Saturday, 15 July 2006

Desorption Kinetics of Boron in Alluvium Derived Soils of North-West India in Relation to Soil Characteristics.

SANJAY ARORA, S.K.Univ of Agricultural Sciences and Technology-J, FoA, Chatha, JAMMU, 180009, India and D.S. CHAHAL, Dept of Soils, Punjab Agricultural Univ, LUDHIANA, 141004, India.

The deficiency of boron (B) in plants is most widespread in high pH soils which are coarse in texture and low in organic matter content. The range between boron deficiency and toxicity being narrow, poses difficulty to maintain appropriate boron levels in soil solution. Emerging deficiency of boron in alluvium derived arid and semi arid soils of North-West India had been reported. The reactions of plant nutrients in soils are dynamic. They can be studied from either an equilibrium or kinetic viewpoint. The data regarding the rate of nutrient transformations in soils are ultimately needed to make sound recommendations concerning nutrient management. This type of information is critical if growers are to utilize plant nutrients so as to maximise crop production, yet enhance efficiency and preserve environmental quality. To study the kinetics of B desorption, surface soil samples were collected from the ten agricultural lands representing alluvial benchmark soils of North-West India. In general, the climate is hot in summers, cool in winters with mean annual rainfall varying from 300 to 1200 mm, seventy per cent of which is received during July to September. There is a considerable variation in temperature with monthly mean maximum of 42oC in June, and minimum of 4.7oC in January. Soil temperature regime is hyperthermic and moisture regime is Aridic, Ustic and Udic. In this study, soils were pretreated with 40 µg B/g soil for 48 hours and were then equilibrated with 0.05 M mannitol at 25±1oC for 1,2,4,8,12,18,24,32,48 and 72 hours with intermittent shaking. These were then centrifuged for 10 minutes and filtered through Whatman No. 42 filter paper. Boron concentration was determined in the filtrate using Azomethine–H. The various kinetic models used to describe rate of B desorption from the soils includes zero, first, pseudo-first, second order, parabolic diffusion and elovich model. These were tested for goodness of fit by least-square regression analysis. The amount of desorbable boron was strikingly higher in the coarse textured soils as compared to the fine textured soils. The increase in desorbable boron with time in sandy soils can be attributed to less number of exchange sites due to the low clay content to retain boron more tightly. Also, desorbed boron when expressed as a percentage of adsorbed was remarkably higher in coarse textured soils as compared to the fine textured soils. This again suggests that boron was held far less tightly in the former than the latter soils and thereby required less time for its desorption from the coarse textured as compared to the fine textured soils. The difference in the desorbable boron appears to have largely resulted from less number of B retentive sites as well as their ability to retain boron with great tenacity in coarse textured soils because of the large variation in physical and chemical properties of these soils. Furthermore, the desorption of B took relatively lesser time i.e. 16 to 24 h in coarse textured soils in comparison to 32 to 48 h in fine textured soils. This behaviour of B desorption reaction with time is due to the nature of the reactions because of variation in clay content and the amount of organic matter and calcium carbonate content which have a substantial role in B adsorption and release by soils. Desorption reaction was initially fast and thereafter, it becomes slow and seemed to have completed in 24 hours in many soils, while it took 32 hours to complete desorption reaction in the rest. The total desorbable boron values ranged between 7.12 and 16.10 mg/g. The data of boron desorption were also fitted to various kinetic models, like Elovich, Power function and Parabolic diffusion and were tested by least square regression analyses for describing boron desorption from the soils. It was observed that Elovich kinetic equation was best among different models, as evidenced by the overall highest values of coefficient of determination and lowest S.E.values over the entire range of time. The power function model was found to be equally good in describing boron desorption kinetics in all the soils. The coefficient of determination (R2) values ranged between 0.906 and 0.996, while error of estimate values varied from 0.0042 to 0.0194. Parabolic diffusion equation could not describe the B desorption rate satisfactorily except in some soils. The study thus suggested the possibility of cropping up of B deficiency in the soils which have already desorbed about 90 per cent of its leachable boron especially in coarse textured soils belonging to high rainfall areas and are being intensively cultivated with high yielding varieties and the use of B- free chemical fertilizers.

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