3.2 Soil Acidity and Liming

See also 2.3 Soil pH

What is soil acidity?

Soil acidity occurs when there is a build up of acid in the soil. The production of acid in the soils is a natural process and many soils in the high rainfall areas of SA are inherently acidic. Acidification is a slow process but it is accelerated by agriculture.

As soils become more acidic, plants intolerant of acidic conditions do not thrive and productivity declines.

How fast does soil acidify?

 The rate at which a soil acidifies depends on:- 

• Soil type 
  Light sandy soils with little clay or organic matter have lower buffering capacity and therefore acidity develops more quickly than on heavier soils. 

• Rainfall 
  Higher rainfall increases leaching of nutrients which in turn increases acidification. 

• Land use
  Higher production increases the rate of acidification. Shallow rooted plant systems also increase acidification compared with deep-rooted plants.

Figure 1:
Examples of acidification rates for different farming systems

*kg lime/ha/year required to neutralise acidity

What causes soil acidity? 

The natural rate of acidification is accelerated by agricultural practices. These include: Use of nitrogen fertilisers The impact of nitrogen fertilisers on acidification depends on the type of fertiliser and what happens to the nitrogen (see Table 2).

Figure 2:

Lime required to counteract acidity caused by fertilisers in acid soils.

Removal of plant products 

The removal of plants products leaves the soil more acidic. Highest acidification rates occur when hay is cut and fed on a different paddock or sold off the property. Similarly, the use of night paddocks transfers alkalinity from the other paddocks to the night paddocks.

Figure 3: 
Lime required to counteract acidity caused by product removal.

*kg CaCO3 per tonne

Animal product removal 

The removal of animal products also cause soil acidification but rates are generally low. 

For example at normal management levels for milk, wool or meat production, the acidity produced is only equivalent to about 1 to 10 kg of lime per ha per year.

Note: Superphosphate does not directly cause acidity, but may increase the process indirectly by promoting better pasture growth or higher productivity.

Symptoms of Soil Acidity

The following symptoms tend to indicate a soil acidity problem.

• Reduced yields

• Poor plant vigour

• Uneven pasture and crop growth (especially acid sensitive plants).

• Poor establishment and persistence of pasture species such as lucerne and phalaris where previously they grew well.

• Poor nodulation of legumes.

• Stunted root growth.

• Persistence of acid-tolerant weeds (eg sorrel and geranium).

• Increased incidence diseases

• Abnormal leaf colours

• A soil pH test is needed to confirm a soil acidity problem.

What does acidity do?

Soil acidity has a negative impact on fertility, biological activity and plant productivity.

Plant tolerance and productivity

Species and varieties with low tolerance to acidity will decline in productivity and persistence. Greater reliance is placed on acid tolerant plants that are generally not as productive.

Soil fertility

Soil pH influences nutrient availability. In strongly acid soils, potassium, calcium and magnesium are depleted due to leaching. Low levels of calcium and magnesium can also cause stock health problems such as milk fever and grass tetany. A lack of calcium can cause soil structural problems.

Aluminium, if present in the soil, becomes available once pH(CaCl2) goes down to less than 5. Aluminium is toxic to plants and severely restricts root growth. Acids attack soil minerals and increase net loss of nutrients from the soil eg. Mn, Cu, Zn.

In some soils Manganese toxicity will develop around pH (CaCl2) 5.0 although this is unusual in most high rainfall SA soils

Figure 4:
Effects of acidic soil on plant nutrients.

Biological activity

Soil acidity reduces and even stops the activity and survival of useful soil organisms such as:

• nitrogen fixers

• decomposers

• nutrient recyclers

Organic mats often form on the soil surface as a result of reduced biological activity and organic matter not being broken down.

Soil Structure/Clay Degradation

The leaching of nutrients and increased availability of clay minerals such as Al and Fe can result in a decline in soil structure and some irreversible damage to the clay content of soil.

Off-site effects

As a result of poor pastures, limited growth and shallow root depth, recharge under acid soils is greater than under productive perennial pastures. This will contribute to rising water tables and an increase in the salinity of streams and dryland salinity.

Streams are also more likely to contain nutrients leached out of the soil due to the acidic conditions.

Benefits of liming

Raises soil pH. 

A well balanced soil pH is important for: soil fertility and nutrient availability plant species that can be grown biological activity of the soil

Pasture vigour and productivity 

Lime application increases pasture productivity. Trials throughout the Mt Lofty Ranges showed increases in productivity up to 35%.

Livestock health 

Increased calcium and magnesium levels in the plant helps to overcome problems such as grass tetany in cattle.

Economics 

Research data shows that responses to lime can be profitable (Fig 1 and 2). Most economic advantage is achieved by liming highly productive or perennial pastures.

Perennial pasture: phalaris/subclover. Most economic rate is 3.5t/ha, which is sufficient to increase subsoil pH. 

Annual pasture: annual ryegrass/subclover. Most economic rate is 1.5t/ha, but is insufficient to improve subsoil acidity.

Figure 5: 
Economics of applying lime to perennial or annual pastures (Data from NSW Agriculture, Wagga Wagga) 

Figure 6: 

Net cash flow after applying lime to perennial or annual pastures. Lime applied at 3.7t/ha at total cost of $235/ha. (Data from NSW Agriculture, Wagga Wagga)

Management of acidity

What can we do?

• Recognise soil acidity

• Monitor soil pH 

• Know crop & pasture requirements

• Keep accurate records for each paddock

Do not assume that every paddock has the same soil pH or acidification rate!

Management options

1. Apply liming material at a rate based on pH, soil type, land use (see next section). 

2. Use acid-tolerant plants eg cocksfoot, some clovers. This is a short term option only as the soil continues to acidify with associated consequences. 

3. Reduce the rate of acidification to a minimum. 

• Sow perennial pasture - deep rooted, more summer active, reduce N leaching.

• Use fertilisers wisely - match plant requirements, monitor plant and soil levels, and use least acidifying N fertilisers.

• Feed hay onto paddock in which it was cut where possible - recycles nutrients and alkalinity. 

• Rotate grazing paddocks. 

• Irrigation with bore water in the Mt Lofty Ranges and SE often applies carbonate which helps to neutralise acidity. 

• Buy in hay/grain where practical.

3.2 Soil Acidity and Liming

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