Soil acidification is a natural process, which is accelerated by productive agriculture. Yield reductions in wheat can be up to 20-30% and the choice of crops may be restricted to acid tolerant species and varieties. In pastures grown on acidic soil some legume species may fail to persist.
Reduced plant growth increases the susceptibility of the land to wind and water erosion, resulting in loss of nutrients and soil organic matter.
When eroded soil and nutrients enter waterways long-term degradation can result. Insufficient water usage can occur when deep-rooted species fail to thrive, increasing the risk of dryland salinity.
Reduced nutrient uptake by crops and pastures grown on acidic soil, can result in more nutrients being leached and potential groundwater pollution.
How acidic soils affect plants
Topsoil
In very acid soils, all the major plant nutrients including nitrogen (N), phosphorous (P), potassium (K) and also the trace element molybdenum (Mo) may be unavailable, or only available in insufficient quantities.
Acid topsoils affect microbial activity, most notably reducing nitrogen-fixing rhizobia bacteria, which form a symbiosis with crop and pasture legumes in favourable conditions.
In acid soils, reduced rhizobia populations may be unable to successfully ‘nodulate’ roots; legumes may be deficient in nitrogen and pastures may become grass dominated.
Subsurface soil
The major impacts of acidity occur in the subsurface soil. When soils acidify, aluminium in the soil becomes soluble.
In this form, aluminium retards root growth, restricting access to nutrients and water deeper in the soil profile. Poor crop and pasture growth, yield reduction and smaller grain size occur as a result of inadequate water and nutrition.
The effect of aluminium toxicity on crops is usually most noticeable in seasons with a dry finish, as plants have restricted access to stored subsoil water for grain filling.
Causes of soil acidity
Soil acidification is an inevitable consequence of productive agriculture. The main cause is the inefficient use of nitrogen fertilisers, particularly ammonium-based fertilisers.
Ammonium is readily converted to nitrate in the soil, which if not taken up by plant roots, can leach away from the root zone, leaving the soil acidic.
Most plant material is slightly alkaline. As plants grow, alkalinity is removed from the soil into the plants.
If the plant material is removed by grazing, harvest, or relocated by the concentration of dung into stock camps, rather than being returned to the soil, there is a net export of alkalinity from the system.
Over time, as this process is repeated, the soil becomes acidic.
Diagnosis and management of soil acidity
Soil pH
The only way to diagnose soil acidity is to test the pH of the soil. Soil pH measures the hydrogen ion concentration (acidity) on a logarithmic scale from 1-14, with seven being neutral.
The lower the pH of soil, the greater the acidity. A soil with a pH of four has 10 times more acid than a soil with a pH of five and 100 times more acid than a soil with a pH of six.
In WA it is standard to measure pH using one part soil to five parts 0.01 Mole calcium chloride (CaCl2). Some soils in WA show large seasonal variation in pH if it is measured in water. pH measured in water can read 0.6-1.2 pH units higher than in calcium chloride.
Soil sampling
Samples for soil pH need to be taken at 0-10, 10-20 and 20-30cm to detect acidity throughout the soil profile. The subsurface pH cannot be estimated from the topsoil pH. Acidic subsurface soils may underlie topsoils with optimal pH.
Sampling sites need to take paddock soil-type variability into account. Sample sites should be re-locatable (ideally by GPS) to allow comparable repeat sampling, about every 3-4 years.
Liming
Plant growth and most soil processes, including nutrient availability and microbial activity, are favoured by a soil pH range of 5.5-8.
Applying agricultural lime is the most economical method of ameliorating soil acidity and liming needs to be an integral part of farming if the system is to be sustainable.
In WA agriculture, target pH levels of 5.5 in the topsoil and 4.8 in the subsurface are recommended (see Table 1).
| Soil depth | pH | Lime amount over 5 years |
|---|---|---|
| 0-10cm | under 5 | 2t/ha |
| 0-10cm | under 5.5. | 1t/ha |
| 10-20cm | under 4.5 | 2t/ha |
| 10-20cm | under 4.8 | 1t/ha |
| 20-30cm | under 4.5 | 1t/ha |
| 20-30cm | under 4.8 | measure pH in 3 years |
Keeping the topsoil pH above 5.5 will treat on-going acidification and ensure that sufficient alkalinity can move down and treat subsurface acidification.
The effects of aluminium toxicity in the subsurface are minimised if the pH is above 4.8.
If pH soil test results are at or above the target levels, only maintenance liming will be required to counter ongoing acidification due to agriculture.
If the topsoil pH is below 5.5, recovery liming is recommended to prevent the development of subsurface acidity, even if the subsurface pH is currently at 4.8.
The application rates of lime will depend on the soil pH profile, soil type, farming system, rainfall and lime quality.
Qualified experts are able to develop liming recommendations to suit individual requirements. Soil pH test results from 0-10, 10-20 and 20-30cm are required for accurate recommendations.
Complementary management strategies
Tolerant species and varieties of crops and pasture can reduce the impact of soil acidity. These should be used in conjunction with a liming programme to recover soil pH to target levels.
If left untreated, the soil will continue to acidify, the cost of amelioration will increase and all the while productivity will continue to be lost.
Farming systems can be managed to reduce the rate of soil acidification but this will not eliminate the need for liming. Using appropriate rates of less acidifying nitrogen fertilisers to reduce nitrogen leaching is most important in higher rainfall areas.
Some rotations are more acidifying and it may be possible to replace them with less acidifying options, for example, replace a legume hay rotation with a less acidifying crop or pasture.
Agricultural lime and how it works
In WA, limesand (from coastal dunes), crushed limestone and dolomitic limestone (usually marketed as dolomite) are the main sources of agricultural lime.
In all of these sources it is carbonate from calcium carbonate or magnesium carbonate that neutralises acid in the soil.
The quality of the lime is important, not the source. Neutralising value, as a percentage of pure calcium carbonate (100%), and particle size distribution are the important factors in lime quality. With a higher neutralising value, less lime can be used.
Lime with a higher proportion of small particle sizes will react quicker to neutralise acid in the soil, which is beneficial when liming to recover acidic soil.
Transport costs are a large proportion of the cost of liming, however since lime sources vary in quality, it is important to compare the cost (delivered and spread) of the equivalent neutralising value of limes.
Acidic soils can cause significant losses in production, restricting crop and pasture choice and restrict root access to water and nutrients. Treat and restore your soils before it is too late.