Soil pH Levels Explained

If you cast your mind back to school chemistry, you might remember the odd lesson on acids, bases and the pH scale. The pH scale ranges from 0-14 and is used to standardise how acidic or how basic a substance is. Acids and bases sit on the low and high end (respectively) of the pH scale. Right in the centre of this scale at pH 7 is water; it is neither acidic nor basic.

However, the pH of a substance (including soil) is not static, it can flux and change depending on what’s added to it. Adding a basic substance to water will make that water become more basic. Similarly, adding equal pH and volume of acid and alkali, will result in a neutral pH substance.

The pH level of soil is a hugely important factor in agriculture, landscaping and horticulture. Soil pH has wide reaching effects on almost all soil chemical processes, plant life and microbial development. To add to this, the pH of soil is always changing, depending on the surrounding ecosystem and its inputs and outputs.

Measuring soil pH

Before getting deep into soil pH and what it means. It’s important for you to understand just how simple it is to measure your soil’s pH. Whether you’re in charge of hundreds of hectares of farmland, or simply your own little plot in your garden, it’s a trivial task that can reap huge benefits.

Rudimentary soil pH testing kits can be purchased online for less than £10, and electronic soil meters can be picked up for less than £20. This allows you to quickly and scientifically assess the quality of the soil in which you’re planting. The electronic kits will also give you more detailed information on the soil and let you test endlessly (until the battery runs out).

What soil pH Means

Most crops require acidic soil – exactly how acidic, depends specifically on the kind of crop you intend to grow. For instance, potato likes particularly acidic ground between pH 4.5 and 6 but lettuce prefers soil closer to neutral of pH 6 to 7.

These may seem like tight margins for error when you’re dealing with hundreds of hectares of farmland. However, pH works logarithmically. This means that a change of 1 pH indicates a 10 fold change in real life. PH 4 is 10 times more acidic than pH 5 and 100 times more acidic than pH 6. With this in mind, there is a relatively large margin for error.

This is because different ground nutrients are available more readily at different soil pHs and since different crops require different nutrients the growth is directly connected to the soil’s pH.

Changing Soil pH

There are a number of ways you can change your soil’s pH. Common materials and soil additives are sulphur, aluminium sulphate and ferrous sulphate. Sulphur is the most common way of reducing the pH of soil and is the least likely harm a plant. Sulphur is converted to sulphuric acid by soil organisms and can take a few weeks before the pH of the soil increases. The rate at which sulphur is converted into acid is dependent on heat and grain size. Finely powdered sulphur distributed in the warmer months being the fastest acting.

If you’re finding that your soil is too acidic, lime is a common additive used to increase the pH and reduce the acidity of soil. It’s applied in much the same way that sulphur is. However, ambient temperature is less crucial here.

When applying substances to change the pH of soil, remember that the changes don’t happen instantly. Ensure that you leave a few weeks between testing the soil pH to make sure that the additive has taken effect.