VARIATION OF CONTENT 37 



helps to dissolve some of the mineral matter in the soil and 

 make it available for plants. The dark color of soils is due 

 largely to the presence of an abundance of humus. As dark 

 colors absorl) the sun's rays more completely than light 

 ones, dark soils, or those rich in humus, are warmer than those 

 which are lacking in it. It is evident that it is important 

 to maintain a plentiful supply of humus in the soil. 



38. Content of the Various Elements at Different Stages. 

 Different plants draw on the supply of the various elements 

 of plant food in different proportions. They also vary in 

 their composition and in their draft on the soil at different 

 stages of growth. The quantity of potash, for instance, in 

 a crop of wheat increases up to the time when the crop is 

 fully headed, after which it decreases till at harvest nearly 

 half the potash the plant contained has been lost. This 

 potash is washed out by rains and dews or it is returned to 

 the soil by way of the roots. The greatest quantity of nitro- 

 gen is also to be found in the cereals and grasses at about the 

 time when the plants are in blossom; later the nitrogen con- 

 tent decreases. The quantity of phosphorus increases as 

 long as growth continues and does not noticeably decrease 

 at maturity. With other crops which do not dry out when 

 ripe, as the potato, there is no loss of any of the elements 

 when the plant is mature. The composition of the mature 

 plant, however, does not necessarily show the quantity of 

 food material which has been used during growth. 



39. When the Different Elements Are Needed. Plants 

 differ in the time at which they need the various elements of 

 plant food just as they differ in the proportion of these ele- 

 ments which they utilize. In general, nitrogen is most 

 largely used in vegetative growth (the production of leaves 

 and stems) and is drawn on more heavily in the earlier 

 stages of growth than toward maturity. Forage crops 

 require a specially hberal supply of nitrogen. Phosphorus, 

 on the other hand, is an important constituent of seeds and 



