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NITROGEN

Form taken up by plant: NH4+, NO3-

Mobility in soil: NH4+: no; NO3-: yes

NO3- water soluble, not influenced by soil colloids

Mobility in plant: Yes

Deficiency symptoms: Chlorosis in older leaves, under severe deficiency lower leaves are brown, beginning at the leaf tip and proceeding along the midrib.

Soil pH where deficiency will occur: None due to nitrate's mobility

Role of nutrient in plant growth: N assimilation into amino acids for protein and amino acid synthesis, component of chlorophyll, vegetative growth

Enzymes that require N: Nitrate reductase, nitrite reductase, nitrogenase

Role of nutrient in microbial growth: Necessary for the synthesis of amino acids

Concentration in plants: Wheat 1.7 - 3.0%

Grain 2.0%

Forage 3.0 %

Straw

Corn 2.7 - 3.5%

Soybeans 4.2 - 5.5%

Grain sorghum 3.3 - 4.0%

Peanuts 3.5 - 4.5%

Alfalfa 4.5 - 5.0%

Bermudagrass 2.5 - 3.0%

Effect of pH on availability:

Precipitated forms (low pH): none

Precipitated forms (high pH): none

at pH>8, no nitrification; at pH>7, NO2- accumulates

Interactions with other nutrients: Si: enhances leaf erectness, thus neutralizing the negative effects of high nitrogen supply on light interception (leaf erectness usually decreases with increasing nitrogen supply); P: symbiotic legume fixation needs adequate P or a N deficiency can result; Mo: component of nitrogenase therefore could have Mo induced N deficiency in N₂ fixing legumes (especially under acid soils conditions); Fe: necessary for nitrogenase and ferredoxin (electron carrier), legume hemoglobin, deficiency reduces nodule mass, and nitrogenase;

Fertilizer sources: ammonium sulfate, anhydrous ammonia, ammonium chloride, ammonium nitrate, ammonium nitrate-sulfate, ammonium nitrate with lime, ammoniated ordinary superphosphate, monoammonium phosphate, diammonium phosphate, ammonium phosphate-sulfate, ammonium polyphosphate solution, ammonium thiophosphate solution, calcium nitrate, potassium nitrate, sodium nitrate, urea, urea-sulfate, urea-ammonium nitrate, urea-ammonium phosphate, urea phosphate.

References:

Burford, J.R., and J.M. Bremner. 1975. Relationships between the denitrification capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Biochem. 7:389-394.

Marschner, Horst. 1995. Mineral Nutrition in Higher Plants. Academic Press, London.

Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil Fertility and Fertilizers. MacMillan Publishing Co., New York, N.Y.

Authors: Heather Lees, Shannon Taylor, Joanne LaRuffa and Wade Thomason

PHOSPHORUS

Form taken up by plant: H2PO4-, HPO4=

Mobility in soil: None; roots must come in direct contact with orthophosphate P

Mobility in plant: Yes

Deficiency symptoms: Lower leaves with purple leaf margins

Deficiency pH range: <5.5 and >7.0

Toxicity symptoms: None

Toxicity pH range: Non toxic (optimum availability pH 6.0-6.5)

Role of nutrients in plant growth: Important component of phospholipids and nucleic acids (DNA and RNA)

Role of nutrient for microbial growth: Accumulation and release of energy during cellular metabolism

Concentration in plants: 1,000 – 5,000 ppm (0.1 –0.5%)

Effect of pH on availability: H₂PO₄ ^– at pH < 7.2

HPO₄ ^2- at pH > 7.2

Interactions with other nutrients: P x N, P x Zn at high pH, in anion exchange P displaces S, K by mass action displaces Al inducing P deficiency (pH<6.0)

Phosphorus fertilizer sources: Rock phosphate, phosphoric acid, Ca orthophosphates, ammoniumphosphates, ammonium poly-phosphates, nitric phosphates, K phosphates, microbial fertilizers (phosphobacterins) increase P uptake

Additional categories:

Mineralization/immobilization: C:P ratio of < 200: net mineralization of organic P; C:P ratio of 200-300: no gain/loss of inorganic P; C:P ratio of >300: net immobilization of inorganic P

P fixation: Formation of insoluble Ca, Al, and Fe phosphates

Al(OH)₃ + H₂PO₄^- -à Al(OH)₂HPO₄

(Soluble) (Insoluble)

