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Chapter 9

Chapter 9. Soil and Fertilizer S, Ca, and Mg. Soil and Fertilizer S. Source of soil S (total content in soils may range from a few 100 to several thousand lb S/acre) Metal sulfides (e.g. FeS, pyrite) Gypsum, CaSO 4 . 2H 2 O (“neutral salt”) Elemental S Atmosphere, SO 2

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Chapter 9

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  1. Chapter 9 Soil and Fertilizer S, Ca, and Mg

  2. Soil and Fertilizer S • Source of soil S (total content in soils may range from a few 100 to several thousand lb S/acre) • Metal sulfides (e.g. FeS, pyrite) • Gypsum, CaSO4. 2H2O (“neutral salt”) • Elemental S • Atmosphere, SO2 • Contributes about 6 lb S/acre/year by rainfall in Oklahoma • Most (70%) is from natural causes, such as volcanoes • Ocean, 2700 ppm SO4= (0.27%) • Irrigation additions: for every 1 ppm of SO4-S (or anything else, for that matter) in the irrigation water, there will be added 2.7 lb/acre to the soil with each acre-ft of irrigation. • 2.7 X ppm S = lb S/acre foot of irrigation 1 acre foot = 325,851 gallons Water 8.34 lbs/gallon= 8.34 * 325,851 = 2,717,597 lbs

  3. Solution S • Present as SO4= in amounts ranging from between 1 to 100 ppm • Form absorbed by plants • Concentration changes rapidly depending upon uptake and leaching • In equilibrium with solid forms, like gypsum in arid and semi-arid soils • Saturated gypsum (CaSO4 2H2O) solution contains about 2,410 ppm gypsum, or about 450 ppm SO4-S. This is more than enough to meet the needs of vigorously growing plants. • Relatively mobile in soils • Leaches in conjunction with cations like K+, Na+, Ca2+, and Mg2+

  4. Exchangeable SO4 • Exchangeable SO42- • Most important in highly weathered acid soils that have a high (or significant) anion exchange capacity • Organic S • In non-calcareous soils, S behaves in soils like N, and organic matter accounts for >90% of total soil sulfur • C, N, and S are closely related in soil organic matter, with common ratio of about 12:1:0.14, and N:S of about 7:1 • For every 7 lb of N mineralized there may be an associated 1 lb of S mineralized • Mineralization and immobilization of S is similar to that for N as far as factors affecting the processes, the end product, and effect of the processes relative to plant available S.

  5. Precipitated sulfate compounds • In calcareous soils SO42- precipitates as sparingly soluble gypsum and epsomite (MgSO4. 7H2O). • Equilibrium solution SO42- concentration far exceeds that required to meet plant requirement • Subsoils, even in humid climates, usually are much higher in SO4= concentration than surface soil, especially if there is an accumulation of clay in the B horizon

  6. S oxidation - reduction reactions • Anaerobic environments produce H2S (rotten egg smell) S response? # of years? • S emissions • Elemental So can be oxidized by thiobacillus sp. in the presence of oxygen as described by the general reaction • So + 1½ O2 + H2O ========> H2SO4 ===> 2 H+ + SO42- • This provides a source of available SO42- as well as an acidifying effect on the soil • The reaction is slow and usually requires several weeks/months to affect a change in soil pH H2S lethal

  7. Soil Test S • Usually measures water soluble and easily exchangeable SO42- - S • Saturated calcium phosphate • Ammonium acetate • Only of importance in humid regions, even then soil test is of questionable value • Often recommend blanket S fertilizer for sandy, low organic matter soils • Fertilizer S • Many minor formulations, most economical is gypsum (17% S) • K-Mag (22% S) • Ammonium Sulfate (21-0-0, 24S) • Slow release fertilizer forms include • animal waste • gypsum • S-coated urea (about 25 %S)

  8. Crop Use and Deposition • SO4 08/09 Cl 08/09 Ca 08/09

  9. Soil and Fertilizer Calcium • Soil Ca • Content depends upon mineralogy, rainfall and CEC to a greater degree than for K and Mg • Ca bearing minerals, except for carbonates and sulfates, are too slowly weathered to supply crop needs as a sole source. However, this is seldom a circumstance. • High rainfall leaches Ca out of soil over geologic time, however, plant growth and the consequent recycling (most plants contain relatively high amounts of Ca (.5%)) continually replenishes the surface where Ca is held on CEC.

  10. Soil and Fertilizer Calcium • Soil solution • May contain from 30-300 ppm. For corn 15 ppm Ca in soil solution is related to max yield. • Mass flow (because solution concentration is usually high) and root interception are major uptake mechanisms. • Deficiency is uncommon • Low supply of available Ca (Ca2+ ) is associated with very acid soil. Correction of acidity (addition of CaCO3) usually supplies more than enough available Ca. • Some crops may have difficulty getting enough Catranslocated to plant parts with high demand under certain special circumstances (e.g. peanuts). • Soil test by determining exchangeable Ca, similar to that for K. • Fertilizer Ca • Use lime or gypsum

  11. Soil and Fertilizer Mg • Magnesium behavior in soils is more like calcium than any other element. As a general rule, Mg salts (compounds) are usually slightly more soluble than Ca salts (also, Mg2+ is less tightly held on exchange complex than Ca++).

