Reclaimed fen soils comprise a considerable part of agricultural production in Karelia. They are chiefly used for creating meadow agrocenoses, as unfavourabl climatic conditions limit the possibility of growing other technical crops. One of the principal factors in the development of plants is temperature regime of the soils. Non-frosty period and the air temperature above 10oC in the area of our investigation lasts about 110 days. The sum of temperatures above 10oC is 1300- 1600oC. Snow cover melts by the middle of April. The virgin fen soils freeze down to about 30 cm, drined fen soils - to about 40 cm. Temperature above 10oC in arable horizons is observed for 3-4 months. The number of frosts (mean for 10 years) from May to August 10, 5, 1, 3, respectively.
Unfavorable climatic conditions, low mineralization of graund-waters conditioned, low degree of decomposition and ash content, biological activity of peat deposit. The processes of accumilation and exchange virgin fen soils are most active in the layer down to 10-15 cm.
Drainage and cultivation of fen soils leads to considerable changes in their properties. As a consequence the changes in hygrological conditions, fertilizers application and liming indices of soil firtility improve: all forms of soil acidity decrease, ash content, degree of decomposition, volume mass, biological activity grow. The processes of accumulation and exchange are the most active in arable horizons and the layer down to 40 cm. But at the same time deficit of a number of usable elements constantly remains. Several years after beginning of cultivation reserve of nitrogen reduces, potassium does not accumulate due to its high migrational ability, phosphorus comes into strong contact with organic matter and iron. The latter is conditioned by high content of iron in ground - waters. The plants lack calcium and magnesium, therefore along with liming the soils to increase pH it is necessary to periodically apply low dozes of lime or dolomite meal. Creation of meadow agrocenoses of prolonged use and receipt of planned crops on fen soils in unforable climate become possible only by regular applying of all forms of fertilizers.
A.A. Strelkova Russian Society of Soil Scientists Centre RAS, street Puskinskaya, 11 Petrozavodsk, Karelia RUSSIA Phone: 7-94-10 E-mail: tols@centre.karelia.suI.M. Nesterenko 185610, Institute of Biology Karelian Research Centre RAS street Puskinskaya, 11 Petrozavodsk, Karelia RUSSIA