Farmers can’t ignore soil sampling when determining which nutrients are deficient in their field and how much nutrients they need to apply to achieve a target yield.
Ken Panchuk with Saskatchewan Agriculture’s crops and irrigation branch says producers can get only a ballpark figure of the nutrient levels in their fields by estimating the nutrients used by previous crops.
This is because it is difficult to know the rate of mineralization, which is the decomposition of crop residue and organic matter that releases nitrogen, phosphorus, potassium and micronutrients back into the soil.
The rate of mineralization may be even harder than usual to predict this year in some parts of the Prairies because of the dry weather in May, June and most of July, followed by average to above average moisture in August and September.
“Given that situation, there is a phenomenon called the birch effect, and that is after you have a prolonged dry period there could be a rapid mineralization of nitrogen and other nutrients,” Panchuk said.
Performing the complete suite of nutrient testing may not be necessary every year, depending on how well the history of the field is known, what kinds of issues have shown up in the past and if there has been a recent change in how fertility is managed over the long term, said Jeff Schoenau, a fertilizer researcher at the University of Saskatchewan.
Immobile nutrients such as phosphorus and potassium are relatively stable, and producers may need to test them only every two or three years to benchmark their levels and watch for an uptrend or downtrend.
However, It’s difficult to rely on benchmarking for the more dynamic nutrients such as nitrogen and sulfur.
Sulfur levels can change dramatically, and there is a high degree of variability in salinity levels in western Canadian soil.
“Whenever you run into those sulfate salts in (saline soils), it throws the soil test for sulfur out of whack, but if you get a low soil test sulfur rating, of course you need to correct it, especially if you’re growing canola,” Panchuk said.
Testing soil as close to the time of seeding and plant nutrient demand as possible will provide the most accurate results in terms of what a crop needs.
However, the logistics of modern production have pushed soil testing back to the fall for many producers.
Fall soil testing is best done when soil temperatures drop below 10 C because cooler temperatures will slow down microbial activity and nutrient levels will remain more constant.
Soil testing labs are now in one of their busy seasons and are turning over soil tests within days to producers who are already banding in nitrogen before freeze-up.
Producers who rely on fall soil testing should also check nitrogen and sulfur levels in the spring in a few fields because these nutrient levels could change from fall to seeding time.
Schoenau said obtaining representative soil samples from fields is a critical step in soil testing. Improper sampling is one of the main reasons for bad fertilizer recommendations, he added.
“Some recent research that we did in a fairly small area around 20 acres, we found that we need as many as 60 cores to come within five percent of the true mean or average of that area,” he said.
“Now that is a lot of cores, but I think for an 80 acre field you would want to be taking at least 15 cores.”
There are four main strategies to use when choosing where to take samples in a field:
- Random sampling take samples where farmers’ intuition tells them they will receive a representative sample and from multiple areas that will average out to be a representative sample.
- Benchmark sampling picks an area of a field that best represents its productivity. This method improves sampling quality because samples are taken from a consistent area year after year, and changes in the field’s fertility can be closely identified.
- Grid sampling can be expensive and time consuming because of the large number of samples needed to accurately cover a field. A specific number of samples are taken per acre across the entire field.
- Zone sampling samples different parts of the field separately based on a known difference that exists in the field, such as soil types and characteristics, elevations, crop protein levels, satellite images, different probable responses to fertilization and past productivity according to a yield map off a combine. Enough samples have to be taken in each of these zones to obtain a representative sample. Panchuk said 15 or more cores are used to make a composite sample.
A sample slice can deal with the micro variability in a field, especially in zero-till production where little soil mixing occurs.
Schoenau said farmers can produce a sample slice by cutting out a section of the soil across the seed and fertilizer rows with a flat bottom shovel and adding it to the composite sample.
Most nutrient turnover and cycling occurs in the top six inches of the soil, which is where many samples are taken from.
Panchuk said mobile nutrients may require two sampling depths, and he encourages two subsets: zero to six inches or zero to 12 inches, and six to 24 inches or 12 to 24 inches.
This is particularly important in fields where high rates of nitrogen are used or where manure was spread and the crops didn’t use all of the applied nutrients.
Schoenau said it’s a good to occasionally sample the nitrate and sulfate levels deeper in the soil profile because they will move with the water to the roots, especially later in the season, and contribute to the crop’s nutrient supply.
It’s important to keep the samples as close as possible to their original state when they were gathered.
“When that sample comes out of the field, ideally it’s chilled or frozen or air dried in order to stop the microbial activity, or greatly slow down the microbial activity and prevent any large changes in the available nutrients in that sample from the time it leaves the field to the point where it goes into the lab,” Schoenau said.
Producers may want to contact the lab about how it wants them to store and transport their samples.