It is very rare to get soils that are made up of just one particle type. Most soils are a mixture of all three particle sizes and can be broadly divided into coarse-, medium-, and fine-textured soils. Coarse soils, which are higher in sand, allow water to drain through them more quickly than other soil types and have the advantage of warming up more quickly in the spring. Their texture gives them resistance to waterlogging and compaction. The downside of coarse soils is that they can be prone to drought and the loss of nutrients. If you have a very coarse soil, your management priority will be to add large amounts of organic matter in order to improve nutrient status and moisture retention. Medium soils are usually soils that are well balanced between sand, silt, and clay, so that none of these particle types dominates the characteristics of the soil. Medium-textured soils are ideal for cultivation and can be very fertile. They do not need any particular remedial management practices. Fine-textured soils are high in clay and are generally more difficult to work. Rainwater tends to collect on the surface in puddles, and the soil is slow to warm up in the spring, due to its higher moisture content. Soils with high clay content should never be cultivated or stepped on for harvesting when they are wet, as this will lead to compaction and permanent damage. When clay soils dry out, they form rock-hard segments surrounded by characteristic deep cracks, making cultivation very difficult. On the plus side, fine-textured soils retain moisture well in the summer. They can be improved by the addition of plenty of organic matter as well as by rough digging in the autumn, leaving large clods over winter to be broken down by the action of the frost.
The relationship between different soil types and which plant species they naturally support is fundamental. Top: Dartmoor; middle: the Scottish borders; bottom: the Jurassic Coast.
There are many parallels between soil types and patient constitutional types in herbal medicine, whether you understand those according to Galenical humoural theory, Tibetan medicine, Ayurveda or other traditions. Each constitutional type has its tendency to moisture, dryness, inflammation, and stagnation, for example. Once we understand our own constitutional type, we can nurture ourselves effectively, staying as close as possible to an ideal healthful balance. Likewise, once we understand the soil in our herb garden, we can nourish and manage it so that we are able to grow the best possible crops of medicines.
Overlaid onto the various soil textures is the effect of its pH. Chalky soils naturally have a higher pH and will favour the growth of certain plant species such as Juniper (Juniperus communis), Lily-of-the-Valley (Convallaria majalis), Thymes (Thymus spp.) and Marjorams (Oreganum spp.), for example. Acid soils naturally have a low pH and favour other plant species such as Blueberries (Vaccinium corymbosum) and Camelias (Camelia spp.). Soils that are naturally quite alkaline, such as chalky soils, can cause essential nutrients like iron to become unavailable, even though in theory there should be plenty in the soil. Iron is an essential nutrient to plants, and when they cannot take it up, they start to form pale or blotchy leaves, often with the small leaf veins remaining green, giving the leaves a chequered appearance. This phenomenon is called ‘limeinduced chlorosis’. Conversely, very acid soils can also cause problems for plants that are not adapted to those conditions. In soils with low pHs, some elements reach higher than optimal levels due to their enhanced solubility, and aluminium, manganese, and iron can be present at toxic levels. Alongside this additional toxicity, essential nutrients such as phosphorus, potassium, magnesium, and molybdenum can become deficient. When soil pH is too low, soil life becomes inhibited. This interferes both with the plants’ ability to form beneficial symbiotic relationships with fungi and the overall health and balance of micro-organisms and invertebrates present in the soil. Adding more organic matter will have the effect of moving a soil closer to the neutral from either tendency. Soils with a pH that is close to neutral can support a very wide range of herbs. If you want to grow specialist herbs that prefer either low or high pHs and you do not naturally have these soils present on your site, it is quite possible to create raised beds or containers filled with appropriate soil.
The pH of the soil will favour certain plant species. Top: Cheddar Gorge; middle: Wild Marjoram growing in alkaline soil at Cheddar Gorge; bottom: upland pasture on acidic soil in the Scottish borders.
Plant growth is affected by the soil not only in its surface layers, but also at deeper levels. If you are establishing a new herb-growing area, or if you do not understand your current one, it is a good idea to investigate the nature of the soil down to a depth of about 1 m / 3 ft. You can either use a soil auger, which is a hollow spike that is driven into the ground and removes a thin core of soil, or you can dig a small soil pit. Once you start digging, you will notice that the soil consists of layers of different colours and textures. Together, these layers form what is called a ‘soil profile’. The layers have been built up over centuries due to the action of vegetation, cultivation, animal or human traffic, and climatic conditions. The uppermost layer, known as the topsoil, is usually darker than the layers beneath it, due to higher levels of organic matter. You will find that the topsoil layer is absolutely teeming with life. As it is nearest the surface, it receives rainfall first, and so over time there is a tendency for nutrients to be washed down into the deeper layers. With time the higher levels of organic matter in this layer tend to increase its acidity a little and increase the solubility of mineral salts that are washed down into the lower layers. On steep slopes this top layer can be very thin, and on recently completed building sites it may be completely absent.
Below the topsoil is the subsoil. This is usually harder to dig when dry and often stickier when wet due to an inherently higher clay content. Iron oxides tend to be washed down from the top layers into the subsoil and may form hard red-coloured concretions or even a continuous hard layer, called an ‘iron pan’, which effectively prevents roots from being able to grow through it. If you have only a thin layer of top soil and a hard iron pan in the subsoil, it is much more difficult to grow crops, because they will have less access to water and nutrients. In wetter areas, you may notice that the soil is blueish or greenish with mottling. This is a sign that there is a lack of aeration and that the iron compounds present in the soil have been reduced to greenish ferrous oxide rather than reddish ferric oxide. Blueish-greenish anaerobic subsoils are called ‘gleys’ and restrict the growth of plant roots to the shallow aerated layers, unless corrected by drainage. You may find that the lower part of your soil pit is filled with water even in the summer. If this happens, you will know that your area has a relatively high water table or that the lower layers of your soil prevent free drainage. The depth of the water table has a significant influence on the plant species that will grow happily at a particular site. Land with a high water table or sub-surface compaction will reveal its true nature by the plants it naturally supports. If your new herb-growing site is covered by Rushes (Juncus spp.) and Creeping Buttercups (Ranunculus repens), you can conclude that it is naturally damp. If it also shows signs of surface damage from the hooves of grazing animals, you can suspect a high water table and compaction. On soils such as these there is much that can be done to improve them with drainage or you may decide to concentrate on growing herbs that enjoy wetter soils, such as Sweet Flag (Acorus