تثبيت پتاسيم در خاكهاي شاليزاري شمال ايران

Potassium Fixation in Paddy Soils, North of Iran

Fixation of potassium in the main rice cultivated areas of North of Iran was investigated through collecting composite soil samples (0-30 cm depth) from 67 randomly selected paddy fields (one sample per field) scattered all across Guilan (33 fields) and Mazandran (34 fields) provinces. Twenty grams of air dried soil was transfered into a polyethylene tube (34-45 cm long x l cm diameter), then a known quantity of K as KC1 solution and enough distilled water added to the tube until the depth of water above soil surface reached 5 cm. The tubes were incubated under this condition for 30 days, then the amount of K fixation measured. The amounts of K fixation were between 4.9 and 65.1 percent, with a mean value of 27.4% . Potassium fixation in Guilan province was of a wider range (4.9-65.1%) than that in Mazandaran province (14.2-49.2%), however, the mean values (28% vs 27%, respectively) for the two provinces were not significantly different. It was estimated that, out of 470.000 ha of rice cultivated areas in North, about 98000 ha had a high (more than 40%), 218000 ha had a medium (between 20 to 40 %), and the remaining 154000 ha were of a low (less than 20%) potential for K fixation. The significantly high potential for K fixation of these soils is a result a gradual K depletion due to the disturbed input-output K balance in these soils. The amount of K fixation was significantly (5%level) correlated with coarse clay, silt+clay, total clay and CEC (r = 0.7), but it was poorly correlated with fine clay and silt content of the soils. In addition, it was revealed that in 54% of the soils, the ammonium acetate extractable K, which was assessed under wet conditions of 30 days after submergence, was below the critical level of 115 mg kg"1 as for rice. The low available K plus a high fixation rate prevailing in these soils, can cause serious nutritional problems for rice in these soils.