Oct. 1. 1921 Effect of Soil Temperature on Development of Nodules 19 



come only within certain limits. As was expected in advance, the rate 

 of nitrification in soil differs greatly at the different temperatures, 

 producing greatly different concentrations within a short time after a 

 series of plants have been started. In addition, there is soon consid- 

 erable difference in the size of the plants at the different temperatures 

 and a consequent difference in ability to absorb nitrates. The varia- 

 tions which arise from these causes can be limited somewhat by the use 

 of soil low in total nitrogen, thus making impossible the accumulation 

 of large amounts of nitrates in any case. Variations in the concentra- 

 tion of nitrates which have been observed in the experimental work 

 described here will be noted later, and their possible effect upon the 

 results will be discussed. 



APPARATUS AND METHODS 



The apparatus used for the control of soil temperature in these experi- 

 ments is that which has been used in the Laboratory of Plant Pathology 

 at the University of Wisconsin for several years and needs no new 

 description.^ In all cases plants were grown in metal cans 6 inches in 

 diameter and 10 inches deep. The number of plants which could be 

 grown in each can vnthout serious crowding of roots was 3 for soybeans, 

 5 for peas, and 10 for clover and alfalfa. The soil used was a sandy 

 loam from a pasture which had never been cultivated. To this was 

 added about an equal weight of sand in order that the total nitrate 

 content should be kept low and that the mechanical condition of the 

 soil should permit the easy removal of the roots. The temperatures 

 noted in the different series were those at which the water was main- 

 tained in the tanks in which the cans were set. Fluctuations of tempera- 

 ture did not often exceed 1° C. from those given in the tables, and 

 were of only a few hours' duration. Although record was made twice 

 daily of the actual temperatures, it is not believed that a computation 

 of the mean temperature from these figures would give a figure more 

 significant than tlie convenient even numbers used here. It should also 

 be noted that although the surface of the soil was insulated to some 

 degree from loss of heat and moisture by the use of mineral wool, never- 

 theless at the higher temperatures the surface soil to the depth of about 

 I inch was usually cooler by i to i}4° than the water. However, it is 

 believed that the larger part of the roots and nearly all the nodules were 

 sufficiently deep in the soil below this cooler layer, so that error arising 

 from this source is not considerable. 



Water was supplied through a glass tube which passed to the bottom 

 of the metal can where it entered an inverted unglazed flower pot 3 

 inches in diameter, which acted as a reservoir. In the last series the 



> Jones, 1,. R. soil temperatures as a factor in phytopathology. In Plant World, v. 20, no. 8, 

 p. 229-237, 2 fig. 1917. lyiterature cited, p. 236-237. 



