• Soil pH is a measure of the concentration of hydrogen ions (H+) in the soil solution. The lower the pH of soil, the greater the acidity.
• pHwater should be maintained at above 5.5 in the topsoil for pastures and 5.2 in the subsoil.
• pHwater should be maintained at above 6 in the topsoil for most crops and above 5.5 in the subsoil. Many vegetable crops do better at pHwater around 7.
• A well-maintained soil pH will maintain the value of the soil resource, maximise crop and pasture choice and avoid production losses due to low soil pH and associated nutrient uptake issues.

Soil acidity is a major environmental and economic concern. Approximately 50% of Australian agricultural land (approximately 50 million hectares) have surface pH values less than the optimum level to prevent subsoil acidification. Most Tasmanian soils are naturally acid with pH in the topsoil as low as pH 4.5 because of our high rainfall and where there are high organic matter contents. Topsoil pH values for agricultural lands in Tasmania show no sign of increased acidity associated with more intensive use in contrast to large areas of cropping country on mainland Australia. Soil pH in Tasmania was found to be greater under cropping systems compared to pasture, which is likely to be due to the application of lime or dolomite (Cotching et al. 2001). Lime and dolomite are readily available in Tasmania and distances from quarry to paddock are not great, which keeps transport costs down.

Acidic soils result in significant losses in production and where the choice of crops is restricted to acid tolerant species and varieties, profitable market opportunities may be reduced. In pastures grown on acidic soils, production will be reduced, and some legume species may fail to persist. If untreated, acidity will become a problem in the subsurface of soils, which are more difficult and expensive to ameliorate.

Soil pH, or soil reaction, is an indicator of the acidity or alkalinity of soil and is measured in pH units. The pH scale is logarithmic and goes from 0 to 14 with pH 7 as the neutral point. The lower the pH of soil, the greater the acidity. A soil with a pH of 4 has 10 times more acidity than a soil with a pH of 5 and is 100 times more acid than a soil with a pH of 6.

Soil pH affects the solubility of minerals or nutrients essential for plant growth. In very acid soils, all the major plant nutrients (nitrogen, phosphorous, potassium, sulfur, calcium, magnesium and the trace element molybdenum) may be unavailable, or only available in insufficient quantities. Plants can show deficiency symptoms despite adequate fertiliser application. Extremely and strongly acid soils (pHwater 4.0-5.0) can have high concentrations of soluble aluminium, iron and manganese, which may be toxic to the growth of some plants. Plants such as lucerne, barley and canola are highly sensitive to aluminium toxicity caused by a strongly acid pH.

Soil pH is normally reported as pH in water (pHwater) and pH in calcium chloride (pHCaCl2). The pH measured in 0.01M calcium chloride solution is designed to more closely resemble the conditions experienced by plants in soil and is often more consistent over time. The pH measured in calcium chloride is on average 0.5 to 0.8 units less than pH measured in water. If pHwater and pHCaCl2 have similar values, i.e. the pH measured in water is nearly as low as that measured in calcium chloride, the soil will most likely be saline.

Soil pH in water is commonly described using the following terms:

Soil pH provides various clues about soil properties and is easily determined. There may be considerable variation in the soil pH from one spot in a paddock to another. Your result is only as good as your sample. To determine the average soil pH of a paddock it is necessary to collect soil from several locations, combine and thoroughly mix these before taking a subsample for testing. The timing for collection of the soil sample and the sampling depth are important as the acidity of soil varies throughout the year, and down the profile. The pHwater in summer is in many circumstances lower than that in winter by up to 0.5 of a unit and this is generally attributed to soil drying, root and bacteria activity, and nitrification of nitrogen fertilisers. This is important when making recommendations for winter crops based on analysis of samples taken over summer. The pH can vary down the soil profile with the surface soil often being less acidic than the subsoil in Tasmania.

The most accurate method of determining soil pH is with a pH meter. A less accurate result can be obtained using an inexpensive pH test kit available at most nurseries or hardware stores. These test kits generally consist of some testing solution, a colour chart and sometimes a test tube. You put a sample of your soil either onto a sheet of white cardboard or paper, or in the test tube, add a few drops of test solution, shake the tube up and leave it for an hour or so to settle. The solution on the card or in the tube changes colour according to the pH of your soil. Compare the colour of the sample with the colour chart that came with the kit. Matching colours will tell you the pH of your sample.

Soil acidification is a natural process accelerated by agriculture. Soils tend to become acidic as a result of (1) rainwater leaching away basic ions (calcium, magnesium, potassium and sodium) (2) carbon dioxide from decomposing organic matter and rainwater forming weak organic acids (3) decay of organic matter and ammonium (including urea) and sulfur fertilisers. Rainfall contributes to a soil’s acidity. Water (H₂O) combines with carbon dioxide (CO₂) to form a weak acid — carbonic acid (H₂CO₃). The weak acid ionizes, releasing hydrogen (H+) and bicarbonate (HCO₃). The released hydrogen ions replace the calcium ions held by soil colloids, causing the soil to become acidic.

Most plant material is slightly alkaline and removal by grazing or harvest leaves residual hydrogen ions in the soil. Over time, as this process is repeated, the soil becomes acidic. Major contributors are hay, especially lucerne hay and legume crops. Alkalinity removed in animal products is low, however, concentration of dung in stock camps adds to the total alkalinity exported in animal production. Most Tasmanian farming systems require inputs of lime to maintain soil pH with cropping systems generally more acidifying than pasture-based systems, although continuous legumes with pastures cut for hay have very high acidification rates. Fertiliser nutrients can also acidify soil. Nitrogen is the main nutrient affecting soil pH with nitrate-based products the least acidifying and ammonium-based products having the greatest potential to acidify soil. Phosphorus and sulfur fertilisers have a small acidifying effect on soil pH and potassium fertilizers have little or no effect on soil pH (Fertiliser Technology Research Centre 2009). The acidification potential of various N, P and S fertilisers can be expressed in terms of kg lime equivalent to neutralise the acidity generated by one kg of nutrient added in different forms (Appendix 5).

