Most farms have some areas affected by soil acidity, salinity, sodicity, waterlogging, compaction, or non-wetting sands. Recognising these issues is always the first step to managing them. When a problem area is identified either through a soil test or a visual assessment, mark it on the farm plan.
Soil acidity (pH) and aluminium (Al)
Soil acidity and alkalinity are measured as soil pH, which is a measurement of potential hydrogen ions within the soil. Soil pH tests can be done in water (H20) or calcium chloride (CaCl2). Results from the CaCl2 test are more consistent so are the recommended measures to be used in lab soil test results. In-paddock pH testing is done in water.
Soil pH affects nutrients available for plant growth. Many plants prefer slightly acidic conditions, however, in highly acidic soils aluminium and manganese can become more available and more toxic to plants, while calcium, phosphorus and magnesium are less available to plants. In highly alkaline soil, phosphorus and most micronutrients become less available.
Salinity
Salinity is measured using electrical conductivity (EC) by passing a current through a water extract from the soil sample. As salt conducts electricity, the more total soluble salt (TSS), the higher the reading, which is recorded in deci-siemens per metre (dS/m).
Plants growing in saline soils may face the combined challenge of high salt levels, waterlogging (which exacerbates the salinity impact) and extreme grazing pressure, because sheep prefer grazing salty areas. This makes saline soils vulnerable to overgrazing and erosion. These soils require land class fencing and careful management to maintain ground cover.
Some naturally saline sites, classed as ‘primary saline wetlands’, will support salt-tolerant native plants, which should be encouraged. ‘Secondary saline sites’, formed by rising salty groundwater tables exacerbated by regional tree clearing, may contain salt-tolerant species, such as spiny rush or sea barley grass, and can be sown to improved salt-tolerant species, such as messina or puccinellia.
Soil structure
Poor soil structure affects root growth and water infiltration. The stability and structure of soil depends on soil organic matter levels, the levels of calcium (Ca), which hold soils together, and elements that disperse soil, such as magnesium (Mg) and sodium (Na).
Sodicity refers to a soil property where a significant proportion of sodium exists in the soil compared with other cations on the exchange complex. A soil is considered sodic when there is sufficient sodium to interfere with its structural stability, often affecting plant growth. Sodic soils tend to suffer from poor soil structure, including hard-setting soils, hardpans, surface crusting and rain pooling on the surface. These soils can cause problems for erosion, water holding capacity, water infiltration, drainage, water run-off, plant growth, cultivation and machinery access to paddocks.
Useful soil test results include the ratio of calcium to magnesium (Ca:Mg) and the total amount of sodium compared with other elements (called the exchangeable sodium percentage or ESP).
Waterlogged soils
Waterlogging occurs when water fills the soil pores and does not drain away, reducing oxygen availability and subsequent plant growth. Waterlogging is generally caused by poor drainage as a result of either too much surface water or sub-surface water. It is important to distinguish the source of the water, as surface drains will have little effect if the issue is caused by too much sub-surface water.
Causes of excess sub-surface water include:
- high clay content of subsoils
- high magnesium within subsoils (Ca:Mg ratio less than 2:1)
- a ‘hard pan’ from excessive cultivation or underlying rock shelf
- rising groundwater.
Plant production losses in waterlogged soils may also result from nitrate deficiency. Lack of oxygen leads to soil nitrates being converted into a form that plants cannot use and fungal diseases, caused by plant roots in waterlogged soils being more susceptible to fungal attack.
Compacted soils
Soil compaction is the compression of a soil into a hard layer and is most likely to occur from driving heavy farm machinery, cultivating or grazing cattle on wet soils. Hard pans can also occur as a result of repeated disc ploughing.
To determine if a compaction layer exists, use a spade to examine the soil to about 30cm depth (a compacted layer will make digging to this depth difficult) or use a backhoe to dig a deeper soil pit. Compacted soil shows up as a hard, solid layer, which can look like bricks rather than loose aggregates. Plant roots may be growing horizontally along the top of the compacted layer or appear as ‘J-shaped’ roots, because they cannot grow through it. Surface soil strength can be measured using a penetrometer, which measures the pressure needed to push the rod through the soil.
Non-wetting sands
Water repellence in non-wetting sands is caused by plant waxes that coat the sand particles and prevent water from infiltrating the soil, particularly when it is dry. The result is poor germination and plant growth, and, with large bare areas, it can greatly increase susceptibility to wind erosion.
Techniques to combat the problem include direct drilling, sowing on the contour and sowing into the bottom of furrows with press wheels to improve soil–seed contact and establishment.