Abstract
The lower four and eight subsurface horizons of the Gujranwala and Satghara soils, respectively were separated by wel sieving into 15-2 mm, 2-0.05 mm (sand) and <0.05 mm (silt and clay) size fractions. Water fractionated material was 66 to 82% silt and clay in the Gujranwala and 88 to 96% in the Satghara. Sand (2-0.05 mm) fraction was 16 to 18% for the Gujranwala and 1 to 8% for the Satghara. The 188-208 cm (deepest) layer itsSatghara series was an exception containing 45% of silt and clay and 54% of sand. Nodule fraction (15-2 mm) was 0.4 to 9% of the whole soil in all the horizons. Potassium and ferrous were mostly concentrated into silt and clay size fraction of both the soils. But manganese was more concentrated in coarser (sand and nodule) fractions. There was a significant positive correlation of total zinc concentration with Fe and Mn concentrations in 15-2 mm fracion.
INTRODUCTION
Some of the changes rclatcd to zinc nutrition of rice are increased concentrations of liCO3, Mn and Fe in solution and increased partial pressure of CO2 and soil pH changes towards neutrality (Ponnamperurna, 1%5; Rehmatullah ci at, 1976). Increased concentrations of Ca, Mg, FICA, Mn and Fe have been observed to antagonize the uptake of Zn by rice plants in solution culture Worn° et al., 1975; Rashid el al., 1976). Nevertheless, ions in solution in saturated soil situation are under continuous gravitational pull to move to lower soil horizons. Sajwan and Lindsay (1986) have hypothesized that Fe in solution diffuses to more oxidized zones in soil and gets precipitated as ferric hydroxide, Fe2(OH)63 which may depress Zn activity through the precipitation of.ZnFei0.1 or a frankliniielike solid material.
In addition the higher partial pressure of CO2 and the lower redo x potential of flooded rice fields are conducive for precipitation fMn as Mn CO3, MnS, (Ca, Fe, MnODA ) and Mn304 etc. (Ponnamperuma, 1972; Jenne, 1968). Adsorption, co-precipitation or occlusion of trace metal ions during crystal growth 01 Fk: and Mn oxides can be further accentuated by the precipitation of carbonates under calcareous soil conditions. Carbonaceous concretions containing some Fe and Mn have been reported in subsurface horizons of two important rice growing soils (Gujranwala & Shaighara series) of Pakistan (Rahmatullah et al, 1990). The present paper reports the extent of these concretions in the subsurface horizons and their chemical composition with possible implications on Zn nutrition of flooded rice.
