In the 1970’s and 1980’s, there was an ongoing debate about the advantages and disadvantages of the various sources of fertilizer nitrogen.

Because 82-0-0 was the least expensive source at the time, much of the marketing was focused on complaints about 82-0-0. There were statements that 82-0-0 made soils hard, killed all of the soil life and was easily lost from the root zone. This debate stimulated a substantial amount of research throughout the Corn Belt designed to evaluate the effectiveness of various nitrogen fertilizers.

As results of the research projects were evaluated and summarized over time, a general conclusion emerged. If applied properly so that loss was minimized, all sources of fertilizer N had an equal effect on yield if the same rate of N was applied in all cases. In other words, “a pound of N was a pound of N regardless of source”.

With today’s soaring commodity and fertilizer prices, the debate about sources of fertilizer N is starting all over again. However, there is a new wrinkle. Those who market products designed for foliar application are making claims that are not supported by research. I’ve heard that some are saying that one pound of N applied as a foliar product is equivalent to 5 to 10 lb. of N applied to the soil. This claim is simply not true.

Most products sold for foliar application contain urea or some derivative of urea. Urea will enter through the leaf tissue in small amounts. Urea is then easily metabolized to proteins and leaves that are light green turn to dark green in one or two days. The same thing will happen with the application of very dilute 28-0-0. One half of the N in 28-0-0 is in the form of urea.

The greener leaves, although they look good do not always translate into added yield. They are, many times, a cosmetic effect that makes the grower feel good with a warm, fuzzy feeling. It’s important to remember that the overwhelming amount of N used by corn must be taken up through the roots. Very small percentages of the total used are taken up through the leaves.

It’s a common practice to apply 28-0-0 with irrigation water. This is a good practice. But, in this situation, most of the N applied is taken up through the roots.

Although the products and marketing claims have changed, the basic principle has not. “A pound of N is still a pound of N, regardless of source.” A higher price doesn’t mean that its a better product.

SOURCE: George Rehm, U/MN

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