The Upper Thracian Plain in Bulgarian Agriculture

Abstract

fT 11 ^HE broad valley of the Maritsa River forms the large lowland plain of Upper Thracia which extends east-west between the Rhodope Mountains and Sredna Gora and slopes gradually and almost imperceptibly from elevations of 100 to 200 meters eastward and east-southeastward. The Upper Thracian plain, a structural feature formed in the beginning of the Pliocene, is delimited on the north by a fold and on the south by a series of faults. The hills of Plovdiv (Phillipopolis) are erosion remnants of a peneplain which has sunk, and after the sinking has been overlayed with sediments, thus assuming the character of a real plain. The process continues even at present, as is indicated by the earthquakes of 1928, and the absence of old diluvial terraces. As a result, the landscape reveals every aspect of a typical lowland plain-such an area as provides fundamental conditions requisite for easy irrigation. The plain of Upper Thracia has excellent soil for varied crop production. Loose and mellow alluvium characterizes the river flats, but elsewhere black earth, or chernozem, has developed over the area, which approximates 7,500 kilometers in extent, almost 7'2 per cent of the whole area of Bulgaria. The proportion of cultivated land is substantially higher than in other parts of the country. The climate of the region is distinguished by a favorable positive anomaly of temperature with the result that frost in springtime or autumn rarely damages growth. The growing season is defined in accordance with Table I. The precipitation is not adequate for normal agriculture. The valley lies within a horseshoe-shaped depression encircled by elevations high enough to condense most of the moisture, leaving the valley within the rain shadow. The precipitation for the whole of Bulgaria is 65 centimeters annually, whereas in the Upper Thracian plain it averages only 50 to 55 centimeters. The inadequacy of moisture may be compensated for in large part by irrigation, for which the conditions are so favorable. Table II showing drought indices