CHAPTER XXXI 



LOSSES OF PLANT FOOD FROM PLANTS 



To determine the amounts of the different elements required 

 for the production of crops, analyses have commonly been 

 made of the mature plants, but there is much evidence that the 

 results thus secured may not always represent the full amounts 

 positively required for the growth of the crop produced. The 

 ultimate purpose of every plant (if we may so speak) is repro- 

 duction; and in the main it is the function of the leaves and stem 

 to contribute toward the formation of seed. The fixation of carbon, 

 oxygen, and hydrogen occurs only in the leaves or other green 

 parts of the plants, and the carbohydrates thus formed by photo- 

 synthesis, as well as the proteids, are in considerable part trans- 

 ported to, and stored in, the seed. The leaf is well called the 

 laboratory of the plant, but in the workshop or factory certain 

 tools are necessary, including potassium, magnesium, calcium, and 

 iron, besides the nitrogen, phosphorus, and sulfur which are formed 

 into living tissue in connection with the carbon, oxygen, and 

 hydrogen. But these four elements (potassium, magnesium, cal- 

 cium, and iron), which we may perhaps call work tools rather than 

 structural materials, are in large part discarded after some use, and 

 they are found deposited to some extent in the old leaves which 

 may become dead or inactive before the growth of the plant is 

 complete and while growth is still very active in the newer leaves 

 and other younger parts of the plant. 



As the older leaves become inactive, more or less of the plant 

 food which they contained and required for their own growth is 

 translocated to the newer leaves, but very considerable amounts 

 may be removed by leaching and thus returned to the soil by an 

 external route. As the plant approaches maturity, appreciable 

 amounts of plant food are thus removed by being washed or leached 



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