Soil Moisture Constants

Rhizodeposition is measured as 0 recovered from soil adhering to the roots or soil not adhering to the roots. Soil moisture content was constant in all treatments. 

The sampling of a suction lysimeter is initiated by applying a vacuum (approximately 40-50 cm of mercury) through the vacuum/pressure line with a hand pump or electric pump. The valve on the sampling line must be closed. A constant vacuum may be maintained on the lysimeter using an electric pump. The time required before collecting a sample from a lysimeter will depend on the method of vacuum application, the moisture content of the soil, and the soil type. 

In an unsaturated zone, the capillary force becomes predominant, and the pressure gradient becomes a suction gradient. Hydraulic conductivity is no longer constant, but is a function of the water content or suction, which is greatest in value when the soil is saturated and decreases in value steeply when the soil water suction increases and the soil loses moisture. 

Table I shows the results of calculating a soil diffusion coefficient and soil diffusion half-lives for the pesticides. The 10% moisture level specified means that the soil is relatively dry and that 40% of the soil volume is air available for diffusion. Complete calculations were not made for methoxychlor, lindane, and malathion because, based on Goring s criteria for the Henry s law constant, they are not volatile enough to diffuse significantly in the gas phase. This lack of volatility is reflected in their low values of X. These materials would move upward in the soil only if carried "by water that was moving upward to replace the water lost through evapotranspiration at the surface. Mirex has a very high Henry s law constant. On the basis of Goring s criteria, Mirex should diffuse in the soil air but, because of its strong adsorption, it has a very large a and consequently a very small soil air diffusion coefficient. The behavior of Mirex shows that Goring s criteria must be applied carefully.

The effects of soil depth and moisture on pesticide photolysis were studied. Moist soil at depth of 3, 2.5, 2, 1.5, 1, and 0.5 mm were each dosed at 2.5 pg/g with 14C-niclosamide and photolyzed under a xenon lamp at constant temperature. Samples were removed after 20, 40, 110, and 153 h of continuous irradiation. The decrease in percent of niclosamide and the appearance of degradates were followed by analyzing the soil extracts by HPLC [76],... 

The 210Pb input from the atmosphere must have been constant over the past 150 years due to the relative constancy in the maritime climate (temperature and soil moisture influences the radon emanation rate) and the resulting constancy in the input source for 210Pb. Therefore, the deviations from a single log-linear relationship of the unsupported 210Pb activity with the dry mass of sediment accumulation must be due to some property of the watershed. The three different relationships shown in... 

The Rothamsted Carbon Model (RothC) uses a five pool structure, decomposable plant material (DPM), resistant plant materials (RPM), microbial biomass, humified organic matter, and inert organic matter to assess carbon turnover (Coleman and Jenkinson 1996 Guo et al. 2007). The first four pools decompose by first-order kinetics. The decay rate constants are modified by temperature, soil moisture, and indirectly by clay content. RothC does not include a plant growth sub-module, and therefore NHC inputs must be known, estimated, or calculated by inverse modeling. Skjemstad et al. (2004) tested an approach for populating the different pools based on measured values. 

There are physical changes constantly taking place in soil. Soil temperature changes dramatically from day to night, week to week, and season to season. Even in climates where the air temperature is relatively constant, soil temperatures can vary by 20° or more from day to night. Moisture levels can change... 

To 50g of soil (as sampled, recoveries of Oxamyl are constant for moisture contents of up to at least 30%) contained in a 450g wide-necked jar fitted with a ground-glass stopper, add 50g of anhydrous sodium sulphate and 200ml of acetone-dichloromethane solvent. Shake orbitally for 4h, ensuring that all the solids are kept in suspension. Allow to settle, and remove a 100ml aliquot of the supernatant liquid by pipette. Evaporate this extract just to dryness in a 250ml round-bottomed flask fitted to a rotary evaporator. 

Incomplete tables of the physical constants and efficacies of a large number of compounds have been prepared (Table I). However, investigations show that much more information is needed to give a complete understanding of soil fumigation. We need tables showing vapor pressure, aqueous solubility, solvency in or solubility of nemic waxes, and efficacy, for temperatures of 20 , 25 , 30 , 35 , and 40 C. Furthermore, efficacy should be expressed in relation to moisture content of soil and stage of nematode. 

On sandy loam soil first-order rate constants for photolytic decline, k = 4.07 x 10 3 lr1 irradiated in moisture-maintained soil, k = 1.07 x 10-3 h 1 irradiated in air-dried soil, k = 1.52 x 10 3 h 1 in dark control moist soil and no degradation in dark control air-dried sandy loam soil from Madia, CA. t,/2 = 120 h for the first 96 h irradiation over all t,/2(calc) = 200 h from 96-168 h but in dark control system t,/2 = 460 h in moist sandy loam soil irradiated metabolism t,/2 = 650 h in dry sandy loam soils. Rate constants due to photolysis k = 2.55 x 10-3 h, and due to moisture k = 3.0 x 10 3 h in moist sandy loam soil (Graebing Chib 2004)... 

The composition of the soil solution changes constantly as elements dissolve, desorb or decomplex from the solid phases, as plant roots and lower plants withdraw nutrients or release organic substances, as rainwater brings in solutes from outside and as the soil water passes down the profile into the drainage water. Figure 1-4 summarises these processes. It should be noted that equilibria are assumed between the solution and the matrix components but there is no certainty that rapidly fluctuating moisture contents will allow true equilibrium to be attained. 

Dielectric constant monitoring probe for the remote measurement of soil moisture. (Courtesy of Automatika Inc.)... 

Given the bacterial populations that utilized p-coumaric acid as a sole carbon source and the physicochemical (e.g., constant temperature, adequate nutrition and moisture) and biotic conditions of these two laboratory systems, utilization of p-coumaric acid ranged from 0.6 to 5.0 pg/g soil/h for the open systems and 8.6 pg/g soil/h for the closed system. The pg values for the open system represent steady-state rates as modified by nutrition, while the pg values for the closed system represent maximum rates. Whether such rates ever occur in field soils is not known, since the physicochemical and biotic environments of field soils are so different from those of laboratory systems. Laboratory soil systems provide potential rates of utilizations, but until field rates are determined the importance of microbial activity in phenolic acid depletion from soil solutions will not be known. 

Moisture-Equivalent—Briggs and McLane (1907) made extensive physical studies on what has been termed moisture-equivalent — that is, the percentage of water retained by a soil, when the moisture content is reduced by means of a constant centrifugal force until it is brought into a state of capillary equilibrium. While King s procedure yields specific information on the relative retentiveness of various soils only after considerable time, the method of Briggs and McLane is rapid,... 

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