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The Soil Profile of Heene Cemetery

Heene Cemetery occupies about one acre of flat land in Worthing that is botanically classified as unimproved old meadow. After the cemetery was declared full it was unmaintained for many years, and the absence of soil disturbance during this time allowed stable communities of underground flora, fungi and fauna to establish.

A photograph that shows a soil cross section, revealing horizons
Soil cross section, revealing horizons, copyright Radosław Drożdżewski.

Undisturbed soil consists of layers parallel to the surface, each layer being known as a soil horizon, characterised by its texture, colour, depth, chemical composition from its underlying geology, flora, fungi, and fauna. A vertical cross-section through the soil horizons, as shown in the illustration above, is called the soil profile. The texture of a soil horizon is determined by the size and relative proportions of the particles making up the soil.

Worthing sits on a Cretaceous greensand/sandstone layer, of which the youngest, or top, part is Upper Cretaceous chalk. Below it are Lower Cretaceous layers of clay and sandstones. Specifically, the clay layer is a sedimentary sandy carbonate mudstone with clay particles. This mixture has a reasonably good balance of drainage and water retention. Soil particle sizes are generally named in three categories, clay, having the smallest particle size range, silt, with intermediate particle size range, and sand, giving the largest particle size range. Soil with roughly equal proportions of sand, silt, and clay is commonly called loam, but the proportions supplied in purchased soil may vary. Sand varies in composition according to the local geology, but may be either powdered quartz (SiO2), as in Worthing, powdered feldspar (Na, K, or Ca aluminosilicates), or powdered Aragonite (CaCO3), the latter having its origin in extinct coral or shellfish. Clay is hydrated aluminium silicates, whose water content varies with the amount trapped in the mineral structure. Silts are fine-grained soils not having the composition of either sands or clays. These descriptions refer to the inorganic compositions of local soils, but all soil in Heene Cemetery has an organic component too, generally referred to as humus.

Soils that have medium to large spaces between the inorganic particles allow water to drain through and are called permeable soils. Soils with small spaces tend to hold water within them and if clay is present become heavy and difficult to work. In prolonged wet weather they soon become saturated. Heene Cemetery soil is neutral to acidic (pH 6 – 7) and of permeable loamy quality. It is therefore a good growing medium for plants.

When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay and plot this information on a triangular diagram, with each size particle at one corner. The soil type can then be determined from the location on the diagram. At the top, a soil would be clay; at the left corner, it would be sand; and at the right corner it would be silt. Soils in the lower middle with less than 50% clay are loams.

Determining soil texture is often aided with the use of a soil texture triangle plot.
Determining soil texture is often aided with the use of a soil texture triangle plot, copyright CMG Lee.

How soils are produced

It takes years for a soil to be produced, by the processes of geological, meteorological, chemical, and biological weathering on solid rock. The rock is broken down (erosion) in stages, initially by forces such as glaciation, ground movement, wave action and tidal surges, then by high winds, solar heat, precipitation, and freezing and melting, combined with temperature changes. Geological and meteorological weathering is essentially physical, but there is also chemical weathering by the action of oxygen, water, and carbon dioxide, including the unwelcome effects of acid rain. Finally there are the effects of the living communities that make the soil their home. The greatest degree of weathering will always be in the top layer, with successively less change as one looks deeper. Each type of climate produces a distinctive soil type that forms in the particular local circumstances. Heene Cemetery soil therefore is in large measure a consequence of the prevailing maritime climate of Worthing. Until recently the temperature and precipitation variations throughout the year were reasonably consistent, and this would have produced steady pH readings and drainage characteristics. As the area is flat leaching of nutrients by percolating water is uniform. Soil communities are partially determined by the plant cover; in the south east sector there is a high concentration of meadow grasses, and elsewhere there are areas dominated by scrub such as brambles, or by trees and shrubs.

In general, where there is relatively little weathering, soils are thin but soluble minerals may be present, having not leached away. Where there is intense weathering, soils may be thick but poorly supplied with nutrients. Soil development takes a very long time, and it may take hundreds or even thousands of years for a good fertile topsoil to form. Soil scientists estimate that in the very best soil-forming conditions, soil forms at a rate of about 1mm per year. In poor conditions, soil formation may take thousands of years! Typically, a soil in a temperate climate like southern England will contain 45% mineral matter, 25% water, 25% air, and 5% organic matter, living and dead.

The content of soil horizons

As the diagram below shows, the simplest soils have four horizons, a top organic humus layer (O horizon), a dark layer of topsoil (A horizon), a layer of subsoil (B horizon), and a partially weathered bedrock layer (C horizon).

The simplest soils have four horizons.
The simplest soils have four horizons, copyright Wikipedia.

The topsoil is where there is most biological activity, for if undisturbed it is a balanced ecosystem of principally microorganisms, fungi, and invertebrates of great variety criss-crossed by plant roots serving to bind the soil as well as provide nourishment for the surface plants and fungi. The percolation of water through the soil carries minerals and other nutrients, and also the smaller soil particles such as clays, both accumulating in the subsoil. The presence of a higher concentration of iron and clay particles means that the subsoil holds more water than the topsoil, but there is less organic material. Because Heene Cemetery has had relatively little soil disturbance for decades this simple profile matches the soil horizons found there.

