Farmers who hire companies to do soil sampling need to ask the following questions:
- What type of soil sampling pattern do you recommend and intend to use?
The answer depends upon the farmer’s need. Traditionally composite samples are taken from representative areas of the field, and bulked into a single sample for submission to the lab. An elaboration of this system is to mark the sample points through global positioning so they can be returned to in the future. However, if you are looking for more detail, such as why certain portions of the field perform differently, other sampling patterns will be required, including grid, landscape and zone sampling.
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- Can you describe landscape positions?
In traditional composite sampling, it is preferred to sample from the mid slope. Generally mid-slope positions comprise 60-80 percent of the field. Sampling knolls or depressions may taint the composite samples. These samples can be collected and submitted separately to the lab. If not, they should be kept out of the composite sample.
- Do you smoke when you sample?
Contaminants such as cigarette ashes, dirty hands from eating salted sunflower seeds, and unrinsed sample pails could taint the soil sample.
- Is there a cooler on your quad or truck?
If the day is long and the weather becomes warm, samples can heat up, especially if stored in the cab of a pickup. Samples should be kept cool to prevent changes in nitrogen content.
- How many cores do you take per field?
Assuming the composite sampling pattern is used, a minimum of 15-20 samples per field are required. Sometimes people mistakenly assume that if 16 samples are adequate for 160 acres, then four samples are OK for 40 acre fields. This is not the case. Also, large fields that had different previous crops should be considered two different fields for sampling purposes.
- Can I ride with you to select the sample sites?
The grower knows best how to select representative areas of the field, based on knowledge of crop performance, cropping history and location of manured areas. This also gives you a chance to see your sampler in action Ð how samples are taken, handled and packaged. Unless you plan to ride with your sampler every year, make sure the sampling locations are recorded, either on a field map or as GPS waypoints.
- How deep do you sample?
The contractor should be able to sample to half a metre. Immobile soil nutrients such as phosphorus and potassium and many of the micronutrients should be measured in the zero to 15 centimetre depth. But nitrogen recommendations in the Prairies are based on the soil nitrate-nitrogen content in the top half a metre.
If sampling depth is less than half a metre, then the adviser making fertilizer recommendations must resort to a “conversion factor” to estimate what is in the remaining, unsampled centimetres.
Ensure the sampler’s cores are the full half a metre in depth. Otherwise, make note of the actual depth on the submission form. This will help the adviser’s estimation exercise.
- What information do you include on the submission form?
The information submitted with the soil sample is critical, especially if you expect to use the fertilizer recommendation from the testing lab. Standard information includes field location, return addresses, sampling date and sample depths. Critical information needed for your fertilizer recommendations are: previous crop and yield produced; expected crop and target yield; fertilizer placement options and other sources of potential nutrients such as pulse or legume crops and manure.
- Which lab will you send my samples to?
It is best to hire a lab that uses chemical extraction methods appropriate for your soil. Fertilizer recommendations may be a useful, but not essential, service offered by the lab. If you plan to use the laboratory’s fertilizer recommendations, ensure they are based on knowledge and research from your area.
- Who will receive, store and organize my soil test results?
Some growers get fertilizer recommendations from their adviser or lab with little appreciation of the actual soil test results. To test simply to generate a fertilizer recommendation for that year and crop is to underuse that information resource. Soil test information can be used in making other assessments, such as tracking whether soil test levels are building or declining, whether the risk of herbicide residues are high or low and what effect changes in crop rotation or tillage systems have on the soil. To gain full value from the soil test exercise, reports should be stored and tracked over time.
