Availability of Liquid Fertilizer
 is no Greater than Dry

Every now and then someone calls the extension office asking if there is a big difference in the availability of different fertilizer sources. Sometimes the question is prompted from curiosity and other times it is prompted from the claim of a fertilizer dealer looking to make a sale. Almost always the comparison is made using liquid versus dry fertilizer and specifically in reference to phosphorus.

There may be good reasons to use liquid fertilizer but improved nutrient availability ISN'T one of them. Most commonly used dry fertilizers are nearly 100 percent soluble in water. Regardless of original form, fertilizers react with soil minerals and soil micro- organisms and are rapidly transformed from their original makeup. Because of these reactions, a 100 percent available fertilizer declines in plant uptake efficiency rapidly after application. This is true regardless of fertilizer form. The recovery efficiency of nutrient application is only 10-30 percent of phosphorus (P), 35-60 percent of potassium (K), and 50-75 percent of nitrogen (N). In the case of P and K, fertilizer nutrients not used the first year contribute to plant uptake in following years as a part of the soil reserve.

Orthophosphates versus Polyphosphates

Without going into details, there are two primary methods to produce phosphate fertilizers. One method results in a phosphate fertilizer with a few more impurities (compounds normally found in the soil and not harmful to crops) and in a form known as orthophosphate. This is the method used to manufacture most dry fertilizers. The other method is energy intense (also more expensive) but forms a fertilizer that is purer. This form of phosphate is referred to as polyphosphate. Polyphosphates are simply long chains of orthophosphates. Most liquid fertilizers contain polyphosphates. When introduced into the soil, polyphosphates react with water relatively quickly, producing orthophosphates again. The majority of plant uptake of P is in the orthophosphate form.

Liquid vs. Dry Fertilizers

As mentioned earlier, there are good reasons to use liquid fertilizers. They're easy to handle and meter more precisely. However, they are not more available than dry forms. Additionally, some liquid fertilizers are grossly overpriced relative to other liquid or dry alternatives. As you determine what fertilizer program is best for you, always first consider the cost per pound of nutrients being applied.