THE RELATION OF COLLOID DISPERSION IN SOILS TO CHEMICAL CHANGES INDUCED BY BIOLOGICAL TRANSFORMATIONS OF ORGANIC MATERIALS

Abstract

Organic materials may influence tha physical condition of soils in 2 ways: directly, by their purely physical properties, and indirectly, by causing microorganisms attacking them to change the electrolyte concentration of the soil solution. A study of this indirect effect is here reported. Where biological transformations of urea to nitrates were complete, decreases in pH and in degree of colloid dispersion (measured in a 1:10 aqueous soil suspension), also increases in specific conductance as well as concentrations of nitrates and cations (especially Mg and Ca) in the soil solution, are attributed to formation of HNOs from urea. Where the transformations in another soil were incomplete, increases in pH and in colloid dispersion presumably had their origin in the (NH4)2CO3 arising from the transformation of urea. Where the biological utilization of sucrose occurred in "nitrated" soils (nitrates buihV up by HNO3 previously added), increases in pH and in colloid dispersion, as well as decreases in specific conductance and in nitrate and cation concentrations of the liquid phase of these soils, are assignable to the withdrawal of HNO3 from the soil solution by micro-organisms and to the entrance of basic cations into the exchange complex to take the places of the H-ions withdrawn. With sucrose added, nitrate and sulphate concentrations declined, presumably to form microbial proteins and similar compounds. Early in the incubation of sucrose-treated soils, NH4-ions began to increase. After nitrates were reduced to a minimum, increases in nitrates, sulphates, basic cations and specific conductance of the liquid phase, and decreases in pH and colloid dispersion are doubtless caused by ammonification and nitrification of microbial proteins formed earlier from sucrose utilization. During incubation the "nitrated" non-sucrose-treated samples slowly increased in pH, presumably in adjustment to the HNO3 added in "nitrating." The decrease in colloid dispersion following increase in nitrates from biological transformations of nitrogenous organic materials is considered as analogous to the action of those H-ions producing amendments (S, Fe sulphate, alum, etc.) known to influence percolation and tilth.