An understanding of soil and the contribution its properties make to the growth and survival of plants is essential to an understanding of farming practices and to the design of experiments related to those practices.
This paper is therefore principally about soil as a growing medium, not as either a geophysical entity or as a material providing archaeological insights of itself (for the latter, see, for example, Courty, Goldberg and Macphail 1989).
The support material used in hydroponics is the ultimate growing medium, providing little but mechanical support for the roots, while all nutrients, including oxygen, are provided by a carefully metered flow of an aqueous solution, within a temperature controlled environment. This description provides the necessary insights into what a soil contributes to plant growth and survival and will help in developing an understanding of why soils of different geological origins and different physical structures affect plants to differing degrees.
A soil supports a plant against wind and gravity, dependent on root anchorage and spread. On a slope, on a flood plain or in a potential gully, the soil may be more or less resistant to movement, both in its own right and in combination with roots. This feature of its behaviour is just as important for the survival of earthworks (Sedimentation, Erosion and Plant Revegetation in Experimental Earthworks), as it is in studies of plant growth.
Another feature of soil is the way in which water and air flow and permeate around the roots. This is crucial to the ability of roots to take up nutrients and for them to reach out as far as necessary to maintain effective growth.
The passageways for water and air are largely dictated by the particle size range and distribution within any given soil but this may be modified by the presence of organic material, dead and decaying plants and deliberately added conditioning material, including manure (Manuring).
The quality of a soil, in this respect, is now called texture and is influenced by such components as clay, silt, fine or coarse sand and stones. The US Department of Agriculture definition for silt is specifically set to incorporate wind-
Clayey soils tend to be poorly drained, to retain water and to provide "perched" water tables. Sandy soils are free draining and may be prone to leaching. The consequences of these effects for nutrient transport and displacement are outlined later.
The originating rocks and deposits from which soil particles are formed dictate the chemical and nutrient qualities of those particles, particularly with regard to the provision of trace elements. The development of a soil also incorporates the process of humification, that is, the incorporation of organic matter through decay and admixture, including the effects of earthworm activity.
Soils can be seen to have a clear vertical profile, as expressed colloquially in terms such as soil and sub-
The passage of water through soils influences the uptake and transport of nutrients from soil particles, according to the specific circumstances prevailing. The complex structure of silicates in clays allow ready exchange (take up or release) of trace elements, such as potassium and magnesium, along with many others, with the soil water. Chalk or limestone based soils tend to have quantities of calcium present. The presence of organic acids in the soil solution modifies these exchanges, as do the processes of evaporation or rainfall, irrigation or flood.
Excessive rainfall or irrigation may lead to leaching, the transport of mineral or organic nutrients out of the local root zone. Where downward transport meets a relatively impermeable zone, such as a clay layer, then elements such as iron may be precipitated in various chemical forms, leading to podsolisation or panning. Such layers are largely impenetrable to roots and can only be broken up by deep, high-
The converse process of aridification occurs where soil and water minerals, in particular, are moved towards the surface by an excess of evaporation over rainfall (or irrigation for that matter). A concentration of such salts near the surface is just as inimical to plant growth. This surface effect is called cementation and parallels podsols in making the soil difficult to work and beneficiate.
Perhaps surprisingly to non-
This presentation is deliberately condensed and for those wishing to explore the subject in greater detail texts such as Cruickshank (1977) and Russell (1961) are suggested.
Courty, Grant and Macphail "Soils and Micromorphology in Archaeology." Cambridge Manuals in Archaeology. C.U.P. 1989
Cruickshank, James D. "Soil Geography." David and Charles 1977 (3rd imp.)
Russell E.W. "Soil Conditions and Plant Growth." Longmans 1961 (9th ed.) [The first seven editions of this text were written by Sir John E. Russell, formerly Director of the Rothamsted Experimental Station UK]