A two- year greenhouse experiment was carried out to study the response of two high yielding varieties of rice i.e. Amol -3 and Khazar to potassium application in paddy soils of North of Iran. For this purpose 51 composite soil samples (25 in the first year, and 26 in the second year) were taken from 0-25 cm depth of 51 randomly selected fields all over the region. Some properties of the soils such as pH,EC,SP,CEC and exchangeable plus soluble K were measured .The amount of ammonium acetate extractable K of the soils were in the range of 41 to 280 mgkg’ with a mean of 133.4
mgkg-1.Further more, the level of this form of K in 94% of the soils was below 200 mgkg-1, in 55% below 135 mgkg-1, and in 29% below 100 mgkg-1 indicating the relatively low levels of easily available K in these soils. The critical level of 135 mg kg-’ of NH4OAc extractable K was obtained for high yielding variety of Khazar by the Cate-Nelson graphical method. It was estimated that in about 100,000 hectars (55%) of Paddies presently under, high yielding varieties, the level of NH4OAc extratable is below the critical level. The response of both varieties to potassium fertilizer was highly significant (0.1% level) .The treatment rate of 112.5 kg/ha of potassium chloride increased the yield of aboveground dry matter by 22% and that of rough rice by 22.7% in Amol-3. In Khazar variety, the increase in yield of the same plant portions were. 16.2 and 17.7% respectively for the treatment rate of 150 kg/ha of potassium sulphate, and 22.6% and 20.12% for the rate of 350 kg/ha of the same fertilizer. However, the grain yield due to the higher rate was not
significantly (5% level) different from the lower K rate .The data indicate that the contribution of the non-exchangeable form of K to the total K uptake in the majority of the untreated soils is significant, suggesting the reliance of plant on the slowly available form of K in these soils. The results also revealed that in almost 70 percent of the soils, the K percentages in the straw of both ‘varieties are less than the reported 1% critical level for rice
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