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Soil pH - 2001

Based on some recent discussions on soil gypsum requirements, there seems to be a need to discuss pH -  what it is, how it is measured and what it means.  The pH scale is a measure of the amount of hydrogen present in a system, in other words, how acid the system is.  The larger the number, the less hydrogen present.  Although the scale runs from 0 (very acid) to 14 (very basic), for most California agricultural soils we are concerned with a range of about 4 to 9.5.  At a pH of 9.5, a soil is classified as an alkali or sodic soil, which is due to an accumulation of sodium carbonate resulting from poor drainage.  Soils with this pH would be an unlikely choice for growing avocados or citrus not just because of the pH, but the poor drainage.  A low pH near 4 indicates a coarse textured soil of granitic origin or a poorly buffered (low ability to resist change) soil that has been artificially acidified.  This might happen through continual use of ammonium-based fertilizers or acidifiers, such as sulfur.  

A common misconception is that saline soils (those high in soluble salts, such as calcium sulfate and sodium chloride) have a high pH.  In fact, most salts lower a soil's pH.  This salt effect, depending on the soil, can be as much as 0.5 to 1.5 pH units.  In severe situations,  saline soil can be recognized by white salt crusts and  damp, oily-looking surfaces devoid of vegetation.

In  calcareous soils which are dominated by calcium carbonate, the pH is typically around 8 - 8.4.  The source of the carbonates can be inherited from the soil itself. Frequently it is possible to see old shells from when the land had been underwater.  The carbonates can also derive from irrigation water high in bicarbonates.  The bicarbonates cause soil calcium to precipitate as calcium carbonate.

Measurement of  soil pH can be highly ambiguous.  This is not necessarily the fault of the instruments that measure pH.  For example, in analyzing water samples there can be accuracy to two decimal places.  The problem is with soil itself.  First of all it is far from consistent - roots, worms, clay lenses, leaves, etc.  all make soil quite a hodge podge.  This means that sampling is extremely important to get a good soil representation; that is, one that will help you manage your farm. 

Once in the lab, procedures are followed that can add further to the confusion.  Two factors which are extremely important in pH measurement are 1)the soil to water ratio used to take the reading and 2)the salt concentration in the soil solution.  Increasing either, lowers the pH reading.  Depending on the lab, a pH measure is made on an extract from  1) a paste (just adding enough water  to make the soil glisten), 2) a 1:1 soil-water ratio, 3)a 1:5 ratio, or 4) a1:10 ratio.  Adding more water dilutes the actual soil pH and usually causes a slight rise in pH.  The amount of increase is a function of soil texture.  The amount of salt in the soil will also affect the pH by lowering it as much as 1.5 units if there is a substantial amount of salt present.  Some labs will add salt to soil in order to standardize readings.  The procedures followed by labs are accepted practices.  However, if you are making comparisons of the same soil done by different labs, it is important to know their procedures to determine if the results are even comparable.

Timing of the sampling can have a tremendous effect on measured pH.  Taking the pH reading of a soil that has been dried out and sitting around will be different from a fresh sample.  Because of the seasonal variability in salts and soil biological activity which are both affected by rains, irrigation and temperature regimes, a measured soil pH can not really be much more accurate than +/- 0.5 pH unit.  This means worrying about the last decimal point in a soil pH of 6.35  is not worth it when the same soil might vary seasonally between 5.8 and 6.8  It's not the pH being wrong, it's just the nature of soil.

Soil pH is one of the most important pieces of information to aid in identifying and preventing problems in the orchard.  Proper sampling provides the best control over getting useful information out of a soil pH.   There are inherent limits to the accuracy of the values, though, just as with most other biological systems.