  12. Magnesium • Soil Mg • Content varies with parent material and climate (rainfall) under which soil developed. • Ranges from a few 1000 ppm to a percent or more. • Acid, highly leached soils are lowest and most likely to be deficient. • Exchangeable Mg is most important available form • Deficiency is more common than for Ca • May be a result of low CEC, high rainfall, and abundant Ca. • Grass tetany (hypomagnesmia) is a disease or malady of livestock that have low Mg blood levels relative to K and Ca. • Most economical remedy is to supply Mg supplement “free choice” to livestock. • Soil test is determination of exchangeable Mg, using same extraction as for K and Ca.

  13. Magnesium • Fertilizer Mg • K-Mag (11% Mg) • Aglime (most aglime contains some MgCO3) • Dolomitic lime (contains significant amounts of MgCO3)

  14. MICRONUTRIENTS • Fe, Zn, Mn, and Cu • All are absorbed by plants as the metal cation • All are immobile in soils • All form relatively strong chelates, both naturally and synthetically • strength of formation (strength with which the metal ion is held) is in the order Cu>Fe>Zn, Mn

  15. Iron • Soil Fe • Total content • ranges from 20,000 to 100,000 lb Fe/acre. • most is present as Fe2O3 3H2O, which may also be written as 2 Fe(OH)3. • Soil solution Fe • Amount of Fe2+ and Fe3+ in soil solution is extremely small in all normal soils and is governed by the following reactions

  16. Iron

  17. Iron At pH of 7.0=Fe+++ = 10-39/(10-7)3=10-39/10-21=10-18 moles/literwith an atomic weight of 55.85 (Fe)Conc. in ppm = 55.85 g/mole * 1000mg/g * 10-18 moles/l =55.85 x 10-15 mg/l = 55.85 x 10-15 ppm pH of 5.0: 55.85 x 10-7 ppm (critical amount = 10-6) why aren’t plants deficient at pH 5?

  18. Iron • Plant uptake. • chelates (claw-like organic chemical structures that hold metal ions tightly, e.g. Fe in heme, Mg in chlorophyll) • Chelates improve the mobility of metal ions because the metal-chelate complex is water soluble. • Chelates are naturally occurring in soils. (fulvic and humic acids) • Synthetic chelates are sometimes used as fertilizer. • Chelate acts like conveyor belt between Fe(OH)3 and plant root surface

  19. Iron

  20. Iron Many plants are capable of causing Fe to become more available if they experience a deficiency. This is most common in dicots and is called “adaptive response mechanism”, triggered by Fe deficiency. • increased production of organic acids • increased production of chelates • Fe Soil test. • Most common is extraction of soil using a synthetic chelate, DTPA. • critical level is 4.5 ppm.

  21. Iron • Crop deficiencies • Crop specific, usually only on high pH soils (>7.5) • Sorghums and sorghum-sudan are most sensitive • Fertilizer • Increase natural chelation by adding organic matter to soil (feedlot manure, rotten hay, etc.) is most effective long-term remedy. • Soil applied compounds quickly becomes unavailable if they are soluble inorganics (e.g. Fe SO4) or are too expensive if they are synthetic chelates • chelates may be economical for high value crops (horticultural) • Foliar application is temporarily effective

  22. Zinc • Zn • Deficiencies are uncommon • Often a result of high pH, low soil organic matter • corn is most sensitive cultivated crop • Soil test • DTPA • Critical level depends upon crop • 2 ppm for pecans • 0.8 ppm for corn • 0.3 ppm for other sensitive crops • 0.0 for wheat! • Fertilize using ZnSO4, 2-6 1b Zn/ac.

  23. Manganese and Copper • Mn, Cu: Deficiencies are rare • Cu deficiency most common in high organic matter soils. • Strong chelate complex formed between organic matter and Cu. • Mn toxicity may be more common than deficiency • Low pH soils (<4.5) • Frequently flooded conditions (rice). • DTPA soil test • Fertilize using sulfate salt

  24. Cloride • Cl: Deficiency extremely rare • Response to Cl is often confounded with disease suppression • Limited to regions that do not receive Cl in rainfall or use KCl fertilizer for correcting K deficiency. • Soil test is water extraction of Cl-, critical level is about 40 lb/ac 2 ft. deep • Fertilizer is 0-0-62, KCl

  25. Boron • B: Deficiency is limited to sandy, low organic matter soils in areas of high rainfall • H3 BO3 is mobile in soils. • Shallow rooted crops are most sensitive to deficiency (e.g. peanuts) • Soil test is hot-water soil extraction • Fertilizers are sodium and calcium borate (borax)

  26. Molybdenum • Mo: Deficiency is limited to areas of low soil Mo or where soils are highly weathered and acidic. • availability strongly linked to soil pH. • deficiencies can often be corrected by liming • Large seeded legumes can receive adequate supply from “normal” seed to meet season requirement. • Deficiencies are so rare that a reliable soil test has not been developed • Fertilization requirement is extremely small • Seed coating of ammonium molybdate is adequate

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