What is Soil pH and Why is it Important

Every atom (the basic bit of every element/compound/material/liquid etc.) contains a nucleus of smaller particles, surrounded by electrons spinning around it in virtual orbits

If an atom loses or gains an electron, it alters its characteristics and becomes an Ion. In its natural state, an atom has no overall charge, but if it loses an electron it becomes positively charged, called a Cation. If the atom gains an electron, it becomes negatively charged- an Anion.  This can be illustrated when you rub a balloon on your jumper and then it sticks to a wall, the balloon has been given a negative charge by gaining electrons from your jumper and attaches to the positively charged wall.

pH is short for 'potential of hydrogen' or ‘power of hydrogen’ and measures the hydrogen ion concentration in a substance.  pH levels range from acidic and neutral to alkaline. The measurement was first used by Danish biochemist SPL Sorensen at the Carlsberg laboratory in 1909.

Water is a neutral chemical, each molecule of it is made of two hydrogen and one Oxygen atom, H2O. A small number can separate and become Hydrogen cations H+   and Hydroxide anions  OH-. However, the number of these remains equal so it sits at pH7 which is neutral.

Acids, however, have a higher proportion of H+ than OH- and alkaline substances have more OH- than H+.

The pH scale that you may be familiar with is this one -

The pH scale is used to measure the acidity or basicity (alkalinity) of an aqueous solution. The scale runs from pH1 (highly acidic) to pH14 (highly alkaline) with pH7 being neutral.

The colour spectrum is gained by using a Universal Indicator- as a liquid or impregnated paper strip. This is a dye obtained from a species of lichen.

Digital pH meters can also be used to give a numerical value, but for testing soil samples a paper strip indicator is usually sufficient.

"For those interested, the pH scale is logarithmic in nature. The concentration of hydrogen ions in a solution is calculated as the negative logarithm of the number of hydrogen ions. -log(H+)

Although it sounds complex, (it is) it represents a factor of 10 change with each incremented number eg. Each decrease in number means ten times as much H+, so pH 2 is ten times more acidic than pH 3, and conversely, pH 14 is ten times more alkaline than pH 13"

Having the right soil pH is key to growing a healthy crop but is often not considered, however, knowing and understanding your soil pH level is essential and probably the single most important chemical measurement you should be aware of.

Ornamental gardeners may appreciate that some heathers need acidic soils, but vegetable growers may not factor it as much as their nutrient levels or soil structure. 

While these are obviously very important, the pH of the soil plays a major role in how well your plants can absorb nutrients that may be present in your soil, but not necessarily available.

Although soil pH levels don't directly affect plants, they will also determine which bacteria and fungi will thrive and which nutrients will be available to your plants. Fungi favour a slightly acidic environment and bacteria more alkaline. Soil pH will also affect earthworm activity and populations. 

It should also be noted, that plants give out substances via their roots, called exudates.

These carbon-rich excretions are used to feed bacteria and fungi in the soil, which in turn feed the plants with nutrients. These exudates, however, can produce acids which can help to alter pH levels in the area around the root tips, more suited to the particular plant.

Therefore, the plant may well be able to control its environment better than the gardener can!