WHAT PLANTS NEED 305 



they need. One of the remarkable effects of phosphates 

 has been shown to be a greater ability to form roots, so that in 

 soils, as clays, in which roots do not develop readily, phos- 

 phates are of great use. Phosphates are also said to hasten 

 the ripening processes. It should be recognized, however, 

 that fluctuations in the supply of phosphates do not show 

 such important results in the plant as fluctuations in the 

 supply of nitrates. In fact, plants show decided symptoms 

 of nitrogen starvation, but there are no recognizable symp- 

 toms of phosphorus starvation. One of the questions under 

 discussion is as to the natural supply of phosphates, some 

 maintaining that the phosphates are disappearing from the 

 soil in an alarming way, and others maintaining that the 

 supply is sufficient for an indefinite time. 



14. Other soil salts. Numerous other salts, as the 

 compounds in the soil are called, are needed by plants in one 

 way or another, as compounds of sulphur, potassium, cal- 

 cium, magnesium, iron, etc. What these substances enable 

 the plant to do is known in some cases, but in others 

 the particular use has not been discovered. But they 

 are all constituents of the soil, and have to be reckoned 

 with. 



15. Toxic substances. It is not only necessary to deter- 

 mine whether a soil contains the substances that plants need, 

 but also whether it contains substances injurious to the plants 

 we wish to cultivate. Such substances are spoken of in a 

 general way as toxic substances. For example, a soil may 

 contain too much acid or too much alkali, and such a soil 

 must be treated accordingly, that it may become more 

 neutral. Compounds of metals getting into the water supply 

 in the waste products drained away from industrial establish- 

 ments working on metals may be very injurious to cultivated 

 plants. Gases of various kinds diffused through the air from 

 manufacturing establishments may be very destructive to 

 plants, or at least prevent vigorous growth. 



