Poor Early Growth of Wheat Under Direct Drilling

Abstract

Poor early growth of wheat under direct drilling on a hardsetting duplex soil was studied in the light of a range of soil physical and biological properties. Two systems of direct drilling were included in the study: one with a short fallow maintained by herbicide (chemical fallow), and another in which a fallow period was absent and herbicide was applied 1 week before sowing (nil fallow). Plant measurements indicated that the poor early growth observed under both direct drilled systems, as compared to that under conventional cultivation, was not due to poor germination or poor emergence. Rather, it was shown to be a consequence of reduced growth after establishment. Weight per plant measured 64 days after sowing for the conventional, chemical and nil fallow treatments was found to be in the ratio of 3.2: 1.8 : 1.0, respectively. Soil physical measurements during the 9 weeks from sowing indicated that moisture availability was unlikely to be an important factor affecting the observed growth difference for the particular season. Much higher bulk density (1.66 versus 1.35 Mg/m3 at 50-100 mm) and vane shear strength values were found in the undisturbed soil between the drill rows in the top 100 mm of the two direct drilled treatments. Vane shear strength measured in the top 50 mm layer of the direct drilled plots was up to 2.9 times higher between the drill rows than in the drill rows. The poor vegetative growth on the chemical fallow plots was probably caused by restricted root growth in the denser and stronger 0-100 mm depth of undisturbed soil. The poor early vegetative and root growth of wheat in the nil fallow could not be fully explained by the soil physical properties, but indicated the presence of other root inhibitory factors. Our results suggest that one such factor is the presence of inhibitory Eacteria on the roots.