Soil water content

Water-retention curve Graph showing soil-water content as a function of increasingly negative soil water potential. 

Another group of effects consists in blocking the channels of losing water from the soil layer, i.e., the hydraulic conductivity responsible for the gravitational flow, and of physical evaporation. All these effects provide an increase of the water content of the soil and, consequently, improve the water supply of plants, which is reflected in the three last columns in Table 8. According to the data of various authors, an increase in the soil water content (AW) in sandy soils lies in the range of 10-35% at doses up to 0.2% in a number of cases [10, 11, 58, 131-133] the dependencies of AW on the doses of the hydrogels added have been studied. 

In first approximation, this dependence can be presented as a linear one until a free volume of the soil pores is attained, i.e., up to 35-45%. Consequently, an instant increase in the soil water content can be written as ... 

In many cases, where one is concerned with the effects of specific environmental factors it is appropriate to replace the general term stress by the appropriate quantitative measure (e.g. soil water content or water potential) together with an appropriate measure of the plant response (e.g. growth rate). 

The diversity of responses to water stress may therefore reflect in part the diversity of signals, from turgor and water content of leaf cells, to root perception of soil water content and strength. 

Turner, N.C., Schulze, B.-D. Gollan, T. (1985). The response of stomata and leaf gas exchange to vapour pressure deficits and soil water content. II. In the mesophytic herbaceous species Helianthus annuus. Oecologia, 65, 348-55. 

Fig. 11. Bulk soil water content and root ABA content in consecutive 10 cm soil layers on days 3, 9,12 and 18 after withholding water (A) from maize plants growing in 1 m deep soil columns. Well-watered plants (A) received water daily throughout the experimental period. Points are means of five measurements. Modified from Zhang Davies (1989).

Fig. 12. Relationships between root ABA content and bulk soil water content for maize plants growing in drying soil columns. Data are from Fig. 11, but do not include soil water contents less than 0.1 g cm in which many roots were non-living. Modified from Zhang Davies (1989).

Taylor, H.M. Ratliff, L.F. (1969). Root elongation rates of cotton and peanuts as a function of soil strength and soil water content. Soil Science, 108, 113-19. 

E. K. S. Nambiar, The uptake of zinc-65 by roots in relation to soil water content and root growth. Aust J. Soil Re.s. 14 61 (1976). 

Numerous factors control AET and thus control the hydrologic performance of an ET cover. Soil-water content, rate of root growth, and total root mass strongly affect the rate of AET. AET is also affected by whether wet soil is available in surface soil layers, deeper in the profile, or in... 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

The solubility of components in the soil solution will be controlled by the innate solubility of the compound in question and the existing soil solution characteristics, particularly salts already present. High salt concentrations will result in salting out and precipitation of some components. Note here that salt concentration is not constant in that as the soil dries, the concentration of salt increases. Precipitation reactions may not be reversible when the soil water content is subsequently increased. 

Soil water content may be reported in a number of different ways but is most commonly reported as the amount of water in grams per gram or kilogram of oven dry soil (see equations in Figure 5.13). It may also be reported... 

TABLE 5.1. Common Instruments for Measuring Soil Water Content in the Field... 

In the laboratory, soil water content is measured by drying in the oven and with a pressure plate apparatus. Drying soil can change the form and species of components present, and for this reason, most soils are air dried carefully or at temperatures only slightly above room temperature before analysis. A number of different field measuring methods are used mostly to determine the amount of water available for plant use. 

Vaughan PJ, Suarez DL. Constant capacitance model computation of boron speciation for varying soil water content. Vadose Zone J. 2003 2 253-258. 

Although all characteristics of electricity have been used to investigate soil and its properties, only a limited number are used routinely. The most common are those used for the determination of pH, salt content, and soil water content. Of these three, pH is the most common measurement and frequently the first measurement made prior to all other determinations. Although pH can be determined by many methods, for soil, the most common is to use a pH meter and electrode. Conductivity or resistance is used to measure soil salt content, while several different electrical characteristics of soil are used to determine... 

There are many additional methods and variations on the methods discussed earlier (see Section 5.13). Most are designed for determination of soil water content in the field and are not generally used in laboratory analysis of soil components such as available plant nutrients or contaminants [18-20],... 

Soil water content can be measured in the field using a number of different types of electrodes and arrangements of electrodes. Describe two of these methods in some detail. 

FIGURE 10.4 Relationship between soil water content and relative air permeability. 

Guenzi. W.D. and Beard. W.E. Picloram degradation in soils as influenced by soil water content and temperature. J. Environ. Qual, 5 185-192. 1976a. 

In partially saturated media, the diffusion coefficient also is a function of the volumetric water content, 9. Calvet (1984) showed that the variation in soil water content influences the apparent diffusion coefficient for organic contaminants in two ways by changing the ratio of gas diffusion of volatilizable pollutants to liquid diffusion, because the air-filled porosity is affected, and by modifying pollutant... 

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