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L904 Soil, Water, and Plant Relationships

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L904 Soil, Water, and Plant Relationships Plant growth depends on two Soil’s Physical important natural resources — Characteristics soil and water. Soil provides the mechanical support and nutri- There are many factors that ent reservoir necessary for plant determine the physical characteris- growth. Water is essential for plant tics of soil. These include soil texture, life processes. Effective manage- soil structure, bulk density, and soil ment of these resources for crop porosity. They all affect the interac- production requires the producer to tion between soil, water, and air. understand relationships between Soil Composition. A unit soil, water, and plants. of soil is a combination of solid Knowledge about available soil material, composed of mineral and water and soil texture can influence organic matter, and open space, the decision-making process, such called pores. By volume, most soils as determining what crops to plant are roughly 50 percent solids and and when to irrigate. This publica- 50 percent pore space. tion provides general information The mineral matter makes up on the physical characteristics of about 45 to 47 percent of the total soil, soil and water interactions, and soil volume. This mineral matter how plants use water, particularly consists of small particles of either as these topics relate to irrigated sand, silt, or clay. Soil, agriculture. However, the informa- Organic matter is made up tion is pertinent to rain-fed agri- of decaying plant and animal Water, and cultural production as well. More substances and is distributed in in-depth information on other and among the mineral particles. Plant related topics is available in other Organic matter can account for up K-State Research and Extension to about 5 percent of the overall publications, including MF2389 soil makeup by volume, but many Relationships What is ET?; L935 Important agricultural soils have less than 1 Agricultural Soil Properties; and percent organic matter. Danny H. Rogers L934 Agricultural Crop Water Use. The pores, spaces that occur Professor, Extension Irrigation Engineer, The basic soil, water, and plant between the mineral particles, are Biological and Agricultural Engineering relationships are important to agri- important because they store air Jonathan Aguilar cultural producers, but especially to and water in the soil. Assistant Professor, Water Resources irrigation users that desire to use Figure 1 shows the approximate Engineer, Southwest Research Extension best management practices such relationship between the substances Center as irrigation scheduling. Irrigation in the soil composition, with the scheduling determines when and Isaya Kisekka pore space shown split between air Assistant Professor, Irrigation Research how much water needs to be added Engineer, Southwest Research Extension to a crop’s root zone to promote Center optimum yields. One climatic- or evapotranspiration (ET)-based Mineral 47% Philip L. Barnes irrigation scheduling option is the Associate Professor, Water Quality, Organic 3% Air 25% Biological and Agricultural Engineering KanSched program (see reference Water 25% list for program availability). Freddie R. Lamm Professor, Irrigation Research Engineer, Northwest Research and Extension Center Figure 1. Typical soil composition by volume. Kansas State University Agricultural Experiment Station and Cooperative Extension Service and water. The amount of water percent clay, and 40 percent silt by together. Structureless soils can and air present in the pore spaces weight is classified as a clay loam. be either single-grained (individ- varies over time in an inverse rela- Soil Structure. Soil structure is ual unattached particles, such as a tionship. This means that for more the shape and arrangement of soil sand dune) or massive (individual water to be contained in the soil, particles into aggregates. Soil struc- particles adhered together without there has to be less air. The amount ture is an important characteristic regular cleavage, such as claypans or of water in soil pore space is essen- used to classify soils and heavily hardpans.) Soil structure is unsta- tial to crop production and will be influences agricultural productivity ble and can change with weather further discussed in the section on and other uses, such as load-bear- conditions, biological activity, and soil water content. ing capacity for structures. soil management practices. Soil Texture. The size of the The principal forms of soil Soil Bulk Density and Porosity. particles that make up the soil structure are platy, prismatic, Soil bulk density expresses the ratio determine