SOIL TEXTURE Botany 3214 - Soils
SOIL SEPARATES • mineral soil is considered as a porous mixture of inorganic particles, decaying organic matter, water and air. • particle size and particle size distribution of these inorganic particles have an important bearing on the physical properties of soil.
mineral fraction of soil is divided into are called soil separates. three major size groups. These groups • Sand - 0.05 mm to 2 mm in diameter • Silt - 0.002 mm to 0.05 mm in diameter • Clay - <0.002 mm • Gravel - 2 mm to 2 cm • Stones - >2 cm • Particles of <0.001 mm are designated as colloids - clay, a colloid is unique because of its large surface area.
SOIL TEXTURE • relative proportion of the various size separates in a soil. • important because it affects and is related to several soil properties such as soil structure, aeration, water holding capacity, nutrient storage, water movement, and bearing strength.
soil separates do not act as individuals, but as partners, or aggregates. Aggregates are the clumps of soil separates. When aggregates are bound together into large masses they are called peds • It is difficult to alter the soil texture on the field level.
12 categories of soil texture but three broad classes: • Sand - coarse texture - light soils • Loam - intermediate texture - happy soils - most productive • Clay - fine texture - heavy soils
Soil texture can be determined by the “Textural Triangle” - based on feel of soils. • Textural grade is not determined or based on the major soil separates but on the separate that exerts the dominant influence. • Sand in the textural name - 50-60% or more of the soil weight.
Clay appears in the textural name - 20-30% or more by weight. • Example - loam in the textural name exhibits a happy medium between the physical and chemical properties of a coarse sand and fine clay textural soil. • Soil textural varies with depth of the soil - generally clay is more abundant in the lower layers of soil (subsoils).
SOIL STRUCTURE • Grouping of primary soil particles, ie. sand, silt, clay into aggregates (peds). The soil separates are attached together. • Structure can be altered by management practices at the field level. • Cultivation, Cover crop, Manuring, • Organic matter
Soil aggregates are cemented soil particles by: • Organic matter - breakdown by fungal and bacterial action cement particles together • Clay minerals • Fe and Al oxides • CaCO3 • Silica (H4Si04) - soluble, glue-like
TYPES OF SOIL STRUCTURE • Granular structure - most beneficial structure for plant growth.
Granular aggregates are formed by the breaking apart of larger aggregates through the physical processes of wetting and drying, and freezing and thawing. • They are cemented together by by-products of microbial decomposition of organic matter, microbial gums. The more the gums the more aggregate stability. • To increase cementing, simply add more organic matter to the soil.
Platy structure - peds are platelike aggregates arranged horizontally in the soil - platy structure is often found in the A and E. Horizon (below the A) where water moves laterally through the soil. Platy structure can be detrimental because it restricts root and water penetration.
Platy structure is often found in agricultural soils in the A horizon where the combination of excessive cultivation and compaction and the accumulation of clay at the bottom of the plow line form a zone called the plow pan.
Blocky structure - Peds are cub-like aggregates that are about the same size and shape in the vertical and horizontal dimensions. Blocky structural peds are found most frequently in the B horizon. They have been created by the wetting and drying and freezing and thawing cycles of the B horizon. The clay films also act as a binding agent for the blocky aggregates. The B horizon can often be determined in a profile by looking for the location of the blocky peds which can be readily seen.
Angular blocky angles on the edges approach 90 degrees • Subangular blocky if the sides of the peds are greater than 90 degrees
Prismatic or columnar structure - where the blocky peds are longer than they are wide. The prismatic or columnar structure is identified commonly only in the B horizon. They are often the first structures formed in a soil, because their formation only requires vertical cracking in the
Structureless C horizons generally lack any structural aggregation. Their lack of structure is termed “massive.” Where very sandy soils lack aggregation, we refer to their structureless conditions as “single grained.”
Granular: Resembles cookie crumbs and is usually less than 0.5 cm in diameter. Commonly found in surface horizons where roots have been growing.
Prismatic: Vertical columns of soil that might be a number of cm long. Usually found in lower horizons.
Columnar: Vertical columns of soil that have a salt"cap" at the top. Found in soils of arid climates.
Textural grade Not based on dominant separate but on the separate that exerts the greatest influence. “Sand” in the textural name = 50-60 % of the soil by weight. “Clay” in the textural name the clay separates are 20-30% or more of the soil weight. “Loam” is a mixture of clay, sand, and silt. A loamy sandy soil is at least 50% sand but the loam has the greater influence on the soil.
SOIL POROSITY • The mineral and organic components of the soil form a solid, but non-rigid, body called the soil matrix • The interstices between the component of the solid matrix form irregular channels called pores.
The geometry of the pore network is a function of the soil separates present, how they form aggregates (peds) and how these aggregates are associated to form the soil body. • The volume of the soil matrix not occupied by solids is referred to as the pore space of the soil.
Volume of soil consists of the volume of soil (Vs) + volume of water (Vw) + the volume of air (Va). • V = Vs + Vw + Va • Vw is inversely proportional to Va. Increasing one necessarily decreases the other. • Pore space = (V - Vs) or (Vw + Va)
The ratio of total pore space to total volume is the porosity. • Porosity = V - Vs = Vw + Va • V Vs + Vw + Va • Porosity x 100 = % pore space.
SOIL PORES: • Macropores - diameter >0.05 mm. Aeration of the matrix and infiltration and conduction of water. They exist between the soil aggregate (inter-aggregate). • Sandy soils - Good aeration, filtration and drainage • Low water storage capacity - Relatively infertile • Droughty - Non-colloidal - having low clay content
Micropores- diameter is <0.05 mm. Capillary pores of the matrix can serve as the region of water storage. Exist within the soil aggregate (intra-aggregate). • Clay soils have micro pore systems - High water and nutrient capacity - high fertility • Difficult to manipulate • Specific surface area of soil depends on both the type and amount of clay mineral present.
Platy: Thin, flat plates of soil that lie horizontally. Usually found in compacted soil.
Single Grained: Soil is broken into individual particles that do not stick together. Always accompanies a loose consistence. Commonly found in sandy soils.
Massive: Soil has no visible structure, is hard to break apart and appears in very large clods.