Soils/ Soil & Water Relationships

Soil The top few inches of the earth’s surface that supports plant growth. Formed from parent material (rocks and minerals) by a process known as weathering. Productivity can be lost by soil degradation, such as erosion and pollution.

Four Principal Components of Soil 25% water 25% air 40-45% minerals 5-10% organic matter Organic matter is material made from living or once living material.

Soil Formation Soil is formed from parent material through a process called weathering. Types of weathering are physical (mechanical) and chemical. Mechanical/physical: Plants & animals Ice wedging Chemical water Acids oxygen

Factors that affect soil formation Climate Living organisms Parent material Time How the soil weathered Topography (the lay of the land)

Soil formation factors

Soil Particles (separates) Soil has three particle sizes: sand – the largest silt - medium clay – the smallest

Soil Texture Soil texture is the relative percentage of sand, silt, and clay in a soil sample. There are 12 soil textures. Different plants prefer different soil textures. Different textures have different relationships with water depending on the percentage of particles making up the soil. Scientists use the soil textural triangle to determine the soil’s texture.

Ribboning soil to determine texture

Weathering results in particles Sedentary soils are weathered from large patches of bedrock, so they remain in place and don’t really move. Transported soils occur when particles are transported. colluvial- moved by gravity (landslide, mudslide) alluvial- moved by water (Delta, flood plains) aeolian- moved by wind glacial till- particles moved by glaciers

Organic matter/humus OM comes from living matter. Dark colored soils indicate greater amounts of OM. Benefits of OM in soil: Makes soil porous. Adds N and other nutrients to soil. Helps hold water. Furnishes food for living organisms. Minimizes leaching. Stabilizes soil structure. Examples of living contributions to soil: Roots Algae Fungi Small animals Insects Slugs Snails Worms Snakes reptiles Examples of non-living contributions to soil: Peat moss Leaf remnants Compost Grass clippings Saw dust manure

How should your soil smell?

Soil Profile

How should your soil look?

The soil profile has at least 4 horizons or layers. Name Colors Structure Process Occuring O Organic Black, dark brown Loose, crumbly, well-broken Decomposition A Topsoil Dark brown to yellow Zone of leaching* B Subsoil Brown, red, yellow, gray Larger chunks, dense, cement-like Zone of accumulation C Parent material Varies by parent material Dense, rocky Weathering of parent material *Leaching: when materials move through the soil profile; often materials like chemicals and pollutants are carried through the soil profile by water.

Soil horizons

Soil structure: soil particles stick together to make 4 types of aggregates Granular Platy Blocky Prismatic/columnar Wedge

Soil Structure Soils with more sand tend not to bind together and therefore do not show much structural arrangement Clay give soil more structure Soil structure is important in the absorption of water and the circulation of air Structure of the A and B horizons should have medium and large particles for a loose structure that is good for water infiltration, root development, and seed germination.

Water below the surface Surface material and soil texture will determine how rapidly water flows through the soil profile. Water that fills up pore spaces between soil particles accumulate below the earth’s surface to create groundwater. Porosity is a measure of the amount of open space compared to the total volume of rock/soil. The ability of material to transmit fluid is permeability. Sandy soils have a greater porosity and therefore a greater permeability than clay and silt. Clay is the least porous and permeable.

Water Relations The size, shape, and arrangements of soil particles in a given texture will determine the ability of the soil to retain water. Large pores conduct water more rapidly than small pores. Water is more easily removed from large pores than small pores. In sandy soils, water is lost due to gravity faster than plant roots can access it.

Plant water After water is lost due to gravity, the remaining water is stored under tension in the pores of various sizes. The smaller the pore, the greater the tension, and the more energy is required to the water. Water cannot be removed by plants from very small pores. Hygroscopic water, that which is water closely bound to soil particles, is also not available for plant use.

Plant water Field capacity is water available to plants. It’s about 24 hours after soil saturation, after gravity has set in. This varies depending on the soil, and can be extended by the irrigation method chosen. The goal is to allow the plant to get as much water as needed, by altering irrigation to overcome the challenges of the soil texture and weather. Permanent wilting point is when the point at which water is no longer available to the plant.

Soil –Water relationships: permeability & drainability SAND FAST SILT MEDIUM CLAY SLOW

Soil structure can change over time Soil structure can change over time. Small and smaller particles make water infiltration more and more difficult.

The Water table Just below the surface is the zone of aeration. Below that is zone of saturation. Below the zone of saturation is the groundwater. The upper boundary of the zone of saturation is the water table. The depth of the groundwater/level of the water table varies with the precipitation and climate as well as what gets pumped out and used. It is essential that the amount pumped out does not exceed nature’s ability to recharge the water source.

The Water Table

Aquifers An aquifer is any underground, water-bearing layer that groundwater can flow through. Aquitards confine the water.