How do you make sound decisions about applying fertilizer with prices at all-time highs? Stan Miles says a good place to start is by returning to the basics of soil fertility. That includes understanding soil sampling and interpreting soil sampling results.

Miles is an agronomist with A&L Great Lakes Labs. Although the lab is in Fort Wayne, Ind., Miles provides insights from his location near Princeton, Ind.

Start with pH. “Look at pH values first when you get results,” Miles says. “The biggest reason is because pH greatly affects availability of major nutrients, including phosphorus and potassium.

“If pH levels are 5.5 or below, availability goes way down, even if there are adequate amounts in the soil. We suggest keeping pH in the 6.5-to-7.0 range to increase availability of these fertilizer nutrients.”

Miles acknowledges that maintaining pH in the proper range can become challenging on rented land. The question always arises about who should pay for lime — the tenant or the landowner? Working out these issues in advance makes sense.

“You don’t want to wind up spreading $1,000-per-ton P and K, and crops can’t utilize it because nutrients aren’t in the soil solution,” Miles says. “That can happen if the pH is too acidic.”

Ask about soil sampling depth. If someone else pulls soil samples, how deep do they pull them? Do they pull samples at the same depth each time?

“Consistent soil sampling depth is critical to having good information,” Miles says. “P and K levels can vary based upon soil sampling depth.”

Here is how P and K concentrations can vary, based upon sampling depth. In this example, soils were pulled at different depths to illustrate how depth can affect results:

Obviously, samples were pulled at these depths to show how nutrient values could vary. No one recommends a 2-inch sampling depth!

Sample in the same season. You can get accurate results any time of year if you pull at the same time of year each time, Miles says. The only exception might be sampling frozen soils. Freezing can cause erratic results. Potassium is the major nutrient most affected by seasonal changes.

Compare new and old results. If you follow consistent sampling methods, including time of year, you can track changes in sampling zones over time, Miles says. The only caution is that some labs, including his lab, switched from the Bray P1 test to the Mehlich 3 testing system recently. If you know which system was used each time, you can convert results, if necessary, and compare. Ask your consultant for assistance.

Consider banding vs. broadcasting. Should some soil zones test low in P and/or K, consider banding so nutrients aren’t tied up with larger amounts of soil, and so roots can reach them.

“If soil tests are at higher levels, it’s likely not as critical whether you’re banding or broadcasting fertilizer,” Miles concludes.