Behaviour and availability of boron in soils under cotton-wheat rotation

Surface (0-15 cm, n=155) and profile (0-15, 15-30, 30-60, 60-90 cm, n=8) soil samples, cotton leaves at pre flowering stage (n=75), wheat flag leaves (n=80) and tube well water (n=75) samples were collected from Bathinda, Mansa and Mukatsar districts of Punjab. About 5, 18 and 20 per cent of soil, cotton and wheat samples were deficient in B, respectively. Hot water soluble B in soil was significantly positively correlated with pH (r= 0.230), ESP (0.680), B concentration in leaves of cotton (r=0.259*) and wheat (r=0.531) but negatively with CaCO3 in soil (r= -0.210 ). About 40 per cent of tubewell waters had >2.0 mg B l. On an average, one cm ha of tube well water may add up to 93g B ha to soil. Boron concentration in tubewell water was significantly positively correlated with HWS-B (r=0.286**) and cotton leaf B (r=0.286). Available B in soil decreased with soil depth (0-90 cm). Two field experiments were conducted to study the response of cotton (Bt cv. RCH 650 BGII; non Bt cv. F 2228; desi cv. FDK 124) and wheat cultivars (T. aestivum cv. PBW 62; T. durum cv. PDW 314; Triticale cv. TL 2908) to B application (0, 0.5, 1.0 and 2.0 kg B ha as borax) using a Typic Ustrochrept, loamy sand (pH 7.2, EC 0.36 dS m, HWS-B 0.36 mg kg soil) soil. A significant response of 2.18 and 1.52 q ha in seed cotton and grain yield of wheat was recorded with an application of 1.0 and 0.5 kg B ha, respectively. Boron concentration in different plant parts of cotton followed the order: leaves>boll squares>petioles>sticks. On the basis of agronomic and B uptake efficiency, cultivars of cotton (RCH 650 BG II > FDK 124 > F 2228) and wheat (PDW 314>TL 2908>PBW 621) responded differentially to B application thus indicating that yield of Bt cotton and durum wheat will be reduced more than the other cultivars under B deficiency. Two pot studies were conducted, using the same soil as was used for field experiments, to study the effect of boron (0, 0.12, 0.25, 0.50, 0.75, 1.0, 2.0 3.0, 4.0, 5.0 and 10 mg B kg soil as borax) and commercial grade calcium carbonate (0, 1.25, 2.5 and 5.0% w/w as) on growth and yield of cotton (cv. RCH 650 BG II) and wheat (cv. PBW 621). In the absence of applied CaCO3, growth of cotton and wheat was reduced when B was applied in excess of 2.0 mg kg soil. Necrosis of leaves from tip to margin in cotton and purple coloration of plumule in wheat was observed with application of 10.0 mg B kg soil. Application of CaCO3 (1.25, 2.5 and 5.0% w/w) significantly improved the growth of both the crops. Irrespective of the levels of CaCO3, a value of 0.57 and 7.67 mg HWS-B kg -1 soil, a concentration of 62 and 940 mg B kg dry matter of leaves, 45 and 210 mg B kg dry matter of petioles and 20 and 51 mg B kg dry matter of sticks, produced 90 and 50% of the maximum dry matter yield of cotton, respectively. The corresponding values for wheat were 0.66 and 6.71 mg HWS-B kg soil, 7.94 and 27.0 mg B kg grain and 15.3 and 170 mg B kg straw, respectively. Twenty one surface (0-15 cm) soil samples selected from those collected from different districts were analyzed to estimate chemical pools of B and available B by using different extractants (NH4OAc, AB-DTPA, hot and cold calcium chloride, tartaric acid, HCl, mannitol and hot water soluble). The amount of B in readily soluble, specifically adsorbed, oxide bound, organically bound and residual mineral fraction varied from 0.17 to 2.71, 0.14 to 1.77, 0.31 to 5.88, 0.56 to 7.42 and 0.17 to 2.71 per cent of total B in soils, respectively. Of the several extractants, NH4OAc extracted the minimum (0.39 to 3.53 mg B kg -1 soil) and HCl extracted the maximum (1.16 to 14.74 mg B kg soil) amount of available B in the soils. HWS-B was significantly and positively correlated with B extracted by other reagents (r=0.772** to 0.905**) thus indicating that these extractants may also be used to delineate the available B status in soils of arid region.