Soils which are flooded for lowland rice culture shift from aerobic to anaerobic organic matter transformations. Anaerobic carbon transformations, involving chiefly rice crop residues, are characterized by the formation of various organic acids. These may accumulate after prolonged incubations in amounts sufficient to be toxic to developing rice seedlings. In these experiments the effects of acetic, propionic, and butyric acids were studied at 1, 5, and 10 mN on the growth and nutrition of 14 day old (Oryza sativa L.) cultivar ‘Earlirose’ rice seedlings. Nutrient solutions were used in the experiments with pH controlled at 6.5 in one experiment and in another the acid concentrations were allowed to attain equilibrium pH with the nutrient solution (1 mN=4.6, 5 mN=3.9, and 10 mN=3.8).
Root elongation of rice seedlings was decreased by increased organic acid concentrations at both pH's. New root initiation was totally inhibited at all organic acid concentrations at equilibrium pH, and at 10 mN with pH 6.5. New root initiation at 1 and 5 mN at pH 6.5 allowed increased seedling dry matter production, whereas it was reduced in all other treatments. Plant height and weight were also decreased by increased acid concentrations. At pH 6.5 the plants showed no specific symptoms of organic acid toxicity except reduced growth. At equilibrium pH values specific symptoms were observed. At 1 mN, the seedlings withered, similar to desiccation; at 5 mN the leaf tips showed symptoms similar to bronzing; and at 10 mN the seedlings died after 24 hours.
Uptake of both P and K by roots were reduced by increased concentrations of all organic acids at both pH's. P concentration and total uptake were reduced in the shoots with all treatments, whereas the effects on K in shoots were not consistent.
The magnitude of organic acid toxicity is a function of the kind, concentration and the degree of dissociation of the acid. Increased media pH reduces the toxicity of the acid concentrations.