In this first part of a series about gardening we discuss the characteristics of your soil, the importance of the pH and organic matter, and how to improve the soil's qualities.

The key to good plant health is in the soil

October 2017
As a garden designer I often get asked why new plants die or fail to thrive. Apart from the obvious cause of inadequate watering after planting, the most likely reason for planting disasters is the state of your soil. 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.

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 air spaces, and increase drainage, and stony soils can be drought-prone.

Soil characteristics

In the garden, sandy soils are free draining, quick to respond to seasonal changes, and low in water and nutrient reserves. Clay soil needs organic matter 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.

Changing pH

Artificially 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 if needed.

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. Dig in organic matter or lay it on top as mulch. Rich organic matter like 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.

Other factors

If life were only that simple I hear you cry! Of course there are other factors that affect plant health, such as soil moisture levels, your garden climate, pests and diseases. Using the correct planting techniques and ongoing maintenance are very important too, but, on the whole, it is your soil that is the key to establishing plant health and vigour. If you don't get this right, everything else is secondary.

Soil pH

The term pH refers to the acidity or alkalinity of your soil. A simple pH-testing kit will confirm the acidity or alkalinity of your soil. Acid soils naturally support 'signal' plants such as Rhododendrons. These ericaceous plants will not thrive on alkaline soils. Horticulturally, 'neutral' soil is pH 6.5, which many plants enjoy.

Extremes below pH 5.5 (acid) and above pH 7.5 (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.

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