Direct drilling and ploughing: their effects on the distribution of extractable phosphorus and potassium, and of roots, in the upper horizons of two clay soils under winter wheat and spring barley

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Abstract
Summary: A study was made in the unusually dry 1975–6 season of the distribution of extractable phosphorus and potassium, and of roots, in the top soil in the second year of direct drilling on a Denchworth series clay (with winter wheat), and in the fourth consecutive year of direct drilling on an Evesham series clay (with spring barley). With both soils there were greater accumulations of phosphorus and potassium in the upper 5 cm with direct drilling compared with ploughing. In the Denchworth soil there were smaller concentrations of phosphorus at all depths below 5 cm with direct drilling and in the Evesham soil this depleted zone was located between 10 and 15 cm depth. There was little extractable phosphorus at depths below 20–30 cm.Measurements of the overall change in concentration of extractable nutrients at each depth between the beginning and end of the season suggested that there may have been contrasting patterns of depletion with cultivation treatments. Depletion was greater between 2·5 and 5·0 cm with direct drilling, and for spring barley on Evesham soil this coincided with the zone in which roots were most abundant. For potassium, there was an increase in the concentration in the 0–2·5 cm zone, apparently due to potassium loss from the maturing crops. At harvest, the total depletion of phosphorus from all zones between 0 and 50 cm depth was appreciably less than data (Cannell & Graham, 1979) on the content of phosphorus in the crop per unit land area, indicating that release from non-extractable reserves in the soil had occurred.Despite the exceptionally dry summer of 1976, crop growth and yield were not adversely affected by the tendency for extractable phosphorus and potassium, as well as roots, to concentrate in the upper layers where desiccation could occur most readily. Possible reasons for this, as well as factors that may contribute to the observed patterns of distribution of nutrients and roots, are discussed.