All-Russia Institute for Agricultural Microbiology, St.Petersburg-Pushkin, 189620, Russia

Background and objectives

An important method of improving soil fertility and creating an ecologically favourable microenvironment within soil is to enhance and improve the store of organic compounds through application of green manures [1].

Results and conclusions
Experiments over several years have shown that green manures stimulate multiplication in the soil of all trophic groups of useful agronomic microflora including ammondiers, bacteria utilizing mineral nitrogen, oligotrophs, filamentous fungi and actinomycetes. Some of these microorgnisms also produce vitamins and auxins. The use of green manures increases the utilization of mineral fertilizers by plants. Immobilization by microorganisms prevent nutrient being washed out during the autumn-spring period.

Green manures can be regarded as regulators of many biological processes in soil but they do not increase the humus concentration. We have therefore been testing the joint application of green manures with cereal straw to increase humification. Experiments were set up in the following crop rotation: autumn wheat, rape, potato, spring wheat. Rape was ploughed either alone or combined with straw in late autumn when the biological processes in soil have slowed down. In spring, NPK (75 ;kg/ha) was added and microbiological and biochemical studies of soil were carried out immediately after potato shoots appeared. Green manures were utilized by microorganisms as an organic substrate which stimulated microbial multiplication and increased the simultaneous degradation of straw. Straw added alone to soil was resistant to degradation.

The total number of microrganisms in soil treated with NPK, rape, straw, and straw with rape was 52, 102, 75 and 194Xl06 ;cfu/g soil respectively. Substrate-induced respiration was 84, 112, 102 and 138 ;mg CO2/kg soil correspondingly. The activity of cellulose degradation and nitrification also increased. The process of humification was also increased, as shown by an increase in the number of microorganisms utilizing humus in soil (from 1.3 to 4.4, 9.3 and 13.5X106 ;cfu/g soil). Phenol oxidase activity, which is responsible for the synthesis of the polyphenols in humus, also increased from 27 to 34, 36 and 38 ;mg/g soil. The proportion of mobile humus increased, demonstrating an increase in soil fertility which in turn affected the metabolism of potato plants, as shown by an increase in chlorophyll and carotenoids in leaf tissue. Long-lasting effects of straw on the concentration of humus were observed. Addition of green manures to soil also reduced the disease potential by influencing soil fungistasis (the ability to prevent or promote the germination of fungal spores, including phytopathogenic fungi). The addition of green manures before the crop is sown encourages the multiplication of microflora antagonistic to plant pathogenic fungi. These antagonistic microorganisms did not increase in numbers when NPK or straw only was added to the soil.

The decrease in disease potential of the soil through application of green manures reduced infection and increased yield and quality in the same and in subsequent years. The yield of potato increased from 15.6 ;t/ha when NPK was added (19 with rape, 18 with straw) to 22.3 ;t/ha with a combined application of rape with straw.

1. Loshakov VG, Elimer F, lvanov CF, Sinix YM, 1995. Izvestia TSHA 1, 3-15.