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



after the acid treatment (1 day before germination), and a number 

 of seedlings were injured. It was evident from the results obtained 

 that these heavy applications of water leached out enough acid mate- 

 rially to reduce acid injury. Leaching is evidently not practicable 

 as a method of preventing injury at most nurseries when germination 

 is prompt. In a sandy soil when the weather is cold and germination 

 requires 18 or 20 days, leaching soon after the application of acid 

 may be a practicable method of preventing injury. 



PREVENTION OF INJURY BY FREQUENT WATERING. 



Fortunately two definite relationships which opened the way for 

 developing a practicable method of controlling the injury to the 

 pines were found. It was found that the amount of water in the soil 

 at the time of germination bore a direct relation to the amount of 

 injury, and that injury seldom occurred after the seedlings had sent 

 their roots down five-eighths of an inch into the soil. The length of 

 root shown in Plate I, figure 3, is typical of injured seedlings in 

 general. The stoppage of growth of root apices in treated beds 

 always occurred at times when the upper soil became relatively dry 

 and while the root tips of germinating seedlings were still in the upper 

 five-eighths inch of soil. Although the nurserymen water the beds 

 often enough to prevent drought injury to the seedlings, great varia- 

 tion in the moisture content of the surface soil occurs. The upper 

 one-fourth inch of soil at this nursery just after watering has fre- 

 quently been found to contain 21 to 25 per cent of moisture, while at 

 the same points the soil when dry has contained but 1.96 per cent of 

 water, the average of 12 determinations made on different occasions. 

 In a single period of 11 hours the moisture content of the surface soil 

 at four different points in the seed beds dropped from 12.02 to 1.85 per 

 cent. This of necessity caused great variations in the concentration 

 of the soil solution. While beds were not ordinarily allowed to 

 become as dry as this during the germinating period, they often 

 became quite dry at the surface. A little below the surface the mois- 

 ture content of the soil is more stable. The most rapid loss of moisture 

 found in the seed beds from 1 to 2 inches in depth during the period in 

 which determinations were made was a drop from 17 to 11£ per cent 

 in a period of approximately 36 hours. This explains the relative 

 safety of roots which have penetrated below the upper half inch of 

 soil. That the root above the tip should resist relatively high con- 

 centrations of acid is in agreement with the results of Heald x and 

 other investigators, who find the tip of the root to be the portion most 

 sensitive to poisons. The difference in resistance between the very 



1 Heald, F. D. On the toxic effect of dilute solutions of acids and salts upon plants. In Bot. Gaz., 

 v. 22, no. 2, p. 130, 1896, 



