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Wichita State University Wichita, KS 67208
Acidity and Basicity of Fertilizers Davld R. Hershey Department of Horticulture University of Maryland College Park. MD 20742 Salts and ions can be classified as chemically acidic, neutral, or basic.' Ca(N03)~is chemically neutral (eq 1). NH4H2P04is chemically acidic (eq 2), and (NHJzHPOa is chemically basic (eq 3). Ca(N0J2
-
CaZt(aq)+ 2N0,Jaq)
(1)
Salts used as fertilizers are also classified as acidic or basic.2 When plant roots absorb most or all of their nitrogen as nitrate (NO3-), they typically absorb more anions than cations. The roots excrete OH- into the soil to maintain charge balance and create what is termed physiological alkalinity or basicity. Alternatively, roots absorbing a substantial portion of their nitrogen as ammonium ion (NH4+)typically excrete H+ tomaintain charge balance andcreate pbysiological acidity. The explanation for these phenomena requires a consideration of the essential elements absorbed by roots. There are 16 elements generally considered essential for higher plants. Carbon, hydrogen, and oxygen comprise most of the plant structure, as H 2 0and organic molecules, and are absorbed in nonionic forms. Roots absorb Hz0 and 0 2 and leaves absorb C02 and 02.The remaining 13 essential elements, or nutrients, are absorbed primarily by the roots as ions, except for boron, which is absorbed as boric acid B(OH)3. The six macronutrients are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. The macronutrients are required in relatively large amounts, from about 0.2 to 4.0% of plant dry mass. The micronutrients are required in relatively small amounts, typically less than 100 mgkg of plant dry mass. The micronutrients are iron, manganese, boron, chlorine, molybdenum, zinc, and copper. The symbols of the essential elements can be remembered with the help of the mnemonic, C HOPKNS CaFe ClZn, MOB CuMn Mg3. In prose form the mnemonic is "C. Hopkins cafe closing; mob coming with machine guns". Root charge balance is determined by macronutrient absorption since macronutrients are absorbed in much greater amounts than the micronutrients. The macronutrient cations are ammonium, potassium, calcium, and magnesium. The macronutrient anions are nitrate, sulfate, and the orthophosphates (HPOa2-,HzP04-). Nitrogen plays a critical
642
Journal of Chemical Education
role in charge balance since it mav occur as a cation or anion. and nitrog&-containing ions areabsorbed in a greater mol& concentration than any other ion.4 If all nitrogen is absorbed as anions, then the root will usually absorb more anions than cations. If an appreciable amount of nitrogen is absorbed as ammonium, then more cations than anions will be absorbed. e more likelv than excess anion u ~ t a k e Excess cation u ~ t a k is because, when nitrogen is not considered, the molar cokentration of macronutrient cations absorbed exceeds that of the macronutrient anions. Fertilizers containing ammonium may also produce Hf when soil microbes convert ammonium ion to nitrate in a process called nitrification (eq 4).
A chemically neutral salt, like Ca(NO?)?,can be physiologically basic, while a chemically basic salt, like (NH,)>HPO,, can be physiologically acidic. Fertilizer labels typically indicate the potential acidity or basicity of the fertilizer in units of pounds of calcium rarbonate (CaCO,) per ton of fertilizer.$ Calcium carbonate is the standard liming material used to raise soil pH. The widely available home and garden fertilizer, Miracle-Gro 15-30-15, has a potential acidity of 1000 pounds of calcium carbonate per ton of fertilizer. Fertilizers are more likely to he acidic than basic, for the reasons discussed previously. The series of three numbers, 15-30-15, is the fertilizer grade or analysis. The three numbers represent the percent by weight of nitrogen (N), available phosphoric acid (P205), and soluble potash (K20), respectively. The fertilizer actually does not contain P205 and K20. This form of expression is based on obsolete methods and persists because of tradition. Scientists generally specify N-P-K content on an elemental basis. The 15-30-15 would he 15N-13P-12K. Conversions are P 2 0 5 X 0.43 = P, and K20 X 0.83 = K. Neutral salt solutions, like 10 mM CaC12, can also lower soil pH when used to prepare soil samples for pH measurement.5 Typically, two volumes of solution are added to each volume of soil sample. The lower soil pH is due to mass displacement, by Ca2+,of H+ from soil cation exchange sites into the soil solution. CaC12is chemically neutral, yet acidic when added to a soil sample for pH measurement.
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Mastelton, W. L.: E. J. Slowlnski. ChemlcalPrlnclpleswlth Oualk tatlve Analysis: Saunders: Phlladelphia, 1978: pp 461-462. Henhey. D. R. Mawland Greenhouse Growers'News 1987, 43). 7-9.
Hershey, D. R. JAgmn. Educ. 1988, 17,141.
'Nye. P. H. Adv. Plant Nutrltlon 1986, 2,129-153.
Russell, E. W. Soil Condltlons and Plant Growih, 10th ed.: Longman: London. 1973: pp 123-124.
