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Dig up some earthly secrets

Breakdown of soil content
Breakdown of soil content

It can seem as though soils are a strange medium and the secrets can only be unlocked by a scientist, brandishing a collection of test tubes and expensive gadgets.

Thankfully, understanding your soil’s texture is relatively easy. It’s a popular part of the RHS courses here at Manea School of Gardening, which we’ve been examining in detail. And the most expensive piece of kit you’ll need to test soil texture is a jam jar!

The texture of a soil is determined by the size of the mineral particles in the soil. Some soils around the Fens will be organic soils and will not have a noticeable presence of mineral particles; they are formed purely by layers of organic matter over a period of centuries.

However, where a mineral soil is present (across the majority of gardens in the country) around 50 per cent of the soil is composed of weathered and eroded parent rock, which forms the mineral particles.

The largest of mineral particles in the soil is gritty sand, silt particles are smaller, and clay particles are the smallest. The size of these particles helps determine the drainage of a soil – large sand particles have lots of macro (large) pores which can retain air and moisture. This means that a sandy soil usually offers the best drainage, but as water and dissolved nutrients have relatively free passage through the soil, it can lose nutrients more easily.

Additionally, sandy soils change temperature more quickly, which is useful for an early start as spring warms up.

By contrast, clay-based soils offer poorer drainage, change temperature more slowly, but they retain the nutrients better.

Clay also holds on to a pH better so it responds slower to an adjustment in acidity or alkalinity but will hold an adjusted pH for longer.

You can investigate your soil’s texture by feeling it; if you can form a ball when damp which can then be squashed into a sausage shape it is likely either a clay or a silt-based soil.

If this ‘sausage’ can be hooked into a ring, this indicates strong clay content. Equally, a clay based soil will feel sticky, whereas a silt-based soil tends to have a silky, smooth feeling – a contrast to a gritty sandy soil.

Your soil likely has a mix of these textures, though one will usually prevail, hence a dominating presence of sand is termed a sandy soil or sandy loam.

Medium loam refers to a fairly even mix of soil particle sizes, which is the ‘ideal’ soil to have, so a sandy loam would have a slight majority of sand particles.

A jam-jar test will help you conclude what soil you have.

To do this, you can fill a jam-jar about halfway with soil, then fill to around three-quarters with water and shake.

Sand particles will settle first, within a couple of minutes; silt settles within an hour or so and then the clay within a few days.

The seemingly unfavourable clay texture will not write-off a garden and even an ideal loam mix far from guarantees perfection. Any soil requires working to prevent compaction, but it is helpful to know what you’re working with, and how well it will retain nutrients, changes temperature or holds water.

Soil texture is incredibly difficult to change so it is best to work with what you have. However, the addition of organic matter, such as compost or manure, and good cultivation techniques can greatly improve the structure – now that’s a different story...

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