Wilt-capacity

Depth (cm) Soil matrix potential, 0.03 MPa (field capacity) Soil matrix potential, 1.5 MPa (permanent wilting point) ... 

Soil strength Water-holding capacity Field capacity/wilting point Hydraulic conductivity Fertility... 

SOL AWC (mm/mm) Available water capacity of the soil layer. The available water in the soil is calculated by subtracting the water content at the permanent wilting point from that at field capacity SOL AWC = FC - WP. 

If it is necessary or desirable, water can be extracted from unsaturated soils in the laboratory. This requires either pressure or suction to move water from the soil. A common laboratory method for removing water from unsaturated soils is the pressure plate (see Figure 7.13). Plates for this type of extractor can be used for extraction of water at field capacity 33 kPa and at permanent wilting point - --1500 kPa.3... 

Different sources will give different values for field capacity and permanent wilting points however, these are the values most commonly used. 

The water fraction of each element of soil can not exceed a certain maximum value, known as the field capacity. If the water content is less than the field capacity, then no water is able to leave the element. If the water content is equal to or greater than the field capacity, then water is able to leave the element at the same rate as water enters. The water is also taken up by the roots of plants. Here it is assumed that the uptake rate of water is proportional to the length of root. If the water content falls below a critical value, the plants can no longer take up water, and they wilt and eventually die. 

Available Soil Moisture (%) = Moisture at Field Capacity (%) - Moisture at Wilting Point(%) If bulk density is known, per cent values can be converted into cm of water a follows ... 

Veihmeyer, F.J. and Hendrickson, A.H. (1949), Methods of Measuring Field Capacity and Permanent Wilting Percentage of Soils, Soil Sci. 68 75-95. 

Most of the absorbers for CO involve reduced copper salts, and cuprous chloride generally is used This can be in acid, which is the case here, or in base. The only difference is in the order that you take out the gases. Cuprous sulfate can be used as an absorber for CO, but it absorbs slowly unless beta naphthol is added to it to act as a catalyst. The capacity of acid cuprous chloride for CO is generally pretty high. One mL wilt absorb 18 mL of CO gas. 

The saturated paste method provides an estimate of salt in soil at normal field moisture equivalents. Saturation percentage is about four times the 15-atmosphere moisture equivalent or permanent wilting point (irreversible plant desiccation), and approximately two times the 0.33 atmosphere moisture equivalent or field capacity (moisture held in soil against drainage by gravity). Soil with a saturated paste EC of... 

Feustel and Byers determined the moisture-holding capacities and wilting percentages of three peats and two soils, and of mixtures of these. A portion of these data is shown in Table 30.1. From these data the available moisture has been calculated and expressed as percentage of the moisture-holding capacities of the various materials and mixtures. 

MOISTURE-HOLDING CAPACITIES AND WILTING PERCENTAGES OF PEAT AND SOIL, AND OF MIXTURES OF THESE (from FEUSTEL and BYERS, 1936)... 

The moisture characteristic (moisture content versus soil suction) of a soil provides valuable data concerning the moisture contents corresponding to the field capacity (defined in terms of soil suction, this Is a pF value of about 2.0) and the permanent wilting point (pF of 4.2 and above), as well as the rate at which changes In soil suction take place with variations in moisture content. This enables an assessment to be made of the range of soil suction and moisture content that is likely to occur In the zone affected by seasonal changes In climate. 

Plants need water and carbon dioxide along with sunlight for photosynthesis. Shortage of water in the soil and low insolation slow down photosynthesis. Crop yield depends also on the availability of minerals (fertilizer). The utiUzable water available to the plant is the difference between the retention capacity of the soil and the limit of extraction, and this capacity is dependent on the type of soil. In arid zones, the water which is available to the plant is only a fraction of the water received by the soil because the latter ends up in different places, for instance, as runoff water, seepage water lost or diverted, or water which is a constituent part of the soil and is not extractable by the roots. If the water extracted by the roots is insufficient, the plant will wilt and may eventually reach the permanent wilting point. Each plant requires a certain depth of soil for occupation by its roots and the water... 

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