As world urea and phosphate prices inch back to 2007 levels, fertilizer prices for the upcoming 2010 vegetable season should be a bargain compared to the unprecedented prices seen during recent years. Current mid-July UAN32 prices hover around the $260/ton mark compared to $830/ton in the summer of 2008. And, with the current price of 11-52-0 ranging around $450/ton, down from over $1000/ton two years ago, preplant phosphate applications should approach near normal rates.

And, despite the comparable prices for 11-52-0 (monoammonium phosphate, MAP) and 18-46-0 (diammonium phosphate, DAP), 11-52-0 is the preferred choice as a preplant Phosphorus fertilizer for desert vegetables. Below is an 11-52-0 preplant fertilizer refresher:

1. Soil Solution Differences
   When 11-52-0 is applied, the soil solution pH surrounding the granule ranges from an acid pH of 3.5 - 4.2. However, the initial pH around the 18-46-0 granule will be alkaline with a pH of 7.8 - 8.2. The high pH soil solution in combination with high pH soils, and extra ammonia added to 18-46-0, can result in zones of free ammonia which could cause seed germination problems, seedling injury and potentially interfere with root development.
2. Phosphorus Uptake
   P is taken up from soil solutions by roots in two forms: H₂PO₄ and HPO₄. Research has shown a trend that plants take up H₂PO₄ more rapidly than HPO₄. This factor is important in the MAP-DAP comparison, because the acid soil solution in 11-52-0 favors the formation of H₂PO₄, thus more potential P uptake.
3. Micronutrients Effects
   Plant availability of micronutrients manganese, iron, and zinc usually increase in acid soil solution environments. The acidic zone (pH 4.0) created by 11-52-0 enhances micronutrients availability while the alkaline zone created by 18-46-0 (pH 8.0) decreases the availability of these micronutrients.

To contact Kurt Nolte go to: knolte@cals.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.