Soil nitrogen. II.—Changes in levels of inorganic nitrogen in a clay‐loam soil caused by fertilizer additions, by leaching and uptake by grass

Abstract

AbstractConcentrations of nitrate‐ and ammonium‐N in both the surface (0–9 in.) and the subsoil (9–18 in.) of a heavy clay‐loam were determined at intervals both on uncropped soil and under rye‐grass. On uncropped land, high levels of nitrate were maintained through wet periods which caused percolation through 20 in. of bare soil. Only persistent and heavy rainfall in late summer reduced surface‐soil nitrate to low levels. Ammonium‐N provided by ammonium sulphate or by decomposition of urea was converted to nitrate quite slowly and the soil contained equal amounts of ammonium‐ and nitrate‐N in July. No ammonium‐N was transported into the subsoil. Little total mineral nitrogen was lost from the topsoil of uncropped land during June and July, but during August and September nitrate concentrations in the subsoil increased rapidly. Total inorganic nitrogen in the top 18 in. of soil remained roughly constant until the end of July, after which there was a loss of nitrogen from the profile as a whole. On unmanured plots, fluctuations in ammonium‐N levels were much smaller than corresponding changes in nitrate.Under grass, soil nitrate levels fell quickly in warm wet weather when growth was rapid. In the same period, on plots receiving ammonium sulphate or urea, grass removed ammonium‐N quickly and there was no increase in soil nitrate. When a first cut of grass was taken in July, four‐fifths of the total supply of nitrate‐ or ammonium‐N present in the soil at the beginning of the experiment had been removed. While the second crop was growing there was little nitrate (or ammonium‐N) in the surface soil, but the harvested grass ‘recovered’ as much of the applied nitrogen as the first crop. It is suggested that nitrogen used in growth by the second crop was either taken up earlier and stored in the plant, or that it was taken from the subsoil.