Several phone calls have recently been made to the UW Extension office by area crop producers with questions regarding claims made by visiting fertilizer sales persons. Here's a sampling of some of these claims and the reasons why you shouldn't fall for them.

"The nutrients in our fertilizer are more available than the fertilizer sold by Company X even though they have the same grade analysis."

State fertilizer laws ensure that labels on all fertilizers list the guaranteed available primary nutrients in order of N, P2O5, and K2O as a percent by weight of fertilizer material. The fertilizer grade is determined on a weight basis regardless of whether the fertilizer product is liquid or dry. The content of each nutrient is determined in laboratory tests using the Official Methods of Analysis of the Association of Official Analytical Chemists. The results are interpreted as the fraction of a particular nutrient from a fertilizer that would be available to plants. Therefore, two different materials that have the same analysis will have equal amounts of available nutrients in the fertilizer itself.

"Our polyphosphate fertilizer will out perform the competitor's orthophosphate fertilizer."

Agronomically, poly versus orthophosphate is a "non-issue". Polyphosphates are composed of a series of orthophosphate molecules connected by the process of dehydration. The polyphosphates convert rapidly to orthophosphate once in the soil. Numerous studies have been conducted to evaluate the relative merits of each type. Despite some commercial claims to the contrary, there is no evidence that one material is superior to another for crop production. About two-thirds of the phosphate in liquids such as 10-34-0 and 7-21-7 is in the polyphosphate form. Most dry fertilizers such as 18-46-0, 11-52-0, and the 9-18-9 liquids have phosphate in the ortho form.

"The white phosphoric acid in our fertilizer makes it superior to the green acid you are currently using."

In general, there are two primary grades of orthophosphoric acid: furnace acid (white) and wet process acid (green or black). The white acid is very pure but requires considerable energy and is more expensive to produce than the green. Higher analysis liquid fertilizers containing the primary nutrients are usually formulated with the white acid because some of the impurities in the green form such as aluminum, iron, and magnesium cause problems in their formulation, storage, and application. With dry fertilizers, these problems are circumvented and the impurities present no major physical problems.

"Liquid fertilizer provides superior crop performance compared to dry fertilizer."

When compared to dry formulations, liquid fertilizers offer the advantage of easier handling, mixing, and application. From a nutrient availability and crop performance standpoint, numerous research trials have concluded no difference between the two forms when the same amounts of plant nutrients are applied. Handling preference and cost per nutrient unit should dictate buying decisions.

Final thoughts

Nutrient availability to crops is ultimately controlled by soil reactions. Don't be "ripped-off" by sales claims with no agronomic research base. All verbal claims should be backed-up in writing. It is interesting to note that many verbal claims are not cited in published company literature to avoid prosecution by the Wisconsin Department of Agriculture, Trade, and Consumer Protection.