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 I, 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 (I 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 p]-I's. New root initiation was totally inhibited at all organic acid concentrations at equilibrium pH, and at 10 mN with pFI 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 bottl pH's. P concentration and total uptake were re= duced 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.

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Rice (Oryza sativa L.) straw disposal by soil incorporation may have adverse effects on subsequent rice seedling growth from toxic anaerobic decomposition products and temporary immobilization of the soil mineral N. Glasshouse and laboratory studies were conducted to study the problem under conditions of direct sowing of rice. Samples of a typical rice soil, Sacramento clay, which is a very fine, montmorillonitic thermic Vertic Hapiquall, were collected. Rice straw in amounts of 0, 0.25, and 0.5% was added to soils which were incubated aerobically for 0, 15, and 30 days in a glasshouse plant growth experiment, and for 0 and 20 days in preparation for a laboratory soil incubation experiment. Nitrogen was applied at the rate of 50 ppm in the glasshouse experiment and 50, 75, and 100 ppm in the laboratory study. N was added after aerobic incubation and subsequently rice was grown for 21 days to measure seeding growth effects. The glasshouse experiment with rice cultivar ‘Earlirose’ also evaluated the production of acetic, propionic, and butyric acids, while the laboratory soil incubation experiment was for N transformation data. The results clearly indicate that when soil and rice straw were not incubated prior to planting rice seedlings, applied N was immobilized, causing inhibition of plant growth and low N content of plants. When rice straw was incubated in soil for 15 to 30 days before planting seedlings, N immobilization was reduced and plant growth was promoted. Nitrogen immobilization was observed to be the principal cause of inhibition of plant growth from added rice straw. Organic acids (acetic and propionic) were produced in amounts considered to be below the toxic concentration of plants. Butyric acid was not found in the soil extracts.

The pH of the flood water in rice fields is largely determined by the chemical equilibria that exist between the CO2 balance achieved by the aquatic biota and the various solutes, solids, and gases in the water. Water pH values undergo diurnal changes, increasing by midday to values as high as pH 9.5–10 and decreasing as much as 2–3 pH units during the night. The pH of shallow flood water is greatly affected by the total respiration activity of all the heterotrophic organisms and the gross photosynthesis of the species present.Ammonium form fertilizers broadcast into a high pH water are highly susceptible to direct NH3 volatilization losses. Nitrogen losses from fertilizer broadcast into flood water on a fertile, neutral‐pH Maahas clay were as high as 20% of the amount applied, but losses varied depending upon water pH, the nitrogen source, and rate, time, and method of application. Losses from an acid Luisiana clay, where the flood water was not conducive to algal growth and did not exceed pH 6.8, produced NH3 volatilization losses consistently less than 1% of the total N applied. Placement of N fertilizer in the soil at depths of 10–12 cm reduced NH3 volatilization losses to less than 1% of the total N applied.

Ammonia volatilization from flooded rice (Oryza sativa L.) is a major mechanism for N loss and poor fertilizer use efficiency. Ammonia volatilization is influenced by five primary factors: NH4‐N concentration, pH, temperature, depth of floodwater, and wind speed. This NH3‐volatilization model is based on chemical and volatilization aspects. The chemical aspects of the model deal with the NH4/NH3(aq) equilibrium in floodwater. Ammonium ions undergo dissociation with a first‐order rate constant, while NH3(aq) and H undergo a diffusion‐controlled association reaction with a second‐order rate constant. The transfer of NH3 across the water‐air interface of flooded soil systems is characterized by a first‐order volatilization rate constant. By utilizing the chemical dynamics of the NH4/NH3(aq) system in association with transfer of gaseous NH3 across the interface, an equation was derived to determine the rate of NH3 volatilization from flooded systems as a function of the five primary factors. The chemical aspects of the model include the derivation of association and dissociation rate constants. The volatilization aspects of the model, which is based on the two‐film theory, allows it to compute the volatilization rate constant for NH3. Expressions are derived to compute the Henry's law constant, gas‐phase and liquidphase exchange constant, and the overall mass‐transfer coefficient for NH3.