32 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 



across the swale and up on the other side. The three or four trees of 

 each row which were in the damp soil of the swale were severely 

 injured, whereas the trees in the dry soil of the upper slope were 

 entirely uninjured. In another case a 16-acre orchard on heavy soil 

 had been abundantly irrigated twice during the month prior to 

 fumigation and was very moist at the time of fumigation. This 

 orchard was severely injured throughout, whereas an adjacent orchard 

 of the same soil type which had been without irrigation for so long 

 that the soil was dry and the foliage of a hardened appearance at the 

 time of treatment showed very little injury to any trees. Many 

 similar instances of greater injury on damp soil have been noted. 



The mere wetness of the soil does not in itself offer full explanation 

 of plant injury due to this factor. For instance, the writer has 

 conducted fumigation of orchard trees immediately following a 

 heavy rain with no more injury than to trees in dry soil. Likewise 

 fumigation frequently follows immediately after an irrigation without 

 noticeable damage to the trees. The writer's own observation 

 inclines him to believe that the greater injury to citrus trees in wet 

 soil is induced especially after the plants have been subjected to a 

 very moist condition for a sufficiently long period to set in action 

 forces which change the general metabolism of the plant and result 

 in foliage or fruit so constituted as to be less resistant to hydrocyanic 

 acid. Support to this is presented by the work of Fowler and Lipman 

 (6) on the effect of soil moisture on young lemon trees. These 

 writers concluded that soil moisture in excess of the optimum leads 

 to depressed growth, light colored foliage, and general lack of vigor, 

 the visible damage being greater than if the moisture condition is 

 below the optimum. Of further interest is the statement of Pfeffer 

 {Id) that the supply of water affects the formation of cuticle. Al- 

 though a dry soil tends to slacken growth and hasten maturity of 

 plants, thereby rendering them more resistant to hydrocyanic acid, 

 it has been observed that protracted situation in soil so deficient in 

 moisture that the plant suffers ultimately leads to a physiologically 

 weakened condition. It has been stated elsewhere in this paper that 

 plants in a state of impaired health are more susceptible to injury 

 than normal healthy plants. 



The soil type also appears so to influence the physiological con- 

 dition of the tree that modified reaction to hydrocyanic acid some- 

 times occurs. A 30-acre lemon orchard which was fumigated 

 experimentally in 1918 was about equally divided between two 

 distinct soil types, one a loam designated as a "barren" soil, the 

 other black adobe which contained about 10 per cent humus. Injury 

 occurred throughout this orchard but the degree was noticeably 

 greater on the loam than on the black adobe. Other instances of 

 injury due to different soil types have been noted. Groups of trees 



