Understanding pH of Soil

Acidity is a major and often overlooked problem in Australia. There is approximately 8-10 times more land affected by acidity than salinity. To correct this problem, approximately 66,000,000 tonnes of lime would need to be spread nationwide.

pH is a measure of the level of hydrogen in the soil. The more hydrogen in the soil the more acidic the soil is. Soil pH has an enormous impact on the availablity of soil nutrients to the land and also greatly affects the organisms living in the soil. Being a logarithmic scale, a pH reading fo 5 is 10 times more acidic than a pH reading of 6. A pH reading of 4 is 1000 times more acidic than a pH reading of 6.

Lime Application Rates

Typical application rates are between 1.5 t/ha – 2.5 t/ha, however they will vary greatly depending on how acidic your soil is and what texture your soil is. The following chart can be used as a rough guide to determine what rate you apply Dry Lime at to increase the pH by 0.5

Sandy Loam2.0 t/ha
Loam2.5 t/ha
Clay Loam3.5 t/ha
Clay4.5 t/ha

Studies of the Impact of Surface Applied Soft Lime to Soil Conditions and Pasture Production

The following information has been supplied from the AFSA /SGS Lime Trial. The impact of surface applied soft lime to soil conditions and pasture production in Western Victoria.

Soil Response

Movement of lime into the soil

Broadcast lime moves more rapidly into the soil than originally anticipated. The commonly held view is that ‘lime moves slowly (0.5 to 1.0cm per year) down the soil profile’ (Acid Soil Action, 1999) is not supported by the measurements from these trials. On all 20 top-dressed pasture paddocks, soil pH change was detected down to 5cm after 16 months and continues to react at depth in the subsequent year irrespective of the lime used.One additional trial established in 2002 using a high quality lime indicated a change in surface pH after only six weeks of application.

Rate of lime reaction with the soil

The speed at which the different limes reacted in the soil varied between suppliers and showed limited correlation with the effective neutralising value (ENV) of the products after 16 months. One likely explanation is that all the limes used were considered ‘soft’ limes, compared to most limes which are insoluble and react differently when wet (ENV is determined on a dry sieving basis). However after 28 months all limes when applied at the same rate had resulted in the same pH change in the soil profile.

Rate of application

Increasing application rates increased the magnitude of the change in soil pH up to about 3.5 t/ha. No significant pH changes were measured at rates higher than 3.5 t/ha although differences may be observed in subsequent years. At low application rates (<1.5t/ha), significant changes in soil pH remained 28 months after application.

Liquid lime

The application of liquid lime did not significantly change soil pH whereas a change was seen in application of dried lime at the same site.

Longevity of lime

These trials have not been established long enough to answer this question.

Soil fertility

The application of lime appears to result in an increase in plant available phosphorous in the soil. Analysis on soils sampled to a depth of 10cm showed an average increase of 13% from an Olsen phosphorus of 12.6 mg/kg to 13.8 mg/kg (7 sites). Further testing of three sites examining the top 5cm of soil (where most of the pH change occurred) showed a more dramatic change in available phosphorous.

Trace elements

The application of lime modified the quantity of some trace elements in the tissue of subclover. Before liming phosphorous, manganese and molybdenum were outside the desirable range for subclover. After liming all elements were within the desirable range. No copper deficiencies were recorded that could be linked directly to the application of lime.


Application of lime, even at low rates, dramatically reduced the amount of available aluminium in the soil. Average soil pH across all sites increased from pH 4.5 (CaCI2) in the no lime treatments to pH 4.9 (CaCI2) in the limed areas and resulted in a correspond 89% reduction in soil aluminium (as a % of total cations).

Pasture Response

Pasture Quantity

There were only minor increases in pasture quantity during the winter period as result of liming. However differences in the order of 15% were recorded in the spring period. This response appears to be linked to the changing pasture composition.

Pasture Composition

Liming improved the component of desirable species in the pasture (perennial grasses and clover) if the pasture composition was below 50% at application. Limed pastures that already has a high proportion of desirable species (greater than 75%) showed no response as a result of liming. The most obvious change was a reduction in annual grasses after liming.

Pasture Quality

The modification in pasture composition as a result of liming was reflected in the pasture quality changes during the spring ‘haying off’ period. Pastures with a high component of annual grasses that were limed showed a less rapid decline in pasture quality compared to the no limed areas. However in the pastures with high proportions of desirable species at application, there was no difference in the lime and no lime areas.

Livestock Responses

The limited work to date has failed to identify any measurable differences in livestock performance between the limed and non-limed areas. This is not surprising given the significant changes in pasture quality and quantity occur during the spring ‘flush’.