The right time for soil sampling

Soil testing indicates soil nutrient reserves available for crop uptake. This is useful any year, and especially after low-yielding or high-yielding years when reserves may be more difficult to predict. Test results will help farms set fertilizer rates specific to the needs and yield potential of each field – a key step in implementing the right rate principle of 4R Nutrient Stewardship practices.

Soil sampling just prior to seeding provides the most accurate measure of nutrients available to the crop, but the springtime slot has practical limitations. Fall soil sampling can be almost as accurate and has various advantages: less time pressure to get samples collected and analyzed, especially if done after harvest; more time for fertilizer planning, including variable rate prescription maps; and the opportunity to buy the right amount of fertilizer at a lower price. (Prices are often, but not always, lower in the fall.)

The best time for fall sampling is after soil has cooled to at least 10°C. Cool soils reduce the microbial activity that can mobilize nutrients. Soil samples collected after this activity slows down will more closely reflect spring nitrate (NO3–) contents. If farms plan to band fertilizer in the fall, sampling when soils drop to 10°C (not too much lower) should allow for fall application before the ground freezes.

Sampling immediately after combining may show what nutrients, if any, were deficient for the current year’s crop, but early fall sampling is not recommended for planning next year’s fertilizer rates. Changes in nitrogen levels often occur after sampling due to moisture-fueled mineralization and losses to leaching, denitrification and immobilization.

Canola Watch video: fall soil sampling

Cost. Custom sampling in two parts (0-6″ and 6-24″) and lab analysis of that two-part soil sample will cost around $100. Cost per sample will be lower when a farm submits more samples. At $1, or less, per acre, soil sampling will pay off if farmers use results to make more refined fertilizer rate decisions.

Composite samples. One composite sample per field can provide a general impression of soil nutrient levels. For the composite, take 15-20 sub-samples from the most productive areas – not hill tops, not low spots, not saline areas. Divide each core into two or three soil depths and put them into separate pails. Suggested depths are 0-6” and 6-24”, or a three-way split of 0-6”, 6-12” and 12-24”. With the 15-20 sub-samples separated by depth, blend those samples to create one composite sample per depth. Submit each depth in its own sample bag.

With one composite sample per field, farmers can create fertilizer blends specific to the needs for each field. If field-specific fertilizer blends are not logistically possible, a compromise is to apply the same blend but at different rates to match the yield goal for each field.

Zone samples. For more precision, collect separate samples from common zones within the field. Zones are generally based on productivity differences that can be fairly predictable based on soil characteristics, drainage or elevation. Three zones could be hilltop, mid-slope and low-lying areas. Five zones would delineate the side slope positions. For each zone, follow sampling methods similar to the composite sample technique. This could mean six or more samples per field (two depths for three zones, for example), but this method can provide meaningful results for fields with higher levels of soil variability, and can point to the value of variable-rate fertilizer applications.