Page 32 - Nutrient Management Planning Guide

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Soil Nutrient Cycling

Slow cycling

Inorganic Fraction Organic Fraction

Rapid cycling

Slow cycling

Primary minerals

Secondary minerals

Occluded

Soil solution

Labile Labile

Moderately labile

Moderately labile

Chemically and physically protected organic P

Decomposer community

Fertilizer P Manure

Plant residue

Created by Len Kryzanowski

Figure 2.1.7 Conceptual Relationship and Interactions Between Soil Phosphorus Pools

The amount of P in soil solution at any given time is usually low. Consequently, to meet crop requirements the P in soil solution is constantly replenished from the labile pools. The rate at which labile P is converted to soluble P is more important than the total P content of the soil. Fertile soils can rapidly replenish P in soil solution, ensuring good crop growth.

The organic P pool comes from microbial, plant and animal residues deposited on or in the soil. Most of the organic pool is found in the top layers of soil and nearly half of this is in the form of phytic acid (Figure 2.1.8). Each molecule of phytic acid has the potential to release six molecules of orthophosphate (H

2

PO

4

- or HPO

4

2- ) to the soil solution.