ISSUE 2 May 14, 1998
UPDATE ON MAP VS DAP
This winter there was some misinformation distributed regarding MAP (monoammonium phosphate, or 11-52-0 or sometimes 10-50-0) and DAP (diammonium phosphate, or 18-46-0). The information was presented showing that DAP increases soil pH and MAP acidifies the soil, therefore MAP is a better product. It is true that when DAP is added to water, the initial solubility reaction increases pH and MAP decreases pH, however, we add MAP and DAP to soil, which is highly resistant to changes in soil pH. Unlike a glass of water, soil will quickly change the soil around the MAP and DAP pellets to the pH of the soil, not the other way around.
An additional consideration is the activity within a few weeks of MAP and DAP application by soil bacteria that transforms the ammonium-N in both products into nitrate. That reaction is acid forming and influences the pH of the soil far more than the initial solubility reaction of either MAP or DAP. If a more acid forming reaction was really important in increasing the availability of phosphate from the applied fertilizer, DAP is much more acid forming than MAP and would win hands-down. However, many studies over many years have shown that is most cases, there is no difference between either product in the plant uptake of P.
Producers should choose MAP over DAP mostly on the basis of cost per unit of phosphate and local availability, not in its pH in a glass of water.
INCREASING SOIL PH LEVELS IN SOME AREAS
Some producers have reported an increase in soil pH in some areas of North Dakota recently. These soils usually have free limestone/carbonates in them and have increased anywhere from 0.2 to 0.5 pH units. Some producers have blamed the use of DAP phosphate, however DAP is ultimately acid forming and does not cause increases in pH in the soil during the season. In addition, it is not uncommon for Central Corn Belt growers to apply 200-300 lb/acre of DAP every other year, and still have to apply limestone to neutralize acidity every 4-6 years.
Environmental conditions over the last four to five years have favored increasing pH levels in some areas. Continually wet conditions increases carbon dioxide solubility, forming carbonate minerals where favorable. Increasing water tables also carries soluble minerals such as magnesium and even sodium towards the surface, where the potential equilibrium pH of soil including magnesium carbonate and sodium carbonate is higher than when calcium carbonate is mostly present. Increases in pH higher than about 8.2 are very difficult in soil without some sodium around.
What can be done to decrease this acidity? Eventually, it will turn dry, the water table will lower and the most soluble salts, sodium and magnesium will again leach downward in the soil. In their absence and in the absence of excess soluble bicarbonate, soil pH will decrease somewhat, however, in soil with large amounts of excess carbonates, pH will probably remain in the high 7's or low 8's for generations to come.
SALINE SOIL RECLAMATION
If I could market a substance to apply to a salty soil and make it not salty, I could retire. But it looks like I will be here awhile, because it is not possible to remedy a salt problem by adding a salt. If youre not sure this is true, take two containers of household table salt, open them and pour one on a table. Then pour the other container on top. Did the first pile go away? I bet not. I bet if you try to apply gypsum on a salty area that already has 2 tons per acre of gypsum, it wont get rid of the gypsum either.
The idea that gypsum will improve salty soil comes from confusion over what is "alkali" and the chemistry of many of our North Dakota soils. In some parts of the world, the word "alkali" is used to describe soils that are generally unproductive. In some areas, these are alkaline soils with high pH that react with certain herbicides to injure the crop or carryover into subsequent crops. "Alkali" also describes soils high in sodium, but low in salts. These soils are also called slick spots, because the sodium prevents stable soil aggregates from forming, causing soil to seal tightly when wet, preventing water infiltration, and then when dry the soils become very hard and impervious to roots. In these soils, and especially when soil sulfate levels are low and not a dominant anion, application of gypsum will reclaim these soils. In soils with a sodic problem and a high level of free carbonates, application of elemental sulfur will acidify and form gypsum from the limestone and sulfate, again reclaiming the soil.
In most North Dakota soils, however, even those with high sodium and low salts, research has shown that more soluble compounds such as calcium chloride are more effective in reclaiming sodic soils than gypsum.
If soils are salty, the problem cannot be helped by the addition of a soil amendment. The problem is high water table. If neighboring fields are being fallowed, encourage continuous cropping. If the problem is beginning next to the road, grow a buffer strip of alfalfa to dry out the area beside the ditch deeply and prevent water from encroaching into the field. Try more salt tolerant crops. If nothing but weeds grow in areas, allow them to grow as long as possible, then mow them to encourage more growth and prevent them going to seed instead of working the soil and creating mini-fallow fields that perpetuate the problem. If there is a market for alfalfa close by, consider throwing the field into alfalfa occasionally to dry the field out.
SURFACE APPLICATION OF NITROGEN
Nitrogen should be incorporated into the soil for the most consistent results. Urea and urea-ammonium nitrate solutions are particularly sensitive to losses from ammonia volatilization if left on the surface without incorporation or rainfall. With soil temperatures in the 60 degree range on sunny and breezy days, incorporation or rainfall/irrigation within 48 hours is important. If soils are in the 50 degree range, days are cloudy and there is little wind, these fertilizers can remain on the soil for a day or two longer before significant losses can occur. The use of the urease inhibitor Agrotain can extend the life of urea about 10 days over an untreated application. Nitrogen from ammonium sulfate can also volatilize, but at a much slower rate than urea. The sulfate component, however, does not volatilize and can wait indefinitely for a rain. Ammonium nitrate is an excellent source of surface applied N, and in areas where it is available, it can be used without little fear of volatility. Rainfall is needed eventually with any product to move the N to the root zone.
Losses from urea volatility can vary greatly, but a rule of thumb is when the weather is warm, sunny and breezy, to expect losses after about 48 hours to be about 5% a day until significant rainfall occurs.
WATER USE BY CROPS
One never knows in North Dakota whether to build an ark or buy an oasis, so although most areas of the state presently have good soil moisture, the following is a list of crops and the threshold soil water + seasonal precipitation needed to make some crop and also the general relationship above that threshold level for calculating what to expect for yield with further precipitation. General soil fertility, management practices, variety and any number of about 40 different factors can influence yield, but this is a general guide to what moisture means in this state.
Crop Response to Water
Threshold for some yield(in.)
Dr. Dave Franzen
NDSU Extension Soil Specialist