Soil structural decline - Soil Essentials: Managing Your Farm's Primary Asset - page 131

The same test can be done by taking a moist sample of the soil and working it
between the fingers for a while then putting a few pellets of this reworked soil in the
water. If it disperses after this treatment it is an indication that the soil is fragile, and
can be easily damaged by cultivation, especially when too wet (Figure 40 on p. 119).
Crusting
Cultivated soil with reasonably high clay content can form surface crusts after
rain, which may inhibit seedling emergence. This crusting is easily observed.
Crusting is usually caused by too much cultivation and/or too low an organic
matter content of the soil. The remedy is to return the soil to a pasture phase, or
increase organic matter content by stubble retention, minimum till cropping or
green manuring. Sodic soils are also prone to crusting (Figure 41 on p. 119).
Responding to soil structural decline
Structurally damaged soils probably mean that farming practice needs to be
critically assessed to find the cause of the problem that should then be addressed.
This may mean shifting from conventional cultivation to minimum till or
direct drilling techniques in cropping, and careful stock management in grazing
enterprises. If stock can be kept off wet paddocks when they are saturated, or at
least rotationally grazed with a system like one week on, and seven weeks off in wet
weather, the soil will have a chance to self repair.
If too much sodium is the problem it may be possible to make the soil more
friable and reduce surface crusting by using top dressings of gypsum periodically.
This should be combined with growing deep-rooted crops or pastures, so roots can
hold the soil open.
How does soil structure decline?
Soil structure can be damaged over time in a number of ways.
By water
Rain falling on bare soil will impact hard enough to break soil aggregates into
smaller particles, which are then easily removed by erosion, or compacted to form
a dense soil that inhibits plant growth and water penetration.
By machinery
Wheels of all types of machinery can compress soil into compacted layers, which
then resist water penetration and inhibit plant growth. This is more so when wet
soil is trafficked.
Cultivating soil can also rapidly destroy soil structure especially if it is
performed when the soil is too wet or too dry, or done too often. Some machines
are more damaging to soil structure than others, with rotary hoes and disc ploughs
being among the most severe.