CHAPTER 2 - SOIL AND WATER2. The soil. 2. 2 Entry of water. Soil moisture conditions. Available water content. Groundwater table. Soil erosion by water. Soil composition. Soil profile. 2. 1. Soil texture. 2. 1. Soil structure. When dry soil is crushed in the hand, it can be seen that it is composed of all kinds of particles of different sizes. Some originate from residues of plants or animals (rotting leaves, pieces of bone, etc.), these are called organic particles (or organic matter). The soil particles seem to touch each other, but in reality have spaces in between. These spaces are called pores. After irrigation or rainfall, the pores are mainly filled with water. Living material is found in the soil. It can be live roots as well as beetles, worms, larvae etc. They help to aerate the soil and thus create favourable growing conditions for the plant roots (Fig. The composition of the soil. If a pit is dug in the soil, at least 1 m deep, various layers, different in colour and composition can be seen. These layers are called horizons. This succession of horizons is called the profile of the soil (Fig. The soil profile. A very general and simplified soil profile can be described as follows. The plough layer (2. This layer is subject to land preparation (e. The deep plough layer: contains much less organic matter and live roots. This layer is hardly affected by normal land preparation activities. The colour is lighter, often grey, and sometimes mottled with yellowish or reddish spots. Field Determination of Permanent Wilting Point E.R. Silvertooth Abstract Water is a vital resource for cotton production in the desert Southwest. One method of managing irrigation water is through the use of a. Modelling and prediction of soil water contents at . CHAPTER 10 Field Capacity, Wilting Point, Available Water, and the Nonlimiting Water Range The amount of water available for plant uptake has been related to a soil’s water budget. The three terms associated with the water. The subsoil layer: hardly any organic matter or live roots are to be found. This layer is not very important for plant growth as only a few roots will reach it. The parent rock layer: consists of rock, from the degradation of which the soil was formed. This rock is sometimes called parent material. The depth of the different layers varies widely: some layers may be missing altogether. The mineral particles of the soil differ widely in size and can be classified as follows. Name of the particles. Size limits in mm. Distinguisable with naked eyegravellarger than 1obviouslysand. The amount of sand, silt and clay present in the soil determines the soil texture. A coarse- textured soil is light because it is easy to work, while a fine- textured soil is heavy because it is hard to work. Coarse textured soil is gritty. Individual particules are loose and fall apart in the hand, even when moist. Fig. Medium textured soil feels very soft (like flour) when dry. It can be easily be pressed when wet and then feels silky. Fig. Fine textured soil sticks to the fingers when wet and can form a ball when pressed. Soil structure refers to the grouping of soil particles (sand, silt, clay, organic matter and fertilizers) into porous compounds. These are called aggregates. Soil structure also refers to the arrangement of these aggregates separated by pores and cracks (Fig. The soil structure. When present in the topsoil, a massive structure blocks the entrance of water; seed germination is difficult due to poor aeration. On the other hand, if the topsoil is granular, the water enters easily and the seed germination is better. By means of cultivation practices (ploughing, ridging, etc.), the farmer tries to obtain a granular topsoil structure for his fields. Some examples of soil structures GRANULARBLOCKYPRISMATICMASSIVE2. The infiltration. FIELD AND LABORATORY DETERMINATION OF FIELD CAPACITY. The point where the curve changes to. Plant and Soil XXIX, no. 1 August 1968 SOIL ORGANIC NITROGEN MINERALIZATION AS AFFECTED BY LOW SOIL WATER POTENTIALS by R. WETSELAAR Division of Land Research, CSIRO, Canberra, Australia INTRODUCTION When. Permanent Wilting Point. The permanent wilting point is the water content of a soil when most plants (corn, wheat, sunflowers) growing in that soil wilt and fail to recover their turgor upon rewetting. 1248 Journal of Food, Agriculture & Environment, Vol.11 (3&4), July-October 2013 Journal of Food, Agriculture & Environment Vol.11 (3&4): 1248-1251. 2013 www.world-food.net WFL Publisher Science and Technology Meri-Rastilantie. Infiltration rate. Factors. influencing the infiltration rate. When rain or irrigation water is supplied to a field, it seeps into the soil. This process is called infiltration. The water seeps into the soil; the colour of the soil becomes darker as it is wetted (see Fig. Infiltration of water into the soil. Repeat the previous test, this time with two glasses. One is filled with dry sand and the other is filled with dry clay (see Fig. The sand is said to have a higher infiltration rate. The same amount of water is supplied to each glass. Fig. After one hour the water has infiltrated in the sand, while some water is still ponding on the clay. The infiltration rate of a soil is the velocity at which water can seep into it. It is commonly measured by the depth (in mm) of the water layer that the soil can absorb in an hour. Soil with an infiltration rate of 1. A range of values for infiltration rates is given below. Low infiltration rateless than 1. The infiltration rate of a soil depends on factors that are constant, such as the soil texture. It also depends on factors that