6 CEC Cations can move on and off particles . . . when one leaves, another replaces itThis process is called cation exchange, and cations involved are said to be exchangeable
7 CECThe number of sites that a colloid (small particle) of charged clay or humus (micelles) contains is measured by the:Cation Exchange Capacity expressed in mEq/100g (older unit) or cmolc/kg
8 CEC may range from: 2.0 mEq/100g for sand to > 50 mEq/100g for some claysandhumus mEq/100gunder certain soil conditions
9 CEC How fertile can a soil be? Does applying more fertilizer always provide more nutrients to plants?How much of the CEC is actually filled with cations?
10 CECThe proportion of the CEC occupied by basic (+) nutrients such as Ca, Mg, K, Na, is called:Percent Base Saturation and is an indication of the potential CEC of a given soil
11 CECEstimations that > 99% of cations in soil solution are adsorbed . . .does not mean that percent base saturation is 99%
12 CECExample:A soil with CEC of 10 mEq/100g has 6 mEq/100g of bases (Ca, Mg, K, Na) occupying exchange sitesWhat is the percent base saturation of the soil?
13 CEC6 mEq/100g bases10 mEq/100g sites= 60 % base saturation
14 Cation Exchange is determined by: CECCation Exchange is determined by:1) strength of adsorption2) law of mass
15 CEC Strength of adsorption is as follows: H+ and Al3+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+
16 CEC Law of Mass the more of one ion available, the greater the chance of adsorption
17 NUTRITIONThere are at least 17 elements recognized as essential nutrients for plants;we will recognize 18 elements:C, H, O, P, K, N, S, Ca, Fe, Mg,Mn, Mo, Cl, Cu, Zn, B, Co, Ni
18 NUTRITIONNutrients grouped into 2 categories according to the relative amount used by plants:Macronutrients – major elements; large amountsMicronutrients – minor elements; small amountsBoth are essential for optimal plant production