| SAMPLING PROCEDURES
In order for a soil test to be an effective monitoring tool, sampling procedures need to be consistent from one year to the next. This is where field histories come in. Factors such as sample depth, area represented, time of year, number of sub-cores and sampling equipment should be the same each time the field is sampled.
SAMPLING DEPTH
Most surface samples are taken from a depth of 0 to 6 to 0 to 8 inches. This is where management of fertilizers and soil amendments occurs, meaning this is the volume of soil that will be affected by a grower's management. The exception to this is the mobile nutrient which has the ability to move with the soil water: nitrate, sulfate and chloride. In addition to the surface it is recommended that these mobile nutrients also include a profile depth of 6 to 24 or 6 to 36 inches. These profiles need to be collected into a separate bag with the depth labeled.
Of all the factors affecting consistency of a soil test system, sample depth has the greatest influence. Below is a table demonstrating the affects of a tillage system and sample depth on phosphorus soil test values:
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Soil Sampling Depth and its affect on Soil Test Phosphorus (Bray I-P) | | Tillage System | | Sample Depth (Inches) | Plow P ppm | Chisel P ppm | No-Till P ppm | | 0-1 | 55 | 104 | 172 | | 1-2 | 50 | 104 | 74 | | 2-3 | 44 | 94 | 42 | | 3-6 | 40 | 38 | 22 |
Stratification in Reduced Tillage
For sampling in no-tillage systems maintaining sample depth is even more important. Immobile nutrient (such as phosphorus) build on the soil surface with broadcast fertilizer applications. At some point in time it becomes important to determine the degree of stratification. It is suggested that a sample unit/area be used as a test, instead of a 0 to 6 inch sample depth, perform a split sampling of 0 to 3 inches and also from 3 to 6 inches. This split sampling allows the determination of stratification. If the second depth (3 to 6) is 50 % of the surface depth (0 to 3), significant stratification has occurred. Further, broadcast applications without tillage will continue the process. It would be advisable to recommend a deep tillage and begin the no-till process over.
Representative Area Sampling
Conventional Sampling System
Several options are available for determining how to divide field/fields into individual soil samples. If the field is less than 20 acres the whole field can be made as one sample. If greater than 20 acres then multiple samples are required. Topography, soil type or even divisions can do this division. It is important to remember that unusual areas (farmsteads, fence rows, ponded areas, blowouts, etc.) need to be avoided or sampled separately. Old farmsteads are still apparent decades after they are abandoned.
When sampling, 15 to 20 sub-cores are needed for providing a representative sample, more would be even better. In addition the laboratory needs at least 8 ounces or 1 pound of material in order to perform all analysis.
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GRID Soil Sampling
Grid soil sampling involves dividing the whole field into small equal sized grids. Most grid sizes comprise 2.5 up to 5 acres. The soil cores from each grid are collected either from the center point or from random points through out the grid cell. A minimum of 10 cores is needed in order to provide the laboratory with enough soil to perform analyses.
Time of Year to Sample
The time of year (Early fall - vs. - spring) can have an affect on analytical results. For monitoring, it is recommended that samples be collected during the same season: late fall, winter, spring and summer. Do not rotate your sampling schedule.
Research conducted at the University of Wisconsin | Month | pH (ppm) | Organic Matter % | P (ppm) | K (ppm) | | September | 6.7 | 2.2 | 18 | 140 | | October | 6.9 | 3.3 | 18 | 110 | | November | 7.1 | 3.7 | 18 | 110 | | December | 7.2 | 4.2 | 25 | 170 | | January | 6.9 | 2.7 | 28 | 130 | | February | 7.2 | 2.8 | 19 | 150 | | March | 7.5 | 3.4 | 19 | 120 | | April | 7.2 | 3.2 | 19 | 105 | | May | 6.8 | 2.8 | 19 | 113 | | June | 7 | 2.8 | 19 | 92 | | July | 7 | 2.3 | 10 | 80 |
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| The variation in the test data is due to the natural variability of: wetting and drying, freezing and thawing, plant nutrient uptake and nutrients released from residue decay. Since these changes occur at differing rates each year it is difficult to predict what changes might occur. For this reason, pick the season most convenient for your operation and stick with it. |
Sample Handling
Proper handling of the sample is important to maintain the integrity of the soil. Soil is a biologically active material, this activity continues even after the sample has been collected. It is necessary to collect and ship no later than the next day. If storage is required, three options are available; dry the soil with low heat (not greater than 120 F), store in a freezer or at least refrigerate. These processes will slow down or stop the biological activity, preserving sample integrity.
Sample Submittal Forms
To streamline the laboratory's handling (to assure fast turnaround) of the received samples include laboratory submittal forms. Laboratory supply order form and submittal forms (sample sheets) are available at AgSource Laboratories.
Essential information includes:
- Your company name and address (this should preprinted from the laboratory)
- Grower Name
- Sample Identification
- Crop and yield goals
- Previous crop
- Desired test options
AgSource Laboratories provides complimentary sampling supplies, simply contact us.
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