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It is the writer's firm conviction that the more we as growers of plants 

 learn about soils and soil-plant relationships, the more interesting and 

 fascinating (and perhaps profitable) this business of grov/ing plants 

 becomes. It's somethiiig like playing a gaine of bridge - one doesn't enjoy 

 the game until he begins to understand something of the rules and principles 

 which are involved. The v;riter doesn't pretend to know a great deal about 

 growing plants - or playing bridge either, for that matter, but he has 

 learned enough to enjoy both and he finds that the more he learns about 

 each form of activity, the more interesting and pleasuresome each becomes, 



Rolo of the Soil in Crop Production . 



The soil serves tv/o important functions in the growth of most 

 crop plants; first, it provides mechanical support and second, it serves 

 as a resei'voir for certain essential plant nutrient elements. The maa'iner 

 in which the soil serves as ci reservoir and a supplier of essential 

 nutrient elements has been the object of intense study for the past 25 

 years, 7Jhile different soils vary tremendously in their capacity for 

 storing and supplying nutrient elements to grov/ing plants, the mechanism, 

 by which this is acccrmplished is similar for all soils, at least so far as 

 the fundai'nental principles are concerned. 



Until conparatively recently it Viras generally thought that 

 nutrient elements must be in some soluble form before they could be taken 

 up by plants. For years, the "soil solution" was considered to be the 

 important part of the soil as plant nutrition was concerned. No one v/as 

 able to either separate the "soil solution" from the soil or to duplicate 

 it artificially. Yet it was generally felt that the difficulty was one 

 of faulty technique rather than one of interpretation, Y/hile v;ater is 

 essential for plant grovrth, it is nov; generally agreed that the movement 

 of v/ater into plants tekes place independently of the uptake of plant 

 nutrition. 



Evidence accumulated which pointed to the colloidal fraction 

 of the soil as the important soil component in plant nutrition. The 

 finely divided particles of clay and organic matter, the material that 

 remains in suspension, giving a cloudy appearance when a soil is shaken 

 in ws.ter is the seat of greatest activity. Here is where important trans- 

 fers took place betv/een plaaits, roots and the soil. 



The Mechanism o f Base Exchange or How Plants Feed . 



17e now- have convincing evidence which shov:s that plants can take 

 up nutrient elements through the small rootlets or root hairs at the points 

 of contact betv/een these root hairs and small colloidal particles of clay 

 and humus. These colloidal particles arc charged electrically and carry a 

 negative charge. Since they are negatively charged, they have an affinity 

 for positively charged elements such as hydrogen, calcium, magnesium, 

 potassium, nitrogen in the ammonium form and others, A single clay or 

 humus particle may have scvereJ of these different elements "attached" to 

 its surface. There might be several calcium ions, several hydrogen ions, 

 a fev; magnesium and potassium ions and perhaps a boron ion, all present 

 on a single colloidal particle, ''Knen a plant root hair comes in contact 

 v/ith such a clay or hunus particle, an "exchange" or transfer can take 

 place v/hereby a positive hydrogen (produced by living processes v/ithin 

 -che plant root) from the root hair can be exchanged for a positive 



