From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 1998, No. 3


What happens to applied phosphate immediately after application? When solid phosphate fertilizer is applied, water in the soil moves to the fertilizer granule until the granule has dissolved. The concentrated phosphate solution formed does not move very far, typically less than an inch away from where the granule was originally placed. Phosphate is relatively immobile in most soils, because it reacts strongly with many soil constituents. Over 60 phosphate compounds have been identified. Calcium compounds typically form at basic pH values, and aluminum and iron phosphate compounds form under acidic conditions. The pH range where phosphate is most available is 5.5 to 7.

How much of applied phosphate is available? Because phosphate forms so many different compounds in the soil, the amount available in the year of application usually ranges from 10 to 30 percent. Much of the phosphate not available in the first year will be available to crops in subsequent years. Some estimates show that after 2, 5, and 8 years, nearly 45, 65 and 75 percent, respectively, of a single phosphate application has been used by crops. Consequently, the phosphate used by a crop in any given year is a combination of the phosphate applied in that year plus the phosphate applied in previous years.

Which is better, liquid or dry phosphate fertilizer? Phosphate undergoes the reactions mentioned above, regardless of whether it starts out as a dry or a liquid fertilizer. Dry phosphate fertilizer dissolves and then reacts with the soil. Liquid fertilizer simply skips the dissolution step. Comparisons using the same forms of phosphate in either the liquid or dry states have shown no significant differences in effectiveness between the two forms. Decisions about which form to use should be based upon preference, price, and local availability not on agronomic effectiveness.

Are there differences among phosphate fertilizers? Some of the most common phosphate fertilizers are triple superphosphate (TSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), and ammonium polyphosphate (APP). The first three sources in this list are in dry forms. The concentrated solutions that form when they dissolve have pH values of approximately 1.5, 3.5, and 8.0 for TSP, MAP, and DAP, respectively. Ammonium polyphosphate, normally a liquid, has a pH near neutral. Little practical difference exists in agronomic effectiveness among the fertilizer sources in most cases. However there are a few considerations:

The more acidic fertilizers, like TSP and MAP, may have slightly more effectiveness under conditions where soil pH levels are basic, soil test phosphorus levels are low, and fertilizer rates are high. Advantages are not usually large enough to produce yield differences.

DAP applied in direct contact with the seed in high pH soils can result in seedling damage. The ammonium in DAP can convert to free ammonia in such soils, causing the damage.

Ammonium polyphosphates must first break down before they can be absorbed by plants. This breakdown, termed hydrolysis, occurs fairly rapidly and does not affect the agronomic effectiveness of APP. Also, a portion of the phosphate in APP fertilizers is initially in the single or "ortho" form and is immediately available for root absorption.

Remember, it's not the form or source that makes the difference. It's the phosphorus each contains that boosts crop yields and grower profits.


For more information, contact Dr. T. Scott Murrell, Northcentral Director, PPI, 14030 Norway Street, NW, Andover, MN 55337. Phone: (612) 755-3444.
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