Soils

David Beeson

Soils drive the ecosystem. The plants live in it and their metabolism is the source of the energy and nutrients that feed the animal food chains and webs. It is always worth scanning any profile one encounters on our explorations.

Soil is in layers, horizons.

The A Horizon is usually a deep, dark colour caused by the presence of decaying organic matter, humus.

One of the first actions my students took when studying soils was to burn some topsoil in a crucible. Once fully heated and cooled the end product was often a grey colour – the natural colour of the mineral components. [By weighing fully dried soil samples before and after the burning will give you the % of humus in a soil sample. Clearly, weighing before and after drying gives the water content.]

Humus is generally great for fertility. It holds water and many minerals stick to it lightly, so can be accessed by the local plants. And, as the humus breaks down its nitrogen, phosphorus, calcium, potassium and other elements will be made available to soil organisms.

In our garden we add organic material to enhance our clay soil. Garden compost, purchased materials (Progrow, biogas or mushroom growing waste) and mulched wood and bark are added in quantity. All the herbage from the meadows is processed through our compost bins and donated to the more traditional parts of the garden.

Unfortunately, the humus is being constantly broken down by bacteria and fungi (in aerated soils) and its released elements leached away, so humus needs constantly replenishing.

B Horizon, often called the subsoil, lacks much humus but will often contain materials washed down from the A Horizon.

The C Horizon will be the weathered (broken down) bedrock (D Horizon) or the natural geology of the place. It is this bedrock that provides the mineral part of the soil and will generally determine its pH – acidity / alkalinity.

Soils contain several elements, in varying amounts. Mineral particles, living organisms, water and humus. Understandably, the mineral particles will vary in size from rocks, to stones, gravels, sands, silts and, the finest, clays.

For convenience, soils are allocated to textural classes on their sand – silt – clay components – so, sandy soils, loams and clay soils.

Sand particles are composed of quartz (Silicon dioxide and 0.05 – 2mm in diameter) cannot be moulded, possesses great drainage, aeration and warm up quickly, but are prone to drought. Clay particles are fine in size (less than0.002mm) have a huge surface area when compared to sand, so hold water and minerals, warms up slowly and can become waterlogged and anaerobic. Clay soils are plastic and stick together to form balls.

Soil spaces, pores, are important in holding air, water and allowing drainage, remembering that water does not only flow through spaces, it also sticks to surfaces.

A loam soil is one with a good mixture of all three mineral types.

A soil, when dried, and be separated into its components using sieves, so can be analysed and classified.

The clay particles are the chemically active soil minerals. Being small and electrically charged they absorb and release minerals (including iron) and can swell and shrink. [Our clay soil is often near waterlogged and sticky in winter, yet like dry concrete in summer, but has been improved with humus, sand and some fine gravel.]

Plants use the soil for anchorage, water supply, air (oxygen for aerobic respiration and nitrogen for nitrogen fixation), mineral nutrients (nitrates, phosphates etc) and as a buffer against changes in temperature and pH. In reality, roots take up a small, 5%, of the soil volume and occur mainly in the top 15cm and seldom deeper than 50cm. Tap roots in trees are uncommon. [So gardeners do not dig too deeply unless you need to break up a poor layer.] The Woodwide Web of fungal hyphae is important in not only woodland but open soils.

Acidic soils can shed iron through leaching, and this may be deposited to form an iron pan, which stops water transmission. The lower parts of the soil, above the pan, then shows a distinct grey colouration. This is common in the New Forest of Hampshire, resulting in nutrient-poor soils that often are dominated by heathers and waterlogging. [Along the main roads, in the past, deep ploughing broke up the pan to encourage better herbage for the semi-wild ponies.]

A gley is a wetland soil (hydric soil) that, unless drained, is saturated with groundwater for long enough to develop a characteristic gleyic colour pattern. … They are found in depression areas and low landscape positions with shallow groundwater.

To explore soils in Southern England I would suggest a weekend in Purbeck. There you will be able to explore the chalky hills of Ballard Down, the acidic sandy heathlands at Arne, sand dune development at Studland and the waterlogged environment of Wareham Forest. Add in cream teas at Worth Matravers, early spider orchids at Dancing Ledge, the Saxon church and walls of Wareham and one has perfection.

A New Year Resolution for you … look at the soil and, perhaps, take a photo or two to build up a small portfolio of different types. It all adds that little touch of interest to a wander.

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How about Botany 1 and 2?

Algae? Slime moulds? Mosses? These are the neglected organisms, but fascinating.