II. WATER AND NUTRITION SUPPLY IN RELATION TO PLANT - MOISTURE SUPPLY AND FERTILIZATION OF POT PLANTS

In a pot experiment with gloxinia five nitrogen rates were compared at three moisture levels. The nitrogen rates were 100, 200, 300, 400 and 500 mg N per litre substrate. The soil moisture levels were 90, 65 and 40% of the substrate water capacity.

The salt concentration of the substrate solution seemed to be much affected by both moisture supply and N rate, implying that total suction rather than matric suction should be regarded as representative of soil moisture availability.

Average yield of aerial parts, highest at 90% water capacity, decreased markedly with lowered moisture supply.

Increase in soil moisture stress also depressed yield increment per unit nitrogen applied. This reduction in nitrogen effect represented a decrease in efficiency of nitrogen utilization of the plant, attributable to a disturbance in nitrogen metabolism in the leaf at rising suction tension in the leaf.

For optimum vegetative growth and nitrogen effect, measures to be taken to control leaf suction tension throughout the growing season, should extend over both soil and climatic conditions.