab.
Labs will either report soil nutrient levels in parts per million (ppm) or pounds per acre. A rough rule of thumb to convert ppm to lbs per acre is to multiple the number by two for every six inches of soil. So, for example, if there are 4 ppm of nitrate-nitrogen, there are 8 lbs per acre in 0-6”, 16 lbs per acre in 12”, and 64 lbs in 24”.
Fertilizer recommendations reported on test results are made for actual nitrogen (N), phosphorus (P2O5), potash (K2O) and sulphur (S). So if a lab recommends 15 lbs of actual P2O5, this means 29 lbs of 11-52-0 per acre.
Soil testing labs also offer micronutrient analysis. Micronutrients are needed by the crop in very small amounts but are essential for healthy plant growth. The micronutrients are: boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni) and zinc (Zn). Ideally, analysis for micronutrients should be conducted on a 0-6” sample. Results are usually reported in ppm, but some labs will report in lbs per acre.
Labs will suggest a micronutrient rating ranging from deficient to excess according to the crop. Where possible, labs will use geographically significant data. But for some micronutrients and crops local databases are lacking, so labs extrapolate from foreign data.
After receiving soil test results, connect with your local DEKALB® agronomist to ask questions and dissect results together. Interpreting the recommendations and results are an integral part of making informed crop nutrition decisions.
Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible.
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