Depths at Which Plants Absorb Water and Nutrients. 93 



well illustrated in times of drought, where the vegetation remains unwilted 

 and crops do fairly well even after the water in the surface 6 inches of soil has 

 been nearly or entirely exhausted. 



Preliminary Experiments. 



During 1919, preliminary experiments were conducted in the greenhouse 

 at the University of Nebraska. White Kherson oats was grown in con- 

 tainers 1.5 feet in diameter and 2.5 feet deep. These were filled with a 

 fertile loam soil at an optimum and uniform water-content. The experiment 

 extended from February 4 to April 1. The soil, both in the containers with 

 plants and those without plants (used for checks), was sampled for water- 

 content at intervals of 14 days. During the period of March 4 to 18, when 

 the plants were 28 to 42 days old, a marked loss of water (3.5 to 8.3 per cent) 

 was determined at depths of 1 to 2.5 feet in the containers with plants, while 

 the controls lost only 1.3 per cent at the 1.5-foot level and none at greater 

 depths. Similar results were obtained for the following two weeks' interval, 

 at the end of which (April 1) the plants, then 12 inches tall, were unearthed. 

 Root distribution in the deeper soils corresponded with the amounts of water 

 removed at the various levels. 



Such experiments, while indicative, are subject to two fundamental criti- 

 cisms. Drought in the surface soil might result in a deeper penetration of 

 roots, while water from lower levels might move to higher ones which have 

 been more or less depleted of their supply, either from evaporation or from 

 absorption by the plants. Thus less absorption might take place at deeper 

 levels than the water-content samples would indicate. Moreover, in its 

 upward or downward movement, nutrients might be carried from one level 

 to another by the water. Although the capillary movement of water in 

 soils has been shown by Alway (1913, 1917), Burr (1910), and others to be 

 much less than formerly supposed, experiments were devised to check out 

 all movements of either water or nutrients from one soil-layer into another, 

 except through the roots. In filhng the containers, the soil, after being 

 brought to the desired water-content, was firmly compacted and then sealed 

 with a layer of wax. This consisted of 85 per cent paraffin or parowax and 

 15 per cent petrolatum (Briggs and Shantz, 1912). The seal was applied 

 hot, so that it penetrated a httle into the soil, and when it cooled clung tena- 

 ciously to the soil particles. It varied from 2 to 3 mm. in thickness. When 

 it had cooled and hardened, another layer of soil, usually 6 to 12 inches 

 thick, was added, and the process repeated until the container was filled. Oats, 

 barley, and other plants were grown in containers where the soil was thus 

 separated into hermetically sealed layers of varying thickness. The roots 

 of plants grown in these soils were distributed evenly throughout the soil- 

 mass, penetrating the wax layers without difficulty. In fact, no external 

 differences could be noted between roots growing through the seal and those 

 in ordinary soil (plates 9 b and 9 c). 



This method furnishes at once a means of determining the amount of 

 water or nutrients removed at any given level to which the roots penetrate. 

 Moreover, by using a series of containers in which plants of the same age 

 are grown, it is possible by opening containers from time to time to determine 

 the absorbing activity of the roots at the various levels at any stage in the 

 development of the crop. 