vary, such as the soil moisture content. Soil Texture. Coarse textured soils have mainly large particles in between which there are large pores. Infiltration rate and soil texture. In coarse soils, the rain or irrigation water enters and moves more easily into larger pores; it takes less time for the water to infiltrate into the soil. In other words, infiltration rate is higher for coarse textured soils than for fine textured soils. The soil moisture content. The water infiltrates faster (higher infiltration rate) when the soil is dry, than when it is wet (see Fig. As a consequence, when irrigation water is applied to a field, the water at first infiltrates easily, but as the soil becomes wet, the infiltration rate decreases. Infiltration rate and soil moisture contentiii. The soil structure. Generally speaking, water infiltrates quickly (high infiltration rate) into granular soils but very slowly (low infiltration rate) into massive and compact soils. For example: when an amount of water (in mm of water depth) of 1. Fig. A soil moisture content of 1. The soil moisture content can also be expressed in percent of volume. In the example above, 1 m. This results in a soil moisture content in volume percent of. Thus, a moisture content of 1. If all soil pores are filled with water the soil is said to be saturated. There is no air left in the soil (see Fig. It is easy to determine in the field if a soil is saturated. If a handful of saturated soil is squeezed, some (muddy) water will run between the fingers. At saturation, no air is present and the plant will suffer. Many crops cannot withstand saturated soil conditions for a period of more than 2- 5 days. Rice is one of the exceptions to this rule. The period of saturation of the topsoil usually does not last long. After the rain or the irrigation has stopped, part of the water present in the larger pores will move downward. This process is called drainage or percolation. In coarse textured (sandy) soils, drainage is completed within a period of a few hours. In fine textured (clayey) soils, drainage may take some (2- 3) days. After the drainage has stopped, the large soil pores are filled with both air and water while the smaller pores are still full of water. At this stage, the soil is said to be at field capacity. At field capacity, the water and air contents of the soil are considered to be ideal for crop growth (see Fig. Little by little, the water stored in the soil is taken up by the plant roots or evaporated from the topsoil into the atmosphere. If no additional water is supplied to the soil, it gradually dries out. At a certain stage, the uptake of water is not sufficient to meet the plant's needs. The plant looses freshness and wilts; the leaves change colour from green to yellow. Finally the plant dies. The soil still contains some water, but it is too difficult for the roots to suck it from the soil (see Fig. Some soil moisture characteristics. The soil can be compared to a water reservoir for the plants. When the soil. is saturated, the reservoir is full. However, some water drains rapidly below. Fig. Saturation. When this water has drained away, the soil is at field capacity. Field capacity. When the soil reaches permanent wilting point, the remaining water is no longer. Fig. Permanent wilting point. The amount of water actually available to the plant is the amount of water stored in the soil at field capacity minus the water that will remain in the soil at permanent wilting point. This is illustrated in Fig. The available soil moisture or water content. Available water content = water content at field capacity - water content at permanent wilting point ... A range of values for different types of soil is given in the following table. They are constant for a given soil, but vary widely from one type of soil to another. The groundwater table. The depth of the groundwater table varies greatly from place to place, mainly due to changes in topography of the area (see Fig. Variations in depth of the groundwater table. In one particular place or field, the depth of the groundwater table may vary in time. It may even reach and saturate the rootzone. If prolonged, this situation can be disastrous for crops which cannot resist . Where the groundwater table appears at the surface, it is called an open groundwater table. This is the case in swampy areas. To keep the rootzone moist, irrigation is then necessary. A perched groundwater layer can be found on top of an impermeable layer rather close to the surface (2. It covers usually a limited area. The top of the perched water layer is called the perched groundwater table. A perched groundwater table. Soil with an impermeable layer not far below the rootzone should be irrigated with precaution, because in the case of over irrigation (too much irrigation), the perched water table may rise rapidly. So far, it has been explained that water can move downward, as well as horizontally (or laterally). In addition, water can move upward. Upward movement of water or capillary rise. The same process happens with a groundwater table and the soil above it. The groundwater can be sucked upward by the soil through very small pores that are called capillars. This process is called capillary rise. On the other hand, in coarse textured soil (sand), the upward movement of the water is quick but covers only a short distance. Climatic factors such as wind and rain can cause erosion, but also under irrigation it may occur. However, it can be continuous and the whole fertile top layer of a field may disappear within a few years.
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