I had a customer, let’s call him “Bob,” back in the 1990s who always said he was “going broke” when it was time to buy inputs for the new crop and needed to cut back on expenses. He was an excellent farmer, highly knowledgeable in agronomy, well-read on the latest technology, and very ROI-conscious.  He did all the right things at the right time, including knowing when to buy inputs.

I pulled his soil samples and made fertility recommendations for all the crops grown on Bob’s farm for many years until he retired. For the first few years, I didn’t realize that he, his fertilizer salesperson, and I were playing a game every year, or he was playing a game with his fertilizer salesperson and me.

Here is how the game was played: Every year, when Bob gave me his crop plan, he would say “things are tight” and that we shouldn’t apply any fertilizer unless absolutely necessary. We pulled soil samples every other year on most of his fields and sampled his “high yield fields” almost every year. His fields had excellent fertility, with several hundred, if not a thousand or more acres, that DID NOT NEED any additional P or K to meet the yield goals he had set.

So, I would “not put on any fertilizer unless we absolutely had to” and give him his recommendations, which included many zero rates. Bob would then visit his fertilizer salesperson, and he would tell him that “You have to put back what you take off” or “Now is not the time to cut back as fertilizer prices are low” or that “Not putting on something would hurt your yields next year” or “You HAVE to put on maintenance.”

Usually, right after this, Bob would call and tell me “You have to put on fertilizer if you want to grow a crop” and that I needed to bump up all those zero rates to something.  So, I would put 50 lbs down on the worksheet and send it back. Later, I would find out he applied 100 lbs because his salesperson told him, “That’s as low as the spreader would go.” Then one day, knowing how Bob liked to read up on things, I brought him some university data and research for him to look over in his spare time. That research would show that after a given level of fertility, you don’t “add yield” and you don’t “hurt yield” by not applying fertilizer.

The “game” changed drastically after that.

Every land-grant university in the Corn Belt has guidelines for what is called build-up, maintenance, and drawdown fertility rates. These terms may be called something slightly different from one university to another, but they are there in their fertility recommendations. Here’s how they are defined:

• Build-up occurs when soil test levels are low, requiring additional fertilizer to get soil test levels up to the optimal level for maximum yield potential.
• Maintenance refers to adding only the fertilizer the crop removes, keeping the soil test levels in the (ideally) optimum level for max yields. In recent years, maintenance fertilizer is just putting back what you take off. Many use “maintenance” in the absence of a soil test.
• Drawdown occurs when soil test levels are so high that you stop applying fertilizer for a time (a year or many years), without hurting yields. Some refer to this as the “savings account” or “bank” of fertility.

There are only three classes of fertility for crop production: Deficient, Optimum and Excessive. To make us feel better, to make a point, or just to have

Source: Ohio State University Extension. ‘Tri-State Fertilizer Recommendations Now Available.’ Ohio Agronomy, November 2020. Retrieved from https://agcrops.osu.edu/node/3478.

more colors on a soil test, the maps industry use has broken this down even further into 4-5-6-8-10 classifications:  Very Low, Low, Med Low, Med, Med High, High Optimum, Ideal etc. You get the picture. The universities break it down into three classes:  Build Up (low), Maintenance (Optimum), and Drawdown (excessive). Let’s focus only on drawdowns because this is the only place where you can save money. When soil test values are above optimum levels, additional fertilizer has a very slim chance of increasing yields. Inversely, not applying any fertilizer will not hurt yields either.

In our example, Bob had P test levels at or over 100 lbs per acre on many fields.  He also had K test levels at or over 400 lbs per acre on several samples.

A quick glance at University of Illinois Agronomy Handbook in Chapter eight shows there is no agronomic advantage in applying phosphate fertilizer when P1 values are higher than 60, 65, and 70 for soils in the high, medium, and low P-supplying regions, respectively.  In other words, you don’t need fertilizer, and yields will not be hurt by not applying. As a matter of fact, from a 4R perspective, it’s not only good for your pocketbook, but also for the environment.

Another glance at Chapter eight shows that no K additions are suggested if test levels are above 360 and 400 for the low-and high-CEC regions, unless crops that remove large amounts of K, such as alfalfa or corn silage, are being grown. Again, we don’t need fertilizer, and yields will not be hurt.

At these high soil test levels, we do not need maintenance fertilizer. Despite what Bob’s fertilizer salesperson told him – “You have to put back what you take off,” “Now is not the time to cut back as fertilizer prices are low,” or “Not applying something would hurt your yields next year” you DO NOT HAVE TO APPLY MAINTENANCE AT HIGH SOIL TEST LEVELS. 

Purdue University research shows that when soil test P and K are optimal or above the highest return to fertilizer (Drawdown), P and K should be at the zero rate of application in that season. Across multiple seasons, the zero rate remains best if soil test levels remain in the optimal range. Additionally, some research shows that fertilizer rate decisions have more potential impact on profits when soil test levels of a nutrient are deficient vs optimum. This is because yield can be negatively affected by nutrient deficiency to an extent that offsets the savings of reduced fertilizer rates. However, when levels are in the optimum range, rates can be cut with little effect on yields due to the expectation of a response. In other words, the closer you are to sufficient or excessive, the less likely you are to see a response to fertilization or see a loss to lower fertilization rates. This approach to fertilization is often called the “Sufficiency Approach,” where the rate applied is targeted to produce the highest return to fertilization in that season without regard to the impact on future seasons.

Now for the bad news:  If you don’t have a recent soil test, none of what I discussed above is relevant. You have no benchmark to know what you can cut, what you can reduce, or what you really need to do. It’s a shot in the dark. Soil testing and fertility guidelines are well researched and well established in the Midwest.  Following these guidelines will lead to the greatest chances of improved ROI and negate yield loss.

You don’t always need maintenance.

References:

University of Illinois Extension. ‘Chapter 8: Fertilizers.’ Illinois Agronomy Handbook. Retrieved from http://extension.cropsciences.illinois.edu/handbook/pdfs/chapter08.pdf.

Purdue University Extension. ‘Fertilizer Price Adjustments to Fertilizer Recommendations.’ Pest & Crop Newsletter, October 2021. Retrieved from https://extension.entm.purdue.edu/newsletters/pestandcrop/wp-content/uploads/sites/2/2021/10/Fertilizer-Price-Adjustments-to-Fertilizer-Recommendations.pdf.

Ohio State University Extension. ‘Tri-State Fertilizer Recommendations Now Available.’ Ohio Agronomy, November 2020. Retrieved from https://agcrops.osu.edu/node/3478.