What is pH?

We have all heard of pH, whether we garden, raise crops, or manage pastures. We know that some plants like a more acidic soil, while some prefer a more alkali soil. But what exactly do acid and alkali mean?

The pH scale measures the concentration of hydrogen ions in a dissolved substance. Pure water can be used as a reference point; its pH is 7, or neutral. Any number below 7 is acid, and any number above 7 is alkali. An acidic substance has a greater concentration of hydrogen ions than pure water, while an alkali substance has a lower concentration.

It is interesting to note that pH also represents a relationship between the positive hydrogen (H) ion and the negative hydroxide (OH) ion. When the pH is neutral (7), hydrogen and hydroxide ions are in perfect balance. An acid has a preponderance of hydrogen ions, while an alkali has more hydroxide ions.

On a practical level, pH dictates what nutrients will be available to a plant. As the pH level moves along the scale, various minerals in the soil are locked and unlocked. Most of the nutrients that plants need to thrive are available when the soil is neutral to ever-so-slightly alkali.

The pH scale is a useful tool in animals, as well. Most animals have a blood pH that is just slightly alkali, while urinary pH is generally a little more acidic, and rumen or stomach fluid is more acidic still. The correct numbers vary depending on the species, but pH can sometimes be used to help identify health problems in livestock.

Improving Your Garden Soil