Soil & Environ. 25(2): 99-104, 2006 Response of wheat (Triticum aestivum L.) to organic amendment and application of urea A. Lodhi, M.H. Sajjad, A. Mahmood and F. Azam* Nuclear Institute for Agriculture and Biology (NAIB), Faisalabad, Pakistan Abstract Response of wheat to application of plant residues and urea was studied under field conditions. Plant residues with wide C/N ratio had negative effect on plant growth, while those with higher N content and narrow C/N ratio had a positive effect. Urea application did not affect the biomass yield of wheat probably because of the higher native content of mineral N in soil. Physiological attributes to biomass accumulation i.e., greenness and size of flag leaves and the photochemistry of photosystem II were not affected by different soil treatments. It was inferred that photosystem II was more relatively resilient to environmental perturbations including application of high C/N ratio plant residues that would have affected the availability of N to plants. The results also suggested that ample level of native soil N will mitigate the negative effect of plant residues that have the tendency of net immobilization of N. Key words: C/N ratio, Leptochloa fusca, N immobilization, Sesbania aculeata, wheat, soil organic matter Introduction Organic matter content of the soil is an important determinant of soil productivity. Hence, low organic matter content of most of the agricultural soils of Pakistan (Azam, 1988) may be one of the reasons for a low productivity of these soils. The reasons for low organic matter content could be i) a rapid loss under the prevailing environmental conditions that are very conducive for oxidative processes, and ii) no or negligible organic matter returns to the soil. In order to sustain agricultural productivity, a desirable level of soil organic matter needs to be maintained through repeated applications and proper management. The improvement is, however, dependent upon the availability and chemical characteristics of the plant residues applied. Production of biomass on lands which are not suitable for normal agricultural crops could augment the supplies of organic matter, while residues having desirable chemical composition can be selected for use. For example, salt affected soils can be used to raise biomass of salt tolerant plants including both leguminous and non-leguminous. Kallar grass (Leptochloa fusca L. Kunth) and jantar or dhaincha (Sesbania aculeata) are tolerant to salinity; the former being much more tolerant than the latter. Both have good yield potential and can be used as a source of organic amendment. Sesbania can also be raised on normal agricultural soils and used as a source of organic matter and N. However, the two differ in chemical composition particularly the content of N and lignins. Kallar grass is relatively high in lignin content but low in N content, the reverse is true for Sesbania. Residues high in lignin content contribute directly and substantially to the stable humus fractions (Haider and Martin, 1975; Kassim et al., 1981; Stott et al., 1983; Azam et al., 1985) that have proven beneficial effects on growth and nutrient acquisition of plants. However, wider C/N ratio of the residues may lead to temporary or long- term locking up of essential nutrients like N (Ahmad et al., 1969; Azam et al., 1985). Likewise, soil incorporation of residues rich in N result in the release and accumulation of N (Ladd et al., 1983; Fox et al., 1990; Palm and Sanchez, 1991; Azam et al., 1993; Soon and Arshad, 2002). Thus application of sesbania and kallar grass may affect soil fertility and productivity in different ways. Objective of the present study was to assess the role of the two types of plant residues in supporting wheat growth under filed conditions. Materials and methods Plant material Chopped plant tops of sesbania and kallar grass were used for field application after drying. A portion of the chopped material was finely ground and analyzed for total carbon (C), total nitrogen (N), 2N H 2 SO 4 hydrolysable C and hydrolysable N (Table 1). The two types of residues generally differed in most of the characteristics with kallar grass having a higher non-hydrolyzable (lignins and related substances) component and lower N as * E-mail: asim6006@fsd.comsats.net.pk © 2006, Soil Science Society of Pakistan (http://www.sss-pakistan.org)