ph Measurement of Soil

IF YOU ARE AFTER INTERPRETATION OF YOUR pH MEASUREMENTS - pH is very confusing, very important and pretty useless all at once. The solubility of both nutrients and toxins and hence how welcoming the soil will be to animals and plant roots are affected.

Aluminium-dominated soil is generally referred to as 'acid' soil as low pH below 5 will dissolve poisonous concentrations of aluminium if possible and for many years it was thought the acidity itself poisoned living things. Occasional mineral soils and many organic soils do not have the reservoir of potentially toxic aluminium and are therefore much more fertile than you'd expect. By the same sort of well-meaning but mistaken speech, sodium-dominated soils are referred to as 'alkaline' because of free sodium carbonate in the soil but the physically nasty properties of so-called alkaline soils are due to too much sodium, rather than at pH near 9. I found a donga, an erosion gulley, in Swaziland that contained 'neutral' soil - actually simultaneously 'acid' and 'alkaline'- because it was 55% saturated with aluminium and 35% saturated with sodium, but this is once-in-a-lifetime rare. It does though emphasise the point that 'acid' and 'alkaline' are not opposites in soils. In the soils most organisms prefer calcium dominates and their pH measures between 5 and 8.

This gives the clue to the amelioration of acid and alkaline soils. Enough lime, mainly calcium carbonate, to neutralise the aluminium in the case of 'soil acidity' but this particularly in tropical soils is something to measure experimentally. If the problem is too much sodium then replace it with gypsum, calcium sulphate; if there is plenty of free lime in the soil the gypsum can be formed in situ by irrigating with sulphuric acid or more safely by scattering powdered sulphur which bacteria will oxidise to sulphuric acid. This approach avoids the horrible dangers of over-liming acid soils but improving alkaline soils is still an art as well as a science.

Briefly, sodium-rich soils tend to erode easily and allow water to pass through them very slowly; they form in dry places where you may only find sodium-rich groundwater; and to get the sodium out it has to flow down the soil into the groundwater - careful lest it comes back up somewhere else to destroy someone else's land!

To conclude, do measure soil pH, and if low measure available Al too. Lime according to the Al measurement and not the pH. If pH is high treat according to how much excess Na needs to be removed.