Effect of pH on nitrogen immobilization in two California soils

Summary The effect of pH on nitrogen immobilization from tagged NH₄Cl and KNO₃ was investigated in two soils. Although the ammonium source was utilized by the soil population preferentially at all pH levels in both soils, there was a marked influence of pH on the quantity immobilized. Nitrogen immobilized from the ammonium source increased, as pH increased, whereas the reverse was true of the nitrate source. These results appear to be related to the physiological acidity or alkalinity of the nitrogen source.Most of the nitrogen incorporated into the organic fraction was present in the amino form.Contribution from the Department of Soils and Plant Nutrition University of California, Davis, and the Department of Vegetable Crops, University of California, Riverside. This work was supported in part by a grant from the National Science Foundation.     

A survey of potato aphides in north-west Derbyshire, 1945

Counts of aphides infesting potato crops in north-west Derbyshire at altitudes between 340 and 1360 ft. were made during the summer of 1945. Mysus persicae was abundant on almost all crops, and peak figures on some reached over 1500 per 100 lower leaves. In many fields two peaks were noted, one towards the end of July and the second in mid-September, and in some fields there were still heavy infestations when the haulms died. The number of aphides varied considerably from field to field; sheltered fields were the least infested. In this area in 1945 there was no evidence to support the contention that the altitude and aspect of the field have any influence on the intensity of infestation. Tuber samples from six fields showed that there was a considerable spread of leaf roll.