Have you ever been talking to another gardener about a problem with a plant that’s growing in your garden and they answer by saying: “Have you checked the pH of your soil?”. Did you try to look like you knew what they were talking about, but really didn’t have a clue? This guide to soil pH and soil testing will help make it all a lot clearer.
What is pH?
Soil pH refers to the chemical structure of the soil. In broad terms soil pH can be acidic, neutral or alkaline. Soil pH is a measure of hydrogen ions in the soil and a simple soil test will reveal your soil’s pH in a matter of seconds.
Soil pH is measured on a scale of 0-14 with 7 neutral. A reading of less than 7 is acidic and above 7 is alkaline and this influences the availability to plants of nutrients in the soil. Most plants prefer a soil that’s neutral or slightly acid (pH 6-7) as most of the nutrients they need to grow are available.
Taking a Soil Sample
To do a pH test you first take a soil sample. To do this, dig a little soil from your garden. Take the sample from just below the surface. To assess the garden’s general pH, take several samples from different parts of the garden and mix them together before testing. To sample a specific area such as a lawn or a problem garden bed, just take samples from the area you are concerned about.
Soil pH Test
Soil pH test kits are available at Flower Power. They contain a reactant powder, a liquid indicator dye and a colour chart. To do the test, take a teaspoon of soil from your soil sample and place it on a clean light surface such as a white board or tile. Add the indicator dye to the soil then sprinkle over a small amount of reactant, which is a white powder. Within 30 seconds the white powder will change colour. Match this colour to the colour chart in the kit to get a pH reading.
Soil pH can also be tested using a pH meter, which is attached to a probe that’s placed into the soil.
What Your Reading Means
If your soil’s pH is extremely acidic or extremely alkaline, plants growing in the soil may show nutrient deficiencies such as yellowing of the leaves and stunting. The nutrients the plant needs may be in the soil but as the pH is too high or too low for optimal growth, the nutrients are locked away so the plant can’t absorb them.
To make the missing nutrients available to the plant it is necessary to change the soil’s pH. This is done by adding lime (as agricultural or garden lime, dolomite or alkaline mushroom compost) to soil that’s too acidic, or adding a soil acidifier to soil that’s too alkaline. Soil acidifiers include sulfur (available as a liquid or granules) and composted animal manures.
Soils may become increasingly acidic if they are given high rates of fertiliser. Soil under lawns for example may be more acidic than the soil in nearby garden beds due to the regular application of lawn fertiliser. Most lawn grasses grow best in soil that neutral to slightly alkaline, but many lawn weeds thrive in more acidic soils. Adding lime may not only improve the lawn’s health and vigour but also reduce weed problems.
If the local soil is naturally extremely acidic or extremely alkaline it may be difficult to significantly change the pH. An option is to grow plants that cope with high or low pH in the soil but grow pH-sensitive plants in containers or raised garden beds filled with potting mix.