MAP 11-52-0 vs. DAP 18-46-0
Agronomic Considerations (MAP vs. DAP)
Both MAP and DAP are excellent sources of phosphorus and nitrogen and have a proven, historical record of yield increases. Differences in fertilizer placement, cropping systems and soil reactions may favor one source over the other in specific locations. The following information examines the broad issues of these differences.
Chemical/Manufacturing
MAP is manufactured by combining one mole (molecular weight) of ammonia with one mole of phosphoric acid. DAP is produced by adding 2 moles of ammonia with one mole of phosphoric acid. The additional ammonia in DAP adds beneficial nitrogen, but can create unfavorable chemical reactions in soil solution.
Soil Solution Differences
When MAP 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 DAP granule will be alkaline with a pH of 7.8-8.2. Why is this pH difference important?
Ammonia formation from DAP
The high pH soil solution in combination with high pH soils and extra ammonia added to DAP can result in zones of free ammonia. These areas in the soil could cause seed germination problems, seedling injury and potentially interfere with root development.
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 MAP favors the formation of H2PO4, thus more potential P uptake.
Micronutrients Effects
Plant availability of micronutrients manganese, iron, and zinc usually increase in acid soil solution environments. The acid zone (pH 4.0) created by MAP enhances micronutrients availability while the alkaline zone created by DAP (pH 8.0) decreases the availability of these micronutrients. For example, research on sugar beets and soybeans has shown Mn tissue levels were higher 5-6 weeks after planting when Mn was applied with MAP than when applied with DAP.
Cropping Factors
Two cropping factors should be considered in a MAP-DAP decision.
Legumes
Research indicates that moderate rates of fertilizer nitrogen inhibits the nitrogen-fixing process of legume bacteria. Also, additional nitrogen may encourage more grass growth in legume stands. Based on these factors if legumes are directly fertilized with P fertilizers, it appears prudent to avoid P fertilizers with higher amounts of nitrogen.
Vegetables
Relatively high rates of P are recommended for vegetables. Recommended rates are high because of the short growth cycle and the limited root system of many vegetable crops. Banding the fertilizers for vegetables continue as a BMP. Because of these higher, banded rates of P, it is advisable to use P fertilizers with low salt indexes and avoid sources that create free ammonia (DAP) near the germinating seed. These conditions favor MAP.
Soil Factors
Soil Test P Level
If soil test levels for phosphorus are low, banding the P fertilizers results in greaer crop response and less soil fixation. This soil factor/fertilizer placement favors MAP.
Soil Texture
If the potential for seedling damage exists from salt injury or ammonia toxicity, the probability of this damage is greater in coarse-texured soils. Hence, in sandy soils MAP will potentially have less seedling damage.
Soil pH Water Solubility
Numerous field research trials have shown the level of water soluble P should exceed 60% in P fertilizers for optimum crop growth. Mosaic MAP contains 90.0% water soluble P. Mosaic DAP has 90.8% water soluble P. Both forms exceed the important 60% water soluble threshold.
Solubility of Soil-Fertilizer reaction products
Both MAP and DAP degrade into various reaction products. For example, MAP products are taramakite, dicalcium phosphate and struvite. DAP produces struvite and colloidial apatite. Both the DAP reaction products are relatively insoluble in soils except acid soils.
Taranakite is a hydrated alkali iron-aluminium phosphate mineral with chemical formula (K,Na)3(Al,Fe3+)5(PO4)2(HPO4)6·18H2O.[2][3][4] It forms from the reaction of clay minerals or aluminous rocks with solutions enriched in phosphate
Taranakite forms small white, pale yellow, or gray crystals, which are typically found in pulverulent nodular aggregates, or crusts. Taranakite crystallizes in the hexagonal system, and is noted as having the longest crystallographic axis of any known mineral: the c-axis of the taranakite unit cell is 9.505 nanometres long
phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry.[2] At elevated temperatures in the solid state, phosphates can condense to form pyrophosphates
Dicalcium phosphate, also known as dibasic calcium phosphate or calcium monohydrogen phosphate, is a type of calcium phosphate that is dibasic. It is usually found as the dihydrate, with the chemical formula of CaHPO4•2H2O, but it can be thermally converted to the anhydrous form. It is practically insoluble in water, with a solubility of 0.02 g per 100 mL at 25 °C. It contains about 29.5 percent calcium in its anhydrous form. On contact with water, it converts to hydroxyapatite, which is insoluble solid, and phosphoric acid
Calcium phosphate is the principal form of calcium found in bovine milk. Seventy percent of bone consists of hydroxyapatite, a calcium phosphate mineral (known as bone mineral). Tooth enamel is composed of almost ninety percent hydroxyapatite
The interaction of P fertilizer and formation of free ammonia causing ammonia toxicity increases when soil pH are high and in calcareous soils.
P Solubility
The topics of water solubility and solubility of various compounds formed from soil applied MAP and DAP are relevant to this discussion. These concerns are raised because of greater levels of impurities in MAP.
These reaction products of MAP and DAP suggest in neutral to acid soils that no differences exist in solubility of reaction products, while in calcareous soils greater immediate availability is indicated with MAP.
Field Trials
Hundreds of field trials have compared MAP and DAP. For example, replicated research trials have been conducted at 42 sites the last three years in seven corn belt states. The average corn yield across all sites was 162.4 bushels per acre with MAp and 159.4 bushels with DAP.
Summary
Although both MAP and DAP are defined as ammonium phosphates, there are soil, crop, fertilizer placement, and nutrient interact factors that assist farmer and dealers’ decision process of handling MAP or DAP. These Agronomy factors should be weighted with pricing, handling, marketing, and supply factors in making the final choice: MAP or DAP.
DAP 18-46-0 MAP 11-52-0
Chemical Analyses Typical (Typical Range) Typical (Typical Range)
Nitrogen (Total) 18% 11%
Phosphate (P2O5)
-Total 46.5% 52.5%
-Available 46% 52%
-Water Soluble 42% 47%
Crude Moisture (H2O) 0.7% (0.3%-1.3%) 0.5% (0.2%-1.3%)
Ground Moisture (H2O) 2.1% (1.5%-2.6%) 1.2% (0.7%-2.3%)
Sulfate Sulfur (S) 1.4% 1.5%
Iron (Fe2O3) 1.7% 1.8%
Aluminum (Al2O3) 1.3% 1.9%
Magnesium (MgO) 0.8% 1.1%
pH (1% Solution) 7.2 4.5