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What is soil pH?

Dr Jay Howes discusses soil pH

What is soil pH?

When you look at a soil test, usually the first entry is pH. So, what exactly is pH and what does it mean for the soil?

The pH levels of most New Zealand soils generally fall within the range of 5.0 to 7.0. The pH of the soil is intrinsically linked to the overall productivity as it governs processes in the soil such as nutrient availability, microbial activity and root elongation.

The term pH in chemistry donates ‘potential of hydrogen’. It is used to measure acidity or basicity of a solution. The hydrogen ion (H+) concentration in soil, the sole cause of soil acidity, ranges in pH from 0.0 to 14.0. Depending on which end of the scale the pH is on, a soil can be either acidic (more H+ on the soil) or alkali (less H+ on the soil). Acidic soils have a pH level between 0.0 and 6.9 and alkali soils have a pH value ranging from 7.1 to 14.0, whereas a pH of 7 is considered neutral (neither acidic nor alkali).

At different pH levels, the various nutrients in the soil become more or less available to plants. An excessively acidic or alkali soil can have a negative impact on the potential yield of pasture or crops grown on it. For example, when soils become too acidic, typically less than 5.5, essential nutrients like Phosphorus (P) and Molybdenum (Mo) may become less available to plants. Likewise, when soil pH gets too high, typically greater than 6.5, P and some trace elements like Iron (Fe) and Manganese (Mn) can start to become less available, while Mo can become too available and potentially cause metabolic issues in stock. The ideal soil pH level is approximately 5.8 to 6.2, depending on the crop grown.

Pastoral and cropping soils are typically slightly acidic. Though this is fine, over time soils naturally become more acidic due to a variety of factors including plant uptake of essential nutrients, nutrient leaching, decomposition of Organic Matter (OM) by microbes and the application of certain fertilisers such as elemental sulphur. Once soils become too acidic, typically, production can suffer.

The way to counteract acidification is to apply lime. Liming materials are usually Calcium Carbonate (CaCO3) or Magnesium Carbonate (MgCO3), and it is the carbonate portion that is active in neutralising acid in the soil not the calcium or magnesium. A well-known ‘rule of thum0’ when looking to increase pH in a soil is that 1 tonne per hectare of lime with 80 percent calcium/magnesium carbonate or better will raise the pH of a typical soil by 0.1 units to a depth of 7.5 cm.

It is important to remember this is only a ‘rule of thumb’ and the pH change is very dependent on soil type and the quality of lime. Once a soil is within an optimum pH it can be maintained with a regular maintenance lime programme. This is the cornerstone of a good soil as it ensures that the widest range of nutrients in the soil are usable for plants for ideal crop and pasture production.

To learn your soil pH on-farm, contact your local PGG Wrightson Technical Field Representative.