Abstract
Cotton is an important cash crop in wheat-cotton cropping system of Pakistan. Most of the farmers apply
phosphorus (P) fertilizer only to wheat and not to cotton crop which might be the main reason for low cotton yield.
A field study was conducted to determine P requirements of three different textured calcareous soils viz. sandy
loam, loam and clay loam. Sorption isotherms were constructed in the laboratory by equilibrating 2.5 g soil
samples with 25 ml of 0.01 M CaCl2 solution containing 0, 5, 10, 15, 20, 40, 60 and 80 mg P L-1 as KH2PO4.
Sorption data were fitted to Freundlich model which were found significant at P = 0.05. P fertilizer doses for cotton
crop were computed against theoretical soil solution P levels of 0.0, 0.10, 0.20, 0.25, 0.30, 0.35 and 0.40 mg P L-1.
It was noted that different P fertilizer doses were required for clay loam (0, 93, 132, 151, 155, 160 and 199 kg P
ha-1), loam (0, 49, 67, 80, 100, 117 and 135 kg P ha-1) and sandy loam (0, 30, 54, 57, 63, 70 and 79 kg P ha-1)
soils to maintain the same soil solution P level. The maximum number of bolls was recorded at 0.30 mg P L-1 soil
solution P level in sandy loam (28 vs. P rate of 63 kg ha-1) and loam (35 vs. P rate of 100 kg ha-1) soils.
Statistically significant response of seed cotton yield (kg ha-1) and lint % age were observed to the tune of 4419
and 35.65, respectively for sandy loam soil, while 4982 and 36.37, respectively for loam soil. Clay loam soil
showed minor response to graded levels of applied P with respect to different cotton quality parameters.
