Soil pH

Our study of soil pH begins with the meaning and significance of soil pH. Figure 112 illustrates a pH scale as it relates to the soil. The scale begins with a low pH of 4.0 and goes up to a pH of 8.0. Actually a soil may have pH either below or above these figures.

A pH of 7.0 is neutral. As the soil pH goes below 7.0 it becomes increasingly acid and it becomes more alkaline as it goes above the neutral 7.0 soil pH, up to neutral 7.0 is a measurement of hydrogen. The amount of hydrogen in the soil determines the acidity of the soil. Hydrogen is the acid element. The scale shows a 6.3 soil pH which is considered to be near ideal for most crops. At a 6.3 soil pH the soil test will usually show a base saturation, or pantry fill, of ten to eleven percentage hydrogen

At the neutral pH of 7.0 there is no measurable hydrogen on the colloid. As the soil pH goes above the neutral 7.0 hydrogen remains at zero, but other alkaline elements such as sodium, calcium or magnesium could increase. Below the neutral 7.0 soil pH measures only hydrogen which is not even a plant nutrient. By studying your soil test result you will observe how the soil pH goes down as hydrogen percent base saturation increases.

The soil colloid will always be completely filled with cations. When cations are removed and not replaced hydrogen moves in to fill the negative sites once occupied by the other cations as calcium, magnesium and potassium. As calcium, magnesium and potassium leave the soil colloid hydrogen fills the void and the soil becomes increasingly acid and less productive. We believe crop production failure is caused more because of lack of nutrients rather than because the soil pH becomes lower.

As plant nutrients are removed from the soil colloid hydrogen continues to fill the void until the cupboard is bare of plant nutrients. Try to visualize a soil pantry nearly filled with the non- plant nutrient hydrogen and practically empty of the essential cationic plant nutrients, calcium, magnesium, potassium, etc.

This whole relationship between soil pH, hydrogen, calcium and magnesium is important because it relates directly to the correct selection of limestone, its rate of application and the frequency of application. Instead of applying limestone only to correct the soil’s pH its application must provide the essential plant nutrients calcium and magnesium.