Alberta Agriculture calls poor soil sampling techniques a major problem that causes improper fertilizer applications.
A repeatable soil sampling technique should begin with farmers recording each field separately in a log book. Include the sampling date, a summary of the topography and note areas to avoid when developing a representative sample.
A field log gives farmers the ability to repeat the test to show changes in soil condition year after year without errors caused by sampling from areas that are not average.
Hillsides, saline areas, depressions, old farm sites, fence rows, corrals, areas with significantly different soil textures, and any area not part of the average growing area should be tested separately or left alone.
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Alberta Agriculture identifies four strategies for sampling: random soil sampling, benchmark soil sampling, grid soil sampling and topographic soil sampling.
In a random sample, take soil in a patternless manner and avoid problem areas. The field should have been evenly cropped in the past and no larger than 80 acres.
Benchmark testing is done by sampling each unique area separately. Farmers should sort areas by topography, soil type and crop growth. Precision farming methods can take advantage of this type of sampling.
Grid soil sampling allows farmers to make a repeatable soil condition map from the data. It provides data for precision agriculture and is based on a 10 percent sample of the field, marked out in a grid. It works well on large fields but farmers may find the high cost of grid sampling a deterrent.
Topographic sampling separates the sampling areas by contours and is effective on hilly land.
“You need at least 15 to 20 sampling sites in each field in order to get a good representative sample,” said Ross McKenzie, of Alberta Agriculture in Lethbridge.
McKenzie recommends probe sampling down to 60 centimetres. Probes are more accurate than augers for deep tests.
Probe samplers take more power and maintenance and must be mounted on a heavy test vehicle, but can produce a sample that is easier to measure at several depth points.
Spring is the best time for soil tests. Fall tests can be rendered inaccurate after changes caused by spring moisture and the microbial action in the soil once the ground warms.
But to spread out workload, most producers compromise and use fall tests in order to buy inputs for the coming growing season.
Producing a clean, representative sample is important. Clean plastic buckets are best for collecting samples. Metal buckets can throw off micro-nutrient levels. Any residue left in the pail – even residue from sweaty hands or cigarette ash – can throw off samples.
Soil samples should be broken and mixed thoroughly immediately after they are taken. If they are to be shipped dry, they should be spread on clean paper and dried at room temperature. Drying in an oven can change nutrient concentrations. A fan may be used to dry the sample but take care not to add dust. Any error is magnified when fertilizer recommendations are made.
Samples should be packaged in new bags provided by the soil lab and be filled to the level indicated. If the sample is too small it can skew the data.
Robert Saik, of Agri-Trend Agrology, suggests if the test results seem inconsistent with the producer’s opinion, the grower should contact the lab for advice and consider re-sampling the field.