Organic P sources: Inositol phosphate (Esters of orthophosphoric acid), phospholipids, nucleic acids, phosphate sugars

Inorganic P sources: Apatite and Ca phosphate (unweathered soils) and Fe and Al sinks from P fixation (weathered soils)

Waste: Poultry litter (3.0 to 5.0%), steel slag (3.5%), electric coal ash (<1.0%)

Total P levels in soil: 50 – 1500 mg/kg

Solution concentration range: < 0.01 to 1.0 ppm

Applied fertilizer: < 30% recovered in plants, more P must be added than removed by crops

References:

Alexander, M., 1977. Introduction to Soil Microbiology. 2^nd Edition. John Wiley and Sons, NY.

Brady, N.C., 1990. The Nature and Properties of Soils. 10^th Edition. Macmillan Publishing Co.,

NY.

Brigham Young University. 1997. The Phosphorus Cycle. http://ucs.byu.edu/bioag/aghort/214pres/geochem.htm

Harrison, A.F., 1987. Soil Organic Phosphorus. A Review of World Literature. C.A.B. p.39.

Pierre, W.H., 1948. The Phosphorus Cycle and Soil Fertility. J. Amer. Soc. of Agron., 40:1-14.

Pierzynski, G.M., Sims, J.T., and Vance, G.F., 1994. Soil and Environmental Quality. Lewis

Publishers, FL.

Stewart, J.W.B., and Sharpley, A.N., 1987. Controls on Dynamics of Soil and Fertilizer

Phosphorus and Sulfur in Soil Fertility and Organic Matter as Critical

Components of Production Systems, SSSA Spec. Pub. No.19, 101-121.

Tiessen, H., 1995. Phosphorus in the Global Environment – Transfers, Cycles and Management.

John Wiley and Sons, NY.

Tisdale, S.L., Nelson, W.L., Beaton, J.D. and Havlin, J.L., 1993. Soil Fertility and Fertilizers.

Macmillan Publishing Co., NY.

Authors: Clyde Alsup and Michelle Armstrong, 1998, Asrat Shiferaw 1994, Jerry Speir, 1996

POTASSIUM

Form taken up by the plant:

K⁺

Mobility in the soil:

Mobility in the plant:

Yes

Deficiency symptoms:

Since K is mobile in the plant, visual deficiency symptoms usually appear first in the lower leaves, and progress to the top as the severity of the deficiency increases. Necrotic lesions on broadleaf plants, chlorotic and necrotic leaf margins on grasses, straw lodging in small grains, and stalk breakage in corn.

Role of nutrient in plant growth:

Enzyme activation, carbohydrate transportation, amino acid synthesis, starch synthesis, water relations, stomatal opening and closing, transpiration, photosynthesis, mass flow in absorpton, energy relations, ATP synthesis, translocation of assimilates, nitrogen uptake, protein synthesis, grain formation, tuber development, nutrient balancing, chlorophyll, disease and insect resistance, strengthening of roots and stems.

Role of nutrient in microbial growth:

Fulfillment of biological requirements similar to other organisms.

Enzymes:

Enzyme activation is regarded as the most important function of potassium. Over 80 plant enzymes require K for activation.

Concentration in plants:

5,000 to 60,000 mg/g (0.5 – 6.0%)

Distribution in the soil:

Mineral:

Non-exchangeable:

Exchangeable:

Soil solution:

5,000 – 25,000 mg/g

50 – 750 mg/g

40 – 600 mg/g

1 – 10 mg/g

Effect of pH on availability:

In very acid soils, toxic amounts of exchangeable Al³⁺ and Mn²⁺ create an unfavorable root environment for uptake of K⁺. The use of lime on acid soils low in exchangeable K⁺ can induce a K⁺ deficiency through ion competition.

Interactions with other nutrients:

K⁺ enhances NH₄⁺, NO₃^- and Cu²⁺ uptake, K⁺ decreases Ca²⁺ and Mg²⁺ in plant tissue, Ca²⁺ and Mg²⁺ decreases K⁺ in plant tissue, K⁺ reduces B uptake, K⁺ reduces Fe²⁺ toxicity, K⁺ enhances Mn²⁺ uptake when Mn is deficient and decreases uptake when Mn is present in toxic amounts, Na⁺ is capable of substituting for K⁺. K⁺ reduces Mo uptake, high NH₄⁺ with inadequate K⁺ may cause toxicity symptoms.

