Page 64 - Nutrient Management Planning Guide

59

Chapter 3.1

A soil aggregate is formed when soil particles adhere to one another such that they behave as a single unit.

s i d e b a r

Based on Canadian Soil Classification Group, 1998

Figure 3.1.2 Relative Size of Different Types of Particles

Sand-sized particles can be seen with the naked eye or felt as grit when rubbed between the fingers. Sand has large and uneven surfaces that limit contact between adjacent particles. Consequently, soils dominated by sand do not form stable aggregates. Rather, these fragile aggregates are easily disrupted, suffer from poor structure, and are prone to wind erosion. The large pores formed between sand particles allow rapid water infiltration, but reduce overall water holding capacity.

Silt-sized particles cannot be seen with the naked eye, or felt when rubbed between the fingers. Silt particles cling together better than sand; however, silt contributes

little to the formation of stable aggregates in soil. The pores formed between silt particles are smaller than those formed by sand, which results in slower water infiltration and higher water holding capacity.

Clay-sized particles are only visible through electron microscopy. Clay’s adhesive properties contribute to the formation of stable soil aggregates that are more resistant to physical disturbance (e.g., cultivation). Pore spaces between clay particles are even smaller than in silt, but the total pore volume is considerably larger. This slows water infiltration but significantly increases water-holding capacity. Clay has a huge surface area to volume ratio relative to larger sand and silt particles (Figure 3.1.3). Soils dominated by clay are prone to swelling when wet and cracking when dry.

Adapted from Brady and Weil, 2000

Figure 3.1.3 Relationship Between Surface Area and Volume

Sand (0.05 - 2.0 mm)

Silt (0.05 - 0.002 mm)

Clay (> 0.002 mm)

Soil Particles Diameter (mm)

Very coarse sand Coarse sand Medium sand Fine sand Very fine sand Silt Clay Fine clay

2.0 - 1.0 1.0 - 0.5 0.50 - 0.25 0.25 - 0.10 0.10 - 0.05 0.05 - 0.002 < 0.002 < .00002

8 cm A

Volume

8 cm x 8 cm x 8 cm = 512 cm 3

Surface area

8 cm x 8 cm x 6 surfaces = 384 cm 2

Surface area/volume ratio

384 512 = 0.75 / ÷ cm cm cm cm 2 3 2 3

Volume

2 cm x 2 cm x 2 cm x 64 = 512 cm 3

Surface area

2 cm x 2 cm x 6 x 64 = 1,536 cm 2

Surface area/volume ratio

1,536 512 = 3 ÷ cm cm cm /cm 2 3 2 3

B

2 cm