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Timely questions on soil fertility

It was a trying fall season — hit-and-miss for harvesting, let alone much fieldwork. Some areas had a good run on fall NH3, and we got some dry fertilizer and lime on, but we have a long way to go. While our NH3 season is likely done with the frost and snow, we can often spread dry fertilizer and lime throug

Timely questions on soil fertility

It was a trying fall season — hit-and-miss for harvesting, let alone much fieldwork. Some areas had a good run on fall NH3, and we got some dry fertilizer and lime on, but we have a long way to go. While our NH3 season is likely done with the frost and snow, we can often spread dry fertilizer and lime through the winter if the snow doesn’t get too deep.

Every fall and winter, we get calls about soil testing and fertility. This year with the grain markets in the tank, farmers are working on dialing in fertility plans and asking great questions. Here are some of the most common questions that have come up recently. My answers contain highlights from ISU’s PM-1688, “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” and PM-0287, “Take a Good Soil Sample to Help Make Good Decisions.” You can get them at your local ISU Extension office or the Extension Online Store at

How often should I sample soil and have it tested for P, K and pH?

Fields should be sampled every two to four years for most crops, or once in a crop rotation. Fields should be in the same crop each time when sampled to reduce variability of test results. Once you have a comprehensive soil fertility map for each field, you can re-sample the entire field following the same pattern, or sample only representative portions of each field and problem areas every two to four years as a check on the fertilizer program.

My personal bias is to keep sampling intervals closer to two years for soils that test toward the lower end of the “optimum” ranges or below. Once levels are in the middle of the optimum ranges or higher, then I feel pretty comfortable stretching out sampling intervals to three to four years, as long as we are keeping an eye on crop removal rates and applying fertilizer accordingly.

What time of year is best to sample soil?

Ideally, the best time is either after harvest and before fall fertilization, or before spring fertilization. We prefer not to sample shortly after a lime, fertilizer or manure application, or when the soil is excessively wet. Sampling at other times such as in the winter or in a growing crop is discouraged for determining pH, phosphorus and potassium because interpretations of results are difficult. Field research calibrations for these soil tests are based on samples collected in fall or spring.

How deep should I pull samples?

I almost left this question out, since for most of my career nearly all soil samples were pulled by agronomists like me at retail locations or independent crop consultants. As I talk with growers across the state, however, there is an emerging sector that likes to take some or all of their own samples. They have the GPS equipment, the ATVs and the computer skills to do the job and write their own prescriptions. And they ask great questions relating to various tillage systems and sampling depth.

Six inches is the sample depth we use for all tillage systems when sampling for P, K, Zn, organic matter, pH and buffer pH. For our no-till acres, we also like to have growers monitor the surface pH by taking some 2- to 3-inch samples in representative areas across fields. Something to watch for in most soils, and particularly in no-till where soil tilth tends to be exceptional, is “soft” soils that compact as they go in the probe. Look at the 6 inches of soil in your probe and then drop a dowel or something else in the sample hole to compare depth. Often we end up going too deep.

We always test for P, K and pH. What about testing soil for micronutrients?

ISU provides micronutrient recommendations only for zinc based on soil testing. Soil test procedures for the other micronutrients have not been calibrated because of either lack of, or inconsistent occurrence of deficiencies. With research and soil analysis technology continually advancing, and yields (and corresponding micronutrient use) increasing, I could see this changing in the future.

What should I look for on the soil test report I get back from the lab?

This is where PM-1688 really shines; it walks readers through what the results mean and what steps to take in understandable terms, so I won’t dive into this question much deeper.

Also, your agronomist is a great resource to sit down with you and go through soil test results. While you build on your understanding of soil testing and fertility, it’s also a great opportunity for your agronomist to learn more about your management and decision-making style. I’m always a proponent of building trust and capacity between growers and advisers; creating a team mentality benefits both operations.

I’m looking to reduce costs. Can I cut back on fertilizer application if I have optimum to high testing soils for P and K?

Here is where I use my favorite agronomy answer: “It depends.” ISU publication PM-1688’s soil test interpretation categories represent a decreasing probability of an economic yield response to applied nutrients. The percentage of P and K applications expected on average to produce a yield response within each soil test category is 80% for very low, 65% for low, 25% for optimum, 5% for high and less than 1% for very high. Based on prevailing input costs and expected yield increases, the optimum category is the one to maintain for long-term profitable crop production.

So can we skip P and K on the optimum testing acres since ISU says there is only a 25% chance of a yield response? It’s a tough call, and a discussion I have had hundreds of times from the perspective of a fertilizer dealer and an ISU agronomist. Some common discussion areas:

Are your soil test levels on the upper or lower end of optimum?

Are we looking at grid/zone samples done every few acres, or larger sample areas that may not give us quite as accurate a picture?

What was the trend for the last few sets of samples; are we stable or heading up or down?

How many seasons are we considering little to no P and K? When will we sample again to check test levels?

Do you use starter fertilizer?

Do you have the farm year to year, or on a longer-term basis?

What do we think the next year or two will bring for grain and input prices?

After talking through issues like these with your agronomist, you’ll probably be able to dial in on fertility decisions that will keep your risk to a minimum.

McGrath is the ISU Extension field agronomist at Harlan in western Iowa and author of this “Corn/Soybean Insight” column. You can contact him at

This article published in the December, 2014 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

Nutrient Management

Soil Analysis

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