WILTING AND WITHERING OF GRASSES IN GREENHOUSE CULTURES AS RELATED TO WATER-SUPPLYING POWER OF THE SOIL

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Abstract
Seventeen species of grasses and two of clover were grown in greenhouse, in boxes of light loam, till well-established turfs had been formed; then watering was discontinued and the subsequent decrease in the water-supplying power of the soil at a depth of about 6 cm. was followed, by means of Livingston soil-point readings taken at intervals, while corresponding records were made of the general condition of the plants, till leaves were withered. Boxes without any plants were included also. After a few days, decrease in water-supplying power was nearly uniform till wilting began, after which it was more gradual. Drought injury generally set in when the water-supplying power of the soil had decreased to about 100, which indicates an initial water-supplying power of about 8.3 mg. per sq. cm. for the first hour of absorption. The onset of "permanent" wilting occurred when the supplying-power value had decreased to about 55 for tall oat-grass (Arrhenatherum elatius L.), to about 43 for timothy (Phleum pratense L.), to about 39 for red-top grass (Agrostis palustris L.). Other forms showed this critical value as between 32 and 55, excepting that sheep''s fescue (Festuca ovina L.) and red fescue (F. rubra L.) still had many apparently healthy leaves when water-supplying power had decreased to below 30. A standardized blackened porous-porcelain atmometer sphere exposed like the cultures showed corresponding corrected water losses, cumulated from cessation of watering to onset of "permanent" wilting, of about 487 ml. for tall oat-grass, of about 662 ml. for Timothy, of about 723 for red-top grass. It is suggested that summed daily water losses from a suitable atmometer suitably exposed (a new summation beginning with each period without considerable rain or irrigation) may offer a means of approximating the water-supplying power of a field soil at any time, without any soil observations. For the Baltimore summer and for lawn grasses this summation amounts to about 375 ml. when drought effects become apparent. Preliminary tests would of course be requisite, to establish the critical summation value in any study, for that should depend on kind of soil, kinds and developmental phases of plants and evaporativity.[long dash]Some similar experiments were performed with potted plants of buckwheat (Fagopyrum esculentum Gaertn.), bean (Phaseolus vulgaris L.), Coleus blumei Benth., hollyhock (Malva sylvestris L.) and sensitive plant {Mimosa pudica L.). In these the decreasing volumetric water content of the soil was recorded from time to time, as well as the decreasing water-supplying power, and these two methods for securing numerical indices of soil-moisture conditions were generally in close agreement. Hollyhock and sensitive plant were peculiarly satisfactory for studies of this type, because of their easily observed leaf responses in early stages of wilting.