Page 36 - Nutrient Management Planning Guide

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Soil Nutrient Cycling

Volatilization

Plant uptake

Soil solution SO

4

-2

Weathering/ Dissolution

Precipitation

Oxidation

Reduction

Immobilization Mineralization

Organic S

S

minerals S /S 0 2-

Fe/Al oxides

Desorption Sorption

Leaching

Erosion

Plant residue

SO gas

2

Crop removal

Adapted from Korb and Jacobsen 2002

Figure 2.1.11 The Agricultural Sulphur Cycle

S Cycling in Soils

Figure 2.1.11 represents the basic processes involving S in soils. Only 1 to 3% of the soil total S is in the plant-available form (SO

4

2- ). The bulk of soil S (200 to 1100 kg/ha) is in soil organic matter. Mineralization of organic S compounds is an important source of S for growing plants.

The weathering of S-containing minerals such as gypsum (CaSO

4

) can be a source of crop available S particularly in dry regions where the amount of precipitation is too low to leach it from the soil profile. Other primary and

secondary minerals will release elemental S (S 0 /S 2- ), which is converted to SO

4

2- when exposed to moisture, oxygen and microbial processes.

Crop available S (sulphate-sulphur) can become temporarily lost from soil solution through precipitation with magnesium or calcium (typical for Alberta) or adsorption to aluminum or iron oxides on clay particles. Sulphate-sulphur is soluble and mobile in soils and will move with groundwater. Leaching losses of S are possible, especially in coarse textured soils, but with average rainfall this type of loss will be low for most

N and S are closely related in soils worldwide with the ratio of N to S typically between 6:1 and 8:1.

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