Unfortunately, big wind events like those on August 18 and 19 as well as August 31 in many districts and paddocks, caused massive erosion.
Losing 1mm of soil via wind erosion, over a hectare, is equivalent to losing 10 tonnes of soil. Such erosion can easily be surpassed in lengthy high wind events. It is the topsoil that erodes and this soil layer generally has the highest levels of organic matter and plant nutrients like nitrogen, potassium and phosphorus.
Lack of ground cover leads to high wind erosion. Low groundcover in areas prepared for crops and where stubble was mostly consumed by grazing were among paddocks badly wind eroded. Pasture and forage paddocks totally grazed out, especially annual pastures with few grass butts remaining, were also often badly eroded.
While the loss of 1mm of soil seems trivial, this represents losing 100kg/ha of organic carbon and 10kg/ha of total N. This loss comes from the native and hard-won organic pool built up via good rotations, which support soil structure and on-going nitrogen mineralisation. While most nutrients can be replaced by fertiliser (can be costly), organic carbon is much harder to build up.
Avoiding wind erosion, and to a fair degree water erosion, is aided by retaining sufficient groundcover. Wind erosion is best controlled by standing cover which lifts the wind velocity above the fragile soil surface. One of the big advances over the last 40 years has been the almost universal adoption of reduced or no till farming, with retention of at least minimal levels of stubble from previous crop or pasture.
In pasture situations, native or introduced perennial grasses generally provide good to reasonable protection from wind and water erosion unless grazed into the ground. Dry perennial plant butts are generally present, providing reasonable soil protection if not totally grazed down.
While droughts lead to pastures and stubbles being overgrazed, long term this has several detrimental outcomes to farm income, productivity, and the farm resource. For example, when rain does eventually arrive, good groundcover acts as a “blotter” and results in better water infiltration with less runoff moving off the paddock. In one of my pasture trials for example we measured a seven-fold higher infiltration rate in a native grass pasture with good groundcover compared to almost bare soil.
Perennial pastures where reasonable groundcover has been retained almost always recover faster than over-grazed and near bare paddocks. Previous grazing management, other than groundcover level, can also impact on perennial grass recovery. If perennial plants have received even a reasonable degree of rotational grazing where opportunities are provided for plant recovery, they have greater root reserves and regenerate faster. Plants allowed to develop reasonable plant mass before grazing will also provide more total feed than those grazed heavily from first rains.
Annual pastures commonly present a grazing dilemma in droughts, especially for sheep grazing. Sub clover seed burr and medic pods for example can provide excellent quality feed when pastures are long dried off and if grazed down heavily and not combined with grass residues are vulnerable to erosion.
Equally important to management is the reality that soil lost from erosion takes several lifetimes to be replaced via normal soil formation.
Part of future drought management plans for a given property I believe needs to consider management that preserves sufficient groundcover. Mini feed-lotting, stored fodder and selling down are strategies associated with groundcover preservation and peace of mind when the inevitable drought arrives.
