THE PRESSURE RESPONSE OF THE SOIL WATER SAMPLER AND POSSIBILITIES FOR SIMULTANEOUS SOIL SOLUTION SAMPLING AND TENSIOMETRY

A soil water sample may be viewed as a tensiometer with a relatively slow response time. An expression for the pressure response of a fixed volume soil water sampler is derived. The criterion for determining when the gage pressure within the soil solution sampler may be used to infer the soil water matric potential in the vicinity of the sampler tip depends upon the cupsoil conductance (K*), the sampler's internal volume (VΣ), the initial absolute pressure (P°) associated with the vacuum set on the sampler, and the elapsed time since application of the vacuum (t). It was found that when the dimensionless time given by [P°K*t](VΣ)−1 is greater than about 5, the sampler pressure may be used in determinations of matric and/or pressure potentials (ψ), or more generally the tensiometer pressure potential which also includes the influence of the soil air pressure potential. The essential difference in the response of tensiometers and soil water samplers arises from differences in the nature of the instrument sensitivities (S*). In the “tensiometer-limited” response, the standard tensiometer approaches an equilibrium reading via an exponential decay of the initial pressure disturbance characterized by a response time τ =(K*S*)−1, where K* and S* are, in principle, constant for a particular equilibration. A nonexponential decay response of the soil water sampler on the other hand arises from time-dependent S* and K*. Only the time-dependent S* effect is considered here. The S* in the case of soil water sampler is equal to P(Vgas)−1), where P and Vgas are the instantaneous sampler gas pressure and gas phase volume, respectively. Under conditions where the ratio P(Vgas)−1) undergoes considerable change during the course of soil water collection, large deviations from the exponential decay response occur. Conversely, when only minor variations in P(Vair)−1 are experienced, the soil water sampler pressure response approaches that of the exponential decay. The time-weighting of the collected soil solution is discussed. One particular instrument design suited for simultaneous tensiometry and soil water sampling is described. Field data illustrating cases where soil and water sampler pressures may and may not be used to obtain ψ data are provided. The possible use of this type of device for both soil solution sampling and tensiometry in deep (>10 m) profiles is explained.