N-P-K FERTILIZERS

1 Michigan State University Extension ! Extension Bulletin E-896 ! Reprint July 1996 Liquid or fluid fertilizer use in Michigan has in-creased steadily over the last 25 years. In 1965, 9% ofthe total fertilizer sales in Michigan consisted ofliquids. In 1988, liquid FERTILIZERS accounted for 28%of the market. Anhydrous ammonia and N solutionsaccounted for 78% of the total liquids FERTILIZERS (solids) still constitute the major partof the FERTILIZERS sold in Michigan. In 1988, 92% ofthe total dry fertilizer was sold as bulk material, withonly 8% in bags. The bulk materials are generally lessexpensive and easier to handle, particularly whenlarge quantities are used.

2 Urea is the most popularsource of dry N fertilizer, accounting for 79% of thetotal dry N used. Ammonium sulfate has risen inpopularity. In 1988 it constituted 14% of the dry Nmarket. The use of ammonium nitrate has decreasedgreatly in recent years, although it is a very goodsource of responses to liquid and dry fertilizer are similar,provided the same amounts of plant nutrients areapplied and the same placement and water soluble Pmaterials are compared. Reactions of these materialsin soils are similar. When placed in the soil, dryfertilizers absorb water and undergo chemicalreactions similar to liquid per unit of plant nutrients, application costs,potential for leaching, ease of handling andcustomer service are the prime factors to considerwhen buying FERTILIZERSby Vitosh Extension Specialist, Crop and Soil SciencesThere are many grades and analyses of nitrogen(N), phosphorus (P) and potassium (K) FERTILIZERS ,but all can be grouped into three classes liquids, solidsand is nothing mysterious about different grades ortypes of FERTILIZERS .

3 For a particular grade, manyfertilizers have the same chemical compounds and areused for the same basic purposes-to supplement theessential plant nutrients in the soil. Usually all that isneeded to compare two FERTILIZERS is the amount and theavailability of plant nutrients in the material. Thisinformation is given on the label or bill of sale and isguaranteed. Fertilizer TypesAlthough it is generally agreed that FERTILIZERS comein three physical forms (liquid, solid and gas), there areactually only two classes of FERTILIZERS : liquid and ammonia (NH3) is a gas, but it is classifiedas a liquid because it is a liquid under pressure. Theterm liquid fertilizer applies to anhydrous ammonia,aqua ammonia, N solutions and liquid mixed N-P-K FERTILIZERS are also known as fluidfertilizer.

4 They include true solutions which require noagitation and suspensions or slurry type mixtures of N, Pand K, which require constant stirring to keep the solidssuspended in the and Uses There are several properties of FERTILIZERS andprinciples of fertilizer application which users shouldbecome familiar with. One important property offertilizers is water solubility. Nearly all N fertilizersare completely water soluble. Because of their highwater solubility, granule size and band placement aregenerally not The two most common forms of N in FERTILIZERS areammonium (NH4+) and nitrate (NO3). Underconditions of good plant growth, NH4+ is rapidlyconverted to NO3 by bacteria.

5 Both forms can be takenup and utilized by plants. However, crops such astobacco, potatoes and tomatoes prefer nitrate as theirsource of N. Because nitrate is much more mobile thanammonium, ammonium forms of N are recommendedwhen the application is made prior to the time ofgreatest need. This practice minimizes potential loss phosphate FERTILIZERS are highly water water solubility is very important for earlyplant growth. Thus, it is important for banded starterfertilizers to contain highly soluble forms of P. Soilsnormally contain very little water-soluble P (less than2 ppm in soil solution) but concentrations as high as100,000 ppm have been measured in the fertilizerband.

6 During cool and wet conditions when plantgrowth is slow and the root system is shallow, bandplacement of P FERTILIZERS becomes applications usually contact less than 2%of the total soil volume. Consequently, watersolubility is of little importance where P fertilizersare availability in the band is generallyimproved by the addition of N to the P starter and byincreasing the granule size. Large granules contactless soil per unit of P than small granules. Thus,initial P fixation is lower, and availability potassium FERTILIZERS are highly watersoluble. Like NH4+, K+ is held in the soil by clayand organic matter. Unlike NH4, however, K is notconverted to a more mobile form.

7 Potassium staysrelatively close to the initial point of of K is not generally a problem, except onvery sandy or gravelly soils, due to insufficientcation exchange sites and on organic soils where thecation exchange bonds are much weaker. Because Kis not subject to the same fixation reactions as P,water solubility is not considered can be fixed in the crystal lattice of someclay minerals; however, high K-fixing soils inMichigan are not extensive. Consequently, bandingK is only important where soil tests for K are ex-tremely following is a discussion of several of themore commonly used N-P-K FERTILIZERS . Table 1contains the chemical analysis of these and otherfertilizer FertilizersAnhydrous ammonia (82%N) is a liquid underhigh pressure and must be injected at least six inchesdeep into a moist soil because it becomes a gas once itis released from the tank.

8 In soil, ammonia reacts withwater to form the ammonium (NH4`) ion, which isheld on clay and organic matter. Anhydrous ammoniais generally the cheapest source of N; however, themethod of application is less convenient and requiresmore power to apply than most other liquid or solutions (28 to 32% N) are a mixtureof urea and ammonium nitrate in water. The solutionhas no ammonia vapor pressure and is generallysprayed or dribbled on the soil surface. The loss of Nfrom surface application of 28% N solution isgenerally not considered to be of great concern inMichigan when it is applied early in the spring. Undercertain conditions, however, N loss due to ammoniavolatilization may be serious.

9 If theconversion of urea to ammonia in the liquid fertilizertakes place on the surface, some ammonia can be lost byvolatilization. The remainder of the ammonia may reactwith water on the surface to produce an alkalinecondition, which also promotes volatile ammonia most favorable conditions for volatile N lossfrom surface-applied urea (solid or liquid) arealkaline soils, warm temperatures, intermediaterelative humidity (50 to 90%) and sandy soils withlow organic matter content and low cation inch of rain will normally move surface applied N solutions deep enough into the soil to prevent ammonia volatilization. Nitrogen solutions should notbe applied in the fall, because one fourth of the N is in nitrate form and is subject to loss by leaching or denitrification.

10 Surface application of N solutions to heavyresidues, which occur in no-till systems, has beenshown to reduce its effectiveness when compared toN that is incorporated or knifed-in. Nitrogen can betemporarily tied up in residues and unavailable to thecrop until the residues decompose. Aqua ammonia (21% N) is a liquid under lowpressure and must be incorporated into the soil toprevent the loss of free ammonia to the atmosphere. It ispossible to lose all of the free ammonia if it is notincorporated. Aqua ammonia has advantages overanhydrous ammonia: placement need not be as deep,and high-pressure applicators are not required. Urea (46% N) is the most widely used dry N applied to the soil, urea is converted to ammoniawhich reacts with water to form ammonium within twoto three days (faster under warm conditions).