When the physical, chemical and biological performance of soils is optimised, they are able to soak up rain and contaminants. But even if the structure is good, soil that is not well managed threatens agricultural productivity and water quality.
Visual Soil Assessment
Soil structure refers to the arrangement of the aggregates and pores within the soil. The structure of soil determines the porosity, strength and stability of a soil which influences:
- Water movement and storage
- The roots ability to penetrate, grow and withdraw water and nutrients
- The ability of a soil to resist erosion
Good vs poor soil structure
A well-structured soil has many stable aggregates in a wide range of sizes. Aggregate size of 2-5 mm diameter is considered to be the best and most wind erosion resistant. Good soils also have a large number of pores within and between these aggregates. These pores maintain the correct balance of air and water in the soil and allow easy seedling emergence and plant root growth.
Main forms of soil damage
Aggregate breakdown - is caused by natural processes such as wet/dry and freeze/thaw cycles. Soils comprised mostly of silt and sand, with a small proportion of clay, are most susceptible as they are loosely bound. As organic matter is a major soil binding constituent, those soils with low organic matter are particularly susceptible. Cultivation significantly reduces soil organic matter content and can rapidly result in serious aggregate breakdown with the risk of erosion, crusting, and poor infiltration of rain and irrigation water. Recent soils and many pallic soils are susceptible to aggregate breakdown.
Soil compaction - is the compression or squeezing of a soil which makes it denser and reduces the number and volume of large soil pores. The wetter a soil is, the more susceptible to damage, because water reduces soil cohesion and lubricates the movement of aggregates. With compaction, the proportion of fine pores is increased and decreases the soils water drainage and air exchange capability. Plant root growth is also restricted, causing a decline in the plant's ability to absorb water and nutrients. Farm machinery and animal treading causes compaction especially in wet conditions. Heavy loads can compact soil to over 50 cm in depth.
Best cultivation practices
- Retain a grass phase in the crop rotation and return crop residues to the soil.
- Keep cultivation to a minimum and use minimum tillage or direct drilling methods to reduce soil damage and to retain organic matter levels.
- Leave the seedbed as rough for sowing to reduce the surface windspeed and work at right angles to the prevailing wind. Use light harrows to leave the sown bed surface rough.
- Plant shelter belts at right angles to the direction of the prevailing/damaging wind.
Soil porosity and water holding capacity
- In a well-structured soil, over half the soil volume consists of pore and about 40% of these pores are very small and nearly always filled with tightly held water which can't be withdrawn by plant roots.
- The volume of the small pores is determined by the proportions of sand, silt, clay and organic matter.
- Medium sized pores retain water, which can be taken up by the roots, after rain or irrigation. The volume of these pores determines the water holding capacity of a soil and its susceptibility to drought.
- Large pores (macropores, >0.03mm diameter) are the structural soil pores, and are important for water infiltration into the soil, drainage of excess water, aeration of the root zone, and act as passages for root growth in the soil. In pasture, there are few macropores >5mm, other than earthworm channels.
Slips, Slumps and subsoil flows
Most soils have a high resistance to movement because of the physical friction and chemical bonds between soil particles. This resistance is tested with the downhill "pull" of gravity, and the pressure of water in saturated soil. In Southland slipping is most common in areas underlain by mudstone such as Blackmount and where loessial soils exist around Mataura and north of Gore.
Prevention is the best way to reduce the likelihood of slips occurring on your property. By draining surplus water from the soil surface and lower levels this slows down water infiltration to the deeper soil levels and makes your soil more stable.
However when slips do occur it is best to act quickly to prevent further slipping. By acting quickly you can also avoid burial of your unslipped pastures and to restore pasture production quickly with a desirable pasture species rather than allow weeds to take over.
Fencing the site
It is absolutely essential to exclude stock from over-sown slipped areas to ensure that the plants get well established. You can either exclude them from the whole paddock or temporarily electric fences the site off. Make sure you monitor pasture growth even after it is established because stock will eat the new pasture before the rest of the paddock and may cause further slipping problems if not managed appropriately.
Soil erosion can be minimised in hill country catchments by large scale afforestation with trees or by indigenous forest. Tree roots, particularly lateral ones that spread sideways, greatly reinforce soil's resistance to downhill movement. They act like reinforcing mesh in concrete and if removed from soils with weak resistance it will put extra stress on the soil.
- On eroded slopes Pinus radiata is a good choice because of its tolerance of a wide variety of conditions, low establishment cost and the possibility of some future cash return from the sale of timber. Recommended spacing is 2.5 x 2.5 m.
- Where erosion is less active, slower growing trees such as macrocarpa or douglas fir are a good choice, however Macrocarpa is suited to higher fertility sites. These longer rotation trees maintain their protective cover for longer periods than with the shorter rotation radiata pine. Eucalyptus nitens, E. delegatensis can also be used for close planting.
- In actively moving wet sites poplar and willow species should be used. Alnus species are expensive but recommended for these wet areas.
- In areas where a permanent grass cover is desirable, such as stream banks, poplars and willows should be planted to minimise shading.
Positioned in the path of the Roaring Forties means it's no surprise that Southland experiences more than its fair share of strong winds. During cultivation of paddocks in readiness for next season's crop or pasture, you'll see enormous amounts of topsoil being picked up and carried off by the accompanying winds. Historically, Southland has experienced its fair share of wind events, but it seems that recently, the wind is getting stronger and it's arriving earlier in the season.
What you can do
One of the most effective tools available for minimising soil loss through wind erosion is shelter. The other is of course maintaining good vegetative cover, which in turn holds and protects bare soil.
The time of greatest risk is when this protective plant cover is lost through cultivation, whether for pasture renewal or crop establishment. But even with well-established shelterbelts protecting paddocks, wind erosion can still occur because of inappropriate cultivation techniques.
Farmers in wind prone areas and with lighter soils should look at their cultivation techniques and employ cultivation practices which maintain maximum vegetative cover on the soil surface and encourage moisture retention in soils. The aim should be to produce an uneven soil surface in as 'rough' a condition as practicable and restrict the cultivation period to a minimum timeframe.The use of the chisel plough or grubbers is recommended as these give a fine deep working while still retaining a cloddy surface. Top-working implements, discing and rolling can create a fine seedbed prone to wind erosion even in well-sheltered situations.
Cultivating and sowing at right angles to the prevailing wind and ridging is also recommended as it presents an uneven surface, which will minimise soil blow.
Winter-feed paddocks can be different as the top few centimetres of soil has often been 'puddled' by stock in wet conditions and is therefore very prone to wind erosion once it has dried out. This is because this top layer of soil has lost all its structure and can lift from the paddock very easily. Turning this type of ground over as soon as soil conditions permit in the early spring will minimise the risk of losing this fine layer to the north-westerly wind.
Environment Southland Land Sustainability staff are happy to come and demonstrate techniques for monitoring changes in soil conditions using the Visual Soil Assessment. Adopting appropriate good soil management practices will keep your soil in healthy condition and ensure your most valuable asset isn't blown over the fence to your neighbour's place or into our riverbeds.