PLANT FOOD 71 



ing seed. The stand of clover is much better maintained 

 if the crop is cut for hay before the seed has ripened, unless 

 the stand is kept up by the growth of new plants from seed 

 that scatters in the field. 



72. Effect of the Time of Harvesting on Composition. 

 Annual plants take up nearly all their nitrogen and min- 

 eral matter in their early stages of growth. But the starch 

 and other organic compounds are more largely accumu- 

 lated in the later stages. This is one reason why plants 

 require so much available food in the soil during the early 

 stages of growth. (See Fig. 90.) 



When spring wheat is half grown, it contains about 85 

 per cent of the total nitrogen that is required for the crop, 

 and 75 per cent of the total mineral matter, but it con- 

 tains only 40 per cent of the organic compounds. 



When clover is in full bloom, it contains as much dry 

 matter as when ripe, and more nitrogen and mineral ele- 

 ments. Slight amounts of these are returned to the soil 

 in the later stages. 1 



Any condition that checks the growth of plants before 

 maturity will, therefore, affect the composition of the crop. 

 If dry weather or a lack of food supply check the growth 

 of wheat, it will have a higher percentage of nitrogen and 

 a lower percentage of starch than if it matured naturally. 



QUESTIONS 



1. Why is the soil about a tree lifted? 



2. How many tons of corn, grain and stalks, is an average crop 

 per acre in your community? Assuming the Wisconsin figures to apply, 

 how many tons of water would be evaporated from the corn leaves 

 on an acre? 



J H. Snyder, Chemistry of Plant and Animal Life. Chapter 26. 



