Now take another example. We want to give the manure the power to receive so much life into itself that it is able to transmit life to the soil out of which the plant is growing. But we must also make the manure able to bind together, still more, the substances which are necessary for plant growth — that is, in addition to potash, also the calcium compounds. In yarrow we are mainly dealing with potassium influences. If we also wish to get hold of the calcium influences, we need another plant, which — if it does not enthuse us like yarrow — also contains sulphur in homoeopathic quantity and distribution, so as to attract through the sulphur the other substances which the plant needs, and draw them into an organic process. (Steiner 2007:45)
STEINER, Rudolf. 2007. The Agriculture Course. (Trans.: George Adams) Shrewsbury: Wilding & Son Ltd.