An Introduction to Soil Bryan McElvany Research Coordinator Warnell School of Forest Resources Patrick Davies

This is the stuff under your fingernails DIRT This is the stuff under your fingernails

Why is soil science intimidating? Whole new vocabulary Hard to visualize Somewhat complex topics like chemistry and physics

What is soil? Several different ways to define soil 1) Geologic definition: Loose surface of the earth as distinguished from solid bedrock (support of plant life not required). 2) Traditional definition: Material which nourishes and supports growing plants (includes rocks, water, organic material, air).

What is soil made of? Mineral Matter Organic Matter Air Water

What is the function of soil? Plant medium Recycling system for nutrients Animal home Engineering medium System for water supply and purification

The ability of soil to perform those functions is dependant upon: Soil Texture Soil Structure Soil Chemistry Soil Age Site Factors (topography, water, etc) Etc.

Soil Formation Factors There are five main factors that influence soil formation processes. Type of Parent Material Climate Topography Biotic Components Time

Parent Material Two major divisions in type of parent material Residual-Formed in place (Residuum) Transported-Parent material was moved by: - wind (Eolian) - ice (Glacial) - gravity (Colluvial) - water: rivers (Alluvial) oceans (Marine) lakes (Lacustrine)

Soil Formation Processes Transformations – modification of soil constituents. Mineral weathering, organic matter breakdown. Translocations – movement up, down, or laterally of soil constituents. Additions – movement of soil material in from outside sources. Organic material from leaves, dust from atmosphere. Losses – movement of soil material out of the soil. Transportation to groundwater, erosion of surface materials.

Soil Formation Processes

Soil Formation in Action The soil formation processes, operating under the influence of the soil formation factors, is the driving force in soil genesis. Important questions to remember are: What transformations and translocations took place in the parent material to form this soil? What materials were removed? What was added? How did the climate and topography affect those processes over time?

Soil Horizons As soils develop they form layers called Horizons. These horizons have distinct characteristics produced by soil forming processes. Master horizons include the O, A, E, B, and C horizons. Not every soil contains every horizon based on the level of development of the soil. Depth of each horizon varies between different soils.

O Horizon Layer of accumulated organic matter such as leaves, grass, twigs Material can be in various states of decomposition Generally dark in color O A E B C R

A Horizon Zone of incorporation of organic matter into the mineral soil Generally it’s the most productive horizon High biological activity Generally dark in color Topsoil O A E B C R

E Horizon E Zone of Eluviation or leaching Eluviation is the movement of soil material out of a layer of soil Soluble minerals and organic material move out of this horizon Generally a lighter “washed” appearance in color O A E B C R

B Horizon B Zone of illuviation or deposition Accumulation of materials “washed” from horizons above This material is commonly clay, humus, sesquioxides, or a mixture of the three Variable in color from reds and yellows to browns and grays O A E B C R

C Horizon C Parent material The unconsolidated material that has been affected little by the soil forming processes O A E B C R

Soil Texture Soil Texture is the relative proportion of the primary particles in the soil. Sand = 2mm – 0.05mm Silt = 0.05mm – 0.002mm Clay = < 0.002mm

Soil Texture This proportion of sand, silt, and clay can be grouped into textural classes. Soil texture has a great deal to do with many other soil characteristics.

Soil Texture Coarse textured soil – soil that has a higher proportion of larger particles. Sand, Loamy sand, Sandy loam. Fine textured soil – soil that has a higher proportion of smaller particles. Clay, Silty clay, sandy clay.

Soil Structure Soil structure is the arrangement of soil particles into aggregates. Structureless soils do occur as either single grained or massive.

Pore Space The voids between soil particles are called pores. These can either be filled with water or air. Soil texture plays a major role in total pore space as well as size of pores. Coarse textured soils (sandy) have larger pores, but less total pore space. Fine textured soils (clayey) have smaller pores, but more total pore space.

Soil Permeability Permeability is a measure of how fast air and water can move through a soil. Soil texture and soil structure both pay a role in soil permeability.

Soil Texture and Permeability

Soil Structure -Permeability Granular Blocky Platy

Soil Water Saturated soil is when that soils pores are full of water. Gravitational water is that water that moves out of the soil due to gravity. This water is generally in the larger Macro-pores. Capillary water is that water that is held in the soil due to adhesion and cohesion against the pull of gravity. This water is generally held in the smaller Micro-pores and as a film around soil particles.

Soil Water After a major rain event, once the gravitational water has left the soil, the soil is at Field Moisture Capacity. The wilting point is reached when soil water levels decline to the point that all remaining water is held too tightly by soil particles to be removed by the plant.

Soil Water

Available Water Capacity Available water capacity is a measure of the water available to plants. Commonly defined as the difference between the amount of water at field moisture capacity and the amount at the wilting point. This is the water a plant has a chance of utilizing.

Available Water Capacity

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