The Importance of Soil pH

September 19, 2010 0 By Richard

Most people are aware that some plants and trees thrive in acidic soils and others will only grow in alkaline soils but why is this the case?

First lets touch on some basic chemistry. pH is a measurement of acidity or alkalinity on a scale of 1 to 14. A number below 7 indicates an acidic substance, above 7 indicates and alkaline and 7 represents a neutral substance. You can buy soil testing kits to find the pH of your soil from most garden centres and (in the UK) you are likely to have a pH from about 4 to 8.

The pH of your soil is determined by the amount of calcium or lime (as lime consists mainly of calcium in various forms) contained within the soil. Calcium is an alkaline element and it is usually lost from the soil over time as it is washed out of the soil. In some environments, such as soil over a bedrock of limestone, the calcium is replaced on an ongoing basis. Other soils, such as sand, are unable to hold on to the calcium and slowly turn more acidic over time.

At this point I have to take you on a tangent and look more closely at what plants need from soil. Soil is full of nutrients which plants need to grow and particles of nutrients are ‘stuck’ to the soil by adhesion at the atomic level. The nutrients stick to both inorganic matter (clay particles) and organic matter (highly decayed plant matter) within the soil known as colloids. The other inorganic parts of the soil such as sand and silt are chemically inactive so nutrients are unable to stick to them. This is why calcium is easily washed from sand and is why clay is so important in healthy soil.

So what does this mean to my plants?

Different nutrients are available at different soil pH levels. This affects the way plants grow in different soils.

Some plants are very sensitive to soil pH, the most well know example are Rhododendrons and heathers which will not tolerate lime in the soil. On the other hand there are plants which greatly prefer a lime soil.  This is because of the nutrients available to the plant at a particular pH.

Soil pH affects which nutrients are available to a plants because minerals will be solid at a certain pH and soluble (able to dissolve into the water) at another pH. If the nutrient is solid it is unavailable to to plant. The nutrient needs to be dissolved before it can be absorbed by the plant or stick to the colloids (clay particles or decayed plant matter) in the soil.

Aluminium is not a plant nutrient and if the soil pH is over 5 it remains as a solid and is washed out the soil by water. However, if the pH drops below 5 aluminium becomes available to be ‘stuck’ to the colloids. If the pH were to continue to fall below pH 4.5 the amount of aluminium becomes toxic to some plants and the aluminium particles are in such great numbers they displace the other nutrients from the colloids. This reduces the amount of other nutrients available to plants.

So acidic soils are damaging because of the amount of toxic aluminium where as alkaline soils affect plants because the nutrients needed by the plant are kept lock up in solid form at the higher pH. For example plants in a high pH soil can suffer from manganese deficiency where as manganese would be in excess in low pH soils. If a plant is suffering from a Iron, Boron or Manganese deficiency in a high pH soil you are unable to correct the deficiency by adding more of the nutrient as it will remain ‘lock up’ in solid form until the pH is lowered.

The pH of acidic soils can be increased by adding lime whereas alkaline soils can have their pH lowered by adding peat or other decayed plant matter.

Soil pH also affects the organisms found within the soil. Earthworms are important as they aerate  the soil and help bind it together but they prefer non-acidic soils and need high amounts of calcium available. Beneficial bacteria will generally tolerate a wide pH but most fungi prefer acidic soil.

So as you can see soil pH has a complex relationship with plants and organism within the soil and that the wrong soil pH could starve your plant (or tree) of essential nutrients or poison it with excess aluminium.