7: SOIL FERTILITY

Soil fertility

The study of supplying plant nutrients from soil.

CEC

Understanding __ is the single most important concept in understanding soil fertility.

Soil fertility

Nutrient analysis of the soil determines the potential of the soil for supplying:

• N, P, K, Ca, Mg, S, plus micronutrients to plants during the growing season.

Soil fertility

The quality of a soil that enables it to provide essential chemical elements in quantities and proportions for the growth of specified plants.

Soil productivity

The capacity of a soil for producing a specified plant or sequence of plants under a specified system of management.

Soil fertility

__ is only one of the factors that make a soil productive;

• other factors include: moisture, aeration, pests and diseases, presence of organisms, management practices, etc.

• C, H, O, N, P, K, Ca, Mg, S

• Fe, Mn, Cu, Zn, B, Mo, Cl, Co, Ni

There are 18 nutrient elements that are considered essential for plant growth. These are:

Macronutrients

The elements that are needed by plants in relatively large amounts than other nutrients.

C, H, O, N, P, K, Ca, Mg, S

Macronutrients/ major elements:

Micronutrients

Elements needed by plants only in very small amounts.

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

Micronutrients/ minor/ trace elements:

Other nutrient elements

Some plants apparently either need/can benefit from other elements such as:

• silicon, sodium, iodine, fluorine, barium, and strontium.

• not considered to be generally essential elements for plant growth.

Essential Nutrient Elements

These elements exist in soil:

• in combination with organic compounds – in the complex structure of minerals

• in salts in the soil solution

When the organic and inorganic compounds decompose and the solutes dissociate into their component ions,

• the nutrients then become available for absorption by plants/are adsorbed on colloid surfaces.

1. Nitrogen – NH4+, NO3-

2. Phosphorus – H2PO4-, HPO42-, PO4

3. Potassium – K+

4. Calcium – Ca2+

5. Magnesium – Mg2+

6. Sulfur – SO42-

7. Iron - Fe2+, Fe3+

8. Manganese – Mn2+

9. Boron – H3BO3, H2BO3-

10. Zinc – Zn2+

11. Copper – Cu2+

12. Chlorine – Cl-

13. Cobalt – Co2+

14. Molybdenum – MoO42-

15. Nickel – Ni2+, Ni3+

Forms that are Available for Plant Use (15)

Essential Nutrient Criteria

1. Plants cannot complete their lifecycle in the absence/deficiency of any one of the nutrient elements;

2. The nutrient is an integral component of a plant structure and/or participates in one or more metabolic processes in the plant; and

3. No other element can substitute for the element if it is absent/lacking in supply.

   • its deficiency can only be corrected by addition of that element.

Nitrogen (N)

• Involved in the building up of plant tissues and protein synthesis

  • determines over-all growth.

• A major component of the green pigment chlorophyll.

• Its deficiency causes stunting of growth and general yellowing of leaves (chlorosis).

Phosphorus (P)

• An important component of phospholipids, nucleic acids, and coenzymes.

• Involved in energy transfer in various metabolic processes as part of adenosine triphosphate (ATP).

• Important in the development of:

  • roots

  • flowering

  • grain formation and development

• Expression of its deficiency:

  • purpling in corn leaves cause by pigment anthocyanin

  • roots are short and branched

  • grains do not fill completely

  • some plants mature too early when P is deficient.

Potassium (K)

• Regulates the important processes such as:

  • photosynthesis

  • translocation of carbohydrates

  • protein synthesis

  • uptake and loss of water

• It is closely linked to:

  • vigor and resistance of plants to diseases

  • quality of fruits

  • strength of fibers

  • synthesis of oil, sugar, starch, and carbohydrates.

• Its deficiency is manifested by plants in many ways;

  • foremost of which is the drying up of leaf tips and margins and general loss of vigor.

Calcium (Ca)

It is believed to form the substance calcium pectate and is important in:

• strengthening of cell structure

• permeability of cell membranes.

