Page 31 - Nutrient Management Planning Guide

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Chapter 2.1

Adapted from Jones and Jacobsen 2002a

Figure 2.1.6 The Agricultural Phosphorus Cycle

Erosion

Plant uptake

Soil solution

HPO

H PO

2 4

-

4

-2

Sorption Desorption

Precipitation

Dissolution

Immobilization Mineralization

Organic P

Fe/Al oxides

P minerals

Plant residue

Crop removal

P Cycling in Soils

Phosphorus exists in different pools in the soil. The P cycle, Figure 2.1.6, illustrates these pools and the pathways by which P may be taken up by plants or lost from the soil.

Phosphorus in soil exists in combination with other elements such as O and H. Plant roots absorb P mainly as orthophosphate (H

2

PO

4

- or HPO

4

2- ). The primary orthophosphate form (H

2

PO

4

- ) dominates in soils below pH 7.2, and the secondary form (HPO

4

2- ) is prevalent in soils above pH 7.2. Plants are able to absorb the primary form more rapidly than the secondary form.

In soils, P occurs in three pools: soil solution, mineral and organic. Plants can only use P from the soil solution pool. The mineral and organic pools have stable components that change little with time (e.g., humus) and labile portions that gradually release P to the soil solution.

Phosphorus does not always “flow” toward the soil solution pool from the labile pool. It can move from the soil solution to the labile pools or even directly into stable soil components. Similarly, labile P can be tied up in non-labile organic and mineral compounds (Figure 2.1.7).