Components of soil fertility - Soil Essentials: Managing Your Farm's Primary Asset - page 88

12

Water held in soil
(g water/100 g soil)

8

4

0

1 2 3 4

Organic matter in soil (%)

Figure 25 Dramatic effect of increasing organic matter content on water held (at field capacity) in a very
sandy soil (Mt Burr sand from South Australia). ( Soil Sense M-03)

A fertile soil is a basic requirement for a viable farm.

Water holding capacity
Plant growth is highly dependent on adequate water supplies in the soil. Sandy
soils have very poor water holding capacities because most of the soil pores are
large and drain readily. Clay soils, on the other hand, have many fine pores that
give clay soils their good water holding capacity. The amount of water available for
plant use has a large effect on soil fertility. Infiltration of water into the soil is also
dependent upon its soil structure, and in poorly structured soil much rainfall can
be lost as runoff. This can represent a big loss of productive capacity in lower
rainfall areas. On the other hand, nutrients can leach out of sandy soils in high
rainfall areas, thereby reducing the productive potential of crops and pastures.

Table 20 Infiltration rates for different soils

(Source: Soil Sense M-03)

Soil texture Infiltration rate (mm/hr)

Bare soil

Vegetated soil

Clay 0-5 5-10

Clay loam 5-10 10-20

Loam 10-15 20-30

Sandy loam 15-20 30-40

Sand 20-25 40-50