-3- 



calcium or riasne^ium or potassiuia ion held on the surface of the soil 

 particle. As the plant continuss to si'C^'^i "the reserve of these nutrient 

 elementtj held by the soil colloids is (gradually depleted tuid their place 

 is tai'en by hydrofjen. 



As hydrogen is "exchanged" for various essential nutrient 

 eleirip;nts, the qucntity of hydrogen lield by the clay and hunus colloidal 

 material tends to increase. This in turn increases the nUi-nber of free 

 hydrogen ions in solution. Since it is the free hydroj^en ions in 

 solution which determine the degree of soil acidity, ;;oilE tend to becoae 

 more acid in reaction as successive crops are roiuoved. 



Soil Acidity and plant Grovrth. 



One of the i^nportant thinir.s to note here is th-j exceedingly 

 vital and useful role ^fhiob the hydrogen ion plays in the mechanisin of 

 plaiit nutrition, 7/e have tended to look upon the presence of liydrogen 

 in the soil solution v/ith disfavor and to consider that the soil acidity 

 was somethinj^ which should be corrected. It now appears that a little soil 

 acidity is a o^<^^ thiniW and that to a certain extent at least, it is not 

 so much the acidity of acid soils but the lack of essential nutrient 

 elements such as caloiuia, nagnesiuiti, potassium and others which is respon- 

 sible for poor plant grov^-th, 1/Vhen the ressrve supply of these essential . ••• ■- 

 elements in the soil is replenished through liming and fertilizing operations, 

 normal crop grov:th results. 



Liming alone '■.•'ould correct acidity and supply calcium and 

 magnesium if dolomite is used but it would not correct deficiencies of 

 other essential nutrient elements. In other v/ords, "Lime alone without 

 mcaiure makes the farm and fanaer poor," 



W, C-, Colby 



Plat 7 HooV 



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ioy 



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(.Co Ic tie) 

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*-'■ • '"t M ci »;.«(« I ; 



Nutrients, like calcium, move from the mineral to the colloidal clay and 



h'omus, a;:d from there to the plant roots (right to left), Hj^drOj^en, or 



acidity, traded by the root for nutrients, goes in the opposite direction 



to break dovm the mineral crystals like limestone and to put the calcium 



nutrisnt into availability for the plant. 



by William A, Alt^recht 



Departvaent of Soils, University of Missouri 



