Like many things in life, soil acidity is all about balance. In this case, it’s about the equilibrium between the hydrogen ions (H+) in the soil solution, and those attached to soil particles.
If the balance is disturbed, and there are more H+ ions in the soil than attached to the soil particles, soil acidity occurs. This creates a problem for plants, because in acidic soils, nutrients may become either unavailable, or toxic.
A healthy soil acidity means healthy soil and healthy animals.
Soil acidity and pH
Soil acidity is expressed as pH. First described in 1909 by Danish biochemist Søren Peter Lauritz Sørensen, the ‘p’ stands for potenz ‘power, or potency’ in German, while the H is the chemical symbol for hydrogen. So pH represents the ‘potency of hydrogen’ in a solution, or more accurately, the concentration of H+ ions in the soil.
We talk about pH using a scale of 0 - 14, where 7 is neutral, less than 7 is acid, and greater than 7 is alkaline. Soils are typically between pH3 and pH9, but optimum levels for healthy growth in most plants is 5.8 - 6.0, so they prefer soil that is slightly acid.
Causes of acidity
A lesser known fact is that under normal conditions, soils become acidic over time. This is due to natural processes like weathering, and the subsequent leaching of the weathered materials.
However, other processes can increase acidity. Plant growth and microbial activity both lower pH. In plants, the roots exchange H+ ions for nutrients from the soil solution, thus increasing the H+ concentration in the soil. Microbes transform compounds into nitrates and sulphates, just two of the many processes which result in adding H+ ions to the soil solution.
Soil is also acidified when nutrients such as calcium, magnesium, and potassium ions are leached from the soil. When this happens, the nutrient balance is disturbed, and the soil becomes more acid.
Another acidification process involves aluminium. Some rocks and soils carry large amounts of aluminium ions (Al3+). When the aluminium reacts with water during a process known as hydrolysis, significant quantities of H+ ions are released into the soil, making it more acid.
Some fertilisers can increase soil acidity. For example, Superphosphate is known to create a very acid environment around the root zone of plants, affecting plant uptake of phosphates. Nitrogen fertilisers containing ammonium cause acidity when microbes break them down. Elemental Sulphur is another fertiliser known to increase acidity, but this is helpful for growers of acid-loving crops like blueberries.
Why soil acidity is a problem
One major problem with soil acidity is that it causes many nutrients to become unavailable to plants. Phosphates for example, which are vital for plant growth and development, become increasingly inaccessible once pH drops below 6.
On the other hand, micronutrients such as iron, copper, manganese, and zinc become more available to plants because they are highly soluble in acid soils. However, this creates a problem, since plants only need tiny amounts of the micronutrients. The increased quantities can become toxic.
Another element that increases availability when acidity escalates is aluminium. Once the pH dips below 5.4, the level of Al3+ ions in the soil solution rises, and plant roots suffer deformity.
Yet another issue with soil acidity concerns clover and the reduction in rhizobial formation of nitrogenase. This harms clover’s ability to fix nitrogen.
Solutions to soil acidity
The traditional way of managing acidic soils is to apply lime. Lime is a source of calcium carbonate, and it’s the carbonate which reacts with the acid in the soil to increase the pH and reduce acidity – it’s not the calcium.
While Ag Lime has been the default choice for many years, there are new options available. Fertco’s KingPhos, for example, is a dicalcic phosphate fertiliser, developed to simultaneously lift pH.
This article was published in the Coast & Country News.
