GROWING PLANTS WITHOUT SOIL JOHNSTON 385 



iron, and magnesium. Boron, manganese, and undoubtedly several 

 more elements should be added to the list. 



In what forms or chemical compounds can these elements be " fed " 

 to the plants? There are numerous combinations of these elements, 

 but perhaps the salts most frequently used are calcium nitrate, 

 Ca(N03)2; magnesium sulphate, MgS04; monopotassium phosphate, 

 KH2PO4 ; potassium nitrate, KNO3 ; ammonium sulphate, (NH4) 0SO4. 

 Iron may be supplied in any one of several compounds such as tar- 

 trate, citrate, phosphate, or sulphate. A trace of boric acid and 

 manganese should be added where all other materials are of the 

 highest purity. To be sure, certain plants require larger amounts 

 of a given element than others and this same thing is true of a given 

 plant at different stages of its development, but in spite of such 

 limitations it is rather surprising to find considerable variation in 

 the concentrations and proportions of these salts that produce good 

 growth. 



Almost every original investigator in the field of plant nutrition 

 has devised a particular solution of his own. The result is that 

 there are many formulae for making plant-nutrient solutions, each 

 of which has some good points. The chief purpose of any of these 

 solutions is to give the plant the elements that it needs for its growth 

 in a readily soluble form. The ones obtained from the soil in larg- 

 est amounts are nitrogen, phosphorus, potassium, calcium, and mag- 

 nesium. Many different nutrient solutions have been made from 

 various combinations of these elements. That proposed by Sachs 

 was the first so-called standard solution. Perhaps none has been 

 more generally used than the one devised by Knop, which is made 

 up of the following salts by weight : Four parts calcium nitrate, one 

 part potassium nitrate, one part magnesium sulphate, one part either 

 mono or di potassium phosphate, trace of iron. 



Knop's solution was greatly improved by Tottingham for the 

 growth of wdieat and then simplified by Shive to a solution com- 

 posed of the following volume-molecular proportions of but three 

 salts : 



KH2PO4 0. 0180 m 



Ca(N03)= . 0052 m 



MgS04 . 0150 m 



To be sure, iron was needed and was supplied in a very small 

 amount, as FeP04. Six of the essential ions— K, Ca, Mg, PO4, NO3, 

 and SO4 — used by plants in the greatest amounts were found in 

 Shive's simple formula. When very pure salts and distilled Avater 

 are used it is necessary to add small amounts (mere traces) of other 

 necessary elements. Our more recent knowledge of plant nutrition 