Magnesium (Mg)

• Occupies the center of the chlorophyll molecule and is an enzyme activator.

• Its deficiency in plants also causes chlorosis;

  • yellowing occurs between the leaf veins (interveinal)

Sulfur (S)

• Important component of amino acids and proteins.

• A constituent of coenzyme A.

• Its deficiency - causes general yellowing of plants.

Micronutrients

Involved mostly as:

• activator of enzyme reactions

• respiration

• various metabolic processes

Iron (Fe)

• Present in several peroxidase, catalase, and cytochrome oxidase enzymes

• Found in ferredoxin

  • participates in oxidation-reduction reactions

  • e.g. NO3- and SO42- reduction and N fixation

• Important in chlorophyll formation

Manganese (Mn)

• Activates decarboxylase, dehydrogenase, and oxidase enzymes;

• Important in:

  • photosynthesis

  • N metabolism

  • N assimilation

Copper (Cu)

• Present in laccase and several other oxidase enzymes;

• Important in:

  • photosynthesis

  • protein and carbohydrate metabolism

  • N fixation

Boron (B)

• Activates certain dehydrogenase enzymes

• Facilitates sugar translocation and synthesis of nucleic acids and plant hormones

• Essential for cell division and development

Molybdenum (Mo)

• Present in nitrogenase (N fixation) and nitrate reductase enzymes;

• Essential for N fixation and N assimilation.

Chlorine (Cl)

• Activates the system for the production of O2 in photosynthesis.

• Has been found to be of critical importance in coconut nutrition.

Zinc (Zn)

• Present in several dehydrogenase, proteinase, and peptidase enzymes

• Promotes growth hormones and starch formation

• Promotes seed maturation and production

Nickel (Ni)

• Essential for:

  • urease

  • hydrogenases

  • methyl reductase

• Needed for grain filling, seed viability, iron absorption, and urea and ureide metabolism

  • to avoid toxic levels of these N-fixation products in legumes

Cobalt (Co)

• Essential for N-fixation

• Found in vitamin B12

1. – OM

2. – minerals

3. – air

4. – water

The essential nutrient elements are derived from: (sources of nutrient elements) (4)

1. – Carbon – CO2

2. – Hydrogen – H2O

3. – Oxygen – O2, H2O

Sources of C, H, O: (3)

Nitrogen

• The air partly supply N:

  • by biological N fixation

  • when lightning converts it to NO3-

  • brought down when rains fall

• The major source of N is OM;

  • about 5% N

  • Most minerals do not contain N.

Phosphorus

• P is released from SOM;

  • – about 1% P

  • – bound in phytin, phospholipids, and nucleic acids.

• The major inorganic sources in the soil are:

  • – acid-soluble P

  • – calcium phosphate

  • – aluminum phosphate

  • – iron phosphate

  • – reductant soluble P

Potassium (K)

• found in minerals like feldspars and micas (90% of soil K);

• fixed inside of clay minerals (9% of soil K);

• on the soil exchange sites (1%); and

• in the soil solution (0.1%)

Sulfur

• Contained in OM;

  • about 1%

• Present in minerals;

  • i.e. gypsum and pyrite

• When released into the soil the forms are:

  • H2S, FeS, and SO42-

Other nutrients elements

• Other nutrient elements are present in OM at very low concentrations.

• Most comes from the weathering of minerals:

  • Ca = found in hornblende, plagioclase, dolomite, and calcite.

  • Mg = occurs in hornblende, dolomite, and biotite.

Micronutrients

are derived from various minerals

Iron (Fe)

• Among the most abundant of micronutrients;

  • from limonite, hematite, goethite, etc.

Chlorine (Cl)

• Contained in minerals

• May also be supplied from:

  • salt sprays from oceans

  • irrigation water

Exchangeable cations

available to both higher plants and microorganisms

By cation exchange

H+ from the root hairs and microorganisms replace nutrient cations from the exchange complex

Cation Exchange and Nutrient Availability

• The nutrient cations are forced into the soil solution.

