Soil Fertility and Plant Nutrition

1. Fertile soil / Productive soil

 Provides all necessary soil factors in balanced proportion: air, water, T, nutrients in needed amounts, forms and proportions.

 2. Essential Elements: 3 conditions

 a. Deficiency or low level results in incomplete life cycle,

b. Specificity

c. Required by most plants

90 can be absorbed, but only 16 are E.

 Macronutrients: >1000 ppm in plant

CHO    NPK          CaMgS

 primary         secondary

Micronutrients < 100ppm

Fe, Mn, Cu, Zn, B, Mo, Cl

See table 8-1, p251: Forms and content

3. Nutrient flow in the biosphere

Three stores:

a. Inorganic (soil)

b. Biomass (organic living)

c. Organic matter (inanimate organic)

4. C, H, O: Manufacturing of CHO’s

 • Respiration: CO2(air), H2O (H)

 CO2 + H2O —–> H+ + HCO3- products exchange for soil ions

5. Nitrogen

 Key element: proteins

NH4+, NO3-

 Sources:

• Athmospheric fixation: chemical and biological (symbiotic and non-symbiotic)

• Organic matter

•Fertilizers: NH₄NO₃, (NH₄)₂SO⁴, NaNO₃, KNO₃, CO(NH₂)₂

Details

Ø  Nitrification: pros and cons

Ø  Denitrification: losses 10%-20% up to 60%

·         Promoted by low O2, OM, warm soil

Ø  Ammonia volatilization:

·         Basic solution +NH4+ (calcareous soils).

·         Affects NH₄⁺ and CO(NH₂)₂

·         Up to 30% losses

·         Prevented by irrigation after application, incorporation.

Ø  Other losses: Leaching, Erosion, Crop removal. 

Ø  Deficiency: Poor yields, chlorosis.

Ø  Excess: Spongy tissue, no ripening, disease succeptibility.

6. Phosphorus

Ø   Nucleoprotein, ATP, flowering, cell division

Ø   In soils, 0.1%, not easily available, origin as apatite (Ca) or Al,Fe complexes

Ø  Phosphorus fixation: with Ca in basic soils and with Fe and Al in acid soils.

Ø  Adsorption on clays, CaCO₃, (Fe, Mn, Al) oxides.

Ø  Organic P: Released by Phosphatase, plant and microbial origin.

§  C:P <200:1: release of P

§  C:P > 300:1 P is immobilized

Ø   Optimal soil pH for P availability is 6.5

Ø   Deficiency: Poorly developed root system. Leaves: purple, retarded flowers. Low absorption in cool weather.

Ø    Management: pH, OM, Banding(20% extra efficiency).

7. Potassium: K+. High in plants (2%), low in OM (except grasses). Important in cell division, cell permeability, disease resistance, CHO’s synthesis, fruit quality.

§  Needed during early growth

§  Sources: Feldspars and Micas

§  In soils, K+ can be found as:

Adsorbed K+: Kx

Solution K+: Ks

Fixed between clay layers: Knx or Knex

Available K: Ks+Kx, but Knex can be released

§  As pH decreases, CEC decreases, and available K decreases.

§  Most non-acid soils will contain sufficient amounts of available K.

§  Luxury uptake of K

§  Deficiency symptoms: burning of leaf edges, marginal necrosis.

 8. Calcium:

 Ca++, seldom deficient except in humid, acid and sandy soils.

§  Fertilizers: Calcium ammonium phosphate (superphosphates)

§  Needed for growth of root tips, cell division, cell wall, fruit and seeds, NO3- absorption.

§  Used as a soil conditionner

§  Excess may supress K and Mg uptake

§  In soil: Cax + Cas = Ca available

9. Magnesium: origin: micas, dolomites

§  Role in chlorophyll and as enzyme activator, sugar formation

§  Grass tetany: Hypomagnesemia

§  Available Mg = Mgx + Mgs

10. Sulfur: origin: pyrite FeS₂, Gypsum

OM, irrigation water, acid rain.

   Important for protein synthesis: methionine

§  Role in nodule formation

§  Soil acidulation S —> H₂SO₄

§  In arid areas precipitates as gypsum

§  Anaerobic conditions: precipitates as FeS, CuS

§  May be lost by leaching in sandy soils

 11. Micronutrients or Trace Elements

 Plants are very sensitive to changes in [MN]. pH affects their availability strongly

 a. Fe: Chlorophyll, but not part of molecule. Deficiency: chlorosis on young leaves, mostly trees and plantation. Correct by pH change, chelates application.

b. Zn: Chlorophyll, but not part of molecule. Deficiency: stunting

c. Mn: Chlorophyll molecule, catalyst, Ca, Mg, P availability. Deficiency: interveinal chlorosis

d. Cu: Redox, vit A. Strongly adsorbed to OM

e. Boron: associated with Ca++ uptk and rhizobia. Improves fruit and flower quality. Deficiency: stop growing point, dwarfing. Toxicity in arid zones.

f. Mo: In N fixation. Opposite of metals: as pH increases, [Mo] increases

g. Cl-: Function? Deficiency rare- Toxicity

12. Deficiency symptoms made easy: Mobility and Immobility of nutrients

§  Ca, Fe, Mn, Cu, Bo, Cl: immobile, symptoms on young parts of plants

§  N, P, K, Mg, S, Zn, Mo: mobile, symptoms on lower or older parts.