The Loam Ranger – Reducing soil pH
Dear Loam Ranger,
My turfgrass soil shows a pH of above 7. I know that the best pH range is 6 to 7. Should I try to reduce it, and how?
It is not always easy to reduce the pH of an alkaline soil; it depends on how much lime is present. Whether you even need to depends on what you are growing.
What is alkalinity?
Alkalinity in urban areas usually occurs when lime (calcium carbonate) is present in or has been added to the soil. Some soils are naturally limey (e.g. coastal sands containing shell grit), and some become limey as a result of too much lime or, quite commonly, the presence of crushed concrete or cement.
Naturally alkaline soils are more common in arid and semi-arid regions, but can also occur in wetter regions on chalk or limestone parent materials or when shell grit is present. Such soils are called calcareous soils.
Nutrient deficiencies are easy to overcome
Problems in both naturally alkaline soils and over-limed soils are due mainly to nutrient imbalances, not to the direct effects of soil pH on plant growth. The nutrient deficiencies are most commonly trace element deficiencies, since the carbonates of iron, manganese, zinc and copper, for example, are highly insoluble and thus unavailable to plants at high pH.
Plants differ in their ability to extract these elements from alkaline soils. Plants that are not good at it were traditionally called “acid loving”, but the term “iron inefficient” is more accurate. Notable iron-inefficient plants are citrus, azaleas and camellias, which grow poorly in limey soils. The ideal pH for the growth of turf and generally for most plants lies between about 5 and 7, because this pH range improves the availability of trace elements. Trace elements can be supplied in chelated form to overcome this insolubility, but applications need to be repeated regularly. Alternatively, the soil can be acidified.
Calcareous soils and buffering capacity
Calcareous soils contain calcite (CaCO3), dolomite (CaMg(CO3)2) or both. These can be destroyed only by acids, releasing carbon dioxide and Ca and Mg ions. However, sufficient acid must be added to completely destroy all the carbonate before the pH will start to decline. This delay is responsible for the soil’s buffering capacity.
How to lower pH
These are two main ways gardeners and farmers can lower soil pH: by adding iron sulphate or elemental sulphur (called ag sulphur). To destroy 100 g of lime requires 32 g of ag S or 278 g of iron sulphate. Iron sulphate is purely chemical in its reaction, whereas acidification by ag S is biological, requiring S-oxidising bacteria, which use the energy released from the S and from organic matter, and in the process release sulphuric acid (H2SO4). This requires 6–8 weeks to occur in good soil moisture conditions.
Alkaline or alkaline + calcareous?
Before trying to reduce the pH of your soil, you must determine whether the soil is just alkaline or is both alkaline and calcareous (calcareous soils will take much longer to correct than plain alkaline soils). SESL can do this for you, and tell you how much S or iron sulphate to use.
You might be surprised to see how much S you need, how many times you’ll have to apply it and how many years it will take to make a difference. Naturally alkaline calcareous soils can easily contain 2% to 3% lime, which can make pH reduction impractical for agriculture. In an urban environment, the cost is not so limiting, but to eliminate 2% lime from 1 m2 of topsoil 200 mm deep will still take about 2 kg of ag S or 16.7 kg of iron sulphate. Applying this much all at once would certainly kill your plants, so it would need to be added slowly over several years, making the task a long-term project.
Where only small amounts of lime and alkali are to be reduced, you can use iron sulphate at no more than 100 g/m2 until the symptoms of iron deficiency, as seen for example in citrus, are permanently eliminated.
But where significant amounts of lime are to be destroyed, laboratory services are essential, as it is easy to apply too much acidifying agents. In turf management, a management plan must be drawn up.
Laboratory testing
Regular soil testing backed up by laboratory monitoring is essential to a successful pH reduction program in severe cases. SESL can advise you of exactly how much S or other ameliorants to add to your soil, when to add it and for how long. Achieving reduction of soil pH in seriously alkaline soils is not a short-term task for the faint-hearted, so you will need a long-term partner in a long-term program.