Residual and Cumulative Effect of Boron Use in Rice-Wheat System in Calcareous Soils of Pakistan.
Abdul Rashid1, M. Yasin1, R. Ullah1, and M.A. Ali2. (1) National Agricultural Research Center, Land Resources Research Program, Park Road, Islamabad, 45500, Pakistan, (2) Adaptive Research, Agriculture Dept, Davis Road, Lahore, Pakistan
Our recent, multi-year extensive field research has identified and established incidence of widespread and severe Boron (B) deficiency in rice grown in alluvial calcareous soils of the major rice growing areas in Pakistan. Fine grain Basmati types, i.e., Super Basmati and Basmati-385, as well as medium-long grain/coarse grain cultivars, i.e., IR-6 and KS-282, are affected equally. Average paddy yield increases with B use (@ 1.0 kg B ha-1) were 21%, over control, in cultivars Super Basmati (24 field trials) and Basmati-385 (8 field trials), 30% in cv. IR-6 (3 field trials) and % 14% in cv KS-282 (3 field trials). These substantial paddy yield increases were primarily the consequence of reduced panicle sterility and increased productive tillers per hill with B application. For example, panicle sterility decreased from 23% to 14% in cv. Super Basmati and 28% to 16% in cv. Basmati-385. Better B nutrition of rice plants also enhanced milling recovery as well as head rice recovery, and improved desireable kernel quality traits, i.e., reduction in stickiness, increase in elongation ratio, and decrease in brusting-upon-cooking. Thus, it was established that B deficiency in calcareous soils not only hampers rice productivity but also impairs its cooking quality. Considering the paddy yield increase alone, the use of boron in rice was highly cost effective, with value-cost ratios of 45:1 in cv Super Basmati, 36:1 each in cvs. Basmati-385 and IR-6, and 26:1 in KS-282. Cooking quality imrovement and residual effect of a single B application on the suceeding wheat crop are additional benefits of economics significance. Consequently, B use in rice is now recommended in Pakistan. The recommended dose is 0.75 kg B ha-1. For economic considerations of the residual effect of B, applied to rice, on the suceeding wheat crop, as well as for investigating the possible of B buildup due to continued B use, our present research aims at studying the residual and cumulative effect of soil B application to rice and/or wheat in the prevalent rice-wheat cropping system. Four permanent layout replicated field experiments are in progress in alluvial calcareous soils of the major rice growing areas of the Punjab province, Pakistan. The field experiments were initiated with the 2004 rice crop, and would be continued atleast for three consecutive crops each of rice and wheat. Boron rates, applied to rice and/or wheat, ranged from 0 (control) to 1.5 kg B ha-1 as borax. Average near-maximum (95% of maximum) paddy yield, associated with 1.0 kg B ha-1 to rice crop, was 20% over control. Residual effect of B, applied to rice, on the sucessive wheat crop was highly appreciable. A dose of 1.0 kg B ha-1, applied to the previous rice crop, resulted in 21% increased wheat grain yield. Interestingly, wheat grain yield increase with the cumulative effect of 1.0 kg B ha-1, applied both to rice and wheat crops, was of the same magnitude. Our previous research has revealed that that only <2.0% of the applied fertilizer B is taken up in the aboveground plant parts of the current rice crop. Thus, even if some of the residual B is fixed in the soil and/or leached out of the crop root zone, an adequate fraction stays in the soil's labile pool for meeting B requirement of the suceeding wheat crop. As this field research is continued, we intend to determine the longevity of the one-time applied B in meeting crop requirements in the prevalend rice-wheat system. This paper also reports on critical levels of B in diagnostic plant parts in rice and wheat.