Crops

The 4 Rs of crop nutrition—the Right rate

Another installment in the 4 Rs of crop nutrition

by Dr. Paul Tracy

How much fertilizer should I apply this year? Talk about an open-ended question! Although the appropriate amount of plant food is spatially and temporally dependent, there are a few general rules to consider. I remain a big supporter of using soil-test values combined with projected crop yield as the core basis for estimating nutrient inputs.

The nutrients most commonly classed in the “stable” category are phosphorus, potassium and some of the secondary and micronutrients such as calcium, magnesium, zinc, iron, copper and manganese.

With the micro and secondary nutrients, we generally apply an amount equivalent to the estimated crop needs. We may or may not try to build soil levels.

For phosphorus and potassium, we use a crop maintenance plus soil building philosophy. The maintenance component is based upon actual crop removal. This differs by crop and by intended crop use. For example, a 200-bushel corn crop removes approximately 50 pounds of potassium. That same corn crop cut for silage would produce 25 tons of biomass that would remove approximately 225 pounds of potassium.

When soil test levels are optimum, we simply apply the estimated crop maintenance value. Any differences that occur between estimated and actual yield removal will be accounted for during the next soil sampling cycle.

The build component is required when soils contain less than the optimum level of a given nutrient. In this situation, we apply the maintenance level for a targeted yield, combined with a soil build factor. Since it is often economically infeasible to build a soil with a single fertilization, we often use a build time scheduled to coincide with the next planned soil sampling.

When soils test high in a given non-mobile nutrient, we generally recommend a fraction of maintenance until the next soil sampling. When soils contain excessive amounts of non-mobile nutrients, we generally recommend no application until future soil testing shows a need.

Maintenance and build factors are interrelated and time dependent. For those who have reduced fertilizer inputs over the past few seasons, crop removal may have depleted your soils into a lower testing and higher requirement category.

The right rate determination is much more difficult for mobile soil nutrients like nitrogen. Lately, many organizations have given up and addressed mobile nutrient needs by using state or area-wide averages combined with the cost of nitrogen fertilizer and grain commodity prices. I do not subscribe to this philosophy. Commodity and fertilizer prices are extremely volatile and do not affect crop growth. Fertilizer prices rarely reach rate-limiting factor status. For example, if fertilizer nitrogen cost $0.50 a pound and you overestimate actual crop nitrogen needs by 20 pounds per acre, that overestimation would cost you $10 per acre. Using an average number of 1.2 pounds of nitrogen needed per bushel of grain produced until crop nitrogen needs are fulfilled, if you under-fertilize by the same 20 pounds, you would lose 16.67 bushels or $50 per acre at a $3 per bushel corn grain value. Although these are gross estimations, at rates within 20 percent of optimum, under fertilization is almost always more economically risky than putting on too much.

I continue to use soil test information as a base for nitrogen recommendations because we know there is a correlation between soil organic matter and CEC (both determined by testing) and the soil’s ability to release some organically bound nitrogen and hold/lose some inorganically contained nitrogen. Although highly variable in many situations, this information can be used to help determine total crop nitrogen needs. Until a better system for determining pre-plant nitrogen application rates is developed, I will continue to use soil organic matter, soil CEC and estimated crop yield for standard nitrogen recommendations.

What can we do to fine tune nitrogen rates and improve recommendations? Good record keeping, good knowledge of within-field and across-year yield variations will help. Split applications may delay some of the nutrient rate choices until you have a growing crop to use as a more accurate gauge of yield potential.

Determine the proper amount of plant foods for crops with as many tools as possible. Consider the basic principles of crop nutrient needs and fine tune your decisions based upon all available information. The goal of any fertility program is to prevent crop nutrition from becoming a yield limiting factor, and by doing so in the most efficient way possible.

Dr. Paul Tracy is director of agronomy for MFA Incorporated.

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