  • can be assimilated by roots and soil organisms

  • may be removed by drainage water

Soil pH and Nutrient Availability

• Availability of the macronutrients (Ca, Mg, K, P, N, S), Mo, and B

  • curtailed in strongly acid soils.

• Availability of micronutrients (Fe, Mn, Zn, Cu, Co)

  • increased by low pH.

• even to the extent of toxicity to higher plants and microorganisms

Soil pH and Nutrient Availability

• Slightly to moderately alkaline soils = Mo and all the macronutrients (except P) are amply available;

  • low levels of Fe, Mn, Cu, and Co

  • P and B = availability is also reduced in alkaline soil;

• commonly to a deficiency level

Factors Affecting the Release of Nutrients to Plants

(1) Percentage saturation of the exchange complex by nutrient cation in question.

• Example: if the percentage Ca saturation of a soil is high, the displacement of this cation is comparatively easy and rapid.

Factors Affecting the Release of Nutrients to Plants

(2) Influence of complementary adsorbed cations

• Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+

• K+ - less tightly held by the colloids if the complementary ions are Al3+ and H+ than if they are Mg2+ and Na+.

  • K+ more readily available for absorption by plants/for leaching in acid soils

Factors Affecting the Release of Nutrients to Plants

(3) Nutrient antagonisms:

• K+ uptake by plants is limited by high levels of Ca.

• High K levels - limit the uptake of Mg;

  • even when significant quantities of Mg are present in the soil.

Factors Affecting the Release of Nutrients to Plants

(4) Effect of type of colloid

• The strength of adsorption of specific cations varies with types of colloids.

  • At a given percent base saturation (%BS), smectites hold Ca more strongly than kaolinite .

  • Smectite clays must be raised to about 70% BS before Ca will exchange easily and rapidly enough to satisfy most plants.

  • Kaolinite clays exchange Ca more readily at a much lower %BS.

1. Percentage saturation of the exchange complex by nutrient cation in question.

2. Influence of complementary adsorbed cations

3. Nutrient antagonisms:

4. Effect of type of colloid

Factors Affecting the Release of Nutrients to Plants: (4)

• Movement to the roots

  1. Root interception

  2. Mass flow

  3. Diffusion

Mechanisms of Nutrient Uptake: (3)

Root interception

exposure to soil and new supplies of nutrients - roots could contact 3% of the soil or nutrients in the soil.

Mass flow

• water absorbed by the root creates a water deficit near the root, more water moves to the root carrying nutrients with the water.

  • Important for nutrients in large quantities in the soil solution - N, K, & Ca.

Diffusion

movement of nutrients due to an imbalance of concentration (diffusion gradient)

1. Actively growing plants

2. Metabolic energy required

3. Root hairs

4. Process is selective

Conditions Required for Nutrient Uptake by Plants: (4)

Actively growing plants

anything that affects the metabolism of the plant will affect nutrient uptake.

Metabolic energy

• is required.

  • – Plant roots must be able to respire.

  • – Soils must have oxygen.

Root hairs

are the most active points of nutrient uptake.

Process is selective

a carrier ion moves from plasmalemma across the plasma membrane into the outer space of the walls of the cells of the cortex and picks up a nutrient ion and moves back across the membrane.

Nutrient absorption

__ results in increased acidity.

Justus von Liebig’s Law of Minimum

“Plant production can be no greater than that level allowed by the growth factor present in the lowest amount relative to the optimum amount for that factor”.

The Concept of Limiting Factor

• Plant growth is constrained by the essential element (or other factor) that is most limiting.

  • the level of water in the barrel represents the level of plant production.

The Concept of Limiting Factor

• P is represented as being the factor that is most limiting.

  • – even though the other elements are present in more than adequate amounts, plant growth can be no greater than that allowed by the level of P available.

• When P is added, the level of plant production is raised until another factor becomes most limiting

  • – in this case N.