The toxicosis of podzolic soils

Abstract

Summarizes the results of studies all the year round in 3 successive years (1952-54), mainly near Chashnikovo, Moscow province, of soils: (1) under various types of forest cover, (2) from grassed-over clearings, (3) from clearings recently brought into cultivation, (4) from clearings long cultivated, and (5) under forest regeneration. Tests were made of the toxicity of soil, soil extracts and pure cultures of members of the soil microflora, against Azotobacter and seed and seedlings of crop plants-chiefly wheat and beetroot. Azotobacter, absent from uncultivated and forest soils, perished very rapidly when introduced into any but long and well cultivated soil, and especially rapidly in (1) and (5), where the life of the cells was only 2 hours. Seed and seedlings treated with aqueous soil extracts showed the same trend, soils (1) and (2) [(5) not mentioned] causing the most inhibition of germination and growth, and wheat being much less sensitive than beet, while peas were unaffected. Of the forest soils, those from under Norway Spruce or Aspen were the most toxic, and those from Birch or Oak woods much less so, Azotobacter and beet seed again being the most sensitive. Peak toxicity occurred from July to Oct., with a marked fall in winter and spring. That this difference was not due to more intense leaching in the latter period was shown by tests in which soil leached 4-5 times was still about as toxic to Azotobacter (as was also the first leachate) as before leaching. On the other hand, freezing at -15 to -20oC. for 2 months greatly reduced soil toxicity towards Azotobacter, though even autoclaving at 120oC. for 30 min. did not effect inactivation of the Chasnikovo soils. Application of CaCO3, bringing the pH from 4.6 to 5.6-6, relieved the toxicity of forest podzols to Azotobacter; with application of MgO or NaOH, bringing pH to 6-7.4, the former enabled the bacterium to survive, in some quantity, to > 14 days but with no multiplication, while the latter gave survivals lasting not more than 5 days, hence the effect of liming cannot be simply that of adjusting the pH. In field conditions liming relieves toxicity to a lower degree, and manuring with stable manure has about the same effect. As micro-organisms were believed to be largely responsible for the toxic effects, microbiological analyses were made of the Chasnikovo soils. Of the organisms showing inhibition of Azotobacter and/or beet seed germination, (a) 50 pure cultures represented bacteria, (b) ca. 300, actinomycetes, and (c) 50, fungi. (a) The most toxic to Azotobacter were cultures of sporebearing bacteria of the Bacterium mesentericum and B. subtilis groups, and, more rarely, B. cereus and some undetermined species. Toxic non-sporebearing types included Pseudomonas pyocyanea, P. flurescens and some others. Of these groups some inhibited Azotobacter, some beet germination, and some, both. The number of such bacteria varied from 5 to 450 thousand per g., depending upon locality and the condition of the soil, forest soils containing the most. The distribution of (b) was more or less uniform in the various soils, but (c)-referred to the genera Fusarium, Cladosporium, Verticillium and others-were most abundant in forest soils (1.4-17 thousand per g.). Of the microorganisms, many more inhibited Azotobacter than those acting upon the higher plants. Some cultures of Actinomyces griseus stimulated germination of wheat while depressing that of beet