188 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



Other results of interest are that wheat phints make a more satis- 

 factory growth with an intermittent food supply than when the 

 supply is continuous, that pure ammonia is very toxic to the growth 

 of wheat seedlings, a fact that has a bearing on the deleterious 

 effects often noted from the use of fresh manure, and that the 

 phosphorus in raw rock phosphate is made available by the presence 

 of carbon dioxid. This last fact furnishes an explanation of the 

 beneficial results of combining manure with raw rock phosphate, 

 the bacteria of the manure liberating the necessary carbon dioxid. 



CHLOROSIS DUE TO MAGNESIUM DEFICIENCY. 



In further work on the significance of magnesium as a constituent 

 of fertilizers for tobacco and other crops it has been found that corn, 

 like tobacco, develops characteristic pathological symptoms on cer- 

 tain soils when magnesium is omitted from the fertilizer. It has 

 been discovered also that a projjer balance between the supply of 

 magnesium and sulphur in the fertilizei- or soil is a factor of im- 

 portance in plant growth and that this balance is likely to be greatly 

 affected by the amount of the seasonal rainfall. In relatively dry 

 seasons symptoms of sulphur deficiency are more evident, while in 

 wet seasons magnesium deficiency becomes more acute, probably be- 

 cause of the sulphur brought into the soil by rain in conjunction 

 with the loss of magnesium by leaching. As a result of field-plat 

 tests carried out in most of the important tobacco-growing sections, 

 in which chemically pure salts w-ere used instead of the usual com- 

 mercial-fertilizer materials, it has become evident that under certain 

 conditions magnesium deficiency may result in serious damage to 

 tobacco, even on newly cleared land, and the crop is subject to 

 injury from this cause in various localities. It has been demon- 

 strated that the necessary quantity of magnesium for correcting this 

 deficiency may be readily supplied by using in the fertilizer potash 

 salts containing appreciable quantities of magnesium or by the 

 application of magnesian limestone to the soil. It is obvious, 

 therefore, that the element magnesium must be taken into account 

 both in the general problem of liming and in the proper choice of 

 commercial-fertilizer materials, particularly in the use of highly 

 concentrated fertilizer formulas. 



DEVELOPMENT UNDER ARTIFICIAL LIGHT. 



Continuing the investigations on the effects of the relative length 

 of day and night on plant growth, fairly extensive experiments have 

 been undertaken on the response of woody perennials to this factor, 

 especially with reference to such problems as the basis of winter 

 hardiness, the natural distribution of plants, and their flowering 

 and fruiting habits. A method has been developed for growing 

 plants to maturity with artificial light as the only source of illumi- 

 nation, thus making possible the better control of all environmental 

 conditions and thereby facilitating plant physiological research. 

 Application of this process to a number of species shows that in- 

 itiation of flowering and fruiting and other characteristic responses 

 to differences in duration of the daily illumination period are 

 brought about as readily with artificial illumination as with sunlight 



