Soil Types and Testing

Local Soil Types

In Brighton we garden over chalk. Much of Hove has clay over the chalk. In areas close to rivers, as in Shoreham and around the London and Lewes Roads and the Steine, where there was once streams, we find silt. The typically hilly thin chalk soil that many of us have in our gardens and allotments does in fact behave very much like silty soil.     Because the underlying rock is chalk the overlying soil tends to be alkaline even if its texture or type is mostly clay or silt.    For more info about local soil types visit www.landis.org.uk.  

Is My Soil Acid or Alkaline?

You can buy a pH test kit from the local garden centre quite cheaply to test for acidity/alkalinity. If your soil is very acidic, you can add lime to neutralise it. The best is Dolomite limestone for organic folks and it contains magnesium as well as calcium.    

If you want to grow acid-loving plants in this area, then you will need to use ericaceous compost in a container. I have a blueberry and a rhododendron and a couple of Acers growing happily in my Brighton garden in pots.    

Here is a recipe for home-made ericaceous compost: one part “Moorland Gold” (reclaimed peat available from www.wrorganics.co.uk). Two parts each of loam, leaf mould, grit and medium grade bark. This recipe comes from the Organic Way, the magazine of Garden Organic, and as our group is affiliated with them I am sure it is OK to pass it on.  

Sand, Silt or Clay?

These terms describe the three main particle sizes of soil, sand being the coarsest and clay being the finest. Soil texture is described by these particles and can be tested by simply rubbing some moist soil between finger and thumb. If it feels gritty it is mostly sandy, if it feels silky or soapy it is mostly silt and if it is distinctly sticky then it will be mostly clay.   

To make a further test, take about a tablespoon of soil (moist but not soggy) and roll it into a ball. If it doesn’t hold the shape then it is probably sandy soil. Roll the ball into a sausage shape and then try to make that into a ring – only clay soil will form a ring without cracking so if it cracks it is it is silty soil.    

Half fill a jam jar with soil. (Take soil samples from several locations about 10cm deep and mix them together). Top up with water, put the lid on and shake well. The particles that settle in the first half hour will be sand or grit. The particles that drop during the next hour will be silt and the clay particles may take up to 48 hours to settle! If you mark the boundary between each type and measure them you can work out the proportion of each.  

What makes a good soil?

The gaps between the soil particles (or soil pores) vary in the different soil types from quite large in sandy soil to medium in silty soil and tiny in clay soil. Large pores contain air, which is needed by both plant roots and soil organisms. Medium pores contain water available to plant roots and the very tiny pores hold onto water so tightly that it just isn’t available to plants. The composition of good soil is about 45-50% particles, 3-5% organic matter, 25% water and 25% air. Chris Beardshaw says that the best ratios of particle sizes are 30-50% sand, 10-30% clay and 30-50% silt.    

A predominantly clay soil with its tiny particles will hold a lot of water but not much air. It gets water-logged easily then bakes like a brick in dry summers making it hard to dig or for plant roots to penetrate. Because it is wet in winter it can be slow to warm up in the spring. On the up side, clay soil can hold lots of nutrients so if it is improved with organic matter, producing larger crumbs to allow better drainage and aeration, really good crops can be grown. Clay soil is best dug in autumn, leaving the clods to be broken down by frost action.    

Sandy soil, at the other extreme with its large particles, is very well drained and hold lots of air. It is easy for plant roots to penetrate and warms up quickly in spring so crops can be planted sooner. It doesn’t hold water or nutrients well however and so is both hungry and thirsty.    

Most of us probably garden on chalky or silty soil which tends to be dry and dusty in the summer. In winter it is well drained, especially on slopes, warms quite quickly in spring and is fairly easy to work and for plant roots to penetrate. Nutrients are easily leached out by rain though (mine had no nitrogen or potassium when tested a few weeks ago). Also the rain tends to form surface capping or crusting which stops both air and water getting to the plant roots and micro-organisms. Keep hoeing if this is your soil type to physically break the cap.    

Whatever our soil type, it can be improved by the addition of bulky organic matter in the form of well rotted manure, homemade compost, composted green waste bought in from local sources and green manures. When this stuff has been thoroughly digested by the worms (incidentally worm casts in my bin were very alkaline) and micro-organisms it is known as humus and will hold water and nutrients needed for healthy plant growth in a from that is easily available to the plants. Humus also binds the soil particles into crumbs, which improves soil structure by producing larger pores allowing better drainage and aeration. Mulching and growing green manures on areas not needed for crops will also help to protect the soil structure and surface.