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Fine-tuning your fertilizer budget

We’re in the midst of harvest after another challenging year. Am I off base thinking it seems like our weather has been more erratic the last few seasons? Let’s hope it cooperates so we can wrap up harvest and get some fall fieldwork done before a brutal winter (according to the Old Farmer’s Almanac).

Fine-tuning your fertilizer budget

We’re in the midst of harvest after another challenging year. Am I off base thinking it seems like our weather has been more erratic the last few seasons? Let’s hope it cooperates so we can wrap up harvest and get some fall fieldwork done before a brutal winter (according to the Old Farmer’s Almanac).

While a fair amount of soil sampling is done and some fall fertilizer spreading has started, most dry fertilizer applications are waiting for harvest to end. As growers look at yields, low prices and tight budgets, we are getting a lot of questions about trimming back fertilizer and lime costs for 2016.

Here are some common questions we’re getting. 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.” The publications are at

How often should you sample soil and have it tested for P, K and pH? Sample fields 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 results. Once you have a comprehensive soil fertility map for a field, resample 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 comfortable stretching out sampling intervals to three to four years.

What time of year is best to sample? 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 soil is excessively wet. Sampling at other times such as in winter or in a growing crop is discouraged for determining pH, P and K because interpreting results are difficult. Field research calibrations for these soil tests are based on samples collected in fall or spring.

How deep should you pull samples? For most of my career, nearly all soil samples were pulled by agronomists like me at retail locations or by independent crop consultants. Today, more growers like to take some or all of their samples. They have the GPS equipment, ATVs and computer skills to do the job and write their own prescriptions. Six inches is the sample depth we use for all tillage systems in sampling for P, K, Zn, organic matter, pH and buffer pH. For no-till acres, we like growers to monitor surface pH by taking 2- to 3-inch samples in areas across fields.

Something to watch for in most soils, and particularly in no-till systems where soil tilth tends to be exceptional, is “soft” soils that compact as they go in the probe. Take a look at the 6 inches of soil in your probe and then drop a dowel in the sample hole to compare depth. Often we end up going too deep; adjust accordingly.

We always test for P, K and pH. What about testing soil for micronutrients in Iowa? ISU provides micronutrient recommendations only for zinc based on soil testing. Soil test procedures for the other micronutrients haven’t 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, we could see this changing in the future.

Occasionally, we hear about some sort of cutting-edge testing technology for micronutrients. For now, given the tight crop budgets, I’d have a hard time being convinced I should try some untested new sampling technique for micronutrients no matter how compelling the sales pitch.

What should you look for on the soil test report you get back from the lab? This is where ISU publication 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. Your agronomist is a great resource to sit down with you and go through soil test results.

Is starter fertilizer part of your program? If it is, be sure to take the P and K credit from that into account.

Do you have the farm year to year, or on a longer basis? I could spend paragraphs on this, but the bottom line is: Take a good look at the soil samples, crop yields and how long you’ll be farming that land. If you’re confident you’re still in the optimum ranges for 2016 and won’t be farming that ground for long, skipping P or K could be a chance to trim costs safely.

If you have this land for the long term, the strategy may shift slightly. Trimming fertilizer application rates rather than skipping is something to consider. If you skip P or K, limit risks next fall by soil sampling to ensure you stay in the optimum categories for P and K.

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 a proponent of building trust and capacity between growers and advisers; creating a team mentality benefits both operations.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Contact him at

Answer to most popular question this fall: ‘It depends’

A farmer asks: “I’m looking for a place to reduce costs this year. Can I cut back on fertilizer application if I have optimum-to-high testing soils for P and K?” That’s the most popular soil fertility question this fall. And 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 category to maintain over time for long-term profitable crop production.

The farmer responds, “Are you saying there is only a 25% chance of a yield response to P and K applications in optimum testing soils? With low odds like that, can we skip or cut back for a year or two on P and K fertilizer application rates on the optimum-testing acres?”

It’s a conversation I’ve had hundreds of times from the perspective of a fertilizer dealer and as an ISU agronomist. Some common discussion areas:

Are your soil test levels on the upper or lower end of optimum? If you have good confidence in the sample results and they are on the upper end of optimum for P or K, my experience and the research concur. That is, you can probably trim back on those acres for a year. But cutting back for longer than that makes me nervous.

What about optimum soil test results that are getting some age on them (4 years old or more)? If you can, get some new soil samples taken and update your soil test results before you set up your fertility plan. This is easier said than done some years, so the next best thing is to look back in time at yields compared to fertilizer rates applied. You and your dealer can walk through this exercise together; the bottom line is if you have roughly “kept up” with crop removal by applying maintenance rates of fertilizer, you are probably in great shape on most acres.

If you skipped an application cycle or two and fell far short of crop removal, you could be running a pretty significant risk. Recall that as soil test levels fall from optimum to low, the odds of getting a yield response from fertilizer application shift from 25% up to 65%. If there is any doubt about having drawn soil test levels down with more nutrient removal than what you’ve applied, I’d recommend fertilizing for the 2016 crop and soil-test ASAP next fall.

What about soils that tested optimum more recently (within the last two to three years) and you skipped P or K last year? Or your soil tests show the field is in the lower end of optimum? I’m not sure there is a rock-solid answer to these scenarios either, but my thoughts here are similar to the ones just above. That is, apply the maintenance rate for the 2016 crop and then sample and test your soil after harvest next fall.

Sit down and run the math comparing your application rates to your crop removal; if overall you’re close, you can probably slip by this year with applying less P or K. If you were well short of replacement, it might be too risky to not put on the P or K needed for 2016.

Are you looking at grid/zone samples done every few acres, or larger areas that may not give as accurate a picture? As sample accuracy goes up, my confidence in being able to trim back on P and K fertilizer application for a year on optimum-testing soils goes up.

After talking through issues like these with your agronomist, hopefully you’ll be able to dial in on fertility decisions that will keep your risk to a minimum. Have a good fall and stay safe as you wrap up harvest and fall work. — Clarke McGrath

This article published in the October, 2015 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2015.

Crop Management

Nutrient Management

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