We must bring calcium into the soil by our manure, But it will not be of use to bring the calcium to the soil by any channels that avoid the living sphere. To have a healing effect, the calcium must remain within the realm of life; it must not fall out of the living realm. Ordinary time or the like is of no use at all in this respect. (Steiner 2007:46)
Now there is a plant containing plenty of calcium — 77 percent of the plant substance, albeit in a very fine state of combination. I refer to the oak — notably the rind of the oak, which represents an intermediate product between plant-nature and the living earthy nature, quite in the way I explained when I spoke of the kinship of the living earth with bark or rind. For calcium as it appears in this connection, the calcium-structure in the rind of the oak is absolutely ideal. (Steiner 2007:46)
Now calcium, when it is still in the living state, not in the dead (though even in the dead it is effective) — calcium has the property which I explained once before. It restores order when the ether-body is working too strongly, that is, when the astral cannot gain access to the organic entity. It “kills” or damps down the ether-body, and thereby makes free the influences of the astral body. So it is with all limestone. But if we want a rampant ethereal development, of whatsoever kind, to withdraw in a regular manner — so that its shrinking is beautiful and regular and does not give rise to shocks in the organic life — then we must use the calcium in the very structure in which we find it in the bark of the oak. (Steiner 2007:46)
STEINER, Rudolf. 2007. The Agriculture Course. (Trans.: George Adams) Shrewsbury: Wilding & Son Ltd.