With precision ag technology available to spread fertilizer at variable rates, soil testing is essential to make fertilizer prescriptions. Soil tests will give a basis for fine-tuning fertilizer and manure inputs so they are used cost effectively on your farm.

Test small enough areas to define the variability in fields. At the start of a precision-ag fertilizer program, it may pay for you to over-sample fields to determine this variability.

After determining variability in each field, you can adjust sampling density field by field. If soils are highly variable, more sample points will be needed. If soils are more consistent, then sampling density can be reduced.

Sampling Approaches.

Soil sample at a uniform depth of 6 inches. As the chart shows, if you vary the sample depth, the levels of nutrients for which you are testing will vary as sampling depth changes. That’s why uniform sampling depth is a must.

Common approaches to soil sampling are whole-field, grid and zone sampling.

How do you choose what’s best? All are acceptable.

Three factors normally determine which method to use:

Fixed or variable-rate fertilizer management

Sampling history — recent or long overdue

Existing fertility level — low and need to know and/or high and ready to fly.

With high fertilizer prices, it makes sense to re-evaluate your testing program and consider more intensive sampling and application. Grid sampling and variable-rate technology may be less expensive than you think if it improves fertilizer efficiency.

Figure 1 is an example of an unaligned, systematic…