In strongly acid soils (pHwater < 5.5) the major plant nutrients (nitrogen, phosphorus, potassium, sulfur, calcium and magnesium) and the trace element molybdenum, may be restricted in their availability to plants. While many plants can tolerate pHwater ranges between 5.2 and 7.8, most plants grow best in mineral soils when soil pHwater is between 6.0 and 7.0 (slightly acid to neutral) (Appendix 6). Earthworms also prefer neutral to slightly acid soil conditions. This general rule applies to most of the commonly grown crops, fruits, vegetables, flowers, trees, and shrubs. Most turf grasses tend to grow best between 5.5 and 6.5. Many evergreen trees and shrubs prefer a pH range of 5.0 to 6.0. Poppies, brassicas and onions are susceptible to Al toxicity which is prevalent at low soil pH, and the contract requirement of growing these crops only on paddocks that meet a minimum soil pH has encouraged lime use in Tasmania. Many cropped topsoils are now at a pHwater of 6.0-6.5. Potatoes tolerate a wide range in soil pH while blueberries require acid conditions between pHwater 4.5 and 5.2. Brassica crops require pHwater of 7.0-7.5 to minimise the risk of clubroot infections. Observations from some vegetable producers in Australia are that a pHwater of above 6.5 reduces the occurrence and or severity of soilborne diseases. However, previous liming has been found to increase the severity of common scab in potatoes to the point of tubers being unmarketable (Sparrow and Salardini 1997). The signs of soil acidity tend to be subtle and may be seen as acid sensitive crops failing to establish or persist e.g. lucerne, or crop production being lower than expected, particularly in dry years. Shallow root growth and poor nodulation in legumes or ineffective nodules are added signs of acidic soil.

Rhizobia bacteria are affected by acidic soil conditions which decreases legume nodulation resulting in less symbiotic nitrogen fixation and availability. The resulting nitrogen deficiency may be indicated by reddening of stems and petioles on pasture legumes or yellowing and death of oldest leaves on grain legumes. Some pasture legumes may fail to persist due to the inability of reduced Rhizobia populations to successfully nodulate roots and form a functioning symbiosis.

As soil acidity increases (the lower the pH), more lime is needed to ameliorate soil acidity. You will have to add more lime to clay soils and peaty soils than you will to sandy soils to achieve the same result because different soil types react in different ways to the application of lime. The amount of lime to apply depends on three main factors: neutralising value, fineness of the lime and soil texture.

It is best to apply at least 2.5 t/ha on all soils to get a good pH response but the upper limit for one application is 7.5 t/ha. Lime must be physically in contact with moist acid soil in order to neutralise acidity. Lime dissolves slowly in the soil, therefore, incorporation in the top 10cm of soil (or deeper if possible) is best to increase the rate of reaction and leaching of lime to a greater depth. Incorporating lime will increase soil pH in the 0-10cm soil depth within 1-3 years. High rates of lime are needed to change the pH of Ferrosols because they are highly ‘buffered” which is caused by a combination of iron and aluminium oxide clay minerals plus organic matter. Ferrosols are also known as having variable charge, which means that as the pH changes so does the cation exchange capacity. Tasmanian research has shown that as organic matter content declines on Ferrosols, cation exchange capacity can be maintained if pH is increased (Sparrow et al. 1999).

The best way to adjust pH is gradually, over several seasons. Lime should be applied only when tests show it to be necessary. Do NOT over-lime. Lime adjusts soil chemistry; it is not a fertiliser.

High pH soil may be acidified by adding elemental sulfur. Sulfur is transformed by soil bacteria to sulfuric acid which will neutralize soil alkalinity. Like lime, sulfur should not be applied indiscriminately. Due to the total amount of lime present in alkaline soils, this is a never-ending battle. As soon as the sulfur is “used up” pH will begin to return to original levels. Sulfur is useful for reversing the effects of over liming or for changing soil pH in a small area for specimen trees or shrubs. Addition of organic matter and use of organic mulches combined with the use of acidifying fertilsers such as ammonium sulfate or urea, can also help acidify soils and lower pH levels over time. The sulphur can be broadcast onto the soil surface, but it must be mixed into the soil to a depth of 150 mm (6 inches). Simply spreading the sulfur on the surface of the soil does very little good, but is not entirely useless, but burying it helps ensure that the sulfurous acid doesn’t simply evaporate. Alternatively, you can make holes about 30 cm deep and 50 mm in diameter and fill them to within about 10 cm of the top with sulfur, then cover them back up. The sulfur will slowly turn into sulfurous acid (H2SO3) and acidify the soil near it. The amount of sulfur (95% S) needed to lower the soil pH to pH 6.5 on clay soils varies depending on the existing soil pH. The requirement is 1kg/10m2 at an existing pH of 7.5, 2 kg/10m2 required at pH 8.0, and 3 kg/10m2 required at pH 8.5. Lowering soil pH by adding sulfur can take over 2 years or more, depending on soil microbial activity. When lowering soil pH, remember that the natural tendency of soils in Tasmania is to acidify. This process is hastened by the use of acidifying fertilisers such as urea or ammonium sulfate.