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The effect of adding clay, moisture and the compaction on the physical properties of peat as well as the effect on transpiration and growth of t o m a t o plants were investigated during short term laboratory experiments. The effect of 6, 12, and 20 cm peat height in beds with constand water level were investigated in a 5 months t o m a t o crop.A d m i x t u r e of clay reduced the porosity of the peat. Adding the clay at moderate pressures (10, 100 g c m -2) decreased the a m o u n t of large pores (equivalent to pores emptied at p F 2.0) and to an increase of smaller pores (equivalent to pores emptied in the p F intervals 2.0-4.2). The reduction in the a m o u n t of large pores was greater with admixture of montmorillonite than with a soil clay. An increase of the soil clay content from 50 to 70 per cent had an insignificant effect on the a m o u n t of large pores. Increasing the moisture content of the peat resulted in that fewer pores were emptied at p F 1.3. A t a pressure of 1000 g cm-~ the decrease of large pores was more marked for pure peat than for mixtures of peat and soil clay. Increasing the pressures from 10 to 100 g cm -2 had only a slight effect.For a mixture of peat and montmorillonite a linear relationship was found between the clay content and bulk density, while a curvilinear relationship existed for a mixture of peat and soil clay.In short term laboratory experiments with tomato seedlings transpiration and growth rate were at a m a x i m u m at a p F equivalent to 20-30 per cent airfilled pores. In a glasshouse experiment with tomatoes an increase was recorded in yield, size of fruit, leaf area and total dry m a t t e r production with increasing peat heights above a constant water level.The experimental results showed t h a t porous root media, such as peat, should be treated to counteract the existence of zones having sub-optima] air content by use of sub-irrigation.