Working with Low-CEC Soils (Below 7 meq) - The Ideal Soil 2014 - page 96

Chapter 10

Working with Low-CEC Soils (Below 7 meq)

Minimum Amounts Needed for Ideal Soil Method in Any Soil or Growing Media
Based on results from a Mehlich 3 soil test
Calcium Ca 750 ppm

(1000 ppm is better)

Magnesium Mg 100 ppm

Potassium K 100 ppm

Sodium Na 25 ppm

Phosphorus P 100 ppm

Sulfur S 50 ppm

Boron B 1 ppm

Iron Fe 50 ppm

Manganese Mn 25 ppm

Copper Cu 5 ppm

Zinc Zn 10 ppm

Asoil with a CEC below 7meq does not have sufficient negatively charged
exchange sites to adsorb and hold onto the minimum amount of nutrient cations
necessary to achieve Ideal Soil results. Nonetheless, the minimums are needed,
even if the soil can't hold them against leaching.

Asandy or leached-out, low-organic matter soil may have a CEC of 3meq or less.
With a CEC of 3meq, at 100% saturation, the soil could adsorb and hold a
maximum of

Calcium: 200ppm x 3meq = 600ppm Ca
or
Magnesium120ppm x 3meq = 360ppm Mg
or
Potassium 390ppm x 3meq = 1170ppm K
or
Sodium230ppm x 3meq = 690ppm Na
(See Chapter 2, Cation Exchange)

If we try to balance the primary cations in a CEC 3meq soil to a standard
68%Ca : 12%Mg : 4%K : 1.5%Na cation saturation ratio , we end up with only
408 ppm Ca, 43ppm Mg, 47ppm K, and 10 ppm Na, far below the Ideal
minimums.

The short-term solution is to apply the amount of minerals needed to achieve the
Ideal minimums, realizing that some part of what is added will not be adsorbed
and held to an exchange site, and will be subject to leaching. Irrigation in a low
CEC soil should be kept to a minimum, adding only what the soil can hold. If there