determination of phosphorus critical level in some of paddy soils in guilan

abstract the increasing of yield per unit area is usually achievable by improving the soil fertility level through using (chemical) fertilizers. the purpose of the present research was to evaluate the 6 extraction methods including olsen, morgan, ammonium bicarbonate-dtpa, colwel, bray and mehlich 3 in order to specify a suitable extractant(s) for estimating available phosphorus through using 27 paddy field soils in guilan province. for farm evaluation, two fertilizer treatments of 0 and 60 kg ha-1 of triple super phosphate in phosphorus pentaoxide were used. the correlation coefficients between extracted phosphorus by different extractants and relative yields, phosphorus concentration and phosphorus uptake by rice shoots indicated that colwell, olsen and bicarbonate ammonium-dtpa extraction methods had an acceptable correlation with relative grain yield, phosphorus uptake and phosphorous concentration. thus, with consideration of economic and applicability aspects, extraction speed and its acceptable relationship with plant indices, olsen extraction method is recommended for available phosphorus.