Acidity and alkalinity in any solution is measured as pH. Soil pH is an estimate of the acidity/alkalinity of the soil solution, which is the water that is held in the soil.
Below pHCa 4.4 most plants, except the very highly acid tolerant plants like oats, narrow leaf lupins and the native pasture grass Microlaena spp, show a significant reduction in production. Even tolerant species productive capacity will be affected by soil acidity. Although plant death may not be the result production will be suboptimal
If the pHCa of the 0–10 cm layer is less than 4.8 then the subsurface soil may be acidic.
In higher rainfall areas subsurface soil layers are more likely to be acidic.
Field pH test kit (shown here) uses the water method and pH values are 0.5 to 0.8 higher than the pH in Calcium Chloride.
The acidity of soil varies throughout the year, and down the profile. The pH in summer is in most circumstances higher than that in winter by up to 0.5 of a unit. Therefore the timing for collection of the soil sample and the sampling depth are important considerations. This is particularly important when making recommendations for winter crops based on analysis of samples taken over summer.
In this publication recommendations are made on the basis that the samples for analysis are taken to a depth of 10 cm, in late summer or early autumn.