1. Soils and Soil Types

2. What is soil? Soil is the mixture of minerals (broken down rock), air, water, and organic matter that together support plant life. Soil performs four important functions: 1)It is a medium for plant growth. 2)It stores, supplies, and purifies water. 3)It is a habitat for organisms that take part in decomposition of organic matter. 4)It modifies the atmosphere of Earth.

3. How do soils develop? Soil development is influenced by organisms, climate, topography, parent material, and time.

4. Factors in Soil Development Organisms, Topography Plants and animals help develop soil by adding organic matter (e.g., manure, remains). Fungi and bacteria decompose this organic matter into a semi-soluble chemical substance known as humus. Larger soil organisms, like earthworms and beetles, vertically redistribute this humus throughout the mineral matter. Topography (slope) affects erosion rates, depth of soil, soil drainage, and creates microclimates (e.g., south facing slopes are warmer).

6. Humus No, not ‘hummus’. Humus is the dark brown or black biochemical substance that provides these benefits to soil:  It improves a soil's ability to hold and store moisture.  It helps retain soluble nutrients.  It is the primary source of carbon and nitrogen for plant nutrition.  It improves soil structure which is necessary for plant growth.

7. Factors in Soil Development Climate influences soil development through temperature (weathering rate, level of soil microorganism activity, rate of chemical reactions) and moisture (balance between gain by precipitation and loss by evapotranspiration). The longer a soil has been forming, the thicker it becomes. Young soils are influenced mostly by their parent material, but older soils can be influenced more by other factors such as climate. Parent material is the rock and minerals from which the soils develop (affects soil texture, soil chemistry, and nutrient cycling). These can be either weathered in place (from bedrock) or be sediment transported into an area by wind, water, or ice.

8. Factors in Soil Development

9. Physical Properties of Soil: Texture Soil type usually refers to the different sizes of mineral particles in a representative sample of soil. There are three main classes of soil particles by SIZE:  Sand particles are relatively large (2.0 - 0.06 millimeters)  Silt particles are medium sized (0.06 - 0.002 millimeters)  Clay particles are small (less than 0.002 millimeters) Clay is probably the most important type of mineral particle found in a soil. This is because clay is highly reactive and can attract and hold positive ions (cations) which are then available to plant roots as food. Because they have a lattice-like structure and are somewhat flexible, clay particles can absorb water and other substances.

10. Physical Properties of Soil: Texture The type of soil depends upon the proportions of sand, silt, and clay particles. Loam is a mixture of all three sizes and is best suited for growing plants because it retains water and stores nutrients well. Sandy soils drain well and plant roots can easily penetrate them, but they dry out quickly. Clay-rich soils drain slowly and stay wet. Plant roots cannot easily penetrate clay-rich or silty soils.

11. Soil Texture The US Dept. of Agriculture has established soil categories based upon the percentages of sand, silt, and clay particles. What are the percentages at Point A? What category of soil has 20% sand, 60% silt, and 20% clay?

12. Physical Properties of Soil: Structure Soil particles usually form clumps that give soils a particular structure. Soil structure determines how easily a soil can be cultivated and how likely it is to erode. How easily water can penetrate a soil and the movement of nutrients to plant roots is also affected by soil structure.

13. Physical Properties of Soil: Porosity Pore space is that part of the soil’s volume of that is not occupied by either mineral or organic matter but is open space occupied by either gases or water. Ideally, the total pore space should be 50% of the soil volume. The gas space is needed to supply oxygen to organisms decomposing organic matter, humus, and plant roots. Pore space also allows the movement and storage of water and dissolved nutrients.

14. Soil Profiles Soil varies in composition, texture, structure, and color at different depths. Soil horizons are zones or layers of soil. A soil profile is a vertical section through all the soil horizons.

15. Soil Profile O horizon: Loose and partly decayed organic matter (0-2 inches). A horizon: Commonly known as topsoil, mix of organics and mineral matter (2 to 10 inches). B horizon: Subsoil that contains clay particles washed out (leached) from the A horizon (10 to 30 inches). C horizon: Between B horizon and bedrock. (30-48 in). Contains weathered rock fragments. R horizon: Bedrock (unaltered parent material).

17. Caliche (adapted from the Arizona Master Gardener’s Handbook) Caliche, or hardpan, is a layer of soil in which the soil particles have been cemented together by lime (calcium carbonate, CaCO 3 ). Caliche is usually found on or near the surface as a light-colored layer in the soil or as white or cream-colored concretions (lumps) mixed with the soil. Layers will vary in thickness from a few inches to several feet, and there may be more than one layer. Caliche occurs worldwide in desert or semi-arid regions.

18. Caliche (from the Arizona Master Gardener’s Handbook) Caliche causes three problems in the yard or garden. 1)The caliche layer can be so tight that roots cannot penetrate through it. The result is that plants have only the soil above the caliche to use as a source of nutrients and water and normal root development is restricted. 2)The same conditions that restrict root penetration also reduce water movement.…The restricted water penetration can contribute to problems arising from inadequate root aeration and can lead to accumulations of salt in the soil…. 3)The pH (acidity or basicity) and free calcium carbonate in a caliche soil are often high enough to cause iron to become unavailable for plants.

19. Analyzing Soil Texture We are going to learn three methods of analyzing soil texture. 1)Given percentages of sand, silt, and clay, use the USDA Soil Triangle to determine the soil texture class and its attributes. 2)By shaking a soil sample with water and an additive to prevent clumping and allowing it to settle, a sedimentation test will provide a visual way to measure the soil texture. 3)By working a small sample of soil in your hand, estimate the soil texture using a flow chart by how the soil feels and behaves.

20. Sedimentation Test The sedimentation test is any easy way to measure the percentages of sand, silt, and clay in a soil sample. It is based on the fact that large heavy particles will settle most rapidly in water, while small, light particles will settle most slowly. Calgon laundry additive, usually used as a water softener for hard water, is used here to dissolve the soil aggregates and keep the individual particles separated.

21. Soil Texture by Feel This is by far the simplest technique and all you need is your hands, a pipette of water, a ruler, and a texture flow chart. Each soil separate has a distinct texture that you can tell apart by touch alone. Sand feels gritty and you can see individual grains with the naked eye. Silt will feel smooth, almost silky, but you can only see individual particles under a microscope. Clay will feel sticky when wet and it can easily be molded into long ribbons.