In this article, we are going to learn about the phosphorus cycle ppt. phosphorus is the basic important nutrient in the soil and plays a major role in the metabolism of plants.
The Phosphorus Cycle ppt
The Phosphorus Cycle ppt
PHOSPHORUS IN THE SOIL:
Table of Contents
- the concentration of phosphorus in the soil solution is very low, generally ranging from 0.001 mg/L in very infertile soils to about 1 mg/L in rich, heavily fertilized soils.
- Plant roots absorb phosphorus dissolved in the soil solution using a cross-membrane transporter specific for phosphate ions (HPO42- and H2PO4-
- In strongly acid soils (pH 4–5.5),
- the monovalent anion H2PO4
- – dominates, while alkaline solutions are characterized by the divalent anion HPO42-
The effect of pH on the relative concentrations of the species of phosphate ions. At lower pH values, more H+ ions are available in the solution, and thus the phosphate ion species containing more hydrogen predominates. In near-neutral soils, HPO42− and H2PO4− are found in nearly equal amounts. Both of these species are readily available for plant uptake.
DECOMPOSITION OF PLANT RESIDUES:
Microorganisms that decompose the residues temporarily tie up at least part of the phosphorus in their cells (microbial biomass-P), but eventually release phosphorus to the soil solution through mineralization.
CHEMICAL FORMS IN THE SOIL:
The bulk of the soil phosphorus exists in three general groups of compounds—namely, organic phosphorus, calcium-bound inorganic phosphorus, and iron- or aluminium-bound inorganic phosphorus.
GAINS AND LOSSES:
The principal pathways by which phosphorus is lost from the soil system are plant removal (5–50 kg ha-1 yr-1), erosion of phosphorus-carrying soil particles (0.1–10 kg ha-1 yr-1 on organic and mineral particles), phosphorus dissolved in surface runoff water (0.01–3.0 kg ha-1 yr-1), and leaching to groundwater (0.0001–0.5 kg ha-1 yr-1).
ORGANIC PHOSPHORUS IN SOIL:
The organic fraction generally constitutes 20–80% of the total phosphorus in surface soil horizons.
Three broad groups of organic phosphorus compounds are quite common in soils:
(1) inositol phosphates or phosphate mono-esters of a sugarlike compound, inositol
(2) phosphate di-esters such as nucleic acids from DNA and RNA;
(3)phospholipids, partially derived from microbial and plant cell membranes.
Net immobilization of soluble phosphorus is most likely to occur if residues added to the soil have a C/P ratio greater than 300:1, while net mineralization is likely if the ratio is below 200:1.
INORGANIC PHOSPHORUS IN SOIL:
Two phenomena tend to control the concentration of phosphorus in the soil solution and the movement of phosphorus in soils:
(1) the solubility of phosphorus-containing minerals, and
(2) the adsorption or fixation of phosphate ions on the surface of soil particles.
Fixation and Retention.
Dissolved phosphate ions in mineral soils are subject to many types of reactions that tend to remove the ions from the soil solution and produce phosphorus-containing compounds of very low solubility. These reactions are sometimes collectively referred to by the general terms phosphorus fixation and phosphorus retention.
Inorganic phosphorus compounds in soils fall into one of
- those containing calcium, and(Ex: APATATITE AND FLOROAPATITE)
- those containing iron and aluminium (and, less frequently, manganese).Ex:strengite (FePO4 · 2H2O) and variscite (AlPO4 · 2H2O)
SOLUBILITY OF INORGANIC SOIL PHOSPHORUS:
Precipitation by Iron, Aluminum, and Manganese Ions
H2PO4– ions with dissolved Fe3+, Al3+, and Mn3+ ions to form insoluble hydroxy phosphate precipitates
Reaction with Hydrous Oxides and Silicate Clays
Reaction with Calcium and Magnesium Compounds in Alkaline Soils
H2PO4– ions with dissolved Fe3+, Al3+, and Mn3+ ions to form insoluble hydroxy phosphate precipitates.
Factors Affecting the Extent of Phosphorus Fixation in Soils:
- Amount of clay present
- amount of clay minerals present
- Effect of soil pH
- Effect of Organic Matter. Organic matter has little capacity to strongly fix phosphate ions