How many soil samples should I take off of a field? How deep? Should I have a pattern? When should I take the samples? What lab do I use? Should I believe them? These are just a few of questions producers ask about soil testing. The key to accurate and useable soil test results is to submit a good representative sample.
1. QUALITY OVER QUANTITY
When I first started soil sampling, the rule of thumb was to grab as many samples as possible to “average” out a field. I would collect 15 to 20 samples per field then mix them up and send in one “representative” sample. But in the east central part of Saskatchewan, where I farm, you can start taking samples too close to the sloughs or hills. When you mix these samples together, you can get results and recommendations that don’t suit the predominant needs of the field.
When I started sampling for Western Ag Labs, we started cutting down the number of samples and geo-referencing them with a GPS. I started with four samples per field, and then actually cut it down to two. It was easier to pick two or three “average” sites than to go out and pick a bunch of samples and hope it would be close to average. The most reliable sampler is you. Having someone go out and grab samples may be a mistake. You know the fields and where average is. Unless the soil sampler has a pile of experience or you have GPS points to guide the sampler, the test may be out of touch with average.
2. SAMPLE THE TOP FOURINCHES SEPARATELY
How deep to sample is another common question. Historically, zero to six inches or zero to 12 inches are the common depths. My problem with these two depths is that the roots and nutrients are not distributed this way in our soils. If you pull out a canola root, most of the secondary rooting occurs in the top three to four inches in the soil. That is where most of the available, or exchangeable, nutrients are located due to the higher organic matter content. Lower in the profile in the Prairies, calcium and magnesium levels start to increase dramatically, which decreases the availability of certain nutrients to plants — especially the positively charged nutrients. Taking only a six-inch or 12-inch sample will artificially reduce the levels of phosphate, potassium, and zinc. Things like nitrate, nitrite, sulphate, boron, and chloride are negatively charged and are highly mobile in the soil moisture. That is why a deeper sample is requested to get a better measure under variable moisture conditions.
We like to take a zero to four inch sample for phosphate, potassium and zinc, and a zero to six or zero to 12 inch sample for nitrogen, sulphur, chloride and boron. If we’ve had a lot of rain, I take the deeper sample.
3. SAMPLE PULSE GROUND INSPRING, OTHERS IN FALL
Timing is the other consideration. For most crops, I like to sample in the fall. For most labs, you’ll want to sample when soil temperatures are less than 4C. Temperatures over 4C will encourage microbial activity that will add or subtract nutrients from the soil. Western Ag Labs uses a method of analysis where the temperature is less important. Ask your lab for its recommendation for sample timing.
For fields that had produced a pulse crop, I wait to sample in the spring as close to seeding as possible. By waiting, this gives the nodules in the soil time to breakdown and release some of the nitrogen to give a better estimate of nitrogen available.
4. SAMPLE FROM TARGETEDZONES
On our farm, we have broken our fields into 40-or 50-acre fields, depending on topography, soil type, cropping history or convenience. Smaller fields allow us to vary our fertilizer blends accordingly without the technological infrastructure. Each field is then soil sampled using GPS benchmarks derived from yield maps. We took a larger soil sample from a field last fall and divided it into three, sending it to three different labs for analysis and recommendations. Very interesting results came out of this. The two that came up with the most realistic recommendations were AgVise and Western Ag Labs.
When soil sampling take a sample from a problem area and see what the problem is. Some areas are presumed to be one problem where they end up another. For example, white spots in a field that are assumed to be salinity could end up being magnesium deposits on the surface. If you do not have a strong background in soil analysis, find someone who does and ask them to help.
Try different labs, but stick to one you are happy and follow the recommendations so you have something to compare to. Develop management zones on fields with defined areas of different soils or topography. It will make you money. Consider doing some tissue sampling during the growing season to see how the soil test is working.
Kevin Elmy operates Friendly Acres Seed Farm, along with his wife, Christina, and parents, Robert and Verene, near Saltcoats, Sask. You can contact him at 306-744-2779 or[email protected]