agr204 KY MG ch 4.pdf

AGR-204 Soils and Fertility By Craig Cogger, Extension soil scientist, Washington State University. Adapted for Kentucky by Edwin Ritchey, Extension soils specialist, and Brad Lee, Extension water quality specialist, University of Kentucky....

AGR-204 Soils and Fertility By Craig Cogger, Extension soil scientist, Washington State University. Adapted for Kentucky by Edwin Ritchey, Extension soils specialist, and Brad Lee, Extension water quality specialist, University of Kentucky. the water held in micropores is available to plants, but some In this chapter: is held so tightly that plant roots cannot use it. Soil that has a balance of macropores and micropores pro- Soil and Water 01 vides adequate permeability and water-holding capacity for Organisms 04 good plant growth. Soils that contain mostly macropores drain readily but are droughty and need more frequent irrigation. Nutrients  06 Soils that contain mostly micropores have good water-holding Understanding Fertilizers 08 capacity but take longer to dry out, and excessive wetness can How Much Fertilizer to Use 12 reduce plant growth. Soil texture, structure, organic matter, and human activity Cooperative Extension Publications 12 affect porosity. You can evaluate your garden soil in terms of Estimating Organic Fertilizer Rates  12 these properties to understand their impact. The only tools you need are your eyes, fingers, and a shovel. When to Fertilize 13 Adding Organic Matter 13 Texture Soil pH 15 Texture describes the relative abundance of sand, silt, and Soils and Fertilizer Terminology 16 clay-sized materials. Anything greater than two millimeters in size is considered gravel or rock and is not soil. The coarsest For More Information 18 soil particles are sand. They are visible to the eye and give soil a gritty feel. Silt particles are smaller than sand—about the size of individual particles of white flour. They give soil a smooth, S oil is a mixture of weathered rock fragments (minerals) floury feel. On close inspection, sand and silt particles look like and organic matter at the earth’s surface. It is biologically miniature rocks (Figure 4.1). active—a home to countless microorganisms, inverte- Clay particles are the smallest—about the size of bacteria brates, and plant roots. It varies in depth from a few inches to and viruses—and can be seen only with a microscope. They eight feet or more. An ideal soil is roughly 50 percent pore space typically have a flat shape, similar to a sheet of mica, and when (50 percent voids between soil particles). This space forms a microscopically viewed from the side, they look like the pages complex network of pores of varying sizes, much like those in a of a book. Soils high in clay feel very hard when dry but are sponge. Soil provides nutrients, water, and physical support for easily shaped and molded when moist. plants as well as air for plant roots. Soil organisms are nature’s Although all of these particles seem small, the relative dif- primary recyclers, turning dead cells and tissue into nutrients, ference in their sizes is quite large. If a typical clay particle were energy, carbon dioxide, and water to fuel new life. the size of a penny, a sand particle would be as large as a house. Soil and Water A productive soil is permeable to water and is able to supply water to plants. A soil’s permeability and water-holding capacity depend on its network of two types of pores: silt Large pores (macropores) control a soil’s permeability and aeration. Macropores include earthworm and root channels. (0.002 - 0.05 mm dia.) Because they are large, water moves through them rapidly by gravity so that rainfall and irrigation infiltrate into the soil, and excess water drains through it. Micropores are fine soil pores, typically a fraction of a mil- limeter in diameter. They are responsible for a soil’s water- sand clay holding capacity. Like the fine pores in a sponge or towel, (0.05 - 2 mm dia.) (