Irrigation and nitrogen studies in s. 23 ryegrass grown for seed

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Abstract
SUMMARY: The effects of irrigation and nitrogen on the soil-water regimes developed by S. 23 perennial ryegrass grown for seed were investigated in 1972 and 1974 by neutron scattering. Boots extracted water from a depth of about 105 cm by early May in 1972 and by early April in 1974. Extraction from greater depths was insignificant in both years. Without irrigation the maximum deficit was about 110 mm in both years. Deficits built up rapidly in 1974 but in 1972 were delayed to the end of the growing season. Irrigation limited maximum deficits in the top 35 cm of soil to less than 25 mm whereas without irrigation deficits reached as high as 40 mm in both years.Transpiration from unirrigated grass was close to that calculated from meteorological data in 1972, but in 1974 measured crop transpiration was only about 60%of the potential throughout the growing season. Roots of grass, whilst undergoing inflorescence initiation, apparently may be impaired in their efficiency of water uptake by deficits as little as 30 mm. For irrigated grass agreement between measured and calculated crop transpiration was poor, principally because of surface run-off. Nitrogen in the absence of irrigation appeared to enhance the rate of crop transpiration in 1972 but comparison with 1974 suggested that this effect was not real. Because of surface run-off resulting in uneven distribution of water, and the restrictions to crop transpiration imposed by limited soil water reserves, actual deficits should be monitored in irrigation experiments rather than relying on estimated values.