One of the most important things to consider when growing herbs, apart from watering, feeding and sunlight, is the soil. This is the engine of any garden or container and it will control how your herbs will grow. Soil is part of the overall ecosystem and therefore, understanding, feeding and maintaining your soil is very important to the health of your herbs.
Most herbs will grow quite happily in typical garden soil, as long as it has good drainage. However, some herbs are native to the Mediterranean and prefer gritty, well-drained soil. Good drainage is crucial because the roots of Mediterranean native herbs are likely to rot in moist soil. If your garden soil is heavy, grow these herbs in containers or raised beds similar to the ones in Jekka’s Herbetum.
As the soil is the engine of your garden, it is important to know its condition before you start planting and how to maintain good soil conditions. 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. This includes both the size of the particles that make up a soil as well as the pH (acidity or alkalinity) of the soil. These aspects will affect the type of plants you can grow and how you manage your soil.
A soil’s pH is a vital factor in the plant’s ability to obtain, via its root system, all types of plant foods and essential chemicals. 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. 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. There are always exceptions, for instance Rumex scutatus, Sorrel, will tolerate acid soils.
There are 4 basic soil types for growing herbs: clay, chalk, loam and sand
• Clay soil, less than 0.002mm, 6.5 pH
Clay soils are heavy, high in nutrients, wet and cold in winter and baked dry in summer. 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. Clay soils contain a high proportion of water due to the capillary attraction of the tiny spaces between the numerous clay particles. Clay soils are easily compacted when trodden on while wet. Even though it can be rich in plant nutrients, as they hold nutrients bound to the clay minerals, 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. Although clay soils are difficult to work with, over time and when managed properly, can be highly rewarding.
• Chalk soil, 0.002-0.05mm, 8.5 pH
Chalky soils are largely made up of calcium carbonate and are therefore, very alkaline. This soil is typically light with lumps of flint or chalk, well drained and often shallow. It is possible to increase the nutrient content by adding loads of compost but it is difficult to lower the pH into a range more tolerable for herbs. It is also hard to adjust the soil to help it retain moisture. Therefore, it may be easier to grow the herbs in a raised bed.
• Loam soil, 5.5–8.5 pH
Loam is the gardeners dream soil and is a mixture of clay, sand and silt that avoids the extremes of each type and are fertile, well-drained and easily worked. There are various types of loam depending on the content of clay or sand. They can be clay-loam or sandy-loam depending on their predominant composition and cultivation characteristics. 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, 0.05-2mm, 4.5 pH
Sandy soils are light, dry, warm, low in nutrients and often acidic. This soil feels rough and gritty when handled and are easy to cultivate and work. These soils are very free draining, which means nutrients are quickly washed away and dry out quickly. To help retain moisture and for an extra source of nutrients sandy soils need to be fed in winter with leaf mould and with well-rotted manure. An advantage is that they warm up quicker than clay soils in the spring so sowing and planting can be started earlier.
Other soil types you might come across are silt soils and peat soils. Silt soils are comprised mainly of intermediate sized particles and are fertile, light but moisture-retentive, and easily compacted. Peat soils are very high in organic matter, usually very fertile and hold much moisture. They are seldom found in gardens.
The best way to tell what type of soil you have is by touching it and rolling it in your hands: Clay soil is moisture retentive and, as a result, has a smearing quality, is sticky when wet, can be manipulated into shapes and can be smoothed to a shiny finish by rubbing with a finger; Chalk soil is high in calcium carbonate and will froths when placed in a jar of vinegar; Sandy soil has a gritty element, will fall through your fingers and does not clump. If it is not a coarse sand and perhaps a sandy-loam it may stick together better.
Knowing your soil's pH
In order to determine the pH of your soil, you can buy a soil testing kit from any good garden centre. 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, neutral turn it green and alkaline turn a dark green.
How to work with, maintain & improve your soils
• Clay soils
These soils are rich in nutrients and very fertile but can suffer from drainage problems and may not suit plants that need free draining conditions. This can be overcome if they can be broken up by the addition of organic matter that breaks down the clay into separate crumbs. This also makes the water and nutrients held within the clay more easily available to plant roots as well as the soil warmer, more easily workable and less prone to compaction.
• Chalk soils
These soils are very alkaline, so will not support ericaceous plants that need acid soil conditions. It is very difficult, if not impossible, to acidify these soils and it is therefore better to choose plants that will thrive in alkaline conditions or used raised beds or containers.
• Loam soils
These soils are the ‘perfect’ balance of all soil particle types but even though they are very good soils, it is important to regularly feed with organic matter, especially if you are digging or cultivating these soils every year.
• Sandy soils
These light soils are usually low in nutrients and are free-draining. Therefore, they require a lot of watering. You can improve the water and nutrient capacity of your soil by adding plenty of organic matter to bind the loose sand into more fertile crumbs.
Jekka's top tips
We feed our plants with liquid seaweed in the growing season. We also add Horticultural Epsom salts weekly for a month in spring to our container grown evergreens, or any evergreen in the garden, that has been stressed by the winter.
One thing to remember is that sowing or propagation compost is often called a substrate as it is different to home made composts. A seed substrate differs from a potting substrate by the amount of nutrients. It is always a good idea to use the right substrate for the right job so that you do not give the young seedlings to many nutrients in its early life.
Soil is very important but whichever soil you have you should not be limited or restricted in growing and enjoying herbs. Whenever you are in doubt, Jekka’s top tip is to reproduce the environment from where the herb originates, which, for herbs from the Mediterranean, is a gritty, well-drained soil and, for UK native herbs, is a moist loam.