The extents to which salts in the soil solution or sodium on the exchangeable fraction of the soil are in excess are measures of the salt problem. Salt problems commonly occur in regions with arid and semiarid climates and lower the productivity of extensive areas of agricultural land throughout the world. Salt-affected soils have excessive concentrations of soluble salts or adsorbed sodium, or both. Accurate diagnosis for purposes of soil reclamation or management generally requires detailed information on soils in addition to the salinity and exchangeable sodium status. Size and distribution of plants often indicate the concentration and distribution of salt in the soil. In general, soil management practices that improve permeability or improve irrigation also aid in the control of salinity. When plowing was followed by ordinary irrigation and cropping practices, the excessive exchangeable sodium was reduced to a safe level in the plant root zone within from 2 to 3 years.
Three field methods—piezometer, auger hole, and tube—and the conventional undisturbed‐core method for measuring soil permeability are compared theoretically and experimentally. Natural channels in the soil, such as cracks, roots, and worm holes, cause extreme variations in results by the undisturbed core method and cause the rate of rise, in small‐diameter piezometers, to deviate from the theoretical. This deviation becomes negligible as the pipe diameter is increased. The size of sample associated with each method differs from one method to the other. In non‐uniform soils this causes differences in measured permeability values. In anisotropic soils the piezometer and auger methods, using the usual inflow cavities which are long compared to their diameter, measure essentially the horizontal permeability; whereas the tube method, where length of cavity equals zero, measures essentially the vertical. The undisturbed‐core method may be used to measure either the horizontal or vertical permeability depending upon the direction in which the cores are taken. The piezometer method appears to be convenient and valuable for general use because of its suitability for measurement of permeability at any depth in both uniform and non‐uniform soils. The auger‐hole method is well adapted for measuring permeability in unstratified soils and appears particularly advantageous in soils having many root holes and other natural channels. The tube method, a special case of the piezometer method, with large diameter and a cavity length of zero, is especially useful in measuring essentially the vertical permeability of anisotropic soils. All the methods as described here apply to water‐saturated soil below a water table. The core method in its application to unsaturated soil, where it is perhaps most useful, is not a part of this study.
Commonwealth Scientific and Industrial Research Organization, Australia1 1 The work described in this paper was carried out as part of the research program of the Division of Soils, of this Organization, at the Plant and Soils Laboratory, George Street, Brisbane, Queensland. Acknowledgment is made to C. S. Piper, chemist in charge, soil chemistry section, for advice and encouragement and for supplying information concerning the original Passon method.
Laboratory data show that sodic, fine-textured soils are impossible to reclaim by applying gypsum to the surface, and that the high-salt-water dilution method is effective within limits. A combination of the two methods appears both effective and practical. Basic principles and calculation procedures involved in the engineering evaluation of a reclamation program are presented.
Summary A study of the effects of pH and of carbon/aluminium ratio on the flocculation of podzol humus by aluminium is reported. Portions of resin‐treated humus extracts were adjusted to selected pH values (which varied from 1·8 to 7·0) and mixed with equal volumes of AlCl 3 solutions at the same initial pH. The C/Al ratios employed were varied from 1·5 to over 20. At C/Al ratios of 16, using B‐horizon humus from an Australian podzol, complete precipitation of C and Al occurred only at pH 4‐4·5, i.e. at about the precipitation pH of hydrous aluminium oxide. As the pH was decreased below 4, increasing amounts of C (up to 25 per cent. of that added) and Al (up to 95 per cent.) remained in the supernatant liquid. As the pH was increased above 4·5, all, or nearly all, the added C and Al remained unflocculated. At C/Al ratios of 20 or more, no precipitation of either constituent was found at any pH employed. At a C/Al ratio of 16, only partial precipitation of humus and Al occurred which was maximal at pH 2·5. Similar results were obtained for humus from a New Zealand podzol although it was less sensitive to flocculation. After mixing the two phases, changes in pH occurred which were independent of C/Al ratio, degree of flocculation, or humus type, but which depended on the mean pH of the C and Al systems before mixing. Negative pH changes were recorded below pH 4·5 while above this value positive differences were found. A detailed analysis of these changes suggested the participation of polymeric hydroxy‐aluminium complexes in humus flocculation at pH 3·5‐4·5, whereas below pH 3·5 flocculation is probably brought about by free Al ions. Evidence is presented for the exchange of humus ‘anions’ for hydroxyl on the surface of freshly formed Al hydrous oxide at pH values over 4·5. A brief study of the sorption of podzol humus by Al‐bentonite and by Na‐H‐bentonite is described. Sorption by Al‐clay was maximal and complete from pH 2·0 to pH 3·0, but above this increasing amounts of humus remained unsorbed. Interpretation of the pH changes in these systems was complicated by lack of equilibrium conditions. Pure H‐clays (prepared by resin treatment of clay suspensions) showed decreasing sorption from 50 per cent. at pH 2 to practically zero at pH 4·5 or greater. It is suggested that sorption of organic matter by clays in acid soils is facilitated by the presence of exchangeable or surface‐sorbed Al. Humus suspensions showed considerable deflocculating properties towards H‐ and Al‐clays. This factor may be important in clay migration in soils.
Measurements of discharge from the drains in the Coachella Valley of California have been plotted, and a simple equation has been developed, to give maximum discharge in terms of drainage area or length of tile. It has not been possible as yet to relate the coefficients or exponents involved to differences in soil characteristics or irrigation practices, but as more information is developed this may be possible. Good data on which to base the design capacity of tile drains in irrigated areas have been lacking. The ‘drainage coefficient’ used in humid areas is not applicable, and the ponded condition, which would permit rational calculation of discharge ( Kirkham , 1949) is not reached. The information here developed is presented as a first step towards solution of the problem.
Sediment inflow into subsurface drains in noncohesive soils causes serious maintenance problems. An impervious hemispherical cover over a drain having inflow openings only at the top was previously proposed to solve this problem. Water flows between the cover and the outer drain surface in an upward direction before entering the openings at the top of the drain. Sediment inflow is prevented if the flow velocity is less than the critical boiling velocity. In this paper the shape of the upflow channel openings and the velocity distribution across the openings are evaluated. Terzaghi's concept of effective stress is the basis for the theoretical velocity distribution. An electric analog model is employed to verify this theory. A special laboratory model is used to determine critical velocities. Critical boiling velocities for fine sand and glass beads are about 1% of Stokes' settling velocity. Within reasonable limits the upflow channel width and shape can be designed to prevent movement of sediment. High water tables have the greatest influence on velocity.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)