| Popis: |
The nutritional requirements of cassava are ill-defined. This paper deals with the P requirements of cassava and the effect of P on other nutrients both in the presence of VA mycorrhizae and in soils when VA mycorrhizae were eliminated by fumigation. Cassava was grown on field plots with ten levels of phosphorus (0.002-1.6 ppm P in solution) on two Oxisols in Hawaii. Six cultivars were compared at one location. At the other location the significance of vesicular-arbuscular (VA) mycorrhizae on the nutrition of cassava was investigated. There was a substantial increase in vegetative growth with increased P fertilization in the cultivar experiment; however, root yields were not greatly affected. Even at very low extractable soil P levels (Bray-1 P, 1 ppm) five out of the six cultivars did not respond to P fertilization. One cultivar, Ceiba, produced 42–48 t/ha of roots when P in solution was in the range 0.003-0.025 ppm. In the second experiment the role of VA mycorrhizae was studied by fumigating one half of each plot with methyl bromide which destroyed VA mycorrhizal fungi. Plants grown on the fumigated plots did not become infected with VA mycorrhizae during the first 80 days of the growing period. Cassava, cultivar Ceiba, was grown along with six other species. One of these was chinese cabbage which does not form mycorrhizal associations. Leaves from non-mycorrhizal cassava plants (fumigated plots) invariably contained less P on a percentage basis than leaves from mycorrhizal plants (non-fumigated plots). On the low P plot, leaf P decreased from 0.30 to 0.11% when VA mycorrhiza formation was precluded by fumigation. Likewise K and S concentrations in leaves decreased by 30% and Zn by 10%. There was no evidence that mycorrhizae promoted Ca uptake. These observations provide a basis for explaining why the P requirements of cassava in nutrient solution culture are very great, whereas cassava is generally unresponsive to P fertilization in the field. |
