Most plant species, including the major crops, form symbiotic mycorrhizal associations between their roots and certain fungi which influence nutrient uptake, especially P, from infertile soils. How well‐supplied the phosphate must be before mycorrhizae cease to enhance P uptake is not known; nor have the effects of mycorrhizae on external P requirements of crops been adequately determined in the field. This study examined the influence of mycorrhizae on the P requirements of crops in a tropical field environment on a Tropeptic Eutrustox. Plant growth and P uptake by non‐mycorrhizal and mycorrhizal plants (methyl bromide‐fumigated and nonfumigated soil) were measured at 10 levels of soil P using seven plant species. Brassica chinensis, which does not form symbiotic mycorrhizal associations, consistently grew better and took up more P from the fumigated than from the non‐fumigated soil. All other species growing in nonfumigated plots formed associations with mycorrhizae. In general, plants growing on fumigated soil did not become infected with mycorrhizae. In P‐deficient situations, plant concentration of P was enhanced by the mycorrhizal associations. The levels of soil P at which fumigation ceased to make a difference in the P percentage in plants of the various species were as follows: Glycine max (L.) Merr. (0.1 µg P/ml), Vigna unguiculata L. (0.2 µg P/ml), Allium cepa L. (0.8 µg P/ml), Leucuenu leucocephala (1.6 µg P/ml), Stylosanthes hamata and Manihot esculenta (1.6 + µg P/ml). We suggest that this listing may be the order in which these species depend on mycorrhizae in P‐deficient soils. As a mean of six species growing on the two lowest soil P levels, P uptake by mycorrhizal plants was 25 times greater than by plants without mycorrhizal associations. Thus, some crops appear to be quite dependent upon a mycorrhizal association for P absorption from a soil of high sorption capacity.

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Background and aims Adequate zinc (Zn) in maize (Zea mays L.) is required for obtaining Zn-enriched grain and optimum yield. This study investigated the impact of varying Zn fertilizer placements on Zn accumulation in maize plant. Methods Two pot experiments with same design were conducted to investigate the effect of soil Zn h etero ge neity by mixing Zn SO 4 ·7H 2 O (10 mg Zn kg −1 soil on an average) in 10-15, 0-15, 25-30, 0-30, 30-60 and 0-60 cm soil layers on maize root growth and shoot Zn content at flowering stage in experiment-1, and assessing effects on grain Zn accumulation at mature stage in experiment-2. Results In experiment-1, Zn placements created a large variation in soil DTPA-Zn concentration (0.3-29.0 mg kg −1 ), which induced a systemic and positive response of root growth within soil layers of 0-30 cm; and shoot Zn content was increased by 102 %-305 % depending on Zn placements. Supply capacity of Zn in soil, defined as sum of product of soil DTPA-Zn concentration and root surface area at different soil layers, was most related to shoot Zn content (r=0.82, P<0.001) via direct and indirect effects according to path analysis. In experiment-2, Zn placements increased grain Zn concentration by up to 51 %, but significantly reduced the grain Zn harvest index from 50 % by control to about 30 % in average. Conclusion Matching the distribution of soil applied Zn with root by Zn placement was helpful to maximize shoot Zn content and grain Zn concentration in maize.

As agricultural development intensifies in tropical countries and as costs of superphosphate production increase, the need for further evaluation of rock phosphates has become apparent. An acid tolerant pasture grass (Brachiaria decumbens Stapf.) was used to evaluate the initial and residual (3 years) availability of P from several phosphate materials. A field study was conducted on a Typic Haplustox of the Cerrado of Central Brazil. Initial availability of P supplied as normal superphosphate, Hyperphosphate, Thermalphosphate, and North Carolina rock phosphate was similar when the soils were not limed (pH = 4.3, Al = 1.4 meq/100g). Where lime had been added and pH was increased to 5.4 the P in Hyperphosphate and N.C. rock phosphate was initially less available than P added as superphosphate and Thermalphosphate. After 10 months of contact, lime no longer significantly depressed P availability in these relatively soluble rock phosphates as measured by forage dry weight. The P in the other phosphate rock of low citric acid solubility, Araxá, was virtually unavailable during the 3 months after application. However, within 13 months, maximum forage yields were attained where this material had been applied to unlimed soil and at 25 months maximum yields were obtained on both limed and unlimed soil. Higher rates of Araxá material were required to obtain maximum yields than with normal superphosphate. Forage yields were 2 to 4 times greater where superphosphate was surface applied each year than where similar amounts were incorporated prior to planting. A comparison of extractant methods for soil P indicated that highly acidic extractants (e.g., 0.05 N HC1 + 0.025 N H2SO4) overestimated available P where rock phosphate had been applied. Where superphosphate or rock phosphate had been applied, P availability was better assessed by extractants such as 0.5 M NaHCO3 and Bray 1. The results emphasize the need to supplement laboratory and short‐term studies with long‐term evaluations of P materials with acid tolerant perennial crop species available in tropical countries.

Cowpea (Vigna unguiculata) and soybean (Glycine max) were grown in the field on a Tropeptic Eutrustox which, for a period of 8 years, had been maintained at 10 different soil P levels by appropriate P fertilizer applications. Effects of methyl bromide fumigation of the soil on mycorrhizal infection and concentrations of P and other nutrients in the plants were compared. Plants grown on nonfumigated soils with soil P levels below 0.025 to 0.05 mg P/liter contained higher Ca and K percentages than plants grown on fumigated, low P‐status soils. Differences in Ca and K percentages in the plants were more closely related to crop growth rates than to vesicular arbuscular mycorrhizal infection levels. Differences in Ca and K percentages were small. In contrast, silicon percentages of mycorrhizal soybean plants were 0.5 to 0.9% Si while non‐mycorrhizal plants contained 0.2 to 0.3% Si. The pattern of Si percentages at different levels of soil P was different from that of other nutrients. The levels of Si in mycorrhizal soybean plants were greater than in non‐mycorrhizal plants at all levels of soil P, while with other nutrients, differences diminished with increasing soil P level. Crop growth rate and P absorption strongly affected the percentage composition of other nutrients with the notable exception of Si. If mycorrhizae consistently enhance Si uptake by plants such as soybean then Si uptake may indicate mycorrhizal activity. Silicon content of cowpea, however, was not altered by the presence or absence of mycorrhizae.