An adventure in applied science - IRRI books - International Rice ...

Early research and training results 127SOIL CHEMISTRYDuring his first 5 years at IRRI, Ponnamperuma conducted literally hundredsof fundamental studies on the chemistry of flooded soils. His work embracedsuch investigations as the influence of flooding on the redox (oxidationreduction)potential, on pH, and on the amounts of ferrous iron, ammonia,phosphorus, silicon, molybdenum, zinc, and copper in the soil solution. Hestudied the losses of nitrogen by denitrification, the formation of toxic substancesfrom the decomposition of organic matter, and the influence ofsubmergence on carbon dioxide levels. He examined the chemistry of the acidsulfate clays that are so extensive in Southeast Asia and that occur also in othertropical rice-growing regions. Using large metal drums in which water levelscould be accurately controlled, he investigated the impact of various waterregimes on the chemical properties of soils.Most of Ponnamperuma’s basic studies were conducted in pots in thegreenhouse, although in 1963 he ran a field experiment, in cooperation withMoomaw, studying the effect on yield of the duration of submergence prior toplanting, with and without added organic matter. He found that 2 weeks ofsubmergence were optimum, even when as much as 7.4 t of a mixture of ricestraw and fresh leaves of Leucaena glauca per hectare was incorporated duringland preparation. Interestingly, no significant differences in yield were obtainedwhen an equal amount of nitrogen was applied as ammonium sulfate,indicating that amount of nitrogen, not source, is the important factor.Ponnamperuma’s thorough and complex studies of the chemistry offlooded rice soils cannot even be touched upon adequately here. In brief,however, his work revealed that when a dry soil is flooded, the followingchanges occur during the first 3 to 4 weeks:• The oxygen supply decreases almost to zero except in a thin layer at thesoil surface.• The pH of acid soils increases, whereas that of calcareous soils decreases,thus tending to bring most soils to harmless degrees of either acidity oralkalinity.• Iron is reduced from the ferric to the ferrous form, and large amounts ofsoluble iron are released into the soil solution.• The supply of available nitrogen, phosphorus, silicon, and molybdenumincreases.• The availability of zinc and copper decreases.• Harmful quantities of toxins, such as organic acids, ethylene, and hydrogensulfide may be produced, and under certain conditions the solubleiron quantities may build up to toxic levels.The findings showed that most of the changes that take place after floodingare beneficial to the growth of rice. This information helped develop thethinking of other IRRI scientists, for several of them questioned the need tofollow the time-honored practice of flooding and even thought it might beharmful.