HUMUS STATUS OF ARABLE PODZOLUVISOLS AND THE ROLE OF ANNUAL LITTERFALL IN ITS FORMATION

In this work comparative research of the quantitative characteristics of the humus status (HS) of Estonian arable Podzoluvisols (PD) and som e very closely related soils were carried out. This study was conducte d to determine soil humus cover (HC) and subsoil humus content (percen tage and pool) and to analyse the role of plant residues in their form ation. The specific objective was to discuss the ecological factors an d mechanisms which influence the flux of soil organic matter (SOM) thr ough soils and to evaluate its contemporary status. For this purpose d ifferent databases (DB) were used: (1) DB of large-scale soil mapping (1958-78), (2) DB PEDON of pedoecological researches (1967-85), (3) DB of HS research transecis (1986-91) and (4) data from two long-term fi eld trials (1985-92). In the conditions of regular agricultural practi ce the mean SOM percentage in the HC of arable PD is within the range of 1.8-2.1%, which is 0.1-0.3 units (%) the below optimal level and is in a critical state. Impoverishment is caused mainly by deep plowing. The dependence of humus retaining capacity (presumed to be a good cri terion for evaluating the current HS of soils) on the mode and intensi ty of tillage, soil water regime, carbonate content and textural class was discussed. Mean annual inflow of organic matter into HC (4-5 Mg h a(-1) yr(-1)) forms an average 5.5-6.7% from its pool in soil. Applica tion of manures increases this percentage to 12-13. It may be conclude d that the HC might be shallower, but must be richer in humus. The ann ual balance of SOM reflects better than annual balance of humus, the i nfluence of fresh SOM on the soil current year productivity.