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Method and Investigation of the Soil Moisture Migration under the Effect of the Temperature Gradient

Stefka Alexieva, Ivanka Stoimenova

Institute of Soil Sciences and Agroecology "N.Poushkarov", Sofia, Bulgaria
Abstract

The non-isothermal rate in root-inhabitable soil layer influences water transfer and soluble compounds in a complicate and versatile way. Many authors have investigated the problem with non-isothermal transfer of salts and water in soil, but because of the complex processes, the purely analytical way for solving it is very difficult. As the water and salts migration under a temperature gradient in the soil profile is an important factor, connected to the agro-technics type, our research consists on one hand in determining parameters, characterizing the diffusion of soil moisture in non-isothermal conditions. On the other hand, the research is focused on finding an analytical connection between these parameters and the change in soluble salts concentration for the examined profile.

Introduction

The introduced method includes a definition of parameters, which directly or indirectly characterize water migration and absorbed forms of the basic feeding elements for the plants (Vishnoi, 1980, Joshua, 1973). During the research, some of these parameters are defined in field conditions, and others - analytically. In the analysis, it is accepted approximately, that there is a linear distribution of temperature in the examined soil layer. A basic quantity in the research is the temperature gradient in the root - inhabitable soil layer grad T = dT/dL [oC m-1], where L is the profile's depth. An information indicator for the moisture gradient grad W = dW/dL [% m-1] is the active electrical resistance R [kW], measured at discreet profile depths by conductometric transducers.

It is known that the moisture increase in the profile and the concentration of dissolved mineral salts C (NH4 + NO3), leads to a change in R. On the other hand, R depends on the environment temperature and this functional dependence of R = f (W, T, C) could be presented as a total differential as follows:
The concentration C of the dissolved salts depends also on W and T and this dependency could be similarly expressed by a differential:
When there is an insufficiency of salts, the soil concentration C in determined limits depends on W or (unsaturated solution); the contrary is under surplus of salts, which is practically rare case (Kanwar, 1980, Thinker, 1981). Replacing (2) in (1) results in equation (3), which gives the full dependency between examined quantities.
A thermodynamic balance in a profile layer (dR / dL = 0). Means a lack of diffusion, which is obtained under the following correlation between grad W and grad T .
The temperature dependence of electric resistance R, measured with conductometric transducer [kW oC-1] , is experimentally defined. It depends on soil type and on measuring conditions. Same applies to the sensitivity of conductometric transducer [kW %-1] and to dissolved mineral salt [kW kg soil mg -1]. The measuring conditions with conductometric transducer depend on the form, constructive dimensions of the electrodes and the measuring range. After replacing the parameters characterizing the conductometric transducer's sensitivity to the measuring conditions in (4), we receive
Expression (5) gives a possibility to define the change in dissolved salts concentration in dependence of grad T and grad W. Reading the positive value of the private derivative in (5) - , it is clear, that the non-isothermal transfer of salts and water in the soil could be stopped when the temperature and moisture gradients have different values.

Results and Discussions

In the process of experimental research, the following values for temperature-moisture concentration sensibility of the system "conductometric transducer - ohmmeter" are received:
Sw = 2 [kW %-1]; ST = 0,2 [kW oC-1]; SC = 0,7 [kW kg soil mg-1]. Table 2 shows: the calculated values for grad W from the alteration DR
in the separate layers, the values of grad T, received for the respective periods and the calculated according (5) values of the quantity .

The results in Table 2, showing a difference in content of easily absorbed forms of N - almost reach those received in the laboratory. An obligatory condition for an exact definition is the graduation of the conductometric transducers for the measuring range. Another important condition is the soil type used for the research. With soil with heavy mechanical content and a strong dry-up in result of cracks, it is possible to destroy the contact between the transducers' electrodes and the soil. The experiment was conducted upon smolnitza (clay content - 72-77%), with very heavy mechanical content and specific gravity - 2.68-2.7%. However, there were no such disturbances of the cultivated layer during the observed period. The results from table 1, for both variants, show that applying herbicides, the maize root system uses easily absorbed N forms from the lower layers. By weeding, with a competition between weeds and crops, the root system is weak and uses feeding elements from higher layers (here the maize biomass in a "tasseling" period is 7 times smaller compared to the herbicide variant).

References

. Vishnoi R. K. & Tripathi R. P., 1980. A comparison of two theoretical models for calculating water flux under non-isothermal conditions. J. Ins. Soil Sci., v. 28, N 3, pp. 267 - 276.
. Joshua W. D. & Jong de E., 1973. Soil moisture movement under temperature gradients. Canad. J. Soil Sci., v. 53, N 1.
. Kanwar R. S., Baker J. L. & Johnson H. H., 1980. Nitrate movement with Zero - order Denitrafication in a Soil profile. Soil Science Soc. Am. I. v. 44, pp. 898 - 902.
. Thinker P. B., 1981. Rothamsted Exp. Stat. Rept., N 1, pp. 243 - 274.

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