Sampling and Sample Preparation
One of the most important and probably one of the weakest steps in a soil testing program is sampling. Soils are very heterogeneous and if samples are not taken properly the test results are of little value to the producer. Consequently, extreme care should be taken during sample collection because soil test results can be no better than the sample submitted to the laboratory for analyses. Below are some guidelines to aid in collecting a representative soil sample.
C. Owen Plank
Associate Professor, University of Georgia
• Collect samples using clean sampling tools and a plastic bucket. Use of metal containers for collecting and mixing samples may result in contaminating the sample with some micronutrients, e.g. manganese (Mn) and zinc (Zn), invalidating these tests.
• Collect subsamples from defined areas in a random pattern. Sample similar areas together with at least 10 subsamples (preferably 15 to 20) from an area to make a composite sample. Thus, each green and tee should be sampled separately. On fairways, dissimilar areas (such as low, moist sites versus upland, drained sites) should each be sampled separately.
• Sampling depths will vary depending on soil conditions and whether the area is being sampled for establishment, maintenance, or troubleshooting purposes. Consequently it is important to follow the sampling guidelines of the laboratory performing the analyses. Some suggested guidelines for sampling depths are:
- A. Fairways and tees 4 to 6 inches
B. Modified soils (e.g., greens) 6 inches
- A. Fairways and tees 4 inches*
B. Modified soils (e.g., greens) 4 inches*
*Discard any thatch. Do not discard the mat (thatch containing considerable soil or topdressing).
3. pH modifications (e.g., use of sulfur to lower soil pH)
- A. Sample to a depth of 3 to 4 inches and subdivide the core into 1-inch increments. Using this technique for incremental sampling reduces contamination as compared to pulling individual 1-inch increments. (Note: label samples according to depth, e.g., 1-inch, 2-inch, 3-inch, etc.)
• Carefully mix the subsamples and obtain a representative sample that contains sufficient soil for analyses. Usually one to two cups of soil is sufficient. Some laboratories now provide "fill lines" on the sample container to insure adequate sample volume.
• Keep careful records of where samples were obtained and the particular code used to identify a sample.
Although samples can be obtained at any time of the year it is suggested that samples be collected before the most stressful part of the year. Also, samples should not be collected within two weeks of phosphorus (P), potassium (K), or magnesium (Mg) fertilization. If results are being compared from one sampling period to another, samples should be taken at the same time each year. This is due to the fact that there can be some seasonal variation in results for some analyses. For example, soil pH values will generally be lower in July or August as compared to January or February.
Highly managed areas such as greens and tees should be sampled annually. Annual sampling is also recommended for areas where the grower is attempting to markedly alter some soil chemical aspect such as pH, P levels, salinity, etc. Routine sampling every 2 to 3 years is usually adequate for fairways.
Air dry the soil in an area free of fertilizer dust; include all necessary information requested by the laboratory - name, address, sample identification, type of grass on the area (green-bentgrass, tee-hybrid bermudagrass, etc.).
The laboratory will grind the sample to break up any aggregates and screen it to uniform size (<2mm, or 10 mesh). Thus, any soil material larger than 2mm in diameter, such as gravel or rocks, is not analyzed.
It is important to note that once a sampling strategy (sampling depth, sampling time, etc.) has been developed one should continue the strategy for subsequent years. This is particularly important when one wants to compare results over several years to determine trends in fertility levels. For example, have the P and K management practices used resulted in an increase or decrease in the soil nutrient levels. If one does not follow consistent sampling strategies each year, such comparisons cannot be made.