180 THE MONTHLY BULLETIN. 



That this method can not be expected to maintain the fertility of 

 the soil indefinitely is becoming more and more evident. The soil, as 

 well as everything else, requires a period of rest. The soil contains 

 innumerable organisms essentially connected with the process of trans- 

 forming the raw materials into available plant food. 



The second most important method takes into consideration a period 

 of summer cultivation and a period of soil rest during which time the 

 growth of some crop to be used as green manure is permitted. This 

 system is growing in favor each year. The organic content is better 

 maintained by this method whereby yearly additions are made to the 

 soil from the winter crop. The physical condition of the soil is also 

 improved when this bulky material is turned under. Cultivation, 

 high temperature and irrigation all tend to break up this material and 

 supply it to the trees in soluble form. This system is giving very 

 good results at Riverside where citrus trees under this treatment are 

 showing excellent growth and yield. Under the clean cultivated con- 

 ditions adjacent, the physical condition has become so bad that water 

 penetrates the soil with difficulty and the beneficial effect of the fertil- 

 izers applied is becoming less marked. 



The next most important system is that which does away with most 

 of the cultivation and carries with it the growing of both a summer 

 and winter cover crop. This system has been tried to quite an extent 

 in this part of the State. Opinions as to its value vary, but in general 

 this practice is extremely questionable with orchards where the trees 

 are in full bearing. The main advantage of this system is its adapta- 

 tion to young orchards where only a fraction of the land is actually 

 occupied by the tree roots. As most of our virgin soils are very low 

 in organic material, it is highly advisable to grow all the organic 

 material possible during the early life of the tree. This is accom- 

 plished by not only growing a winter crop but by keeping the ground 

 constantly occupied by a crop which may be used as green manure. 

 With orchards of mature age there is frequently insufficient water and 

 plant food in the soil during the summer to permit of the optimum 

 conditions for the tree when in competition with a vigorously growing 

 cover crop. Some of the advantages of this system, however, are that 

 it tends to keep the soil cooler and cuts down the cost of cultivation. 

 There is no question but that these advantages exist, but it remains 

 to be determined whether they are sufficient to overbalance the losses 

 resulting from the competition of the cover crop. 



Another system is that in which a mulch of organic material is 

 employed to prevent evaporation and keep the soil cool, furni.sh organic 

 material and incidentally do away with cultivation. Theoretically, 

 this system is the ideal one, and if all conditions are ideal, success is 

 practically assured. We are, however, dealing with many conditions 

 Avhich are not ideal and which are practically impossible to change. 

 For instance, the temperature and the low Inunidity during the summer 

 months are factors which can not be controlled. It has not seemed 

 practicable to api)ly water in the natural and ideal method, such as 

 might be done l)y an overhead spray. Thus with certain uncon- 

 trollable factors, it is often necessary to do away with other ideal fac- 

 tors because of the changed conditions. In the first place the cost of 

 the material necessary to properly mulch the surface is very high. 



