Linking the Nonliving to the Living Soil Linking the Nonliving to the Living

Soil is a Mixture Soil is a mixture of minerals, organic matter, water and air. There is a relationship between the composition of the soil and the nutritional needs of plants Decomposers breakdown organic matter and free the nitrates, phosphates, and sulfates that plants require.

Introduction to Soils Terrestrial life is supported by soil Without soil there would not be enough plant growth to feed all of the people and animals on earth How is soil formed? Wind, rain, and gravity all act to break up rock (A.K.A Weathering) Moss and lichens attach to help break down rock Then grass, then shrubs, and then trees All of these leave behind organic matter that mixes with the broken down rocks to form soil

Soil Profile Soil is generally found in layers called horizons. These layers make up the soil profile. A-Horizon – the topsoil Zone of Leaching – materials from soil dissolve in water and percolate down to the subsoil Usually gray or black because of the humus Humus – Decomposed plant and animal residue Valuable layer – good for growing crops Thickness – anywhere from 1 inch to 2 feet B-Horizon – subsoil Zone of accumulation – Place where all of the dissolved materials from the topsoil accumulate Poor in organic content Some organic content has been leached from A-horizon C- Horizon- parent material Not a part of the soil but a part of the soil profile The rock that the soil of the region originated from No distinct lower limit

Soil Texture Soil Texture = %Sand, Silt & Clay in a soil. Soil texture is the single most important physical property of the soil. Knowing the soil texture alone will provide information about: 1) water flow potential, 2) water holding capacity, 3) fertility potential, 4) suitability for many urban uses like bearing capacity

Sand < 2 mm to > 0.05 mm Visible without microscope Rounded or angular in shape Sand grains usually quartz if sand looks white or many minerals if sand looks brown, Some sands in soil will be brown, yellow, or red because of Fe and/or Al oxide coatings.

Sand Feels gritty Considered non-cohesive – does not stick together in a mass unless it is very wet.

Sand Low specific surface area Sand has less nutrients for plants than smaller particles Voids between sand particles promote free drainage and entry of air Holds little water and prone to drought

Silt < 0.05 mm to > 0.002 mm Not visible without microscope Quartz often dominant mineral in silt since other minerals have weathered away.

Silt Does not feel gritty Floury feel –smooth like silly putty Wet silt does not exhibit stickiness or plasticity or malleability

Silt Smaller size allows rapid weathering of non quartz minerals Smaller particles – retains more water for plants and have slower drainage than sand. Easily washed away by flowing water – highly erosive. Holds more plant nutrients than sand.

Clay < 0.002 mm Flat plates or tiny flakes Small clay particles are colloids If suspended in water will not settle Large surface area spoonful = football field

Clay Wet clay is very sticky and is plastic or it can be molded readily into a shape or rod. Easily formed into long ribbons Shrink swell – none to considerable depending on the kind of clay.

Clay Pores spaces are very small and convoluted Water holding capacity Movement of water and air very slow Water holding capacity Tremendous capacity to adsorb water- not all available for plants. Soil strength- shrink/swell affects buildings, roads and walls. Chemical adsorption is large

USDA Textural Classes Sandy soils (coarse) Silty soils (medium) Do not hold water Nutrient poor Hold a lot of air Silty soils (medium) Hold a sufficient amount of water Nutrient rich Hold a sufficient amount of air Clayey soils (fine) Hold a significant amount of water Nutrient rich/organic material poor Holds an insufficient amount of air

Textural Triangle: 50% clay, 30% silt, 20% sand => Clay Soil