Soil acidification is a natural process which occurs in some soil types or as a result of normal agricultural production.

As well as the natural "fixing" of atmospheric nitrogen (N) by legumes, the organic matter in the soil is a source of N for plants.

As organic matter is broken down by soil microbes, the N contained is eventually converted to more simple N forms – ammonium and nitrate.

Both are available for plant uptake but the ammonium form is attracted to soil particles while the nitrate form remains "free" and will move down the soil profile with soil water if not captured by plant roots. Additionally, ammonia not taken up by plant roots can also be converted to the nitrate form, and so be leached from the system.

As nitrate moves down the soil profile, it takes important elements with it, such as calcium and magnesium, leaving the soil in a more acid condition.

There are many factors influencing nitrate production and loss, including rainfall, temperature, soil type, pasture type and cultivation.

Nitrogen cycling which results in the leaching of soil nitrate from the root zone into the subsoil is only part of the acidification process.

In today’s farming environment, more rapid soil acidification can be attributed to other factors, often operating together. The main ones are:

The addition of ammonium and elemental sulphur (but not sulphate sulphur) can make soils more acid.

• Nutrient transfer

The transfer of nutrients such as calcium and magnesium in dung and urine from grazed areas to "stock camps" increases soil acidity.

• Nutrient removal in farm products

Removal of elements, such as calcium and magnesium, sold from the farm in livestock and plant products, can also increase soil acidity. Some typical examples of equivalent lime removal are shown in the following table:

The pH scale is used to define the degree of acidity or alkalinity which exists in the soil. The pH scale ranges from 0-14. Soils with a pH of less than 7 are acid, while soils above 7 are alkaline.

Known or potential acidity problems may not call for corrective action unless the soil pH falls below 6. This will depend on crop and pasture requirements.

Soil pH is easily measured using a soil test. Two test methods are used: Water and Calcium Chloride. The Calcium Chloride test is preferred, although agronomists can interpret either. pH measured in water will read higher than when measured in Calcium Chloride (generally around 0.7 to 1.0 units higher).

Soil pH influences the availability of nutrients to plants. As a result, nutrients such as phosphorus, molybdenum, calcium and boron can be "tied up" and become unavailable to plants as soil pH falls.

Toxic quantities of aluminium and manganese may become more available to the plants as soil acidity increases. Aluminium and manganese toxicity will adversely affect the growth of many plant species, especially legumes.

For this reason the soil aluminium level test is essential.

The ideal pH for plants varies with species and variety. Lucerne, for example, will not grow as well as subterranean clover under low soil pH conditions, especially where aluminium is high.

Similarly, pasture grasses differ in their tolerance to acid soil conditions, with species such as cocksfoot showing greater acid soil tolerance than phalaris.

Wheat varieties vary in their tolerance of soil acidity. Triticale and oats are more tolerant to lower pH than most barley and wheat varieties.

Lucerne and most vegetable crops and orchards require soils with pH levels greater than 6.0 (water).

Soil analysis is the most accurate method of determining lime requirements.

The soil analysis service measures both soil pH and exchangeable aluminium, which provide a sound basis for assessing the lime requirements of a soil.

In the case of a pasture, lime application is preferably undertaken when cultivating or renovating the paddocks.

A soil testing program will also provide the basis for pasture species selection, establishment technique and determination of optimum application rates of both lime and fertilizer.

The effect of lime on pH will vary according to soil type. At a standard application rate of 2.5t/ha, it can be expected that pH will rise by a factor of 0.25-1 on the pH scale in the first year.

When lime is not incorporated into the soil (applied to the surface only), it moves down through the soil profile slowly. As a result, the pH change will normally be smaller and take place over a longer period.

An application of Kurdeez Lime offers the following benefits:

• Better establishment of crops, Lucerne and pasture - particularly perennials

• More vigorous pastures, and lucerne

• Higher crop and pasture production

• Increased livestock carrying capacity

• Greater persistence of improved pasture species and therefore less weeds

• Improved response from fertilizer application

• Greater management flexibility in terms of cropping and pasture production choice

• Greater security and sustainability for the future

• A – NITROGEN            E – CALCIUM        I – COPPER & ZINC
• B – PHOPHEROUS       F – MAGNESIUM   J – MOLYBDENUM
• C – POTASSIUM          G – IRON
• D – SULPHUR              H – BORON
• (The width of the band indicates the degree of nutrient availability)

Rates of application will depend on soil pH and the exchangeable aluminium level as determined by soil testing. Normal applications are between 1.5t and 5t per hectare and is best done at, or prior to the time of pasture re-sowing. If this is not practical, topdressing onto pastures at any time will have an effect.

On relatively acidic soils such as those below pH (Calcium Chloride, CaCl₂ ) of 5.0, lime application ranging from 2.5 – 5.0t/ha should be applied.

For pasture establishment on moderately or strongly acid soils, or where pH sensitive species are to be sown, sowing with a source of nitrogen supply will stimulate early plant growth and result in improved Autumn/Winter establishment.

Depending on the results of soil tests and the type of crop, lime may be required at rates of 2.5 to 5.0t/ha.

Apply the lime at least 4 to 6 weeks prior to planting, and much earlier if possible, to allow time for it to have an effect.

In potatoes, the disease powdery scab may affect the crop if lime is applied too close to planting, or if high rates of lime have been applied.

The finer the lime, the more effective it is.

Dried Agricultural lime must be used in blends with seeds and other fertilizers. Both Dried or Screened Agricultural lime is suitable when lime is applied alone. The latest analysis of the Kurdeez Limes is available on the website at www.kurdeezlime.com.au