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Phosphate rocks (PRs) are suitable for direct application as a possible alternative to more expensive soluble phosphate fertilizers in agricultural fields. But the ability of the PRs to release phosphates in the plant available forms depends on the particle size and chemical and mineralogical characteristics of the PRs as well as the properties of the soil in which they are applied. So an experiment was conducted with four sources of phosphatic fertilizers namely Triple super phosphate (TSP-21.75%P)), Partially acidulated phosphate rock (PAPR-12.97%P)), Morocco rock phosphate (MORP-14.87%P)) and Mussoorie rock phosphate (MRP-8.12%P) whose solubility were tested in six different extractants namely 2% Citric acid, 0.002N Hydrochloric acid, N-Ammonium citrate, Bray-2P extractant, Olsens's extractant and Morgan's reagent under seven periods of incubation (1, 2, 3, 7, 10, 15 and 30 days), with and without soil. The results revealed that release of P were increased on addition of soil irrespective of fertilizers or extractants used. TSP released maximum P (3.05%-3.27% with soil, 2.11%-2.22% without soil) by the 7th day of incubation. The partially acidulated source was found to release P, higher than rock phosphates but lower than TSP, for the initial periods of incubation (1-3 days) (1.31%-1.34% with soil, 0.46% without soil) with an increase in the later periods (7th day onward) (1.27%-1.92% with soil, 0.55%-0.66% without soil). The PRs released maximum P after the 7th day of incubation. Among the different solvents, maximum release of phosphorus was observed by 2% citric acid followed by Bray 2P and Olsen's extractants.

The Relative Agronomic Effectiveness (RAE) of rock phosphates as compared to water soluble Triple Super Phosphate was measured on direct, residual, and cumulative application of the P fertilizers in a field experiment with rice on an Oxic‐Rhodustalf in the eastern plateau region of India. The fertilizers were Morocco Rock Phosphate (MORP), Mussoorie Rock Phosphate (MRP), Partially Acidulated Rock Phosphate (PARP), and Triple Super Phosphate (TSP). The RAE of the rock phosphates were lower for direct application (54–80 %) and cumulative application (70–93 %) of P but roughly equal or larger for the residual effect (92–142 %) as compared to TSP. The P adsorption characteristic of the experimental soil conformed to the linear relationship of both Freundlich and Langmuir isotherm equation. The adsorption data when plotted according to Langmuir equation deviated from a single linear relationship at higher concentration (10 μg ml–1), thereby giving two adsorption maximum values ( 68.49 μg g–1 and 256.41 μg g–1) and binding energies ( 2.86 ml μg–1 and 0.089 ml μg–1) for the soil. Two populations of P adsorption site with widely different affinity for P probably existed in the soil.

The P-fixing capacity of a soil governs the P-nutrition of crop plants. P-nutrition of the crop plant is more a soil problem and a higher dose of phosphatic fertilizer is necessary for soils having high P-fixing capacity. The phenomenon of P-fixation and the great variation in the Pfixing capacity of different soils has thus important bearing on crop response to P-application. The eastern plateau region of India with acid lateritic soil is chronically deficient in available phosphorus resulting in very low productivity. An experiment was thus carried out to estimate the P-fixing capacity of soil collected from two depths, 0 -20 cm and 20 -50 cm, from the Agricultural experimental farm of Indian Statistical Institute, situated at Giridih, Jharkhand, in the eastern India. The soil was acidic in reaction (pH-5.4) with presence of Fe (1.60%) and Al (17.2%). The P-fixing capacity of the soil was estimated to be 59.60% and 64.94% for the surface and the subsurface soil respectively showing lower P-fixing capacity of the surface soil as compared to the subsurface soil which may be due to presence of more organic matter in the surface soil as organic molecules released on decomposition of organic matter complexes with Fe and Al in the soil thereby blocking the P-fixing sites in the soil.

The nitrogen content of weeds was significantly higher than that of Dular rice-straw, but with Taichung Native i rice in winter there was no significant difference between the nitrogen in weed samples and rice plants at the tillering stage. Rice straw contained a higher proportion of nitrogen where there were three hand weedings. The total dry matter of rice was much reduced by weed competition. Weed species contained much nitrogen at the vegetative, flowering and post-flowering stages, indicating severe competition for nitrogen by weeds throughout the growth of rice under dry land conditions. Competition between cereal crops and weeds is principally for nitrogen in years of normal rainfall, with the most intense competition in the early stages of crop growth (Blackman and Templeman, 1938), though Singh and Singh (1939) found that maximum uptake of nitrogen by weeds occurred at the pre-flowering stage. Legumes are generally rich in nitrogen, but Campbell (1924) observed that of all the weeds examined under average field conditions, Amaranthus retrojlexus. L. (pig-weed) contained the highest proportion (3-7 per cent) of nitrogen. Asana (1951) reported the uptake of 19-5 kg. nitrogen/ha, by wheat (Triticum aestivum L.). Mukhapadhyay and Verma (1967), conducting crop/weed competition trials for wheat, stated that the elimination of weed competition at the early stage of the crop (first 4 weeks) led to higher uptake of nitrogen and heavier yields of wheat compared with later weeding at 6 weeks from sowing. The present experiment was therefore undertaken to study the extent of weed/rice competition under different weed control methods in relation to nitrogen. MATERIALS AND METHODSThe field trials were conducted at the Sriniketan Farm, Visva-Bharati, in laterite sandy loam soil at pH 5-5-6-O. The soil had 0-038 per cent nitrogen and 0-3 per cent organic carbon, and was poor to very poor in phosphate. The trials were conducted with various combinations of chemical and mechanical methods of weed control in the rainy (kharif) season of 1967 and 1968 on autumn rice, variety Dular (a photo-insensitive, short-duration (90 days) upland rice variety of West Bengal) under dry upland conditions. There were twelve treatments (Tables 1 and 2) replicated three times in a randomized block design. In 1968 a similar experiment was repeated except that the MCPA, MCPA -f MCPA, MCPA + one hand weeding and 2,4-D treatments were omitted because of poor results in 1967,

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