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?

1. 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.

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 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.

1. 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.

2. 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

1. 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.

2. 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.

3. Soil pH
   1. 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.

   2. 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.

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.

Typical Analyses

	DAP 18-46-0 Specifications	MAP 11-52-0 Specifications
Chemical Analyses	Typical (Typical Range)	Typical (Typical Range)
Nitrogen (Total)	18%
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

Agronomic MAP/DAP Matrix

+ Positive | – Negative | N No factor

Factor	MAP	DAP
Low P Soil Test	+	N
Acid Soil	N	N
Alkaline Soil	+	-
Sandy Soil	+	N
Fine Texture Soil	N	N
Starter band	+	N
Seed Placed Fertilizer	+	-
Broadcast	N	N
Water Soluble P	N	N
Low Levels of MN, FE, Zn	+	N
Legumes	+	-
Vegetable banded	+	-
Small Grains	+	-
Canola	+	-

Please contact on of your IFA Certified Crop Advisors for more information

Click the following PDF link to read the complete article
Alfa Boost

Adequate phosphorus (P) is necessary for higher yields and improved
grain quality. Phosphorus is often referred to as “the energizer” for its role in
converting the sun’s energy into food, fuel and fiber. Some of the benefits that
phosphorus provides growing plants with are improved root growth, earlier
maturity of grain, higher crop quality, better water use efficiency and increased
yields.
The amount of phosphorus needed is dependent on existing soil levels.
Yield losses can be severe as the soil P levels drop below 20 ppm, which is the
critical level for phosphorus in the soil. For example, a field testing 10 ppm that
did not receive phosphorous fertilizer would be expected to yield 80 percent of
a field that was above the critical level. It is important to note that it takes
approximately 18 pounds of P2O5 to raise soil test levels 1 ppm.
Adequate phosphorus absolutely contributes to a balanced soil fertility
program. In corn, without phosphorus fertilizer added, research shows that
adding more nitrogen fertilizer may dramatically increase soil nitrate levels that
are subject to loss due to leaching. In contrast, where phosphorus fertilizers
were added, the corn crop utilized the additional nitrogen added and soil nitrate
levels only increased moderately with increases in nitrogen applied.
In addition, phosphorus plays a key role in photosynthesis, the
metabolism of sugars, energy storage and transfer, cell division, cell
enlargement and the transfer of genetic information. It is a vital component of a
balanced soil fertility program.
For more information on proper soil fertility, visit www.back-to-basics.net
or call your local IFA Crop Advisor or the IFA Agronomy Center nearest you.

March 2009

Those involved in growing commercial crops are very familiar with the primary macro-nutrients, nitrogen, phosphorus and potassium, and would not expect to harvest a profitable crop without assuring proper levels of these nutrients. The need for these three nutrients are long established and well known and always considered in a crop plan. They are commonly part of most any fertilizer program because they are used in the largest amounts of the mineral nutrients by our crops, but what about the other elements. Plants depend on water, carbon dioxide, sunlight and thirteen essential nutrients that are commonly divided into three groups, macro-nutrients, secondary nutrients and micro-nutrients. These names reflect the amount of the nutrient the plant requires, not the importance of the nutrient to the plant. Micronutrients are those elements essential for plant growth which are needed in only very small quantities. The micro-nutrients are boron, copper, iron, chloride, manganese, molybdenum and zinc. Though secondary and micro-nutrients are used by plants in smaller amounts than macro-nutrients they are just as critical and in any production focused crop program can often be the limiting factor for the desired crop yields and profitability.

When the level of secondary and micro-nutrients are the limiting factor this can directly reduce production and also reduce the efficiency and cost effectiveness of the primary macro nutrients that are in the soil and a part of your fertilizer program. Assuring the availability of micro-nutrients to the plants brings greater efficiency and economic return from your fertilizer investment. Plant growth and production is limited at the lowest level of whatever nutrient the plant is in need of, regardless of whether it is needed in large or small amounts. All of the essential nutrients are critical to all plants and even if just one is deficient, it affects the health, quality and yield of all crops.

There is now an increased level of knowledge about the practical importance of assuring availability of micro-nutrients to the plant and the plant functions they influence.

A complete soil test is a great place to start to determine the soil fertility needs of your fields. Plant tissue samples can also be very helpful to understand micro-nutrient requirements. Your local IFA Crop Advisor has access to multiple options to be able to fit your conditions and supply the needed nutrients to maximize your opportunity in raising healthy, high yielding crops.

IM POLIZOTTO, PH. D.

PotashCorp Chief Agronomist

The source of phosphorus in most fertilizer blends today is monoammonium phosphate (MAP 11-52-0) or diammonium phosphate (DAP 18-46-0). It is common in many crop production systems to apply phosphorus (P2O5) and potassium (K2O) fertilizer needs in the fall, prior to fall- and/or spring-planted crops. In the US Corn Belt, it is fairly common for farmers to apply their fertilizer requirements for the next corn crop as well as the soybean crop that will follow it. These applications are often made soon after soybean harvest and when soil temperatures are greater than 50 F.

