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Fertilizing crops in tough times


Some might say that’s shortsighted, but not Dave Mengel. The longtime Kansas State University agronomist, soil fertility expert, and now professor emeritus says in today’s environment, where crop returns barely cover expenses, frugal and fertilizer are two words that can go together.

“If you’re pushing the pencil hard this year – and, of course, you are – you need to know where you are on soil nutrients,” he says. “If you’ve been on a good fertility program for a few years, this may be the year to draw on the bank.”

There are two main approaches to fertilizer recommendations, he continues, and understanding them may help in your decisions. One is the nutrient-sufficiency system; the other is the build-and-maintain system. Some state Extension services, such as Kansas, use both. Others, like Missouri, recommend only the build-and-maintain system.

Mengel thinks there’s a time and place for both.


“This is the traditional system, where a lot of farm fertility programs start out,” Mengel says. “It estimates the amount of phosphorus (P) and potassium (K), for instance, needed in that particular year and the amount for that crop to achieve maximum economic returns based on current soil test levels. It gives no consideration to the future or to building up nutrient reserves to any critical levels.”

At high soil-test levels, fertilizer rates are very low, or zero, since the soil can supply the nutrients needed, he says. At low soil-test levels, fertilizer rates are high, since the soil supply is inadequate to meet crop needs. Over the long run, rates equilibrate at just about crop removal rates.

“Over the long haul, nutrient sufficiency gets about 95% of maximum yield, which is normally the economic optimum yield,” says Mengel.


The build-and-maintain system fertilizes based on the actual soil tests, focusing on getting soil tests above critical or threshold levels so the soil can supply all the nutrients crops need.

“With this approach, you’re investing in the long-term benefits to the land, plus you’re buying yourself the flexibility to not fertilize every year,” Mengel says.

Because this system tends to add nutrients at high soil tests to maintain levels, it also tends to produce yields that are a little better – closer to 100% of maximum.

Once the build-and-maintain system moves soil tests above the critical levels, the annual costs of the two systems tend to be similar. With the build-and-maintain system, this adds the flexibility that fertilization may not be called for every year.

Mengel uses his own Kansas farm as an example of a build-and-maintain strategy. It took him 10 years and an overall investment of about $100 an acre to build P levels above the critical levels. Now it costs about the same as the nutrient-sufficiency system, and he can focus P applications to crops like wheat and corn that are most responsive.

Tenants, he says, tend to like the nutrient-sufficiency approach better than the build-and-maintain system, especially if they are on a short-term lease, because the nutrient-sufficiency system invests for right now rather than for the future.

“If you only have a short-term lease, you may not get to reap long-term benefits,” Mengel says.

“I tell farmers that if they’ve been on a good build-and-maintain program of fertility, now they may want to go back to nutrient sufficiency and fertilize for what this crop needs,” he says. “You can do that for a year or two and ride out these tough times.

“Not everyone agrees with me, but I think it’s a viable choice,” he says.


So can better nitrogen (N) fertilizer management drastically slice nitrate runoff in waterways?

Some. When it comes to preventing nitrate loss in waterways, however, better ways exist, says Michael Castellano, an Iowa State University agronomist. That’s because N mineralization from soil organic matter is a larger nitrate contributor to waterways than is N applied to crops.

“We lose nitrogen when the soil is warm and wet and when crops are not using large amounts of nutrients” says Castellano. “Microbes transform organic nitrogen to nitrate, which is leachable.”

A better method for scavenging nitrates are cover crops, he adds.

Granted, forming a nitrogen management plant – such as supplementing fall and preplant N applications with a sidedressing trip – helps prevent nitrate runoff. It also makes agronomic sense. Still, tweaking N application strategies is a minor factor when it comes to solving water quality problems in states like Iowa.


Dave Mengel offers four tips for improving the accuracy of your soil fertility samples.

  1. When you soil-sample, move your grids from year to year. “You don’t get perfect distribution of fertilizer,” he says. “Fertilizer particles can segregate out in the truck, and then when the drivers get into your fields, they tend to drive the same pattern. If you change your sampling patterns, you’ll get more accurate results over time.”
  2. The best sampling depth varies by what you’re testing for. If your field is in continuous no-till and you are testing for pH only, 3 to 4 inches deep is sufficient. For phosphorus and potassium, 6 to 8 inches is sufficient. For nitrogen and sulfur, take core samples down to 24 inches, says Mengel.
  3. When you are taking soil cores to be submitted as a composite sample, the more cores, the better. Fertility variability within a field grid or management zone can be high. In one study, Mengel says, they took core samples just 1 inch apart, and the phosphorus variability was as much as 100%, possibly caused by starter fertilizer bands.
  4. Keep records! It’s the only way you can know the response to fertilizer on your farm, Mengel concludes.

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