Michigan State University Extension
Soils & Soil Management - Fertilizer - 06109726
07/10/97
N-P-K Fertilizers Extension Bulletin E-896, October 1990
by M.L. Vitosh Extension Specialist, Crop and Soil Sciences
There are several properties of fertilizers and principles of fertilizer application which users should become familiar with. One important property of fertilizers is water solubility. Nearly all N fertilizers are completely water soluble. Because of their high water solubility, granule size and band placement are generally not important.
The two most common forms of N in fertilizers are ammonium (NH`+) and nitrate (NO3-). Under conditions of good plant growth, NH`+ is rapidly converted to NO3 by bacteria. Both forms can be taken up and utilized by plants. However, crops such as tobacco, potatoes and tomatoes prefer nitrate as their source of N. Because nitrate is much more mobile than ammonium, ammonium forms of N are recommended when the application is made prior to the time of greatest need. This practice minimizes potential loss by leaching.
Most phosphate fertilizers are highly water soluble. Phosphate water solubility is very important for early plant growth. Thus, it is important for banded starter fertilizers to contain highly soluble forms of P. Soils normally contain very little water-soluble (less than 2 ppm in soil solution) but concentrations as high as 100,000 ppm have been measured in the fertilizer band. During cool and wet conditions when plant growth is slow and the root system is shallow, band placement of P fertilizers becomes extremely important.
Broadcast applications usually contact less than 2% of the total soil volume. Consequently, water solubility is of little importance where P fertilizers are broadcast.
Phosphorus availability in the band is generally improved by the addition of N to the P starter and by increasing the granule size. Large granules contact less soil per unit of P than small granules. Thus, initial P fixation is lower, and availability is improved.
Most potassium fertilizers are highly water soluble. Like NH`+, Kit is held in the soil by clay and organic matter. Unlike NH`+, however, K is not converted to a more mobile form. Potassium stays relatively close to the initial point of application. Leaching of K is not generally a problem, except on very sandy or gravelly soils, due to insufficient cation exchange sites and on organic soils where the cation exchange bonds are much weaker. Because K is not subject to the same fixation reactions as P. water solubility is not considered important. Potassium can be fixed in the crystal lattice of some clay minerals; however, high K-fixing soils in Michigan are not extensive. Consequently, banding K is only important where soil tests for K are extremely low.
The following is a discussion of several of the more commonly used N-P-K fertilizers. Table 1 contains the chemical analysis of these and other fertilizer materials. ==================================================== Table 1-Primary and Secondary Nutrient Composition of Some Selected Fertilizer Materials (see 1)
Percent
Fertilizer Water Nutrient Composition
Materials Solu- N P2O5 K2O Ca Mg S
bility
----------------------------------------------------
Nitrogen N
Ammonia, 100 82 -- -- -- -- -- anhydrous Ammonia, 100 16-25 -- -- -- -- -- aqua Ammonium 100 33.5 -- -- -- -- -- nitrate Ammonium 100 20.5 -- -- 7.3 4.4 -- nitrate-- limestone Ammonium 100 21 -- -- -- -- 23.7 sulfate Ammonium 100 26 -- -- -- -- 15.1 sulfate-- nitrate Calcium 100 21 -- -- 38.5 -- -- cyanamide Calcium 100 15 -- -- 19.4 1.5 -- nitrate Nitrogen 100 21-49 -- -- -- -- -- solutions Sodium 100 16 -- -- -- -- -- nitrate Sulfur- Variable 35 -- -- -- -- 21 coated urea Urea 100 46 -- -- -- -- -- Ureaform Variable 38 -- -- -- -- --
Phosphate P
Ammoniated 35 3-6 18-20 -- 17.2 -- 12 super- phosphate Ammoniated 100 27 15 -- -- -- -- phosphate nitrate Ammonium 90+ 13-16 20-39 -- -- -- 15.4 phosphate sulfate Ammonium 100 10-15 34-62 -- -- -- -- poly- phosphate Bone meal -- 2-4.5 22-28 -- 20-25 -- -- Diammonium 95+ 16-21 48-53 -- -- -- -- phosphate Mono- 90+ 11 48 -- 1.1 -- 2.2 ammonium phosphate Nitric 40 14-22 10-22 -- 8-10 -- 1-3.6 phosphates Phosphoric 100 -- 52-60 -- -- -- -- acid Rock <1 -- 30-36* -- -- -- -- phosphate Super- 85 -- 18-20 -- 20.4 -- 11.9 phosphate, normal Super- 87 -- 42-50 -- 13.6 -- 1.4 phosphate, concentrated Super- 100 -- 69-75 -- -- -- -- phosphoric acid
Potash K
Nitrate 100 15 -- 14 -- -- -- of soda- potash Potassium 100 -- -- 60-62 -- -- -- chloride muriate Potassium 100 -- -- 22 -- 11.2 22.7 magnesium sulfate Potassium 100 13 -- 44 -- -- -- nitrate Potassium 100 -- -- 50 -- 1.2 17.6 sulfate ---------------------------------------------------- * Relatively unavailable to plants in most soils. (1) From "Fertilizer Handbook," The Fertilizer Institute ====================================================