Nitrate and ammonium in the rhizosphere of wheat crops and concurrent observations of take-all

Summary In 1973 the concentration of nitrate nitrogen in the rhizosphere of winter wheat (NO₃-N _r , extracted withN-K₂SO₄ from soil from within 5 mm of the roots) growing in slightly acid, flintly loam decreased progressively from January until early April, when it became less than that of ammonium nitrogen in the rhizosphere (NH₄-N _r , extracted withN-K₂SO₄). An April application of ammonium nitrate (as Nitro-chalk) augmented NO₃-N _r more than NH₄-N _r , but because NH₄-N _r was depleted less quickly it exceeded NO₃-N _r from early June onwards. From January to April pH _r remained between 6.9–7.2, but then decreased progressively to a minimum of 6.3 in June. The differences in soil pH and forms of N amongst plots that had grown three, six or fourteen consecutive cereal crops susceptible to take-all were small; a first wheat crop differed only in having less extreme changes in NO₃-N _r during April-June.First winter wheat crops were grown on a similar soil in 1974–5, either with farmyard manure applied in autumn (FYM plot, bulk soil pH 7.4) or Nitro-chalk applied in April (NPK plot, pH _b 6.7). Soil nitrate decreased considerably in May and to a lesser extent in June and was always at a lower concentration in the rhizosphere. NH₄-N _r was more than NO₃-N _r in water extracts after May in the FYM plot and after June in the NPK plot, but NH₄-N _b never exceeded NO₃-N _b .There was, therefore, a tendency for more of the dwindling mineral N to exist as NH₄-N close to roots in spring and summer unless temporarily disrupted by spring fertilizer. Take-all developed against this background was mostly slight and never severe. In 1973 it was least in the first wheat, and most in the third, but the differences were small and disease increased steadily in all plots after mid-February. In 1975 it developed quickest and most in the FYM plot. re]19760928