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The only way to increase the low CEC of sandy tropical soils over the long term is to apply high CEC materials such as 2:1 clay minerals. Acid activated bentonite is used in Thailand in the vegetable oil industry during the clarification process. The waste bentonite is discarded afterwards. The aim of the study was to compare the effects of the addition of these oil bentonites (OB) with the addition of cation beneficiated bentonite (BB) on soil properties and plant growth. Palm, rice and soybean OB, and bentonite beneficiated with calcium, magnesium, and potassium were applied at rates between 5 and 40 t ha −1 to an Arenic Acrisol. Three consecutive crops of sorghum were grown in pots. Biomass and plant nutrient content were determined at each growth phase, and selective soil properties were measured at the start and the end of the study. Beneficiated bentonite was not water repellent, but the addition of OB resulted in soil water repellency. The application of bentonite at the rate of 40 t ha −1 increased the cation exchange capacity (CEC) from 0.6 cmolc kg-1 in the control to 1.9 and 0.7 cmol c kg −1 in the BB and OB, respectiveley. The lower value of the CEC for OB compared to BB was probably due to the activation process and oil coating. OB applications at rates higher than 20 t ha −1 did not increase biomass, and biomass decreased with increasing water repellency. The other treatments produced a higher biomass than the control. However biomass was below potential because of widespread nitrogen deficiency. Exchangeable K was exhausted in two crops, whatever the initial level, stressing the issue of K management in this soil type. Soybean OB is a promising material for soil chemical properties and biomass production, probably because of its low oil content.

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Plants have evolved many physiological, morphological, The potential for foliar dehydration tolerance and maxanatomical, and phenological characteristics for imum capacity for osmotic adjustment were compared responding to and resisting drought stress. Certain characamong 12 temperate, deciduous tree species, under teristics and responses to drought do not occur at random standardized soil and atmospheric conditions. but appear to coincide, leading to the sometimes loose Dehydration tolerance was operationally defined as but useful categories of perennial plants as drought lethal leaf water potential (y): the y of the last avoiders or drought tolerators (Ludlow et al., 1983). remaining leaves surviving a continuous, lethal soil Drought avoiders have tissues that are very sensitive to drying episode. Nyssa sylvatica Marsh., and dehydration. They tend to have characteristics that allow Liriodendron tulipifera L. were most sensitive to them to avoid tissue water deficits when soil moisture dehydration, having lethal leaf y of −2.04 and limitation occurs: e.g. deep roots to maximize water −2.38 MPa, respectively. Chionanthus virginicus L., uptake and sensitive stomates to minimize leaf water loss. Quercus prinus L., Acer saccharum Marsh., and Drought tolerators have leaves that can tolerate dehydra-Quercus acutissima Carruthers withstood the most tion, and they tend to have poorly developed responses dehydration, with leaves not dying until leaf y dropped for avoiding dehydration. Tolerators rely on osmotic to −5.63 MPa or below. Lethal leaf y (in MPa) of other, adjustment to survive drought (Ludlow et al., 1985). intermediate species were: Quercus rubra L. (−3.34), Ludlow (1989) has profiled the mechanistically linked Oxydendrum arboreum (L.) D.C. (−3.98), Halesia characteristics that classify these two drought resistance carolina L. (−4.11), Acer rubrum L. (−4.43), Quercus strategies. alba L. (−4.60), and Cornus florida L. (−4.88). In the spectrum of responses ranging from extreme Decreasing lethal leaf y was significantly correlated drought avoidance to extreme tolerance, perhaps the with increasing capacity for osmotic adjustment. C. single most important determinant of drought resistance virginicus and Q. acutissima showed the most osmotic strategy is the dehydration tolerance of the species, which adjustment during the lethal soil drying episode, with is a measure of tissue capacity for withstanding desiccaosmotic potential at full turgor declining by 1.73 and tion (Ludlow, 1989). Dehydration tolerance has been 1.44 MPa, respectively. Other species having reducoperationally defined as the water potential (y) or relative tions in osmotic potential at full turgor exceeding water content (RWC) of the last surviving leaves (called 0.50 MPa were (in MPa) Q. prinus (0.89), A. saccharum the lethal value) on a plant subjected to a slow, continuous (0.71), Q. alba (0.68), H. carolina (0.67), Q. rubra (0.60), soil drying episode (Ludlow, 1989). Drought avoidance, and C. florida (0.52). a strategy possessed by both annual...

We compared the potential for foliar dehydration tolerance and maximum capacity for osmotic adjustment in twelve temperate, deciduous tree species, under standardized soil and atmospheric conditions. Dehydration tolerance was operationally defined as lethal leaf water potential (Ψ): the Ψ of the last remaining leaves surviving a continuous, lethal soil drying episode. Nyssa sylvatica and Liriodendron tulipifera were most sensitive to dehydration, having lethal leaf Ψ of –2.04 and –2.38 MPa, respectively. Chionanthus virginiana, Quercus prinus, Acer saccharum, and Quercus acutissima withstood the most dehydration, with leaves not dying until leaf psi dropped to –5.63 MPa or below. Lethal leaf Ψ (in MPa) of other, intermediate species were: Quercus rubra (–3.34), Oxydendrum arboreum (–3.98), Halesia carolina (–4.11), Acer rubrum (–4.43), Quercus alba (–4.60), and Cornus florida (–4.88). Decreasing lethal leaf Ψ was significantly correlated with increasing capacity for osmotic adjustment. Chionanthus virginiana and Q. acutissima showed the most osmotic adjustment during the lethal soil drying episode, with osmotic potential at full turgor declining by 1.73 and 1.44 MPa, respectively. Other species having declines in osmotic potential at full turgor exceeding 0.50 MPa were Q. prinus (0.89), A. saccharum (0.71), Q. alba (0.68), H. carolina (0.67), Q. rubra (0.60), and C. florida (0.52). Lethal leaf Ψ was loosely correlated with lethal soil water contents and not correlated with lethal leaf relative water content.