pH image2Fiona Smith – AB Lime Environmental Manager

BSc. MSc. Soil Chemistry


Soil pH, reserve acidity and buffering capacity

pH is a measure of the concentration of Hydrogen ions (H+).  High levels of H+ means greater acidity and this is reflected in a low pH reading.  The pH scale goes from 1- 14, with 1 being very acidic.  It is important to remember is that this scale is logarithmic so this means that a pH value of 5.6 is 4 times more acidic than a pH of 6.2, so a small variation is soil pH has a huge impact on soil acidity.

Soil pH is the measure of the H+ ion concentration in the soil solution (active acidity) however this only represents a small proportion of the total H+ ions in the soil.  H+ ions are also held on exchange sites on soil particles and this is called reserve acidity and may be much greater than soil solution pH.

When the H+ concentration in the soil solution is increased or decreased in soil solution there is a shuffling of H+ between soil solution and the reserve sites.  This means that there is a smaller change in soil pH as it is being buffered against changes in pH by the reserve acidity.

All soil types have their own ability to buffer soil pH changes – that’s why the application of specific quantities of lime to different soils result in different pH changes.

Why is soil pH important?

Soil pH affects the soil's physical, chemical, and biological properties and processes, as well as plant growth. The nutrition, growth, and yields of most crops decrease where pH is low and increases as pH rise to an optimum level.  Many plants optimum growth occurs in the pH range of 6-7.  Soil biological activity is also affected by pH.  Most soil organisms also function best with a pH range of 6-7.

Soil pH influences:

  • Soil microbe activity
  • Nutrient availability
  • Toxic levels of Aluminium
  • Gaseous exchanges in the soil

Factors affecting soil pH include:

  • Parent Soil Material
  • Rainfall
  • Nitrogen Applications
  • Cropping sequence
  • Organic Matter Breakdown

How does Lime work?

Liming soil involves the use of calcium carbonate (CaCO3) to neutralise soil acidity.  This is a chemical reaction that occurs in the soil.  It is the carbonate fraction of lime that does most of work by reacting with H+ and producing carbon dioxide and water, thereby lowering the total H+ concentration in the soil.  The calcium (Ca) fraction of lime can displace H+ on the surface of soil particles and they move into the soil solution.  This is how a soil “buffers” lime applications and this is why all soils respond differently to lime applications.

Not all lime is created equal though!  The efficiency of lime to neutralise soil acidity over time is dependent the percentage of CaCO3 is present in the rock, and the range of particle sizes – More on this in our next blog!  Lime also helps to maintain soil structure which makes it more resilient to compaction, water drainage and cultivation.

Why aim for a soil pH of 6.2?

There is a lot of information out there that will tell you that a pH of 5.8 is good enough.  Studies have been done to show that for some species of pasture, production levels out around a soil pH of 5.8.  However this pH does not take into account about the need to create an environment in your soil that maximises nutrient availability, soil biology, soil resilience and long term soil health.  Farmers have come a long way from just being concerned about production at the cost of the long term soil health of their farm.  If you aim for a pH of 6.2 not only will you maximise production, but you will also be maximising soil health which means a productive, healthy and resistant farm soil for years to come.

Our next blog will be on how not all Lime is Created Equal – See you then!