In spite of the great economic importance presented by the cassava crops to Brazil and to other subtropical and tropical countries, to our knowledge no work has been done to study the mineral nutrition of this plant. Cassava roots are consumed either as such (after being cooked) or as manihot meal. The starch which is extracted from the roots and exported represents an important source of foreign currency to Brazil.The present experiment was carried out to study the effects of the macronutrients nitrogen, phosphorus, and potassium on the yields as well on the composition of the roots. Preliminary data were presented elsewhere (5).Plants tive growth during the early stages of development probably accounts for this high response to P addition. Such a response may also in part reflect the amount of P in the cuttings when they were set out in the sand culture. Leaves of the plants grown in the absence of P treatment showed the red purplish color characteristic of P deficiency. The response to N was as large as that observed from the addition of P in the fertilizer. The effect of K in the root yields was much less marked. In the absence of K treatment, the weight of roots decreased whereas that of the shoots increased. Since there is considerable evidence supporting the role of K in the translocation of carbohydrates (1) it seems reasonable to assume that sugars produced in the leaves were consumed in the building up of more green matter rather than being translocated down to accumulate as starch in the roots. Doubling the doses of any element produced a significant increase in yield, the largest response being due to N. However, the discussion following table II indicates this large response is not justified from an economic point of view.
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