The soil is the engine of your garden, so it is important to know its condition before you start planting. Good plant growth is not only dependent on how much you feed the soil but it is also dependent on the structure of the soil. Soil can vary from acidic (pH 3.5) Sphagnum moss peat to alkaline (pH 8.5) Fine loam. Most herbs will tolerate a range of between 6.5 and 7.5 pH which is fairly neutral. There are always exceptions, for instance Rumex scutatus, Sorrel, will tolerate acid soils.
The pH of the soil refers to its acidity or alkalinity. It is a vital factor in the plant’s ability to obtain, via its root system, all types of plant foods and essential chemicals. For example, an alkaline soil can produce stunted plants with yellowing leaves. This is because the minerals, especially iron, have become locked up in the soil. At a neutral pH of 7, most of the essential chemicals and plant foods become available to the plant so producing healthy plants.
The following 4 basic soil types are the most suitable for growing herbs:
- Clay 6.5 pH: This soil is composed of tiny particles that, when wet, stick together making the soil heavy and difficult for the roots to penetrate and in summer, when dry, sets rock hard. Even though it can be rich in plant nutrients, because of its characteristics it is improved by working in extra well rotted leaf mould or compost. This will improve the structure and allow young plants to become more easily established.
- Chalk 8.5 pH: This soil is light with lumps of flint or chalk, well drained and often shallow. It has a high pH making it very alkaline. It is possible to increase the nutrient content by adding loads of compost but it is difficult to lower the pH. A large number of herbs will tolerate chalk. However, considering the characteristics of this soil, to give it depth and help it retain moisture it may be easier to grow the herbs in a raised bed.
- Loam 5.5–8.5 pH: This is often considered the ultimate garden soil in which most herbs will grow. There are various types of loam depending on the content of clay or sand. A sandy loam is the best soil for growing the largest range of herbs as it is rarely waterlogged in winter, is dry in summer and it is naturally high in nutrients.
- Sand 4.5 pH: This soil feels rough and gritty when handled. It is very free draining, which means that the plant’s nutrients are quickly washed away. A plus point to this soil is that it is quick to warm up in the spring so sowing and planting can be started earlier than in clay soils. To help it retain moisture it needs to be fed in winter with leaf mould to retain moisture and with well-rotted manure for an extra source of nutrients.
Checking the pH of the soil
To test your soil buy a soil testing kit from any good garden centre or store. The majority of amateur soil testing kits are very simple and rely on colour rather than a numerical scale. Acid soils turn a solution yellow- orange, neutral turn it green and alkaline turn a dark green.