SOIL-WATER CULTURE TECHNIQUE 21 



lator and a place of reduction and synthesis so that heavy metals 

 are kept in solution. A supply of organic growth factors seems 

 to play a role also. 



These conditions and relations have the greatest favorable 

 influence in cultures of species from polytrophic v^aters, whereas 

 those from oligotrophic lakes and running waters are adapted 

 to lower concentrations and cannot be grown in dense popula- 

 tions in a restricted volume of water. It would be wrong, how- 

 ever, to conclude that the soil-water technique is only useful in 

 dealing with algae from polytrophic habitats. By using soil poor 

 in soluble substances and by reducing its quantity (compared 

 with that of the water) the technique can be adapted to species 

 from oligotrophic habitats. 



A suitable kind of soil can be found only by trial. Recently- 

 manured loam will seldom be favorable, whereas soil from a gar- 

 den bed with seasoned compost or leaf mould, or from a well- 

 worked arable field generally will prove to be satisfactory. With- 

 out being able to give an explanation, I mention the experience 

 that clay soils in most instances give excellent results that are not 

 obtainable with other soils, just as ponds on clay seem to be rich 

 in interesting forms of algae. Sand without humus cannot replace 

 soil. 



The properties of a given soil can be changed by making ad- 

 ditions, calcium carbonate being effective in raising the pH, peat 

 in lowering it. But the latter also increases the humus content. 

 Ammonium-magnesium phosphate is useful for its low solubility 

 so that without causing too high a salt concentration, a constant 

 supply of N, Mg, and P is furnished. With acid soils, ammonium- 

 magnesium phosphate is of course immediately dissolved. Clay 

 soil with NH4MgP04 is exceedingly fertile and causes luxuriant 

 growth, for instance, of most Euglenophyta. A few chlorophyll- 

 bearing algae and most of the colorless form are favored by supply- 

 ing organic substances, such as barley grains or starch covered 

 with soil and water. 



All these ingredients should not be mixed with the soil nor 

 put on top of it, but placed at the bottom of the test tubes or 

 other containers. Like the soil itself they are introduced dry so 



