The relationship of soil mineral NO3-N with stem NO3-N concentration, and of fertilizer-N with the amount of nitrogen taken up by winter wheat, in experiments testing nitrogen fertilizer in combination with aphicide and fungicides, from 1980 to 1982

Abstract

SummaryExperiments on winter wheat were made from 1980 to 1982 to test fungicide and aphicide sprays in factorial combination with four amounts of nitrogen fertilizer, applied in either one or two dressings in spring. The wheat was grown on three farms with contrasting calcareous clay soils from three soil series; each year it followed a 2-year break on one farm, a cereal rotation on the second and continuous wheat on the third. Soils were sampled to a depth of 0·9 m at seedling emergence in autumn, and again in February and April, to determine the NO3-N and NH4-N in each 0·3 m horizon. Crops were sampled for growth analysis at monthly intervals from March onwards and analysed for nitrogen content. Measurements of stem sap NO3-N concentration were also made at 2-weekly intervals from February or March to late June.Measurements of soil mineral N were used to calculate the fertilizer nitrogen dressings used in the experiments. The concentration of NO3-N in the stem sap was related to NO3-N in soil; concentiations remained high until most of the soil NO3-N had been removed by the crop. The time at which stem sap NO3-N concentration declined therefore acted as an index of soil N supply, and the data showed that fertilizer-N was needed when the NO3-N concentration fell below a 200 μg/ml threshold. Yields benefited from N applied in February or March only when stem sap NO3-N concentration fell below the threshold at this time.Apparent fertilizer nitrogen efficiency exceeded 70 % where yields were very large, but ranged between 53 and 64% where yields were smaller because either soil physical problems or disease restraints were present.A severe attack by take-all (Gaeumannomyces cerealis) caused premature senescence at one centre in 1980; this apparently prevented previously assimilated nitrogen from moving into the grain.