Studies of the dispersion of compound soil particles by sonic vibration and cation-exchange resin techniques indicate that the difficultly dispersible particles in mineral soils of high base status are microaggregates (< 250p d i m ) consisting largely of clay and humifid organic material linked by polyvalent metals. The inter-particle bonds in these microaggregates can be disrupted by application of mechanical energy (sonic vibration or prolonged shaking with water), the amount of energy required for dispersion of clay-size mineral material being reduced by treatments that weaken or destroy these bonds (e.g. treatments leading to replacement of polyvalent metals by monovalent metals or to destruction of organic matter). A theory is proposed depicting microaggregate formation as a solid-phase reaction involving linkage of electrically neutral clay mineral and organic matter particles by polyvalent metals on exchange sites, and microaggregate disruption by sonic vibration as a reversal of this reaction. Experiments to evaluate this theory are described. Introdwtion THE importance of the interaction between the inorganic and organicconstituents of soils and the need for more extensive and realistic investigations of this interaction and of the nature and properties of the clay-organic matter complexes in soils have been emphasized by Greenland (1965 a, b) in a recent admirable review. Apart from the comlexity of the soil system, the main difficulty in such investigations has teen the lack of methods of separating the organic and mineral soil colloids and of isolating the clay-organic matter complexes in soils without seriously affecting the properties of these materials. We have recently developed two techniques which permit complete dispersion of soil particles without drastic modification of organic or inorganic material, and the pu ose of this article is to review our findings using these dispersion is achieved by sonic or ultrasonic vibration of soil suspended in water (Edwards and Bremner, 196 196 ). In the other (Edwards and Bremner, 1965), dis ersion is e8ecteJ by shaking soil with an aqueous suspension of a &-saturated cation-exchange resin (carboxylic, sulphonic, or iminodiaceticacid e). The findings relevant to this article surface soils can be summarized as follows:I. The vibration technique permits complete dispersion of soil particles without use of oxidants, acids, or peptizing reagents. It is effective with hi hly calcareous soils, Latosols, and soils containing sta % le suspensions. It does not cause dissolution of significant amounts of

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There is a great interest in wide band-gap semiconductor devices for power electronics application. In this letter, vertical GaN p-n diodes fabricated on bulk GaN substrates are discussed. The device layers are grown by MOCVD on low defect density (10 4 cm −2 ) bulk GaN substrates. The measured devices show breakdown voltages of 3.7 kV with an area differential specific on-resistance (R sp ) of 2.95 m -cm 2 .