Agriculture Reference
In-Depth Information
Box 3.2
Slaking and Dispersion of Soil Aggregates in Water
If dry aggregates do not slake in rainwater, the macroaggregate structural
stability of the soil is good. Typically, aggregates from the A horizon of soils high
in sesquioxides or formed on calcareous parent materials, and under permanent
pasture, do not slake. Other soils may slake slowly or rapidly, depending on their
organic matter content, sesquioxide content, and cultivation history.
Of the aggregates that slake, we differentiate types on the basis of whether
they
disperse
, as shown in figure B3.2.1. Where there is no dispersion, a sample of
dry aggregates is moistened and remolded into small balls, approximately 5 mm in
diameter. These are placed in rainwater and observed for any dispersion. A
description of the main types of aggregate behavior, identified by the test, is as
follows.
Type 1
. Complete dispersion, very poor microaggregate stability. The soil is likely
to have no consistence when wet and to set hard when dry, and is also likely
to erode easily. This soil is not recommended for vineyards.
Type 2
. Partial dispersion, poor microaggregate stability; will probably crust and
need to be protected by vegetative cover from erosion. Gypsum is required if
the soil is to be cultivated and planted to vines.
Type 3
. Complete to partial dispersion after remolding indicates moderate
microaggregate stability. Soil structure will deteriorate under repeated
cultivation, especially if cultivated when too wet; requires careful management
in a vineyard.
Type 4
. No dispersion after remolding indicates good microaggregate stability. The
soil will not crust or erode very readily and is most suitable for vineyards.
Figure B3.2.1
Diagram of a modified Emerson dispersion test for soil aggregates (Cass 1999, after
Emerson 1991). Reproduced from Soil Analysis: An Interpretation Manual (K. I.
Peverill, L. A. Sparrow, and D. J. Reuter, 1999) with permission of CSIRO Publishing.
Air-dry aggregate
Rainwater
Complete dispersion
Partial dispersion
No dispersion
Type 1
Type 2
Remolded
moist aggregate
Rainwater
Dispersion
No dispersion
Type 3
Type 4
