12 BULLETIN 169, U. S. DEPARTMENT OF AGRICULTURE. 



Bureau thermometer under a shelter 4 feet from the ground, 

 from 37° F. at 8 a. m. to 98° F. at noon of the same day in April. 

 The evaporation from white porous-cup atmometers set in the seed 

 beds has varied from 14 to 59 c. c. for 24-hour periods 10 days apart, 

 and still greater variations are to be expected from the darker 

 soil surface. Hot, dry days increase the danger from acid injury 

 both by increasing water loss and consequent acid concentration 

 and by hurrying germination before the acid solution in the upper 

 soil has had much time to decrease in strength. In view of the 

 variability of weather conditions, the system now followed in prevent- 

 ing acid injury is to water daily in ordinary spring weather, every 

 other day or even less often in misty or rainy weather, and twice 

 daily when the temperature exceeds 80° F. In clear weather, 

 waterings are to approximate 0.3 of an inch, while in cold and cloudy 

 weather 0.2 of an inch is to be used. This watering system has proved 

 practicable, and has been entirely successful in preventing injury 

 to pines from acid applied at the time of sowing. 



RELATION OF STRENGTH OF TREATMENT TO EXTENT OF INJURY. 



The degree of dilution of the sulphuric acid in applications at 

 sowing had no apparent relation to the amount of injury likely to 

 result to the seedlings; that is, if 0.25 ounce of acid per square foot 

 was applied, it made no difference, so far as noticed, whether it was 

 dissolved in 64 or 192 volumes of water. There was not a sufficient 

 number of tests with this factor as an independent variable to estab- 

 lish an entire lack of relation, but it is quite certain that within the 

 limits given the amount of water used in making up the solution is 

 not an important variable. 



The first results indicated a rather surprising lack of constant rela- 

 tion between the amount of acid used per unit of soil surface and 

 the amount of injury. In an early test of varying amounts of acid, 

 all of which caused considerable losses of seedlings, the final stands 

 in the plats were as follows: 



Series 501. — Jack-pine plats; all except the check plats were treated with acid at 

 sowing. 



Eight check plats untreated. Final stands ranged from 71 to 163 per square foot; 

 average, 122. 



One plat, 0.125 fluid ounce of acid per square foot at sowing. Final stand, 216. 



One plat, 0.141 ounce of acid. Final stand, 118. 



Two plats, 0.188 ounce of acid. Final stands, 191 and 143; average, 167. 



Three plats, 0.234 ounce of acid. Final stands, 107, 110, and 80; average, 99. 



Two plats, 0.250 ounce of acid. Final stands, 23 and 153; average, 88. 



One plat, 0.313 ounce of acid. Final stand. 94. 



Two plats, 0.375 ounce of acid. Final stands, 11 and 116; average, 64. 



In this series, as in those reported in the remainder of this paper, 

 the plats received weights of seed proportional to their area. 



