45 2 Mineral Nutrition of Plants 



Soils by themselves appear to he a very minor cause of the variation 

 in yields obtained. In order to account for the variations it is necessary 

 to look to the total environment of the plant, both external and internal 

 (6). Such an external environment will include temperature, radiant 

 energy, moisture, wind, humidity, and soil. The internal environment 

 is the physiological status of the plant as related to its age, vigor, and 

 composition, with particular reference to moisture, the critical elements, 

 carbohydrates, and, of course, the plant's heredity. The problem from 

 a practical viewpoint then is to attempt a measurement of all the factors 

 influencing growth of sugar cane and to devise a method which will 

 enable the grower to culture his crop taking full advantage of all favor- 

 able factors and lessening or overcoming all the unfavorable ones. 



At the outset, it was assumed that to obtain a measure of the well- 

 being of a crop, it was necessary to find this reflected in the crop itself. 

 Criteria based solely on soil chemistry except in a few cases have been 

 of little value to an understanding of the physiological requirements 

 of a crop. Just so, it appeared unlikely that the mineral nutrient levels 

 of leaves would solve the problem. 



SELECTION OF TISSUES FOR TESTS 



The first important task was a thorough study of the several organs 

 or tissues of the plant in relation to the various materials and processes 

 pertinent to growth in order to select that tissue which is the most re- 

 liable index to use in following the welfare of the crop being grown. 

 The plant was separated into its several parts: the meristem, the elon- 

 gating portions of the stem, the mature portion of the stem still bear- 

 ing living leaves, the old stem from which the leaves had already fallen 

 (divided into three internode units), the unfolding leaves, the young 

 mature leaves divided into blades and sheaths, and the old green leaves 

 also divided into blades and sheaths. Each of these tissues or organs 

 was analyzed for weight, moisture, dry matter, total sugars, hydro- 

 lyzable carbohydrates, total nitrogen, potassium, phosphorus, calcium, 

 and magnesium. 



Plants grown for these preliminary studies were collected in the early 

 morning at five-week intervals from each of sixteen crops grown at two 

 places, started at four different times of the year. After all the analytical 



