In order for your soil test results to be useful the soil you submit must be truely representative of the field. To make sure your sample is representative there are a number of published protocols you can follow. Some use random sampling, some use grid patterns, while others use some combination of both. It is essential you pick the sampling method most appropriate to your needs.
However, the sampling protocol alone won't guarantee your sample is representative. There are other factors that affect how representative the sample is that are rarely discussed in published sampling protocols. These factors are more apparent to the soil chemist receiving, processing and testing the samples. This note will highlight these factors.
Depending on the sampling protocol and field size, you may collect several kilograms of sample. It might seem natural to submit the entire amount to the lab, however this is not actually a good idea. When the lab receives the sample, a portion, or sub-sample, will be weighed and placed into a drying oven.
Even assuming the sample received is well mixed, how representative the sub-sample is will depend, in part, on the ratio of sub-sample mass to sample mass. The closer to unity the more representative the sub-sample. Since the lab takes 200 to 250 grams as a sub-sample we recommend the grower submit 400 to 500 grams of field moist soil.
The upshot of all this is that for best results the grower should mix the sampled soil well and submit only the recommended amount.
An often overlooked factor in soil sampling is how wet the submitted sample is. Sample nutrient results are returned in units that reflect the 'field moist' or 'as received' condition of the soil. For example, parts per million (ppm) or milli moles per kg (mmol/kg) in the sample 'as recieved'. This makes it important that the sample submitted contains approximately the same amount of moisture normally found under average growing conditions. This will not only guarantee the test results will be most representative, it will make the results comparable year over year and make ammendment calculations more accurate.
We suggest that under excessively wet conditions your sample be drained of excess moisture to a state similar to your average growing condition before submitting the sample for testing.
Soil will almost always contain rocks, plant roots, life forms, crop residue and other foriegn material. How this affects your sample test results varies depending on the material in question and the particular test involved. For best results we suggest you remove as much foriegn material as possible either by sieving through 13mm screen (great for draining and mixing as well) or by picking out the excess by hand.
Sample storage can also affect test results. Samples kept for 24 to 48 hours are best refrigerated. Frozen samples can be kept longer, perhaps as long as two weeks, safely. Dried samples (overnight at 50C or 48hrs at 20C with air circulation) may be kept up to two years.