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.
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.
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 for legumes should be considered in a MAP-DAP decision.
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.
- 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.
The interaction of P fertilizer and formation of free ammonia causing ammonia toxicity increases when soil pH are high and in calcareous soils.
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.
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.
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.
Source: Intermountain Farmers Association