SOLUTION AND SAND CULTURES 427 



known to have a number of pronounced effects upon plants. Except in very 

 dilute concentrations aluminum is distinctly toxic to plants. Its toxicity to 

 such species as corn and barley may become evident in concentrations as lovv^ 

 as one part per million in culture solutions. The detrimental effect of soils 

 with a pH of 5 or less upon the growth of some species is undoubtedly due, 

 at least in part, to the toxic effect of the relatively high concentration of 

 aluminum ions in such soils. The beneficial effect upon the growth of some 

 species of adding lime or phosphates to acid soils is at least partly due to a 

 reduction in the solubility of the aluminum compounds present. 



Symptoms of Mineral Element Deficiency. — Absence or deficiency of 

 any of the necessary mineral elements (including nitrogen) in the soil or other 

 substratum in which a plant is rooted will sooner or later become apparent in 

 the development of that plant. An insufficient quantity of any of the essential 

 elements in a plant in an available form will result in the production of 

 growth aberrations which are symptomatic of lack of an adequate internal 

 supply of that element. In a general way such deficiency symptoms are com- 

 mon to all species of plants. Certain of these deficiency symptoms assume, 

 however, more or less distinctive aspects in many species. For example, 

 manganese deficiency results in the development of a characteristic mottled 

 chlorosis in the leaves of many species. In maize and other cereals, however, 

 this chlorosis assumes the pattern of an alternate yellow and green striping 

 running lengthwise of the leaves. For this reason it is important that the 

 symptoms of mineral element deficiency be studied for each economic species 

 of plants individually. Once such symptoms have been distinguished for any 

 species they are of assistance in diagnosing abnormal development of plants of 

 that species under natural or cultural conditions. 



In Table 41 are summarized the more easily recognized deficiency symp- 

 toms for the nine principal essential mineral elements. In such a summary 

 table these symptoms can be described in only a very generalized way, and 

 in many species some deviations from these symptoms are to be expected. 

 The effects of mineral element deficiencies upon the development of the 

 tobacco plant are illustrated in Fig. 96. 



Solution and Sand Cultures. — Much of our knowledge regarding the 

 role of mineral elements in plants has been obtained by means of solution 

 culture experiments. The growing of plants in solution and sand cultures 

 is today one of the most widely used experimental techniques employed by 

 plant physiologists. The necessary solutions, often rather inappropriately 

 called "nutrient solutions," are prepared by dissolving salts in certain definite 

 proportions in distilled water. A multitude of combinations and concentra- 

 tions of salts have been suggested for use in solution culture work. In gen- 