soil texture. The tradi- columnar, blocky, and granular. of the mass weight of dry soil to tional method of determining soil These soil structure descriptions its total volume. The total volume particle size consists of separating indicate how the particles arrange includes both the solids and the the particles into three convenient themselves into aggregates. pore spaces. Soil bulk density is size ranges. These soil fractions or Aggregated soil types are generally important because it is an indicator separates are sand, silt, and clay. the most desirable for plant growth. of the soil’s porosity. The porosity Generally, only particles small- Soil structure terms also are used in of a soil is defined as the volume of er than 2 mm (1/12 inch) in size conjunction with descriptive words pores in a soil. A compacted soil has are categorized as soil particles. to indicate the class and grade of low porosity and thus a greater bulk Particles larger than this are cate- soil. Class refers to the size of the density. A loose soil has a greater gorized as gravel, stones, cobbles, or aggregates, while grade describes porosity and a lower bulk densi- boulders. how strongly the aggregates hold ty. Like soil structure, a soil’s bulk Sand particles range in size 100 from 2 mm to 0.05 mm. There are 10 subcategories assigned to this range 90 that include coarse, medium, and 20 fine sand. 80 Silt particles range in size from 30 70 0.05 mm down to 0.002 mm. The clay percent silt physical appearance of silt is much 40 like sand, but the characteristics are 60 percent clay more like clay. Clay particles are 50 50 less than 0.002 mm in size. silty clay 60 sandy Clay is an important soil 40 clay silty fraction because it has the most clay loam 70 clay loam influence on soil behavior such as 30 sandy clay loam water-holding capacity. Clay and 80 20 silt particles cannot be seen with loam sandy loam the naked eye. silty loam 190 10 Soil texture is determined by loamy silt 100 the mass ratios, or the percent by sand sand weight, of the three soil fractions. 100 90 80 70 60 50 40 30 20 10 The soil textural triangle, Figure 2, shows the different textural classes percent sand and the percentage by weight of each soil fraction. For example, a Figure 2. A soil textural classification triangle, showing a clay loam soil composed of 30 percent sand, 30 percent clay, and 40 percent silt. soil containing 30 percent sand, 30 2 K-State Research and Extension — Soil, Water, and Plant Relationships Table 1. Average water-holding capacity for Kansas soils, depths greater than 12 inches (NRCS National Engineering Handbook Part 652 Irrigation Guide). Percentage by Mass Fraction by Volume Available Available Soil Bulk Field Wilting Field Wilting Water Water Texture Density Capacity Point Capacity Point Capacity Capacity Sand 1.70 7.0 3.0 4.0 0.12 0.05 0.07 Loamy Sand 1.70 10.0 4.2 5.8 0.17 0.07 0.10 Sandy Loam 1.65 13.4 5.6 7.8 0.22 0.09 0.13 Fine Sandy Loam 1.60 18.2 8.0 10.2 0.29 0.13 0.16 Loam 1.55 22.6 10.3 12.3 0.35 0.16 0.19 Silt Loam 1.50 26.8 12.9 13.9 0.40 0.19 0.21 Silty Clay Loam 1.45 27.6 14.5 13.1 0.40 0.21 0.19 Sandy Clay Loam 1.50 26.0 14.8 11.2 0.39 0.22 0.17 Clay Loam 1.50 26.3 16.3 10.0 0.39 0.24 0.15 Silty Clay 1.40 27.9 18.8 9.1 0.39 0.26 0.13 Clay 1.35 28.8 20.8 8.0 0.39 0.28 0.11 density and porosity can be affected the water is free to drain or perco- When the soil is oven dried, all soil by weather-related factors, biologi- late due to the force of gravity. This water has been removed from the cal activities, and soil management excess water is referred to as grav- soil. The amount of water at any practices. Table 1 lists typical bulk itational water. Since this percola- soil-water content varies by soil densities for Kansas soils. tion takes time, some of this extra type. Specific water-holding capac- water could be used by plants or ities can be obtained from various Soil and Water lost to evaporation. Field capacity sources; however, NRCS County Interactions is defined as the amount of water Soil Surveys are probably the most remaining in the soil after rapid extensive and readily accessible. Soil acts like a reservoir that percolation has occurred. This is not Figure 3 illustrates typical amounts holds water and nutrients plants a definite soil water point; there- of water held at the defined soil need to grow. Some soils are large fore, field capacity often is defined water content for sand, loam, and reservoirs with more holding capac- as approximately one-third atmo- silty clay loam soils. The reasons for ity that release water and nutrients sphere tension. Tension is defined the differences between soil types is easily to plants, while other soils in a following section.
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