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A series of short‐term greenhouse experiments and laboratory incubations were conducted to evaluate the effect of macadamia (Macadamia integrifolia Maiden & Betche) nut shell (MNS) charcoal with varying volatile matter (VM) content on soil properties and plant growth in two tropical soils. Lettuce (Lactuca sativa L.) and corn (Zea mays L.) were planted in an Andisol amended with four rates of MNS charcoal (0, 5, 10, and 20% w/w) containing relatively high VM content (225 g kg−1) with and without N fertilizer. Increasing rates of charcoal without N caused a significant decline in both lettuce and corn growth. Corn growth declined significantly with or without N at the two highest charcoal rates. In a third experiment, corn growth also declined significantly in an Ultisol amended with the MNS charcoal (5% w/w) with and without fertilizers. In a fourth experiment, charcoals with high VM (225 g kg−1) showed negative effects on plant growth while the low‐VM (63.0 g kg−1) charcoal supplemented with fertilizer showed a significant positive effect on corn growth. Results from the 2‐wk incubation experiments showed that high‐VM charcoal caused a significant decline in soil NH4+–N and a significant increase in soil respiration compared with the soil amended with low‐VM charcoal and the soil alone. We propose that phenolic compounds and other products in the high‐VM charcoal stimulated microbial growth and immobilization of plant‐available N. Our results demonstrate that VM content appears to be an important property of charcoal that has short‐term effects on soil N transformations and plant growth. Longer incubation experiments and field trials are needed to further elucidate the role of charcoal VM content on soil processes and plant growth.

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The difference in pH of a soil suspension prepared with 1N KCl and with water was used to determine net charge of colloids with constant potential type surface. The quantity (pHKCl‐pHH2O) called delta pH had a positive, zero, or negative value corresponding to the net surface charge. Negative and positive adsorption of chloride or nitrate ions were measured in soil suspensions with negative and positive delta pH, respectively. Increasing the nitrate ion concentration increased sulfate adsorption in suspensions with negative delta pH values. Negative adsorption of nitrate and chloride ions was measured when sulfate ions were added to a soil colloidal suspension which was initially net positively charged. This supports the present belief that specifically adsorbed anions render a surface more negative by displacing the zero point of charge to lower pH values. This was substantiated by a measured increase in CEC from an initial value of 26 meq/100 g in a phosphated soil. Each millimole of adsorbed phosphate increased CEC by 0.8 meq.