Fertilizer sources:

Potassium Chloride (KCl); Potassium Sulfate (K₂SO₄); Potassium Magnesium Sulfate (K₂SO₄, MgSO₄); Potassium Nitrate (KNO₃); Potassium Phosphates (KPO₃, K₄P₂O₇, KH₂PO₄, K₂HPO₄); Potassium Carbonate (K₂CO₃), Potassium Bicarbonate (KHCO₃), Potassium Hydroxide (KOH); Potassium Thiosulfate (K₂S₂O₃), Potassium Polysulfide (KS_x).

References

Alexander, M.A. 1977. Introduction to Soil Microbiology. 2^nd Edition. John Wiley & Sons, Inc. New York, NY pp. 382-385.

Dibb, D.W. and W.R. Thompson, Jr. 1985. Interaction of potassium with other nutrients. pp. 515-533 in R.D. Munson (ed.) Potassium in agriculture. Am. Soc. Agron.- Crop Sci. Soc. Am.- Soil Sci. Soc. Am. Madison, WI.

Kramer, P.J. and J.S. Boyer. 1995. Water relations of plants and soils, 2^nd Edition. Academic Press, Inc., San Diego, CA. pp. 263-264.

Raven, P.H., R.F. Everet, and S.E. Eiichhorn. 1986. Biology of Plants, 4^th Edition. Worth Publishing, Inc., New York, NY. p. 519.

Salisbury, F.B. and C.W. Ross. 1985. Plant physiology, 3^rd Edition. Wadsworth Publishing Co., Belmont, CA. p. 108.

Tisdale, S.L., W.L. Nelson, and J.D. Beaton. 1985. Soil fertility and fertilizers, 4^th Edition. Macmillan Publishing Co., New York, NY pp. 249-291.

Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil fertility and fertilizers, 5^th Edition. Macmillan Publishing Co., New York, NY pp. 230-263.

Authors: Dallas Geis 1994, Michael Goedeken 1996, Todd Heap and Matt Barnes 1998

IRON

Forms taken up by plants: Fe⁺² (Ferrous), while Fe⁺³ (Ferric) is reduced to Fe⁺² at the root surface before it is absorbed

Mobility in soil No

Mobility in plant No

Deficiency symptom in plant Interveinal chlorosis

Role in Plant nutrition Iron is a component of cells, proteins, and enzymes. It is involved in nitrogen fixation, respiration and photosynthesis.

Typical concentration in plant tissue 20-300 ppm

Fe Soil Test Chelation with EDDHA (ethylenediamineedi-o-hydroxyphenylacetic acid)

Fe is 100% complexed with EDDHA over a broad range of soil pH.

Fertilizer sources Foliar application of FeEDDHA or FeSO₄^.7H₂O

Oxidation/Reduction Oxidation Fe⁺² + 1/4O₂ + H⁺ àFe⁺³ + ½ H₂O

Reduction Fe⁺³ + e^- à Fe⁺²

Fe⁺³Forms of Iron Fe(OH)₃ amorphous

Fe(OH)₃ (soil)

Hematite Fe₂O₃ Goethite FeOOH

Soil Fe(OH)₃ is usually the most soluble form of iron in soils and, therefore, typically controls the solubility of iron in aerobic soils.

Fe⁺² Forms of Iron A common iron mineral in nature is pyrite (FeS₂). Pyrite is often associated with bituminous coal and other ores. Bacterial oxidation of pyrite generates acid and is the cause of acid mine drainage.

FeS₂ + 31/2O₂ + H₂O à Fe⁺² + 2SO₄^-2 + 2H⁺ . Fe⁺² hydrolyzes to form hydrolysis products common under reduced conditions. FeOH⁺predominates in solution at pH< 6.75, while Fe(OH)₂⁰ prdeominates at pH >9.3. Magnetite (Fe₃0₄) is a stable mineral under reduced conditions

Microbial use of iron Many organisms use Fe⁺³ as an electron acceptor such as some fungi and and chemoorganotrophic or chemolithtropic bacteria. This bacterial reduction of ferric to ferrous is a major way iron is solubilized. Reduction takes place under anaerobic conditions (waterlogged). Shewenella putrefaciens is one organism capable of reducing iron. Oxidation occurs under aerobic conditions. At neutral pH, organisms such as Gallionella ferruginea or Leptothrix oxidize iron. Under acidic conditions, Thiobacillus ferrooxidans is the primary organism responsible for iron oxidation. This organism is typical in acid mine drainage areas.

