Advice
RHS Help & Advice
Soil type, pH and soil improvement
Knowing your soil type and whether it is acidic, neutral or alkaline will help you choose the right plants for your garden and maintain them in good health.
Soil type
The mineral constituents of soil are sand, clay and silt. The soil’s texture can be described in terms of its proportions of these components, loam containing a mixture of all three. Chalky soils also contain calcium carbonate or lime.
Stones in the soil will increase pore spaces, and therefore increase drainage, and stony soils can be drought-prone.
Texture testing
How does the soil feel between your fingers? Sandy soil has a gritty element - you can feel sand grains within it, and it falls through your fingers. Clay soil has a smearing quality, and is sticky when wet. Pure silt soils are rare, but occur on river flood plains. They have a slightly soapy, slippery texture, and do not clump easily. If soil froths when placed in a jar of vinegar, then it contains free calcium carbonate (chalk) or limestone.
Characteristics
In the garden, sandy soils are free draining, quick to respond to seasonal changes, and low in water and nutrient reserves. Clay soils drain slowly, holding water well, but this may not be available to plant roots. Clay reacts slowly to temperature changes, staying cold for longer in spring. Silt and chalky soils often behave similarly to sand, although silt has a higher nutrient reserve than chalk.
Soil pH
The term pH refers to the acidity or alkalinity of your soil. A simple pH-testing kit can be obtained from many garden retailers, which will confirm the acidity or alkalinity of your soil. Acid soils naturally support ‘signal’ plants such as rhododendrons, azaleas, knapweed and sheep sorrel: these ericaceous plants will not thrive on alkaline soils. Horticulturally, ‘neutral’ soil is pH6.5 (true neutral is pH7) which many plants enjoy. Extremes below pH5.5 (acid) and above pH7.5 (alkaline) can be problematic, with certain pests, diseases and nutritional disorders becoming more prevalent. Magnesium deficiency and clubroot are more prevalent on acid soils, while trace element deficiencies are common on especially alkaline soils. Indicator weeds for alkaline soils include bladder campion, old man’s beard and black henbane.
Changing pH
Acidifying soil is difficult and expensive. It is best to avoid growing ericaceous plants on alkaline soil; grow them instead in containers of ericaceous John Innes compost. Most plants will survive in acid soil, but garden lime is cheap, and will raise pH effectively where necessary.
Soil improvement
Any soil type can be productive if handled appropriately. Organic matter improves all soils. Any organic substance - compost, leafmould, well-rotted manure, wood and bark chippings, feathers - will in time turn your basic soil type into a darker, crumbly soil. This is known as the soil structure, and it is under your control. You can dig in organic matter or lay it on top as mulch. Rich organic matter (for example manure) is ideal for dry, ‘hungry’ soils (such as sand). Dry, fibrous organic matter (such as composted bark) might be better on clay, which is already rich and wet. Whatever you use, it is best applied when well rotted, and added at least twice a year for maximum benefit. Organic matter improves the drainage and workability of clay, and the water- and nutrient-holding capacity of sands, silts and chalky soils.
Maya Albert