Using slow- and controlled-release nitrogen fertilizers, adding nitrogen fertilizer additives and/or timing nitrogen applications will all improve nitrogen efficiency.

The top questions that many dealers and farmers have about this practice are: How much of this fall-applied N is available to my spring-seeded crop? How much should I credit my regular nitrogen program with N from DAP or MAP?

Nitrogen Soil Reactions

Regardless of the fertilizer source of nitrogen, it eventually gets converted to nitrate (NO3-) in the soil. Urea and anhydrous ammonia nitrogen is quickly converted to ammonium nitrogen, and then the ammonium nitrogen is converted to nitrate nitrogen. These conversions are carried out by soil bacteria and take place very quickly when soil moisture is just below field capacity and temperatures are above 60 F. Nitrification is the term used to describe the conversion of ammonium to nitrate nitrogen.

Based on this research, it appears that dealers or farmers might want to make adjustments to their nitrogen fertilizer programs based on when MAP, DAP or AS nitrogen is applied.

Unfortunately, nitrate is the nitrogen form most easily lost in soils. Nitrate N can be leached during periods of heavy rain and can be denitrified when soils are saturated. Denitrification is the term used when soil bacteria strip oxygen from the nitrate nitrogen (NO3-). When this occurs, nitrogen gases N2 or N20 are formed and can then escape into the air.

Because of these soil reactions, nitrogen management can be a challenge, but slowing nitrification and minimizing denitrification can improve nitrogen utilization greatly. Using slow- and controlled-release nitrogen fertilizers, adding nitrogen fertilizer additives and/or timing nitrogen applications will all improve nitrogen efficiency.

The question to consider is when a small amount of nitrogen is applied with fall P and K fertilizer programs before soils get cold, and nitrification inhibitors are not used, how much nitrogen is left for the next crop?

Recent Research

Recent research conducted by Dr. Robert Hoeft at the University of Illinois and Dr. Gyles Randall at the University of Minnesota has specifically addressed these questions.

For the years 2004 through 2006, they fall- and spring-applied 0, 40 and 80 lbs/acre of N using DAP, MAP and ammonium sulfate (AS) as the N sources. The trials were conducted at sites in Illinois and Minnesota each of the three years. A laboratory incubation study was also conducted in Illinois to determine the rate of nitrification and recovery of ammonium and nitrate N from DAP and MAP.

The laboratory incubation study showed that DAP nitrified just slightly faster than MAP, which agrees with previous work in this area. Also, at moisture levels below field capacity, the amount of nitrate in the soil remained pretty constant over 14 weeks, for both MAP and DAP, indicating no denitrification was occurring, where soils were not saturated. When field moisture exceeded field capacity (saturated soils), however, denitrification was rapid for both MAP and DAP, and as much as 50 percent of the nitrate was lost in two weeks at temperatures of 70 F.

In the field studies, Hoeft and Randall were able to quantify nitrogen losses during the fall, winter and/or spring, following fall and spring MAP, DAP and AS applications. As might be expected, N losses varied from year to year, depending on the soil temperatures and rainfall. Because total applied N rates were low, corn grain yields reflected soil N differences among the treatments.

Observations and Conclusions

After three years of research in two states, the following observations and conclusions were reached:

1. Source of N, including MAP, DAP or AS, had little influence on soil inorganic N concentration in Illinois or Minnesota field trials.
2. Source of N had no effect on corn grain yields at either location.
3. Most years, plots with spring-applied N had significantly higher yields than those with fall-applied N. In 2006 in Minnesota, fall applications outyielded the spring treatments.
4. As much as 70 percent to 80 percent of the fall-applied N from MAP, DAP or AS might be lost in years where nitrification is complete and soils are warm and heavy spring rains contribute to spring leaching and/or denitrification.
5. In years where soils cool quickly in the fall and/or where soils are not saturated in the spring, there is little N loss from fall- or spring-applied N.

Based on this research, it appears that dealers or farmers might want to make adjustments to their nitrogen fertilizer programs based on when MAP, DAP or AS nitrogen is applied. For fall-applied MAP, DAP or AS, don’t assume that all of the N will be available for a spring planted crop. If it is applied on cold soils and soils do not become saturated during spring rains, most of the N should be available. If applied in the fall when soils are warm and/or spring rains leave the fields saturated, N loss from any of these sources can be significant. If in doubt, it is probably safe to assume that there will be a benefit from about 50 percent of the N from these sources when fall applied. When spring applied, assume the same availability as your primary N fertilizer source.Although some nitrogen from fall-applied MAP or DAP fertilization programs is normally lost before spring planting, we shouldn’t assume that this always translates to reduced yields. Because nitrogen from these types of applications is just a small part of the total nitrogen applied to most crops, it isn’t likely that this N loss will affect yields. Nitrogen from the primary N fertilizer source plus N from in season mineralization of soil N will normally be adequate. Yield only will be affected if there is significant N loss from all of your fertilizer N sources.

Hoeft and Randall are presently preparing their research results for publication. This work will be very valuable in assessing, adjusting and explaining nitrogen additions in nutrient management plans. Agriviews and the PotashCorp website www.potashcorp.com will post an announcement when the results are summarized and reported.