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Soil Types & Improving


Clay soils

  • Characteristics - A Clay Soil has over 25% clay content and are also known as heavy soils. These are potentially fertile as they hold nutrients within the clay minerals in the soil. But they also hold a high proportion of water due to the capillary attraction of the tiny spaces between the numerous clay particles. They drain slowly and take longer to warm up in spring than sandy soils. Clay soils are easily compacted when trodden on while wet and they bake hard in summer, often noticeably cracking. These soils often test the gardener to the limits, but when managed properly with cultivation and plant choice, can be very rewarding to work with.
  • Improving - as with all soils adding organic matter is always beneficial, with clay soils this can help break up the cloddiness, breaking the clay into separate crumbs. This makes the water and nutrients held in the clay more readily available to plant roots. Breaking up the clay will also make the soil warmer, easier to work and less prone to compaction and baking hard in the summer.

Sandy soils

  • Characteristics - Sandy soilc have a high proportion of sand and little clay and are also known as light soils. These soils drain quickly after rain or watering, are easy to cultivate and work. They warm up more quickly in spring than clay soils. But on the downside, they dry out quickly and are low in plant nutrients, which are quickly washed out by rain. Sandy soils are often very acidic.
  • Improving Sandy Soils - these free draining soils will tend to be low in nutrients and lose water very quickly. Adding organic matter will help to bind the loose sand into more fertile crumbs, enabling more nutrient and water retention within the soil. It is often beneficial to also use plant fertilisers in sandy soils to give your plants an extra boost.

Silt soils

  • Characteristics - Silt soils are comprised mainly of intermediate sized particles, are fertile, fairly well drained and hold more moisture than sandy soils, but are easily compacted. They can be prone to washing away and wind erosion if left exposed.
  • Improving Silt Soils - these soils tend to be fertile, but adding organic matter will help to bind the silt particles for a more stable crumb, preventing erosion and helping to stop compaction.


  • Characteristics - Loams are comprised of a mixture of clay, sand and silt that avoid the extremes of clay or sandy soils and are fertile, well-drained and easily worked. They can be clay-loam or sandy-loam depending on their predominant composition and cultivation characteristics.
  • Improving Loam Soils  - although loam soils are seen as the gardener’s best friend, is remains important to add organic matter regularly especially if you are digging or cultivating these soils every year.

Peat soils

  • Characteristics - Peat soils are mainly organic matter and are usually very fertile and hold much moisture. They are seldom found in gardens.
  • Improving Peat Soils - although fairly uncommon in gardens, addition of organic matter and farmyard manure will be beneficial with peat soils, which can be quite light and will tend to be slightly acidic. Adding lime would help to raise the pH reducing the acidity.

Chalky or lime-rich soils

  • Characteristics - Chalky or lime rich soild may be light or heavy but are largely made up of calcium carbonate and are very alkaline. Many chalky soils are shallow, free-draining and low in fertility, but variations exist, and where there is clay present, nutrient levels may be higher and the water holding capacity greater.
  • Improving Chalky or Lime-rich soils - these soils are alkaline, so will not support ericaceous plants that need acid soil conditions. Very chalky soils may contain lumps of visible chalky white stone. Such soils cannot be acidified, and it is better to choose plants that will thrive in alkaline conditions. Adding plenty of compost and manure annually will help to lower the pH redudcing the alkalinity.


Soil pH

When designing and planning your garden, you will need to know if your soil is acid or alkaline as different plants thrive in different soils. The soil pH is a number that describes how acid or alkaline your soil is.  A pH of 7.0 is considered neutral.  An acid soil has a pH value below 7.0 and above 7.0 the soil is alkaline.

There are many reasons why you need to be aware of your soil’s acid/alkaline balance.  The solubility of minerals can be compromised if there is an imbalanced pH.  Minerals become locked-up in the soil making them unavailable for plants to absorb.  Essential and valuable micro-organisms become less active in the soil (earthworms hate acid conditions) and certain pests will be active at specific levels like the bane of brassicas, Club Root, a fungal disease which thrives in acidic soil.  Most vegetables and fruit grow very healthy in a neutral to slightly acid pH, the ideal being around 6.5.  There are exceptions like the acid-loving blueberry and the leafy brassicas that thrive in a more alkaline soil. Be aware before you sow or plant of the specific soil pH requirements of the crop or family of vegetable types that you are planting as cultivating the area to a specific soil pH may be necessary.

Most fruit and vegetables will grow in a pH of between ph 5.5-7.5 but achieving the optimum growing conditions will ensure healthier growing conditions for your crops.
You can test your soil pH yourself using a Soil Testing Kit or a Soil pH Meter, both of which are easy to use and relatively inexpensive. 

For more detailed results, the RHS (Royal Horticultural Society) offers a Soil Analysis Service which has been developed specifically with gardeners in mind, providing an analysis of soil texture, pH, organic matter and three major plant nutrients (Potassium, Phosphorus & Magnesium).

pH Reading Your Results

Soil pH Grid

pH 3.0 - 5.0

  • Very acid soil.
  • Most plant nutrients, particularly calcium, potassium, magnesium and copper, become more soluble under very acid conditions and are easily washed away.
  • Most phosphates are locked up and unavailable to plants below pH 5.1, although some acid tolerant plants can utilise aluminium phosphate.
  • Acid sandy soils are often deficient in trace elements.
  • Bacteria cannot rot organic matter below pH 4.7 resulting in fewer nutrients being available to plants.
  • Action: Add lime to raise the pH to above 5.0. The addition of garden lime can help break up acid clay soils.

pH 5.1 - 6.0

  • Acid soil.
  • Ideal for ericaceous (lime-hating) plants such as rhododendrons, camellias and heathers.
  • Action: Add lime if other plants are grown.

pH 6.1 - 7.0

  • Moderately acid soil.
  • A pH 6.5 is the best general purpose pH for gardens, allowing a wide range of plants to grow, except lime-hating plants
  • The availability of major nutrients is at its highest and bacterial and earthworm activity is optimum at this pH.
  • Action: It is not usually necessary to add anything to improve soil pH at this level.

pH 7.1 - 8.0

  • Alkaline soil
  • Phosphorus availability decreases.
  • Iron and manganese become less available leading to lime-induced chlorosis.
  • One advantage of this pH level is that clubroot disease of cabbage family crops (brassicas) is reduced so you will be able to grow lots of Brassicas!
  • Action: Add plenty of compost and manure annually, iron sulphate and other acidifying agents can sometimes be added to reduce pH.  Clay soils often require very large amounts of acidifying material and soils with free chalk or lime are not usually treatable.  Consider raised beds with an improved top soil added.

Meet the Author: Jo Blackwell
Jo  Blackwell

Jo Blackwell is new on the Harrod Horticultural block and has recently taken over her post as Horticultural Advisor and Kitchen Gardener in Stephanie's Kitchen Garden. She caught the gardening bug when she bought her first home 18 years ago.  Her first greenhouse soon followed and she later gained an allotment, where she grows her own organic fruit and vegetables.

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