Nutrient Management

Cation Exchange Capacity

The cation exchange capacity (CEC) is a major measure of potential for the soil’s inherent fertility.  The CEC indicates the soil’s ability to store cations, which include some of the major plant nutrients;  potassium, (K+) for example.

The CEC depends mostly on a soil’s colloidal content.  Colloids in soil are gelatinous substances made of clay particles, raw organic matter and humus.  They have a large surface area relative to their weight and many negatively charged anion sites.  The positively charged cations are held in place at these exchange sites (opposite charges attract one another) safe from leaching but available to the plant roots. 

Cation exchange capacities are expressed in milliequivalents (mEq) (how many thousandths of a gram of hydrogen or its equivalent charge, can be held by 100 grams of dry soil).

The major cation (+) nutrients are calcium, magnesium potassium and sodium.  To assure adequate plant nutrition they must be present in certain minimum amounts, which must also be balanced.  The excess of any one mineral, for example magnesium, may interfere with the availability of another, such as potassium;  even if the lab tests show adequate potassium levels. 

Base Saturation Ratio (cation balance)

Base saturation refers to the percentage of a soil’s CEC occupied by the base elements (cations other than  hydrogen or aluminium).

Some efforts have been made to find the ‘ideal’ cation balance or base saturation ratio, they are: 

• Potassium 4 to 6%  
• Magnesium 10 to 15%
• Calcium 65 to 70%
• Hydrogen 5%

In most cases it is not necessary to completely saturate the exchange complex (exchangeable sites) with these exchangeable base elements.  Usually an 80% to 90% saturation of the CEC with a balanced ratio of exchangeable bases will be adequate for high yields of most crops.

MOST BOOKS WILL TELL YOU A PLANT ONLY NEEDS 14 OF THESE –

THE FACT IS, IT NEEDS AT LEAST 22 OF THEM