HOW SOILS L08E WAT1 i: 101 



of cultivation, one or more of the different foods, but 



to renew the supply is not so easy. The two plant foods 

 most easily exhausted tad mod difficult to replace are 

 water and nitrogen, and as they are to a certain extent 

 dependent on each other, a soil lacking in one is apt 

 to be lacking in both. 



99. Moisture Often Lacking in Soils. — The plant 

 food most often larking in soils is moisture The great 

 areas of land known as deserts are deserts because they 

 have no supply of water. The oases of deserts owe 

 their existence to a supply of water from some spring 

 tt wdl. Provide i desert with ■ supply of water, and 

 <es to be a desert, and in time may become fertile. 

 In many parts of our country are great stretches of 

 land, which, while not reduced to the condition of a 

 . are so poor that the crops produced are hardly 

 worth tin- gathering. These lands were once clothed 

 with a dense growth of magnificent forest trees; now 

 they produce for those who cultivate them the mbsi 

 meagre crops. What has caused the change? lias 

 the mineral food of the soil become exhausted? Bat 

 the nitrogen supply been used up, or has the water 

 supply failed? If left uncultivated these lands grow 

 up again in trees, and as time passes they become 

 clothed sgaio in great forests, Evidently there is 

 snongb plant unuleted to produce great crops 



of trees; then why not enough for farm crops? There 

 is an ample store of mineral plant food in these poor 

 SoQl, hut two tilings are lacking — water and organic 

 matter. But if they are lacking how can great crops 

 of trees grow up? Growing trees add to the soil's 



