Basic Soil Plant Relationships Fundamentals of Nutrient Management Training Course Dec. 14, 2005 Jim Gorman West Virginia University

Nutrient Availability Plant availability of nutrients is controlled by: Physical Chemical Biological properties of soils

Reactions Controlling Nutrient Availability Nutrient uptake by plants Adsorption and exchange reactions Precipitation and dissolution reactions Nutrient additions –Fertilizers, Manure, Crop residues –Atmospheric additions Organic Decomposition Oxidation and Reduction reactions

Arnon’s Criteria for Essential Plant Nutrients Plant cannot complete its life without that element An element cannot be substituted by another element It is required for a specific metabolic function

Essential Plant Nutrients Sixteen Mineral Nutrients (13) Carbon, Hydrogen, and Oxygen from Air or Water Macronutrients (6) Micronutrients (7)

Macronutrients Fertilizer Elements –Nitrogen (NH 4 +, NO 3 - ) –Phosphorus (H 2 PO 4 -, HPO 4 2- ) Potassium (K + ) Secondary Nutrients –Calcium (Ca 2+ ) –Magnesium (Mg 2+) –Sulfur (SO 4 2- )

Micronutrients Cations –Iron(Fe 2+, Fe 3+ ) –Copper (Cu 2+) –Manganese(Mn 2+ ) –Zinc(Zn 2+ )

Micronutrient Anions Molybdenum(MoO 4 2- ) Boron(H 3 BO 3 ) Chlorine(Cl - )

Elements Required by Some Plants Sodium(Na) Cobalt(Co) Vanadium(V) Nickel(Ni) Selenium(Se)

Plant Nutrient Concentrations

Nutrient Concentrations

Leibig’s Law of Minimum Production can never be more than that allowed by the most limiting plant nutrient

Essential Nutrients Both macro and micronutrients are equally important

Ion Exchange in Soils Ion are concentrated on the surfaces of clays Mostly cations are concentrated on negatively charged clay surfaces Exchange is reversible Ion exchange replenishes soil solution when nutrients are taken up by plants

Reversible Reactions Clay-2Al Ca 2+ => Clay-3Ca 2+ +2Al 3+

Cation Exchange Capacity The total amount of exchangeable cations that can be held on the surfaces of soil particles. Cation exchange capacity of soils is expressed as milliequivalent per 100 g of soil at neutral pH.

Milliequivalent An equivalent is a mole of charge and a mole is = X A milliequivalent is 1/1,000 th of an equivalent

Anion Exchange Capacity Source of AEC –Oxides and Hydroxides of Iron and Aluminum –Kaolinite Clay AEC increase as pH decreases Strength of adsorption –H 2 PO 4 - >SO 4 2- >NO 3 - >Cl -

Basic Cations Ca 2+ Mg 2+ Na + K +

Base Saturation Base Saturation % = (Total Bases/CEC) X 100 Or [(Ca + Mg + Na + K)/ CEC] X 100

Sources of Negative Charge on Clays Isomorphic substitution --Si replaced by Al e.g., Mica Vermiculite --Al replaced by Mg e.g., Montmorillonite pH Dependent Charge --Oxides of Iron and Aluminum --Kaolinite --Organic Matter

Structure of Clay Minerals Basic Units Are: Silicon Tetrahedron: A single cation (Si) bonded to four oxygen --Tetrahedron sheets are formed by sharing of Oxygen Aluminum Octahedron: A single cation (Al or Mg) bonded to 6 oxygens

Silica Tetrahedron

Tetrahedrons Sharing oxygens

Aluminum Octahedron

Sharing Octahedrons

Tetra Sharing Octahedron

Types of Clay Minerals Kaolinite (1:1) Mica (2:1) Illite (2:1) Vermiculite (2:1) Montmorillonite/ Smectite(2:1) Chlorite (Mg in the interlayer) (2:1:1)

Kaolinite 1:1 Clay Mineral No Isomorphic substitution All charge is pH dependent No internal surface for adsorption CEC is meq/100 g AEC (40 meq/100g at pH 4.7)

Kaolinite

Mica/Illite 2:1 clay mineral Isomorphic substitution in tetrahedral layer Charge balanced by potassium No internal surfaces Low pH dependent charge CEC 20 to 40 meq/100g AEC less than 5 meq/100g

Mica

Vermiculite 2:1 Clay mineral Isomorphic substitution in both tetrahedral and octahedral layers Low pH dependent charge Large internal surface area CEC 120 to 150 meq/100g AEC is less than 5 meq/100g Expanding clay minerals

Montmorillonite/smectites 2:1 Clay Minerals Large internal areas Substitution in the Al octahedral layer No K fixation Expanding type clays Low pH dependent charge CEC 80 to 120 meq/100g AEC less than 5 meq/100g

Chlorite 2:1:1 Clay Mineral High pH dependent charge Interlayer has magnesium hydroxide CEC 20 to 40 meq/100g No internal surface for exchange No K fixation

Organic Matter All pH dependent charge COOH, OH, and NH2 groups responsible for charge CEC meq/100g

CEC of Soils Sands3-5 meq/100g Sands (dark colored)10-20 meq/100g Loams10-15 meq/100g Silt Loams15-25 meq/100g Clay and clay loams20-50 meq/100g Organic Soils meq/100g

Lyotropic Series (Strength of Adsorption) Al 3+ >Ca 2 +>Mg 2+ >K + =NH 4 + >Na + --Charge of the ion --Size of the ion

Root CEC Grasses meq/100g -- Prefer monovalent Ions e.g., K + Legumes meq/100g -- Prefer divalent e.g., Ca 2+

Nutrients Levels in the Plants Deficient: When the concentrations of an essential element is low enough to severely limit yield and deficiency symptoms may or may not be visible. Critical Range: Concentration in the plant tissue below which response to added nutrients occurs.

Nutrients Levels in Plants Sufficient: The nutrients concentration range in which added nutrient will not increase yield but can increase concentration in the plant tissue. (Luxury Consumption). Excessive or Toxic: When the concentration of the essential nutrient is high enough to decrease yield.