Fall soil test provides portrait of field fertility

Soil testing creates many options, but the primary reason most growers soil test is to take a snapshot of the nitrogen levels in their fields.

This is because nitrogen levels can fluctuate greatly due to environmental and crop growth conditions from year to year.

Other nutrient levels are important, but nitrogen is the largest contributor to yield and part of most fertilizer budgets.

Nitrogen recommendations based on soil tests are most accurate when samples are taken as near to the time of seeding as possible.

However, spring seeding on the Prairies is a busy time, and soil testing doesn’t always work out if it is a wet or late spring.

The database used to create recommendations by the soil test lab are based on either spring or fall soil samples, and more than 80 percent of prairie soil samples are taken in the fall.

That’s because it usually has the widest window for sampling and allows time for analysis and purchase of fertilizer when the price is at its lowest level.

The factors that affect the differences in fall and spring nutrient changes also determine when one should start soil sampling at the end of the growing season. It is important to understand the issues that influence nitrogen soil test levels between early fall and spring seeding.

Fall soil testing and nitrogen application can start earlier than once thought. Studies done by North Dakota State University and Westco Fertilizers found few changes in nitrogen levels from September to October.

Soil testing can begin any time in September, and fall banding can start in mid-September on well drained soil.

On poorly drained soil, wait until soil temperatures are less than 5 or 6 C, usually by early to mid-October.

Early sampling has its advantages:

  • Growers are more likely to use the test results to direct fall nitrogen application if soil test results are in hand and available soon enough to consider before hitting the field to prepare for next year’s crops.
  • Soil sampling before fall tillage will result in a more consistent and reliable zero to 15 centimetre sample core.
  • Fields are more likely to be sampled in the first place.
  • Regrowth, germination and second growth of the harvested crop will not hide available nitrogen from the test results.

Sampling depth should be zero to 15 cm and 15 to 60 cm where possible. This is especially true this fall, when variable rainfall might have resulted in nutrients showing up at varying depths.

A zero to 15 cm depth is particularly important for accurate phosphorus, potassium, soil pH, percentage organic matter, zinc and other micronutrients that are tested only on the surface core.

It can be unreliable to compare year to year results if previous samples were taken at zero to 30 cm depths.

Research has shown that the most accurate portrayal of the large fields is accomplished using management zones derived from satellite imagery, topographic maps and previous yield maps. Electrical conductivity mapping can be used to augment this information.

The information can also be used to direct samplers to a location to either sample or avoid.

The tools aid producers in developing a reliable composite sample of a zone. Grid sampling can be done, but the North Dakota research suggests that the grids should be no bigger than one acre to paint an accurate portrait of the nutrient reserves.

At this size, the cost is generally considered prohibitive by most producers.

Farmers who don’t do the sampling themselves should look for a reputable sampler who employs trained staff and who works with an established soil analysis lab.

The lab should use a recommendation engine that is based on prairie yields and conditions.

One of the most important things farmers can do is actually use the recommendations when making their fertility plans for next year.

Far too often, farmers take soil tests but then use the same fertilizer blends and rates that were used in the previous year.

Why? That, I don’t know.

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