Chapter 8 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, 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 U-Cal
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+
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.
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
S oxidation - reduction reactions • Anaerobic environments produce H2S (rotten egg smell) (S link) • 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
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)
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.
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 (Ca 2+ ) 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 Ca translocated 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
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++).
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.
Magnesium • Fertilizer Mg • K-Mag (11% Mg) • Aglime (most aglime contains some MgCO3) • Dolomitic lime (contains significant amounts of MgCO3)
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
Solubility (review) Solubility of a substance:quantity that dissolves to form a saturated solution (g of solute/L) Solubility product: Equilibrium constant for the equilibrium between an ionic solid and its saturated solution Solid AgCl is added to pure water at 25C. Some of the solid remains undissolved at the bottom of the flask. Mixture stirred for 2 days to ensure an equilibrium is reached. Ag+ conc. Determined to be 1.34x10-5M. What is Ksp (solubility product constant) for AgCl? AgCl Ag+ + Cl- Ksp = [Ag+][Cl-] At equilibrium, conc of Ag+ = 1.34 x 10-5 conc of Cl- = 1.34 x 10-5 Ksp = (1.34 x 10-5)(1.34 x 10-5) = 1.80 x 10-10
Aluminum • The “apparent solubility” product constant (Ksp) for Al(OH)3 in soils is about 10-30. From this, the concentration of Al+++ in the soil solution and its change with change in pH can be calculated.
Solving the above at pH of 5, OH- would be equal to 10-9 The concentration of Al+++ (10-3) is moles/liter. Since the atomic weight of Al is about 27, a mole/liter would be 27 grams/liter (g/L) and the concentration of 10-3 is equal to 0.027 g/L, or 27 ppm. 27 ppm at a pH of 5
Solubility • Critical to the management and growth of plants in acid soils is the knowledge that Al+++ in the soil solution increases dramatically with decrease in pH below about 5.5. When solved for a soil pH of 4.0 (OH- is equal to 10-10), we have A concentration of 1.0 mole/L is equal to 27 g/L or 27,000 ppm. While there may not be a 1000-fold increase in soil solution Al 3+ concentration when pH changes from 5.0 to 4.0, these calculations should make it clear why Al 3+ concentrations may be significant at pH 4.5, for example, and immeasurable at 5.5.
Al toxicity • Soluble Al is toxic to winter wheat at concentrations of about 25 ppm. • Adverse effect of soil acidity on non-legume plants is usually a result of Al and Mn toxicity. • In winter wheat, Al toxicity inhibits or “prunes” the root system and often causes stunted growth and a purple discoloration of the lower leaves. • These symptoms are characteristic of P deficiency, and are likely a result of the plants reduced ability to extract soil P. Laboratory exercise, applying P to decrease Al toxicity?
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
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?
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
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.
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
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.
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
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
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)
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