References:

Brock, T. D.; M. T. Madigan; J. M. Martinko; J. Parker. (1994). Biology of Microorganisms. Prentice Hall Englewood Cliffs, NJ.

Lindsay, W. L. (1979). Chemical Equilibria in Soils. John Wiley & Sons, NY.

Raun, W. R.; G. V. Johnson; R. L. Westerman. (1998). Soil-Plant Nutrient Cycling and Environmental Qualtiy. Plant & Soil Sciences 5813 class notes.

Tisdale, S. L.; W. L. Nelson; J. D. Beaton; J. L. Havlin. (1985). Soil Fertility and Fertilizers 5th edition. MacMillan Publishing Co. NY.

Walsh, L. M.; J. D. Beaton. (1973). Soil Testing and Plant Analysis. Soil Science Society of America, Inc. Madison, WI.

Authors: Fred Kanampiu 1994, Jing Chen, Jason Yoder 1996 and Libby Dayton 1998

SULFUR

Form taken up by plants: SO₄^2-, SO₂^- (low levels adsorbed through leaves)

Mobility in plant: Yes

Mobility in soil: Yes

Deficiency symptoms: Leaves chlorotic (upper leaves), reduced plant growth, weak stems

Role of nutrient in plant

and microbial growth Synthesis of the S-containing amino acids cystein, cystine, and methionine; Synthesis of other metabolites, including CoA, biotin, thiamine, and glutathione; Main function in proteins is the formation of disulfide bonds between polypeptide chains; Component of other S-containing substances, including S-adenosylmethionine, formylmethionine, lipoic acid, and sulfolipid; About 2% of the organic reduced sulfur is in the plant is present in the water soluble thiol (-SH) fraction; Vital part of ferredoxin; Responsible for the characteristic taste and smell of plants in the mustard and onion families; Enhances oil formation in flax and soybeans; Sulfate can be utilized without reduction and incorporated into essential organic structures; Reduced sulfur can be reoxidized in plants

Enzymes needing sulfur: Coenzyme A, ferredoxin, biotin, thiamine pyrophosphates, urease and sulfotransferases

Concentration in plants: 0.1 and 0.5% of the dry weight of plants

Effect of pH on availability: pH<6.5, AEC increases with decreasing pH

Interaction with other nutrients: Associated with salts and exchangeable cations, can be replaced by phosphorus on exchange sites

Fertilizer sources: Organic matter, ammonium bisulfite, ammonium nitrate-sulfate, ammonium phosphate-sulfate, ammonium polysulfide, ammonium sulfate, ammonium thiosulfate, ferrous sulfate, gypsum, magnesium sulfate, potassium sulfate, pyrites, potassium-magnesium sulfate, potassium thiosulfate, potassium polysulfide, sulfuric acid (100%), sulfur, sulfur dioxide, single superphoshate, triple superphosphate, urea-sulfur, urea-sulfuric acid and zinc sulfate

References:

Hartmann, H.T., Kofranek, A.M., Rubatzky, V.E., Flocker, W.J. (1988). Plant Science. 2nd ed. Prentice Hall. Englewood Cliffs, N.J.

Marschner, H. (1995). Mineral Nutrition of Higher Plants. 2nd ed. Institute of Plant Nutrition Univ. Hohenheim. Academic Press. San Diego, Ca.

Tisdale, S.L., Nelson, W.L., Beaton, J.D., and Havlin, J.L. (1993). Soil Fertility and Fertilizers. 5th ed. Macmillan Pub. Co. New York, NY.

Vaughan, D., Malcolm, R.E. (1985). Soil Organic Matter and Biological Activity. Martinus Nijhoff/Dr W. Junk Publishers, Dordrecht.

Authors: Xin Li, Dale Keahey and Jeremy Dennis

CARBON

Form taken up by the plant: CO₂

Mobility in soil: CO₂ mobile in soil pore space.

HCO₃^- mobile in soil solution.

