Soils may be acid, alkaline or neutral, with the pH scale ranging from 0 to 14, with all readings on the low scale of 7 being increasingly acid as the figures move towards zero.

The soil becomes increasingly alkaline as the figures move higher from the neutral base of pH 7.

Plants need to be able to access a range of different chemical elements from the soil if they are to make satisfactory growth and at pH 6.5 to pH 7 all of those elements are readily available to the plants. As the level drops down towards pH 5.5 and 5, some of the elements become less available while others become more readily available, the most dominant of these being iron, which at these levels is in plentiful supply.

Because of the increased quantity of iron in the soil, elements such as phosphorus and molybdenum become tied up with the iron and therefore plants can't get much of them.

This is fine for azaleas and all the other acid lovers that require lots of iron and for the proteacea plants such as grevilleas that need very little phosphorus. It is not good though for the leguminous plants, which include peas and beans, that need the trace element molybdenum.

In all alkaline soils, which are more often found in coastal sands and in areas where salty bore water is used, a similar situation exists. As the soil pH becomes increasingly alkaline at around 8 to 8.5, phosphorus again becomes locked up and the availability of iron and most of the trace elements is significantly reduced.

In terms of nutrient availability, the soil pH plays a most important role but in these days when we are using more and more organic fertilizers the significance of soil pH increases.

The nutrients in organic fertilizers do not, as is the case with chemical fertilizers, dissolve in water - they must be broken down by soil organism, worms being just one of these, into a form that the plant can take up.All of these soil organisms prefer to work in soil with a pH just above or below neutral.

So, the smart gardener will get a pH test kit and use it regularly - you'll be amazed at the improved growth that results, simply because the fertilizers being applied are now producing the growth that they should.

If you test the soil and find the pH level is too high there are a few things that can be used to lower it.

Powdered sulphur is usually available at a fairly reasonable price from produce agents and this is the preferred method, but there are other products that contain aluminium sulphate, such as Hydrangea Bluer, which also will do the job, if the pH is quite high. Use only the powdered sulphur as aluminium sulphate products will seriously damage the soil structure if used at high rates.

To raise the soil pH I always recommend dolomite in preference to garden lime, even and magnesium is a most important plants nutrient.