Hi Mike,

You describe some challenging soil conditions, although I believe they can be overcome in time.

My understanding is, when you have clay soil/subsoil getting nutrients to release from the clay is the challenge and opportunity.

The key is that different minerals adsorbed to the clay colloid cause the colloid to behave differently and either hold nutrients more tightly, or release them more readily.

The magnesium ion has the smallest hydration radius. That is to say, it has the smallest diameter when wet.

Calcium and potassium both have a much larger hydration radius, more than 2x larger if I recall correctly.

Why this matters, if your clay colloids are loaded with elevated levels of magnesium, it pulls the two colloid surface layers tightly together, which has the effect of trapping all the potassium that is now ‘stuck’ between the two colloid layers. You could almost refer to this as ‘colloid level compaction’.

The remedy for this is to add enough calcium that the calcium ions adsorb to clay colloids in large enough numbers to overcome the magnesium pulling the colloid together. This is why calcium has the effect of flocculating clay. Since calcium has a much larger hydration radius, it moves the clay sheets farther apart, which releases the K held between the sheets, as well as the magnesium and calcium.

If I understand what you are describing correctly, the solution is not to add more K, as there is a reasonable probability whatever you add will also become locked up between the clay sheets.

The solution is to add materials which will flocculate the clay, and release the minerals contained between the clay sheets.

Obviously, calcium is a fit here. With your lower pH levels, limestone should be a fit. As pH increases, you may want to look at gypsum as well. In addition, humic substances have been demonstrated to improve clay flocculation, although the quantities required are unrealistic on broad acre crops.