THE EFFECT OF pH ON THE WET STRENGTH OF SOIL CRUMRS

Summary The clay‐clay attractive forces were greater in crumbs from acid soils than crumbs from similar soils but Containing free CaCO 3 . This has two causes: ( a ) positive charges on the edge faces of the clay crystals; ( b ) exchangeable trivalent cations on the clay. Using the improved sodium‐saturation method these two causes were distinguished by measuring the reduction in the strength of the crumbs by ( a ) raising the pH of the percolating NaCl to 7.4 with para‐nitrophenol buffer, and ( b ) preliminary NaCl‐HCl leaching. The effect of trivalent cations was dominant in very acid soils where Al +++ can exist in the soil solution. The soil conditioner‘Flotal’strengthens soil crumbs, initially at least, by substituting Fe +++ ions on the exchange complex. No evidence was found for cementing by precipitated Al(OH) 3 , or Fe(OH) 3 . The same concentration of CaCl 2 6 × 10 −3 M was required to flocculate suspensions of moderately acid surface soils both at the natural pH of the soil and after buffering the suspensions to pH 7×4. However a Callite and an acid subsoil (no organic matter) flocculated at a CaCl 2 , concentration of 3 × 10 −4 M at pH 5. Thus there appears to be sufficient organic matter present in normal cultivated acid soils to neutralize the positive charges on the edges of any clay particles dispersed from the crumbs. It is suggested that, although liming an acid soil decreases the attractive forces between clay particles within soil crumbs, the presence of free CaCO 3 , in the soil helps to maintain a sufficiently high concentration of electrolyte in the soil solution to reflocculate dispersed clay and so keep drainage waters clear.