Mobility in plant: --

Deficiency symptoms: --

Toxicity symptoms: --

Role in plant growth: Basic energy source and building block for plant tissues. Converted through photosynthesis into simple sugars. Used by plants in building starches, carbohydrates, cellulose, lignin, and protein. CO₂ given off by plant respiration.

Role in microbial growth: Main food of microbial population. Utilization by microbes is closely related to C:N ratio.

Concentration in plants: --

Effect of pH on availability: None

Interactions with other nutrients: 10:1 C:N ratio needed for stable soil organic matter. High C:N ratios lead to nitrogen immobilization. Low C:N ratios lead to nitrogen mineralization. N rates in excess of those required for maximum yield can lead to increased soil organic carbon.

Fertilizer sources: Crop residues, green manures and animal wastes can be significant sources of soil organic carbon.

References:

Detwiler, R.P., and C.A.S. Hall. 1988. Tropical Forests and the global carbon cycle. Science. 239:42-47.

Dixon, R.K., S. Brown, R.A. Houghton, A.M. Solomon, M.C. Trexler, and J. Wisniewski. Carbon pools and flux of global forest ecosystems. Science. 263:185 190.

Gillis, A.M. 1991. Why can’t we balance the globe’s carbon budget?. Bioscience. 41:442-447.

Schlesinger, W.H. 1998. “Chapter 2: An overview of the carbon cycle”. Soil processes and the carbon cycle. Boca Raton, Fla: CRC Press, c1998.

Wallace, A., G.A. Wallace, and J.W. Cha. 1990. Soil organic matter and the global carbon cycle. Journal of Plant Nutrition. 13:459-466

Author: Tyson Ochsner

Role of nutrient in plant growth: Required for cell wall rigidity, cell division of meristems and root tips, normal mitosis, membrane function, acts as a secondary messenger, aids in storage of phosphates in vacuoles, actively involved in photosynthesis and found in the endoplasmic reticulum

Concentration in plants: Fresh weight of plants typically contains 0.1-5.0%, can contain up to 10% dry weight in leaves before plant experiences toxicity

Deficiency symptoms: First seen in the younger leaves of plants, loss in plant structure, under extreme deficiencies gel-like conditions, root development no longer takes place, stunted plant growth

Interactions with other nutrients: Since Ca⁺² is so directly related to pH in solution, it effects all of the other nutrients. When NO₃-N is applied to soil, Ca⁺² absorption increases in the plant. Increases in Ca⁺² in soil decreases Al⁺³ in acid soils, as well as decreasing Na⁺ in sodic soils. Increases in Ca⁺² taken up by plants cause deficiencies of Mg⁺² and K⁺. MoO₄^-2 and H₂PO₄^- availability increases with increases in Ca⁺² concentrations.

Sources of Calcium: Lime (CaO) (Ca(OH)₂), Calcite (CaCO₃), Dolomite (CaMg(CO₃)₂, Gypsum (CaSO₄^.2H₂O), any Phosphorus fertilizer, Anorthite (CaAl₂Si₂O₃), biotite, apatite, augite & hornblende.

Amjad, Z. (ed.) 1998. Calcium Phosphates in Biological and Industrial Systems. Klower

Lindsay, W.L. 1979. Chemical Equilibria in Soils. John Wiley & Sons. New York, NY.

Marschner, H. 1995. Mineral Nutrition of Higher Plants. Academic Press. New York,

Tisdale, S.L., Nelson, W.L., Beaton, J.D. and Havlin, J.L. 1993. Soil Fertility and

Deficiency Symptoms: Interveinal chlorosis, necrosis, general withered appearance, leaves are stiff and brittle and intercostal veins are twisted.

Deficiencies: pH 5.0 is best for Mg availability. A higher or lower pH depresses Mg uptake. High K and Ca levels also interfere with uptake.

Where deficiencies occur: Highly leached humus acid soils or on sandy soils which have been limed heavily (due to Ca²⁺ competition). sometimes on soils high in K; Mg deficiencies are indicated by soil test index values less than 100 lbs/A.

Role of Mg in Plant Growth: Responsible for electron transfer in photosynthesis; Central element of chlorophyll molecule (6-25% of total plant Mg); Required for starch degradation in the chloroplast; Involved in regulating cellular pH; Required for protein synthesis; Required to form RNA in the nucleus; Mg-pectate in the middle lamella

Role of Nutrient in Microbial Growth: Important for phosphorus metabolism; Helps to regulate the colloidal condition of the cytoplasm.

