THE EFFECT OF CLAYING CARRIED OUT IN 1923 UPON THE THERMAL CONDITIONS IN CULTIVATED SPHAGNUM PEAT SOIL IN THE SUMMER OF 1939 Yrjö Pessi Society of Peat Cultivation Experimental Station Leteensuo Received October 12. 1959 The addition of mineral soil to cultivated peat soil as a soil improving agent produces changes in the thermal conditions prevailing in the soil (4, 3,5, 6, 12, 6, 2, 13, 7). However, the experiments in the study of this question have mostly been carried out with only a few years at the most having passed since the soil improving agent was added. Now the addition of mineral soil as a soil improving agent on cultivated peat soil has a prolonged effect, as has been made evident by the crop yield results obtained in a number of soil improving tests (1, p. 12, 11, 8, p. 16). At the experimental Station Leteensuo a claying test was established in 1923, and the claying has not been repeated since in the test area. This test consequently provides an opportunity to study the significance of claying from the point of view of the thermal conditions in the soil after a long period. The investigation was car- ried out in the summer of 1959, 36 years after clay had been applied to the test area. Test area and plan of experiment The peat of the test area mainly consists of Sph. fuscum moss. The peat layer has a thickness of about 3 metres. In connection with its clearing for agriculture, the test area was drained by open ditches so that strips 20 m in width were formed. The clearing was done with a hand-hoe in 1921. The investigation was carried out in connection with a Sphagnum bog liming and claying test established in 1923. The detailed plan of the experiment has ben described before (8). Similarly, an account has been given of the effect of claying upon the settling of the soil surface (9) and of the effect of liming upon the soil temperature in the unclayed area (10). One half, in the longitudinal direction, of the 20 m wide test strip was clayed, adding clay in a quantity of 300 m 3 per hectare. 2 The liming was combined with this test by applying varying quantities of lime to areas stretching across the layed and unclayed strips (for details, see 8, p. 3.). The annual fertilization was equivalent to 100—300 kg superphosphate, 200 kg 40 % potassium salt and 100—300 kg calcium nitrate per hectare. Stable manure was given in a total quantity of 51 tons per hectare. The test plant in 1959 was barley, which was sown 12. V. and harvested 6. IX. Results The studies were carried out with the aid of a thermometer specially designed for the measuring of soil temperatures, which has been descriped in detail in a previous publication (10). Measurements were made during the summer months at intervals of about two weeks. The measuring depths were 10, 20 and 50 cm. As measurements were taken only once on each day of measurement, at about 14.00 hours, it was not considered necessary to measure the temperature close to Temperature on days of observation in clayed peat soil (t f), temperature differences between clayed and undayed soil (tj tg) and P-values of temperature differences. Date 10 cm 20 cm 50 cm tj t,~-t0 P-value t, t, —lO P-value t, t, —l O P-value 15. V 13.00.8 0.00910.7 1.2