Soil-Moisture Determination

Using any available resources, find out how pressure plate methods for determining soil moisture levels work. 

Figure 8.2. Neutron probe for determining soil moisture, where nf is fast neutrons and ns is slow neutrons.

The order of the mobilities of alachlor, butylate, and metolachlor in columns of various soils was metolachlor > alachlor > butylate. This correlates directly with the water solubilities and inversely to the adsorption coefficients and octanol/water partition coefficients of these compounds. Diffusion of these compounds in soil thin-layers was as follows butylate > alachlor > metolachlor, which correlates directly with the vapor pressures of these compounds. Significant soil properties affecting diffusion appeared to be bulk density and temperature. Soil moisture is also probably important, but its effect on the diffusion of these compounds was not determined. 

At this point, the samples are ready for analysis. However, before analysis, it is important to determine the moisture content of the air-dried soil. This is reported as the percent water on a dry-weight basis. Air dry soil contains water, usually 1-2%, although as organic matter increases, so does the air dry water content. In extreme cases, limited to organic or high organic matter soils, the percent water may approach 100%. 

Method 7.1. Determination of Moisture in Air Dry Soil on a Mass Basis... 

The adsorption of contaminants on geosorbents also is affected by climatic conditions reflected in the subsurface temperature and moisture status. Calvet (1984) showed how the soil moisture content may affect adsorption of contaminants originating from agricultural practices. The moisture content determines the accessibility of the adsorption sites, and water affects the surface properties of the adsorbent. The competition for adsorption sites between water and, say, insecticides may explain this behavior. Preferential adsorption of the more polar water molecules by soil hinders... 

Water is among the most important compounds on earth. It is the main constituent of the hydrosphere, which along with the mantle, crust, and the atmosphere are the four components of our planet. It is present everywhere on earth and is essential for sustenance of life. Water also determines climate, weather pattern, and energy balance on earth. It also is one of the most abundant compounds. The mass of all water on earth is l.dxlO i kg and the total volume is about l.dxlO km, which includes 97.20% of salt water of oceans, 2.15% of fresh water in polar ice caps and glaciers, 0.009% in freshwater lakes, 0.008% in saline lakes, 0.62% as ground waters, 0.005% in soil moisture 0.0001% in stream channels and 0.001% as vapors and moisture in the atmosphere. 

A variety of studies have shown that as temperature increases, sorption may increase, decrease, or remain the same, with isosteric heats of sorption being very low (Table 21.7). These studies have used the batch slurry technique, so the impact of temperature on water-triazine interactions may mask surface-triazine interactions. In contrast, at 10% soil moisture isosteric heats of atrazine sorption ranged from -10 to -12kcal/mol determined with the SF technique (Koskinen and Rochette, 1996). Sorption coefficients in field-moist soils were much greater than are typically obtained with the batch slurry system, while heats of sorption were much more negative, indicating greater sorption at low moisture contents. 

The temperature dependent algorithms used to predict natural sulfur emissions do not account for all of the variation in observed emissions. Other important environmental parameters may include, but are not limited to, tidal flushing, availability of sulfur, soil moisture, soil pH, mineral composition, ground cover, and solar radiation. A more accurate estimation of the national sulfur inventory will require a better understanding of the factors which influence natural emissions and the means to extrapolate any additional parameters which are determined to be important. 

A supercritical extraction procedure was developed to determine naled, methyleugenol, and cuelure in soil samples [28]. Recovery of methyleugenol was reported as 91-101% after spiking the sample with standard at concentrations of 0.25-45 pg/g. The supercritical fluid was carbon dioxide (pressure of 27.6 Mpa), and the method worked for 5-30% soil moisture. 

Soil moisture can have a significant effect on decomposition (Swift et al. 1979). This is due, in part, to the fact that soil moisture can affect the metabolism of decomposer microorganisms. This effect can be modified by soil texture because bioavailable moisture is determined, in part, by the suction with which water is held between soil particles (matric potential). Thus, the calibration of soils to a known matric potential can lead to the assessment of the effect of bioavailability of moisture in soil (Hillel 1982) and allow for the comparison of process rates between soils at the same matric potential (Orchard and Cook 1983). 

Once the soil has been sieved, as much plant and animal residues must be removed as possible. This is best done with a pair of forceps, one small portion of soil at a time. The bulk soil should be covered to prevent any further drying. When the soil has been picked over, subsamples can be taken for determination of moisture content and water-holding capacity (WHC see following section) the soil should be stored at 4°C until use. It is advisable to use the soil as quickly as possible, and if storage is unavoidable, it should not normally exceed 3 months. 

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