Interactions with other nutrients: Uptake of K⁺, NH₄⁺, Ca ²⁺ , Mn²⁺ by plant limits Mg²⁺ uptake; H⁺ (low pH) can limit Mg²⁺ uptake; Mg salts increase phosphorus adsorption

Fertilizer Sources: Dolomite (MgCa(CO₃)₂) (most common); Magnesium sulfate (MgSO₄ x H₂O) (Kieserite); Magnesium oxide (Mg(OH)₂) (Brucite); Magnesite (MgCO₃); Magnesia (MgO); Kainite (MgSO₄ x KCl x 3H₂O); Langbeinite (2MgSO₄K₂SO₄); Epsom Salts (MgSO₄ x 7H₂O)

Enzymes that require Mg++: Magnesium is a co-factor for many enzymes. This includes enzymes involved in glycolysis, carbohydrate transformations related to glycolysis, Krebs cycle, the monophosphate shunt, lipid metabolism, nitrogen metabolism, “phosphate pool” reactions, photosynthesis, and other miscellaneous reactions.

Examples: ATPase (phosphorylation), phosphokinases; RuBP carboxylase (photosynthesis); Fructose 1,6-phosphatase (starch synthesis in chloroplasts); Glutamate synthase (ammonia assimilation in the chloroplasts); Glutathione synthase; PEP carboxylase

Jacob, A. 1958. Magnesium - the fifth major plant nutrient. Staples Press Limited, London.

Johnson, G.V., W.R. Raun, and E.R. Allen. 1995. Oklahoma Soil Fertility Handbook. 3rd ed. Okla. Plant Food Educational Society and Okla. State Univ. Dept. of Agronomy, Stillwater, OK.

Lauchli, A. and R.L. Bieleski (editors). 1983. Inorganic Plant Nutrition. Springer-Verlag, Berlin.

Marschner, H. 1986. Mineral Nutrition of Higher Plants. 2nd ed. Academic Press, London.

Mengel, K. and E.A. Kirkby. 1978. Principles of Plant Nutrition. International Potash Institute, Bern.

Deficiency symptoms: Boron deficient plants exhibit a wide range of deficiency symptoms, but the most common symptoms include necrosis of the young leaves and terminal buds. Structures such as fruit, fleshy roots and tubers may exhibit necrosis or abnormalities related to the breakdown of internal tissues

Interactions with O.M.: Boron is complexed by O.M. and can be a major source of B to plants. Mineralization of O.M. releases boron to soil solution. The mineral source of boron in soils is Tourmaline, which is a very insoluble borosilicate mineral.

Effect of pH on availability: Boron availability decreases with increasing pH. Overliming acid soils can cause boron deficiency because of interaction with calcium.

Role of Soil characteristics Boron is generally less available on sandy soils in humid regions, because of more leaching. This is especially true in acid soils with low O.M. Boron availability increases with increasing O.M. Most alkaline and calcareous soils contain sufficient Boron because the primary boron minerals have not been highly weathered and, more important, B products of weathering (H₃BO₃) have not been leached out as in humid region soils.

Role of Boron in plants: Cell growth and formation. The action appears to be in binding sugars together. Indirect evidence also suggests involvement in carbohydrate transport.

Toxic levels in soil & water: Boron can be toxic on some alkaline soils when soil test or extractable boron exceeds 5 ppm. Irrigation water that contains > 1ppm boron can also produce toxicity.

Other Sources of B: Animal wastes: 0.01 to 0.09 lb/ton of waste @ 72-85% moisture.

Mortvedt, J.J. 1972. Micronutrients in Agriculture. Soil Science Society of Americia, Madison, Wisconsin.

Philipson, Tore. 1953. Boron in Plant and Soil with special regard to Swedish Agriculture. Acta Agriculturae Scandinavica. III:2.

Raun, W.R., G.V. Johnson, and S.L. Taylor. 1996. Soil-Plant Relationships, Oklahoma State University Agronomy 5813 class notes.

Tisdale S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil Fertility and Fertilizers. 5th ed. MacMillan Publishing Co. New York, NY.