O and A Horizons

The O Horizon is the humus layer where plant and animal debris begin their passage from the surface to the A Horizon topsoil layer by the action of earthworms, plant roots, fungal mycelia, and herbivores, detritivores, and scavengers from the soil community such as millipedes, a variety of insects and their larvae, and microorganisms. Their predators, such as centipedes, arachnids, and carnivorous insects, will also be present. The A Horizon, a mixture of organic and inorganic components, is thus a vibrant, multi-organism layer, whose wildlife list characterises the soil biota (the combined total of all living species) as a whole, and whose mineral content reflects the bedrock.

B Horizon

This is the subsoil layer, which will have characteristics of the topsoil layer if the soil is dug. From the point of view of the organism communities in the topsoil this is undesirable, because the members of these communities live, feed, and reproduce at a specific depth range that supplies their needs. An area of soil that is cultivated will have mixing of communities and the A and B Horizons, which reduces the fertility of the soil and its ability to recycle.

Minerals such as iron oxide and metal salts are present in the subsoil because they have made their way down from the topsoil or have been brought up from the C Horizon by earthworm or geological action, by percolation, or by digging.

C Horizon

In Heene Cemetery this unweathered bedrock layer is almost certainly intact having had plenty of time to settle after the last period of grave digging. C Horizons can serve as anchors for tree roots, but otherwise they lack organic matter and the communities dependent upon it. Only in very wet conditions will water-soluble minerals leach this far down.

Heene Cemetery’s soil wildlife

Currently the cemetery is being surveyed in order to compile lists of resident flora, fungi, and fauna, in some cases with the help of specialists who have most kindly offered to share their expertise. Soil organisms are the most difficult to survey; many are very small indeed, and they can respond rapidly to sound and vibrations and disappear with astonishing speed. Many have been seen but not identified, but gradually the list of known residents is getting longer.

The non-specialist is most likely to be able to identify the soil fauna in horizons A and B. We can distinguish three categories by size, called microfauna, mesofauna, and macrofauna (or megafauna). One cubic metre of topsoil may contain several hundred thousand individuals of the microfauna group and several hundred individuals of the mesofauna group. Below 40 cm populations are very small indeed. The microfauna mostly occupy and move in the spaces in the soil not filled by water, or they occupy the films of water around soil particles. In the cemetery we have protozoans, rotifers, and nematodes, which may be terrestrial or aquatic (occupying films of water). Small arthropods such as mites (Acari arachnids), Collembola (‘springtails’), Symphyla (myriapods that are tiny, distant relatives of centipedes), Pauropoda (another group of myriapods), Pseudoscorpions (tiny arachnid predators), other small arachnids, and also potworms (small Annelid worms that are members of the Enchytraeid group) move in leaf litter and upper soil spaces. Examples of mesofauna are earthworms (larger Annelids), Opiliones (an arachnid group that includes the harvestmen), larger myriapods such as centipedes (Chilopoda) and millipedes (Diplopoda), and insect larvae (such as cockchafers, click beetles, ground beetles, burying beetles, flies (Diptera), and lacewings (Neuroptera)). These and other adult insects populate the plant life immediately above the topsoil layer, notably the groups mentioned plus bugs (Hemiptera), thrips (Thysanoptera), ants, bees, and wasps (Hymenoptera), booklice (Psocoptera) in leaf litter, Diplura and Protura in topsoil, and of course the familiar woodlice (Isopoda crustaceans). Burrowing vertebrates such as moles, or reptiles and amphibians able to use larger soil channels such as slowworms and newts, constitute examples of the macrofauna in Heene Cemetery, and to these we can add slugs and snails (Gastropod molluscs). Geobionts are organisms that live permanently in the soil, like earthworms, geophils live part of their life cycle in the soil, like insect larvae, and geoxenes use the soil as a refuge, like slowworms, or to overwinter as do newts. In their way, each of these organisms influences the properties of the soil, be it by aeration, changes in porosity and permeability, transport of humus, decomposition of humus, alteration of pH, and general mixing and recycling of components. The nutrient cycling of nitrogen, carbon, and sulphur depends on soil populations. Soil fauna are also adept at changing their feeding strategies according to available food, with carnivores often becoming omnivorous if prey is scarce. Many geobionts are very ancient, dating from the Devonian period, more than 350 million years ago.

What is particularly interesting about geobionts is the way their bodily structures have evolved for survival underground. They often have reduced visual capacity, or may even be blind, and colouration may be cryptic or pigmentation lost altogether. Appendages may be reduced in size to assist mobility through the soil, and bodies may be slimmed or elongated. They also have specially adapted water and chemical receptors for life in reduced visibility and proximity of soil and percolated chemicals. The virtual absence of light below the soil surface means that few photosynthetic organisms are important in soil food webs. This attuned sensitivity to special conditions makes soil organisms vulnerable to introduced chemicals, especially horticultural and agricultural chemicals, to which they have no adaptation. Compared with conditions above ground the soil environment, if undisturbed, is remarkably constant within each season, and it makes soil populations and species behaviour good indicators of soil quality. It is difficult to estimate the populations of Heene Cemetery’s soil biota, but with a substantial soil covering they must collectively run into millions of individuals.

Most of the biodiversity of Heene Cemetery is below ground, and any factors affecting soil quality will affect the populations that inhabit it and therefore have consequences for the sustainability of the cemetery ecosystem. Maintenance of the cemetery is always carried out with this possible outcome in mind, so that every stage of the natural cycles can proceed unhindered. Badly maintained, or regularly disturbed, soil will have a preponderance of species able to bear stress and adapt, and a decline in species that are less capable of adjusting to a changing environment. This will affect key food webs in the soil, and such a community is unnatural and will need intervention to restabilise. Heene Cemetery has a structured maintenance programme designed to protect its biodiversity.

Written by Brian Day