I mean the fluid phase of soil when I mention "solution" and from chemical composition, the kind of dissolved ions and the percentage of them is a matter of importance to me.
to analyze the composition of a biomass sample we send it in to a analytical laboratory dealing with fodder, wastwater and waste quality management. We could determine the share of different macro-and micronutrients such as potassium, phosphorous, sulphur, calcium, iron but also different heavy metals. I think that the analytics of monitoring wastewater and water quality might work for your purpose as well. You may try to search of such laboratories in Auckland. I am sure that the New Zealand government has regulations on water quality and its strict control. Try to contact the WWT plant in Auckland. They might provide your with a contact since they have to commission these water analytics.