Is it time to sample soil?

When I was working in retail ag, fall dry fertilizer spreading season always had a certain rhythm to it: Every year fertilizer left our plant as we spread it and came back in, as delivery trucks refilled our stocks. The cycle went on for weeks, as we handled thousands of tons of fertilizer. Several times each season, we had to get out tape measures and calculators to physically inventory our stocks so we could be sure that what was in the shed matched what the books said.

While less easy to see, our soils have a similar cycle of fertilizer “ins” and “outs” as we grow crops on them year after year. And much like a retail agronomy manager, we need to take “inventory” of the plant nutrients in our soils and match that to our cropping system and fertilization plan. If we don’t understand our soil inventory of phosphorus and potassium, we can waste money on overfertilization or, worse yet, limit our crop yields with under-fertilization.

The place to start is soil testing. Iowa State University recommends soil sampling every two to four years, or once in a crop rotation. In today’s economic climate, many of us are prone to sample once every two years. We recommend trying to sample about the same time of year each time soil samples are taken.

Fall sampling advantages

Fall is typically the best time of year to take samples, get results back from the lab and apply any needed fertilizer and lime. Fall application has several advantages over other times of the year. First, compaction issues are typically limited, as soils are often drier in the fall. And if an application does create minimal soil surface compaction, the winter freeze-thaw cycle can often address this.

Second, there is typically more time in the fall compared to spring. And, while the frozen soils of winter work well for spreading fertilizer and lime, snow can limit the number of application days. Finally, my favorite advantage to fall application is the cost. While it isn’t as sure a bet now given the wild market fluctuations of the last few years, fertilizer prices are typically lower in the fall than in the spring.

This fall, fertilizer prices look like they will be a lot closer to 2006 levels than to the 2008 or 2009 levels that gave growers and dealers sticker shock. The sky-high fertilizer prices we all suffered through the last few years caused many growers to reduce fertilizer application rates or skip P, K and lime altogether for a season or two. With the more agreeable price outlook this fall, now is a great time to look at new soil tests and see if you can catch up on lost fertility before it impacts crop yields.

There are many soil sampling strategies available, from management zones and grids to points or other configurations that give a true picture of the variation within a field. ISU Extension has a helpful resource on how to take a good soil sample. Ask for publication number PM-287, “Take a Good Soil Sample to Make Good Decisions.”

Reading soil test results

When you get results back from the soil testing lab, compare them to tables found in another ISU Extension publication, “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” (PM-1688).

If your soil test results fall into “high” or “very high” categories, no additional fertilizer is needed. However, banded fertilizer may be helpful if drainage of the field is poor or if there’s a lot of crop residue on the surface.

If soil test results come back as “low” or “very low,” fertilizer application will be economically beneficial. With today’s crop budgets and razor-thin margins for error, the production penalties for not putting on needed fertilizer can be huge.

If the soil test category comes back as “optimum” for your field, whether or not to apply fertilizer can become a lengthy discussion.

Do you need to apply lime?

Soil pH is another pretty involved topic that’s better suited for its own column. But in relation to this column’s discussion of fertilizer management for P and K, it’s important to know that soil pH has a lot to do with how efficiently your P and K fertilizer can be used by the crop. Best bet is to work with your fertilizer supplier on lime recommendations and applications if they are needed.

To sum up: What you’ve read in this column is a snapshot of fertilizer management. Having worked as a retail agronomist for nine years, I’m a proponent of taking a team approach to helping farmers deal with issues like these.

Treat your local seed, fertilizer and chemical suppliers, and your financial adviser and local ISU Extension experts as your “advisory board.” As the CEO of your operation, you can take their input, pull out what fits best for your situation and, hopefully, proceed profitably.

McGrath is the partner program manager and Extension agronomist for ISU’s Corn and Soybean Initiative.

What if soil tests ‘optimum’ for P, K?

According to ISU research, there’s a 25% chance of getting a yield response from adding fertilizer when the soil tests “optimum” for these nutrients. ISU researchers agree maintaining soil test levels in the optimum category, by fertilizing at the crop removal rate, is most profitable.

On the flip side, with these optimum soils, in an individual year in an individual field, research shows there’s a good chance no yield would be lost from not applying fertilizer for a season.

For soils testing “optimum,” if there’s a positive yield response to fertilization, it generally doesn’t take the full rate to achieve it. So it would also be possible to apply a partial rate of P and K with a minimal chance of yield loss.

But remember: Soil test levels will slowly decline if we go this route. Keep in mind that as yields increase, so do fertilizer removal and the impact on soil test levels. These are both great reasons to test your soil regularly.

To learn more about the Corn and Soybean Initiative, contact your local ISU Extension office, or visit

This article published in the September, 2010 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.