The movement, adsorption, and desorption of sulfate was studied, using samples from the A and B horizons of three soils from Costa Rica. Water, equivalent to 509 mm, was applied to small lysimeters and the distribution of radiosulfate in the columns was determined for untreated soils and samples previously incubated with one of the following substances: (NH 4 ) 2 SO.i, K,SO 4 , simple, or triple superphosphate. Little sulfate movement was observed. This was enhanced slightly by phosphates.Sulfate adsorption from solutions containing 100 to 1,000 ppm sulfate fit Freundlich isotherms. Very concentration-dependent adsorption was indicated for alluvial soils but little concentrationdependent adsorption was noted for latosols. Sulfate desorption by water depended greatly on soil type but was less variable when phosphate solutions were used as desorbants. Higher phosphate concentration increased desorption but the latosols retained appreciable sulfate amounts even against extraction by a 1,000ppm phosphate solution. Some hydroxyl displacement on sulfate adsorption occurs as shown by higher pH values for K 2 SO 4 , soil suspensions in comparison with soil suspensions in KC1 of equal normality.

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The efficiency of oxidation of organic C by the Walkley‐Black method was studied by comparing with determinations using dry combustion equipment and gravimetric determination for the evolved CO2. Fifty surface soils (0–30 cm) were studied, in groups of 10, each one belonging to one of the following taxonomic suborders: Fluvents, Andepts, Tropepts, Udults, and Ustults. Samples from 17 Andept profiles were also analysed to study changes of oxidation efficiency with depth.The results for the surface samples indicated that the commonly used conversion factor was somewhat too small. It is recommended that 75% oxidation be used as the base for conversion to total C, as 86% of the studied samples deviated in < 2% from this value.It was observed that the organic matter in the deeper horizons is more easily oxidized through the Walkley‐Black procedure, perhaps because plant residues are less abundant and there is an accumulation of fulvic acids. In some cases the oxidation is almost complete. This oxidation is rather variable and it was not correlated with the texture or pH of these soils. It is believed that the extent of oxidizability can allow conclusions on the relative age of horizons in Andept profiles and indicates the presence of buried horizons.

Zinc is known to be involved in several enzyme systems in plants and to play a role in protein metabolism 12. It its also required for the synthesis of tryptophane, a precursor of indoleacetic acicl 22. It is thought to be of intermediate mobility in plants 4, and since zinc deficiencies in many plants are common a more detailed knowledge of its function and intracellular localization would be useful. W o o d and Silby 26 report that up to half of the total zinc in leaves can be located in the chloroplasts, depending on the plant species, and that dialysis of an aqueous brei of oat leaves results in no loss of zinc.The purpose of the following work was to localize the zinc of various plant tissues. Materials and methodsAresan-treated corll seeds were germinated on moist filter paper in petri dishes. In Experimertts A and B and for the solubility study, 50 #c of Oak Ridge high specific activity zinc-65 were added per 100 seeds, live days after germinatioi1 commellced. In Experiment A, after two days, and in t3 after three days of the zinc-65 addition, the plants were thoroughly rinsed in tap water, separated into roots, grains and leaves, and weighed. In Experiment C, the isotope was added after four days of germination and the plants cropped after all additiollal five days. Each ~issue was groulld in 0.067 M sodium phosphate buffer (pH 6.7) in all Omnimixer at full speed. The homogellizer was kept in an ice bath during the

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Abstract. The transformations of urea, ammonium sulfate and dicyandiamide (DCD) were studied in an Inceptisol and three Andisols from Costa Rica, considering the influence of temperature and DCD as a nitrification inhibitor.Nitrification was very slow with or without DCD in the strongly acid Inceptisol. A higher urea dose resulting in higher pH was well nitrified without DCD and appreciably retarded by DCD.In Andisols nitrification was retarded as long as a higher DCD level existed. Higher temperatures accelerated the DCD-breakdown and were followed by a quicker nitrification. The decomposition of DCD was slower compared with the Inceptisol. According to these experiments DCD is suitable as a nitrification inhibitor in tropical soils.

The immobilization of P is a significant fertility limitation of Andisols in Central America. It is believed that soil Al and Fe fractions have an important influence on P availability. This study was conducted to obtain information on the various forms of Al and Fe in ten pedons derived from volcanic ash in Panama and Costa Rica. Correlations between these Al and Fe fractions and P immobilization were measured by different methods.The Al and Fe fractions, extracted by acid ammonium oxalate, 4M KOH, sodium pyrophosphate and dithionite-citrate, were determined, and the correlations between these fractions, P immobilization, and other soil properties made.It was observed that oxalate extractable Al correlated significantly with P immobilization, which had values of over 85% by the New Zealand method and 96% by the isotherm method. The Al concentration corresponding to this immobilization was over 2%. The difference between oxalate extractable Al and pyrophosphate extractable Al (inorganic Al fraction) correlated with P immobilization also. The pyrophosphate extractable Al fraction had the lowest concentrations and did not correlate with P immobilization. The NaF-pH and dithionite-citrate extractable Al correlated significantly with P immobilization.Oxalate extractable Fe correlated with the P retained by the isotherm method, but dithionite-citrate extractable Fe was negatively correlation with P immobilized. No correlations were found between the pyrophosphate extractable Fe and other soil properties.

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