Abstract
Among factors that influence wheat yield, phosphorus (P) plays an important role and plants require adequate P
from the very early stages of growth for optimum crop production. Currently, no site-specific fertilizer
recommendations are available for Pakistani soils. The P sorption isotherms were used to determine internal and
external P requirements of wheat cv. Inqalab 91. Three field experiments were conducted on three different
textured soils during Rabi (winter) 2004. Taxonomically, these soils were classified as: i) Chromusterts known as
Kotli series that has clay texture (S,), ii) Haplargids known as Hafizabad series that has sandy clay loam texture
(S2) and iii) Torripsamments known as Thai series that has sandy loam texture (S3)' Eleven P treatments (4 to 96
mg P kg-' soil) were arranged following a randomized complete block design with three replications. Adsorption
studies were conducted and isotherms were constructed by plotting the amount of Padded (mg kg-') vs. P in soil
solution (mg L-') at equilibrium. The soil solution-P levels were determined against different quantities of fertilizer
P for S" S2 and S3 soils. Internal P requirements associated with 95 % relative yield were 0.355 % (S, soil), 0.385
% (S2 soil) and 0.350 % (S3 soil) at booting stage or 0.108 % (S, soil), 0.066 % (S2 soil) and 0.100 % (S3 soil) in
straw or 0.295 % (S, soil), 0.215 % (S2 soil) and 0.315 % (S3 soil) in grain. It ranged from 0.215-0.315 % at
booting stage or 0.066-0.108 % in straw or 0.350-0.385 % in grain. External solution P requirement (EPR; mg L-')
of wheat for its 95 % maximum yield was 0.23 (S, soil), 0.25 (S2 soil), and 0.28 (S3 soil). Corresponding amounts
of fertilizer P (mg kg-') required for these EPR levels were 38 (S, soil), 26 (S2 soil) and 13 (S3 soil). Soil texture
affected the P rates to obtain 95 % of maximum wheat grain yield. It was concluded that P recommendations in
future should be made keeping in view soil texture to make the P application.
