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 calamus), Meadowsweet (Filipendula ulmaria), and Marshmallow (Althaea officinalis), for example. If you have a very wet area, then you could consider creating a pond or bog garden and grow Bog Bean (Menyanthes trifoliata).
Bupleurum (the yellow umbels), growing in Amdo, China.
Some soils are stony, or have stony layers. Provided that the water table is not too high, stones help to improve aeration and can be a helpful source of minerals for the soil as they weather. Large stones can however make cultivation or the harvesting of root crops more difficult. If you are planning mechanized cultivation and harvesting on your land you may need to consider some stone removal. This is not usually necessary for herb growing on a small scale. If you find yourself with a particularly stony area, you can make the most of it by growing alpine plants or those that require very free-draining conditions such as Pulsatilla (Pulsatilla vulgaris), Bupleurum (Bupleurum chinensis), Arnica (Arnica montana), and Gentians (Gentiana spp.).
If you have a large site, differences in the soil profile in different parts of your growing area may help you to decide which species to grow where. While over time most cultivated sites move towards more versatile cropping, it is usually much better to let the choice of species be influenced by the soil conditions than to have a very fixed idea of what you intend to grow where. If you try to mould a site to your theoretical plans rather than working with it, you could find yourself expending a huge amount of energy and resources in trying to adapt the conditions to fit. The influence of varied soil types on your plot is a perfect opportunity to cultivate acceptance as well as to cultivate herbs. If you are lucky enough to have a medium-textured well-drained soil with a neutral pH, you will be able to grow pretty much any herb that you want, where you want.
Slope and aspect
It is easiest to manage a herb-growing area that is level or nearly level, but if you have a steeply sloping site, that need not be a negative attribute – it can be useful. Slopes lend themselves to the creation of different habitats; for example, you can create areas to grow wetland plants at the base of the slope and areas for plants that need warmer soils and more sunshine at the top. In temperate climates with high rainfall, provided the soil is free-draining and you can replicate woodland conditions, it may be possible to grow scarce woodland herbs that require free-draining, shaded conditions. Species to consider include Golden Seal (Hydrastis canadensis), Beth Root (Trillium erectum), and American Ginseng (Eleutherococcus senticosus).
South-facing sites will have more sunlight and will tend to get warm more quickly in the spring. If you are growing aromatic herbs for their volatile oil content, you will find that south-facing sites will be preferable. North-facing sites can be better for herbs that prefer cooler, moister conditions. Although they receive less sunlight, they are less prone to drought and the burning-off of crops.
As cooler air sinks below warm air, sloping sites can influence the likelihood of frost damage in cooler climates. In winter, icy air tends to gather and stay in the lowest part of your garden. The longer this icy air stays, the more frost damage will be inflicted on
