SOIL CONDITIONS 275 



resistance of the crop to attacks by the insect in question. More 

 recently the same subject has been re-investigated in CaUfornia 

 by Nougaret and Lapham (1928), since it appeared possible that 

 if certain soil types in that state are inimical to Phylloxera 

 propagation, a certain degree of control might be achieved by the 

 wider planting of vineyards under such conditions. These observers 

 found that infestation appears to be particularly frequent and 

 pronounced in flat or depressed localities, where inferior soil 

 drainage is aggravated by the occurrence of shallow, compact and 

 impervious subsoils which readily cake, giving rise to a condition 

 of shallow-rooted vines and stagnated soil drainage. Where 

 infestation occurs in deep friable soils, with porous subsoils, 

 destruction of the vines is less rapid than in more shallow soils. 

 The texture of the soils appears to be a factor which exerts itself 

 in a number of ways. It is suggested that in those of a heavy 

 compact type, which cake, shrink and crack when dry, conditions 

 are rendered more favourable for the development and migration 

 of the root-forms of the insect. Facilities for movement, it is 

 claimed, are afforded by intersecting cracks, and along the courses 

 of the roots, from which the soil tends to withdraw slightly in 

 shrinking. If forced to the highly heated surface of sandy soils, 

 which do not cake and crack, and which may be more uniformly 

 in close contact with the roots, the delicate and minute insect 

 would not long survive under the hot arid conditions to which 

 many of the interior valleys of California are subjected. Various 

 soil types in relation to Phylloxera infestation are discussed and 

 the whole area shown in a coloured map. Their bulletin is too 

 lengthy for detailed discussion here, but it may be said that the 

 authors conclude that the correlation between susceptibility to 

 Phylloxera infestation and soil types is a tolerably close one. In 

 certain areas of light-textured soils it is recommended that 

 planting can be extended with a wide margin of safety, while 

 in others, at present free from infestation, the soil types are such 

 that if the insect became established, immunity from attack 

 would not probably be wholly maintained. Loams and soils of 

 heavy texture are favourable to general infestation, and on such 

 lands areas of widespread and long-standing attacks prevail. 



