354 Journal of Agriculture. [fo June, 19 io. 



factor 3 partly, in so fa.r as the air contains oxygen, but the presence or 

 absence of carbon dioxide, is immaterial so long as factor 4, exposure of 

 the leaves to light, is not fulfilled. 



If the first four factors are satisfied, that is to say, if the seed is kept 

 u-arm, provided with water, air and ca.rbon dioxide, and if the leaves of 

 the seedling are exposed to light, then the seedling will be found to increase 

 considerably in weight. This is the case when its roots are grown in dis- 

 tilled water and it mav then even carry its development as far as the 

 production of flowers. In this case, the fifth factor is unsatisfied so far as 

 the supply of mineral salts derived from the .soil is concerned. Growth 

 is then onlv possible at the expense of and by virtue of the mineral salts 

 that were stored up in tbe seed during ripening. In this respect, all 

 plants show considerable powers of accommodation, since the smallest 

 amount of a particular essential salt necessary to permit a plant developing, 

 is very much smaller than the amount it would like to have. To put it 

 into plain English, we might say that the 7>lant is like a hov who prefers 

 to have a pound of chocolates a dav but could get on with an ounce. Tn 

 the case of the plant, however, its j)Ower of accommodation is partly due 

 lo the fact that it is able to go on absorbing and accumulating an essential 

 salt which is only present in the soil in extremely small amount. For 

 this to be possible, a large root system and a good supply of water are 

 necessary. For instance, very manv plants can obtain all the salts thev 

 jequire from ordinary tap water, if the roots are grown in it and it is 

 frequently changed, although in such water, certain essential elements such 

 as potassium, magnesium and phosphorus mav occur in the merest traces, 

 almost incapable of estimation bv analysis. 



Water is the plant's most important requirement, hence, under ordinary 

 cultivation conditions, the physics of the soil, including its aeration and 

 its power of storing and supplying water, is far more important than its 

 chemistry. Plants can adapt them.selves to minimal amounts of salts, 

 without undergoing agricultural deterioration, far more readilv than they 

 can to minimal amounts of water. 



The essential elements which an ordinary plant obtains from the .soil 

 through its roots are calcium, potassium, magnesium, sulphur, phosphorus, 

 and nitrogen. A trace of iron is also necessary, but all ordinary soils 

 contain this element in greater amount than the plant requires it. These 

 essential elements are usually absorbed by the plant in the form of salts, 

 such as phosphorus in the form of phosphates, sulphur in the form of 

 sulphates, nitrogen in the form of nitrates, while the calcium, magnesium 

 and potassium are absorbed as salts of these acids or of others. Hence, 

 three salts such as calcium nitrate, magnesium sulphate, and potassium 

 phosphate will provide the plant with these six es.sential elements, and any 

 ordinary plant can be grown to full adult size, if provided with nothing 

 but these salts and distilled water, and exposed to light and air. 



A point of great importance is that, to .some extent, an essential element 

 can be replaced l>y another which is not es.sential. For instance, a limited 

 substitution of strontium for calcium is po.ssible, that is to say, if strontium 

 is present in the soil, the plant can grow with a less supply of calcium than 

 it otherwise needs. As strontium is, however, rarer and more costly than 

 calcium, this is a fact of theoretical importance alone. The same applies 

 to the fact that calcium mav be partly replaced by magnesium. In the 

 case of cereals, however, it has been found that the presence of soluble 

 silica, or of silicates in the soil, enables the plant to get along with a less 



