Site data from 291 forest inventory plots in Mixed Dipterocarp Forest in centralSarawak were simplified by factor analysis. The distribution of the 33 commonest canopy tree species was related to site factor scores by 't' tests. There appears to be a degree of edaphic influence on the distribution of many species. Fairly constant soil characteristics related to the lithology of the parent material appear to be more important than ephemerally variable properties such as organic matter or exchangeable cations. Magnesium appears to be particularly important, possibly because of an effect on the capacity of mycorrhizal root systems to absorb phosphorus.

Sum ma rySixteen topsoils from Denmark and the UK were subjected to two wetting and drying treatments: (i) moist incubation (wet). (ii) eleven wetting and drying cycles (W/D).The WID treatment resulted in larger P adsorption and resin extraction ofsoil P than the wet treatment. The differences in P adsorption at the final concentration of 800 PM P were mainly above 20 per cent, whereas the differences in amount of resinextracted P were mainly less than 20 per cent. The effects were positively correlated with the cation exchange capacity of inorganic components. Furthermore, the increase in rapidly released P was positively correlated with pH. It is suggested that wetting and drying effects on P adsorption and desorption are associated with changes in soil structure caused by rewetting ofdry samples. IntroductionThe effect of wetting and drying on soil structure has been extensively studied (Baver et al., 1972). Studies of the effect on plant nutrients in soils have also been carried out. For example, Scott and Smith (1968) studied the release and fixation of potassium in soil, and Birch (1958) the mineralization oforganic carbon and nitrogen in soil. Wiklander and Koutler-Anderson (1966) found that drying caused the extractability of fertilizer Padded to soil to decrease. Moser et al. ( I 959) reported that the extractability of soil P by CaClz solution, Bray's reagent, and anion exchange resin in some cases increased and in others decreased when air-dry soil samples were pre-moistened. Bartlett and James ( 1 980), however, found that water-soluble and ammonium-acetate-extractable P of 24 moist soils decreased during 3 months of storage. Beckett and White (1964) observed that the P buffer capacity of an Upper Greensand soil increased due to air drying. Thus the reports on the effect of wetting and drying on P in soils are few and inconclusive. This paper presents the results from wetting and drying experiments carried out on a wide range of soils. The effect on P adsorption isotherms and resin extraction of soil P was investigated. Because a large number of contrasting soils was used, it was possible to carry out correlation analyses between wetting and drying effects and selected soil properties. Based on these analyses, reactions of soil P which may be caused by alternating wetting and drying cycles will be discussed. 710Soils and treatments SoilsSixteen topsoils from Denmark, England, Scotland, and Wales were used. Some data and key properties of soils are given in Table I . All soils were initially air-dried and crushed to pass a 2-mm sieve. Wetting and drying treatmentsThe experiment consisted of two treatments: (i) moist incubation (wet), and (ii) eleven wetting and drying cycles (W/D). The water content of the air-dried samples was brought to the predetermined value corresponding to 80 cm water tension by addition ofdemineralized water. The moist samples were incubated in polythene containers. For treatment (i) the incubation time was 22 days. For treatment (ii) the moist samples were dried after I day b...

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WITH I PLATE SummaryPlant damage and reduced yields were associated with delayed ammonification or nitrification of added urea-nitrogen. In a neutral sandy loam, the adverse effects of urea application were associated with the accumulation of toxic levels of first ammonia and later nitrite. Addition of hydrogen-ion as phosphoric or sulphuric acid alleviated the damage thereby supporting the conclusion that ammonia and nitrite accumulation were the causes of phytotoxicity. In an acid sand, toxicity was associated first with the presence of large quantities of urea remaining in the soil though the evidence was insufficient to show in what way the urea was toxic. There was then a second phase of toxicity associated with the presence of considerable quantities of ammonium-nitrogen under conditions where toxic levels of ammonia were likely to have occurred. The later phase (nitrite toxicity) did not occur. Phytotoxicity arising from urea addition to the soil could be due apparently to the persistence of urea as such in the soil and also to the accumulation of significant quantities of free ammonia and nitrite. The occurrence of one or more of these phases of urea toxicity depended on differences in soil conditions.

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