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There was a small decrease in grain yield (from 10.8 to 10.5 t ha-') when wheat straw (3 t dry matter ha-') was incorporated into a silty clay loam soil sown to winter wheat. In the absence of straw, 60% of autumn-applied '5N-labelled nitrate was lost from the crop:soil system. Straw incorporation decreased this loss to 47%. There was little overall effect on the uptake of N by the crop, presumably because straw immobilised inorganic N that would otherwise have been leached from the soil during winter. Only 12% of the N in "N-labelled straw was recovered by the crop; 78% still remained in the soil one year after incorporation.

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The recovery of autumn-applied labelled fertilizer N in winter wheat and in the soil and roots was measured in five experiments on three soil types in eastern England. In four of the experiments, crop recoveries of fertilizer N ranged from 11 to 34 % in years when drainage during winter and early spring was close to, or less than, the long-term average of about 200 mm. Crop recovery was higher (42 %) at a site where the soil was heavier and winter drainage was less. Total recoveries (in crop and soil, 0-50 cm) ranged from 22 to 61 %. Fertilizer N was at least risk to leaching when there was a large soil moisture deficit at the time of application. There was a linear relationship between fertilizer N lost and drainage (but not rainfall) between the time of N application and the end of March of the following year. Autumn-applied fertilizer N increased grain yield slightly in two of the experiments and decreased it in a third.

A new method is described for reducing nitrate prior to Kjeldahl digestion of soil and crop samples. Zinc is used to reduce CrIII to CrII, which in turn reduces N03-N. Zinc powder is mixed with the sample, a solution of CrK(S04)2 in H2S04 added and the mixture allowed to stand at room temperature for 2 h. The usual Kjeldahl reagents (K2S04, CuS04 and H2S04) are then added and the digestion completed in a digestion block at 360°C. Nitrate is quantitatively converted to NH4-N, whether on its own, in plant material or in soil.

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We studied the properties of the soluble and dispersed compounds of Cu, Mn, Co, Ni, Pb, Zn, and Cd formed by the action of aerobically decomposing plant matter on the respective metal oxides. The metals were mobilized partly in association with colloidal humified organic matter, and partly in true solution as complexes that seemed to be anionic.In the presence of a clay soil there was no net mobilization of colloidally bound Cu, but the dialysable Cu complex was not appreciably sorbed by the mineral colloids and was leached from the reaction mixture.The metals were not precipitated under alkaline conditions from the dialysable complex forms. Material with similar complexing properties was found in the dialysable fractions of a soil organic matter extract, of water squeezed from a raw peat, and of laboratory lysimeter solutions from a podzol under Calluna.Below about pH 6 the exchange of Cu on a soil clay was not affected by the presence of colloidal decomposition products of lucerne. With Co, Ni, and Zn the corresponding pH value was about 4, and the critical value for Cu in the presence of colloidal soil organic matter was also about 4. Below these values the metal and organic matter sorption curves were diametrically opposed so that under these conditions Cu is apparently not strongly bonded to colloidal organic matter.