LECTURE 1 What is soil? What are its functions? What does it consist of?

Definitions… Nutrient cycle….. (R. Manchip) Solum…. (A. Graham)

What is meant by the term “soil”? “The collection of natural bodies occupying parts of the earth’s surface that support plants and have properties due to the integrated effect of climate and living matter acting upon parent material, as conditioned by relief over periods of time.” – Brady & Weil (1999)

Why should we study soils? We are very dependent on soils We will be even more dependent on them in the future! Nearly all food Much of our fiber Medicines (incl. anti-cancer drugs) Biomass as a source of energy, especially as petroleum supplies are

So what are the functions of soil in our ecosystem?

SOIL Medium for plant growth Recycling system Engineering medium Habitat System for water supply & purification

Medium for plant growth Root systems can be even larger than the part of the plant that we can see! What do plants obtain from soil? Physical support Ventilation Water Temperature moderation Protection from phytotoxic substances Supply of inorganic mineral nutrients in the form of dissolved ions

Regulator of water supply Quality and quantity of water in rivers, lakes and dams is a concern! All water has traveled through or over soil Nature and management of soils will have a major influence on the purity and amount of water in aquatic systems Soil absorbs some water, other water flows through the soil to reach groundwater Water cleansed as it flows through upper soil layers Rate of water movement in catchments controlled (link with Prof. Rowntree’s work)

Recycling system Re-use of nutrients very important to ecosystem functioning Soils play a major role in geochemical cycles Soils assimilate organic matter and convert mineral nutrients into forms that can be used by plants and animals Plants and animals return carbon to the atmosphere as carbon dioxide that becomes part of living things again via photosynthesis Accumulation of large quantities of CO 2 can affect global changes, e.g. greenhouse effect

Habitat for soil organisms A cupful of soil can contain billions of organisms belonging to thousands of species There is a vast range of niches and habitats even in a “uniform” soil Soils harbour much of the earth’s genetic diversity Soils are vital components of larger ecosystems

Engineering medium Concept of “terra firma” Most structures rest on soil, most construction projects require excavation into the soil Some soils not as stable as others Reliable construction requires knowledge of the diversity of soil properties 1 location not necessarily the same as another E.g. swelling properties of clays

Working with natural soils/soil material is not the same as working with manufactured material Bearing strength Compressibility Stability

How can we think about soil? “The” soil or “soil” refers to soils collectively, like the term “vegetation” refers to plants. “A” soil is an individual, 3-d body that can be classified as a particular soil type.

How does the story of soil start? Parent material weathers layer of unconsolidated debris (regolith) overlying hard, unweathered rock living organisms act on regolith physically and biochemically SOIL FORMATION BEGINS!

Soil occurs at the meeting place of: LITHOSPHERE ATMOSPHERE HYDROSPHERE BIOSPHERE Soil is the product of both destructive and creative (synthetic) processes

Soil development processes Destructive processes: Weathering of rock Microbial decay of organic residues Synthetic processes: Formation of new minerals and new stable organic compounds Formation of soil horizons in upper regolithWhat does soil consist of?

What does soil consist of? Air Minerals Water Organic matter The relative proportions of these constituents greatly influence the behaviour and productivity of soils

Mineral constituents… Various mineral particles (particles of weathered parent material) Range of sizes Sand (2,0 – 0,05mm) Silt (0,05 – 0.002mm) Clay (<0.002mm) Different mixtures of these result in various soil textures

Organic matter… Wide range of carbonaceous substances including living organisms, remains of organisms and compounds produced by current and past metabolism in the soil Organic matter is lost to the soil over time as CO 2 which is produced by microbial respiration Repeated additions of new plant/animal residues is necessary to maintain a reasonably constant level of soil organic matter The influence of organic matter on soil properties is far greater than its low % by weight and volume would suggest

Soil water… Essential for the survival and growth of plants and other soil organisms The movement of water (and the substances dissolved in it) through the soil profile is of great consequence to the quality and quantity of local and regional water resources

Soil Water (cont.)… Held within soil pores according to the amount of water and the size of the pores within the soil matrix This restricts the ability of the water to flow freely Not all of the water in the soil is available to plants Never pure water but contains hundreds of dissolved organic & inorganic substances More accurate to call it the “soil solution” Contains soluble inorganic compounds essential for plant growth Acidity / alkalinity of the solution is NB!

Soil air… Consists of a changing mixture of gases Differs from atmospheric air Varies greatly from place to place in the soil –Local pockets where some gases are consumed by plant roots and chemical reactions, whilst others are released Generally has higher moisture content than the atmosphere % CO2 higher, % O2 lower Amount and composition of soil air is largely determined by the soil water content