10 STATE BOARD OP FORESTRY. 



is not of first value for agricultural purposes. These abound in the 

 valleys and on the slopes of the lower hills throughout California. The 

 chemical composition of the soil is of little importance, so long as an 

 excess of injurious chemicals does not occur. The physical properties 

 of the soil, such as permeability, retentiveness, etc., are vastly more 

 important. 



Temperature. No one factor has so much influence in governing the 

 extension of Eucalyptus as temperature. The fact that its planting 

 range is practically restricted to California is wholly attributable to 

 the frost-tenderness of the genus. During the seedling years the danger 

 from frost is greatest. The susceptibility of the sugar gum seedlings 

 to frost injury is particularly marked. 



Thermometer readings of temperature are not always true indices 

 of the influence of frost on a tender plant. Much depends upon the 

 condition of the plant at that particular time. If a period of warm 

 weather is followed by a sudden drop in temperature, the plant suffers 

 more than it would if an equally low temperature had come gradually. 

 The exposure of the plantation is likewise important. If the plantation 

 has an eastern exposure where it is reached by the first rays of the sun, 

 the damage will be greater than if it remains in partial shade while 

 the temperature ascends slowly. 



The following order indicates the frost-hardiness of the timber 

 eucalypts very closely : Red, gray, manna, blue, sugar, and lemon gum. 



Moisture. A rapid growing plant, like anything else which grows 

 rapidly, requires lots of food. This is taken up by the roots in the 

 form of mineral salts in solution and elaborated in the leaves. Hence, 

 the tree can not get its food unless its roots can obtain water. There- 

 fore, other conditions being equal, the rate of growth depends directly 

 on the amount of water the plant secures. While eucalypts will grow 

 where the soil moisture is deficient, the rate of growth will be much 

 slower than where the tree gets all it needs. For commercial purposes 

 it is unwise to attempt to produce Eucalyptus where the soil moisture is 

 markedly deficient. A fairly retentive, deep soil which receives an 

 annual rainfall of from 25 to 30 inches will produce good trees. 



Deficient soil moisture is supplemented by atmospheric moisture in 

 the fog belt, where the fog is condensed by contact with the foliage. 

 During a heavy fog the foliage of gum trees drips as if from a shower, 

 and the ground under the trees becomes soaked overnight. The 

 occurrence of fog accounts largely for the excellent growth of Eucalyp- 

 tus on sandy soil at the Presidio, on the Piedmont Hills, and elsewhere in 

 situations, otherwise inhospitable, throughout the Bay counties. Fogs, 

 likewise, make the Los Angeles plain one of the favorable regions in 



