A field experiment was conducted under rainfed environment of Pothowar region of Pakistan to assess physical properties of soil as influenced by various green manure legumes (sesbania, cluster bean and rice bean) and different P levels (0, 30, 60, 90 kg P 2 O 5 /ha). Highest fresh biomass was observed in sesbania (23 t/ha) followed by cluster bean (19 t/ha) and lowest in rice bean (17 t/ha). Maximum crude protein content was found in sesbania (17%) followed by rice bean (10%) and cluster bean (8%). Green manuring crops, on average reduced soil bulk density (5%), enhanced total porosity (8%), and macropores and large mesopores (28%). Maximum reduction (7%) in soil bulk density, and an increase (11%) in total soil porosity and available water (17%) was observed in plots where sesbania was incorporated as green manuring crop. The order of effectiveness in improving bulk density, macro and mesopores was sesbania > cluster bean > rice bean. Sesbania produced greater number of macro and large mesopores about 41% increase over control followed by cluster bean (29%) and rice bean (16%). Phosphorus application showed meager positive impact on various soil physical properties but did not significantly increased porosity or reduced bulk density. Significant differences in volume fraction in pore space suggest that pore space on a volume basis was much higher in sesbania-amended soil than in other treatments.

The very significant yield increases obtained by adding NH 4 to N0 3supplying hydroponic cultures have, for unknown reaso1~s, not been duplicated in soil systems. This research was conducted tu consolidate and extend our knowledge of the effects of added NH. on plant processes, and to define the plant and culture characteristics requisi1te to optimal utilization of mixed N sources. Wheat (Triticum aestivum L. cv. Anza) and tomato (Lycopersicon esculentum Mill. cv. 6718 VF) were grown in solution cultures providing variables of N source (N0 3 , NH 4 , and NH 4 N0 3 ), culture buffering, and concentration of NH 4 and NH 3 (aq). Treatment effects on plant growth and composition were evaluated. Wheat grew best in NH 4 N0 3 culture, equally well in ]110 3 or NH 4 cultures buffered at pH 6, and poorest in unbuffered NH 4 cultures. Growth enhancement with NH 4 N0 3 was accompanied by increased N intake and synthesis of protein. Lowered yields in unbuffered NH 4 cultures were associated with H + injury to the roots and reduced intake of N and nutrient cations. Toxic effects of NH 4 pel' se occurred when culture concentrations exceeded 16 mmol L -• for nots. Severity or toxicity was associated with abnormally high root and shoot amide concentrations, and tissue levels of unassimilated NH 4 exceeding 50 mmol kg-'. In alkaline cultures, NH 3 (aq) concentrations greater than 0.028 mmol L·' for tomato and 0.15 mmol L·' for wheat reduced top growth, and destroyed the structure and functioning of roots. These detrimental effects of NH 3 (aq) were related to its concentration in the culture, not to tissue NH 4 • Y IELDS OF SEVERAL CROPS have been increased by 40 to 70% by adding NH 4 to NOrsupplying hydroponic cultures (Weissman, 1964;Cox and Reisenauer, 1973;Blacquiere et al., 1988; Herberer and Below, 1989). In contrast, enhanced NH 4 supply in greenhouse soil cultures has increased crop yield an average of only 11% (Malhi et al., 1988;Camberato and Bock, 1990;Hagin et al., 1990), and in field experiments with corn (Zea mays L.) summarized by Hoeft (1984) by a mere 7%. The reasons for this wide discrepancy in data from hydroponic and soil systems are not immediately apparent. Both N0 3 and NH 4 are readily absorbed by plants and their N utilized as a constituent of identical organic compounds. They differ in that their absorption and metabolism require different ionic equilibria in which both organic and inorganic cations and anions have a part (I

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The study was designed to investigate the effect of cold irrigation water on rice growth and on uptake of N, P, K and Zn applied ca. 50, 20, 35 and 5 mg kg−1 soil, respectively. Cold temperature of irrigation water reduced rice shoot and root dry weight and plant height, significantly compared to hot temperature treatment. Under low temperature stress N was a major rice growth determinant. Increased shoot concentrations of both P and Zn allevated the low temperature stress. The uptake of N, P, K and Zn reduced significantly at low temperature (16.5–20 °C soil and 20–24 °C flood water) compared to high temperature (24.5–27 °C soil and 26.5–29.0 °C), with the strongest effect being noticed for N, followed by P, K and Zn. Application of N, P, K and Zn increased their uptake in rice shoots. Nitrogen and K had synergistic effect on their uptake. Responses to N and K application and their uptake behavior were well marked at higher than at low temperature whereas reverse was true for P and Zn.