When most people think of soils in terms of plant growth, they consider soil moisture, how hard or loose the soil is for plants to grow through and the nutrient status of the soil. Too often, whether in production agriculture or not, one factor is often overlooked. This factor plays a role in all aspects of the soil environment. That factor is soil acidity which plays a huge role directly and indirectly in plant growth. This week’s column starts the examination of what soil acidity is with following columns devoted to its effects on the soil as a plant growth medium. Understanding what acidity is the first order of business.
In general terms acidity is the measures the concentration of hydrogen ions in solution. Hydrogen ions are cations with a positive charge of +1. The next logical question is where do they come from? Water, H2O, is formed when oxygen, an anion possessing a -2 charge combines with 2 hydrogen ions each with a +1 charge. The net result is a 0 charge although due to various factors the oxygen atom is partially positive while the hydrogen atoms are partially positive due to an unequal sharing of electrons. For pure water at 20o C under standard pressure (sea level) a very small percentage of water molecules break into H+ and OH- (hydroxyl) ions. Acidity, the concentration of hydrogen ions in solution, is measured using a pH scale. For the pure water under the standard conditions listed above, the number of hydrogen and hydroxyl ions is equal and is considered neutral; i.e. there is no “acidity.” This is a pH of 7. As the pH moves lower (toward zero) the concentration of H+ ions increases and the solution is more acid. As the number increases (toward 14) the solution is not termed acidic but basic. A brief explanation of this number scale is necessary.
The pH number is a log scale, the number ten raised to a power. For example 10 raised to the first power, 1, is 10. Raised to the second power it is 100. The log of 1000 is 3. Just to confuse everyone further, pH is a negative log scale that is they take the log and change the sign to make it easier to read. For example, the log of 0.01 is -2 and for 0.0000001 is -7. The negative log of those examples are 2 and 7 respectively. So for a neutral pH of seven the concentration of H+ ions is 0.0000001. This is why as you move from a pH of 7 towards 0 the concentration of hydrogen ions increases even though the number is smaller.
One more explanation for this week, a pH 6 doesn’t seem much more acid than pH 7 but because it is a negative log scale, it is actually 10 times more acid. Finally for perspective, black coffee has a pH of around 5, your stomach acid between 2 and 3, with battery acid essentially a 0. Milk is a tad below 7 while your blood is a little over 7. Ammonia is close to 11 and household bleach over 12. Next week discusses relating this to soils.