THE GROUP COMPOSITION OF HUMUS AND FRACTIONS OF FINE-DISPERSED PHASE IN THE SODDY-PODZOLIC MEDIUM-LOAMY SOIL IN A LONG-TERM EXPERIMENT
The group composition of humus in microaggregates (colloidal, pre-colloidal, fine, medium and coarse silt) of the soddy-podzolic medium-loamy soil has been first studied in three five-course grain-tilled crop rotations in the long-term experiment of V.V. Dokuchaev Soil Science Institute. It is shown that in the trial with a higher dose of fertilizers (N150P150K180 kg/ha/year + 120 t/ha of manure) the humus content increases in soil after the first rotation as compared to that with average fertilizer doses (N60P60K60). This is explained by the content of carbon bond to microaggregates of medium and coarse silt. At the same time both experimental variants reveal deterioration of the microstructure including the decrease in the content of medium silt, accumulation of pre-colloidal and fine-dispersed fractions. It becomes evident that the best microstructure is observed in the variant enriched with fertilizer: it contains a great amount of silty fractions and humus components in the form of humates and nonhydrolyzed residues. Moreover, both experimental variants show a tendency towards decreasing the content of humates bond to silty fractions. The intensive system of fertilization in three rotations allowed maintaining the stable content of humus in the fraction of medium silt, the latter being disturbed to a lesser extent than that in the variant without manure application.