The castor bean cake is a residue from the biodiesel production, which has a great potential as organic fertilizer, but it also presents sodium contents in its constitution, as a result of the use of NaOH, as a catalyst for the transesterification reaction, to produce biodiesel. This study aimed to evaluate the potential use of castor bean cake resulting from biodiesel produced directly from seeds as an organic fertilizer in the cultivation of castor bean (Ricinus communis L.). The cake was washed with water, in the 1:4 (m/v) ratio, in order to reduce sodium contents. Plant height, fresh and dry shoot weight, and total shoot and root nutrients contents were evaluated, in addition to residual elements after cultivation. The study was carried out in the Universidade Federal Rural do Rio de Janeiro, in Seropédica, Rio de Janeiro State, Brazil. The Al Guarani cultivar, grown in a greenhouse, for 70 days, in an Inseptisol substrate from Chapada do Apodi, Rio Grande do Norte State, Brazil, was evaluated in a randomized blocks design, in a 6x2 factorial scheme (six cake doses: 0 Mg ha-1, 5 Mg ha-1, 10 Mg ha-1, 20 Mg ha-1, 40 Mg ha-1, and 80 Mg ha-1; and two cake states: in natura and treated), with three replications. The sodium contents present in the cake are not harmful to castor bean plants at short term. In general, the highest dose of in natura or treated castor bean cake, equivalent to 80 Mg ha-1, promoted the highest nutrients release into the soil.
Ricinus communis L.; sodium; biofuels; mineral nutrients
