Soil - Agroecology: The Ecology of Sustainable Food Systems - page 103

100

80

Clay

60

Percent Clay Percent Silt

Silty day

Sandy
day

40

Silty day
loam

Clay loam

Sandy day loam

20

Loam

Silty loam

Sandy loam

Silt

Sand

0

Percent Sand

FIGURE 8.2 Soil textural names. The best type of soil is determined by the crop and local conditions; generally, however, soils
containing relatively equal amounts of clay, sand, and silt — called loams — are best for agricultural purposes (From USDA diagram).

Platy Prismatic Columnar

Can occur in any
horizon

Common in subsoils of arid and semi-arid
regions

Blocky Blocky

(subangular)

Granular Crumblike

Common in heavy subsoilds of humid
regions

Characteristic of surface soils
with good tilth

FIGURE 8.3 Patterns of soil aggregation (Modified from Brady, N. C., and R. R. Weil. 1996. 11th ed. Prentice Hall: Upper Saddle
River, New Jersey).

structure, but structure is usually more dependent on soil
organic matter content, the plants growing in the soil,
the presence of soil organisms, and the soil's chemical
status. Structure of the crumb or granular type is of
the most benefit for agriculture, since good “crumb

structure” improves soil porosity and ease of tillage,
which together are known as tilth. When a lump of soil
is crushed in the hand, and easily breaks into the crumb
or granular structure noted in Figure 8.3, good crumb
structure is present.