Let's talk Calcium (Ca), as it's a mineral nutrient that is often underestimated. Calcium is an essential element in plants, as it's a constituent of cell walls and membranes which are the ultimate physical barriers against pathogens.
Calcium deficiency makes plants vulnerable, while high quantities of calcium bring increased resistance to pests and diseases.
As importantly, Calcium governs the cellular delivery of all minerals, because it's involved in the transport of minerals across cell membranes. Calcium also determines crop quality, in particular in fruit crops (including tomatoes).
Because of its close link to cytokinins (hormones that regulate root growth), during the formation of fruit, abundant Calcium ensures that root growth doesn't stop and shoot growth doesn't become spindly (this can be seen in Tomato plants that show spindly new growth after the first truss starts ripening).
We have recently also discovered that Ca acts as a biochemical messenger, helping the different parts of a plant to communicate about nutrient distribution, pathogen attacks and environmental cues. Microorganisms (protozoa in particular) also use Calcium in this way.
So, how do we know if we have enough calcium, how do we source it cheaply and sustainably and how do we apply it?
One way to measure the calcium in plants is to use a refractometer - a tool that can indirectly measure the concentration of solids in the sap. A blurry line indicates good calcium levels and also good overall mineralisation because calcium is the carrier of other nutrients.
Calcium is often abundantly present in our soils. When it isn't one could apply Lime or Gypsum, which are two soluble forms that plants can take up. However, Lime and Gypsum are only as sustainable as the industry that produces them, and in some cases transports them a long distance from places where they naturally occur in sedimentary rocks.
Moreover, Calcium storage and delivery from soil to plants are linked to the presence of mycorrhizal fungi. Their microscopic threads extend the root's surface area tenfold and release acids that unlock calcium and phosphorus, delivering both to the plant. In a no-dig system, you have millions of fungi in your soil, provided that it's well drained.
In case you need to supplement it, an easily available source of calcium is the calcium carbonate present in eggshells and bones. To make it soluble, though, it must be removed from its protein matrix, via a series of steps that require only a pan and some vinegar. The result is called Water Soluble Calcium (WSCa) - see youtube or Living Soil Garden courses for details. Due to its poor mobility in soil and plants, WSCa is more effectively applied as a foliar spray, with results visible within a few hours. Vinegar and eggshells (as opposed to Lime and Gypsum) have the huge advantage that they can be made from garden outputs. Laying hens can be kept in an agroforestry system and used to cycle cover crops in a rotation, without grain feed. Vinegar can be made from spoiled or excess fruit or grain or whatever is available. WSCa not only has a much lower impact on the gardener's pockets and the environment, but is more inherently part of the garden's internal nutrient cycles - for those who eat eggs and drink vinegar! (Obviously vegan growers might consider WSCa a viable option).
Incidentally, calcium is most effective in the presence of Boron (, which is also connected to resistance to fungal pathogens (mildews in particular). This connection was first noted by Rudolf Steiner way before it was confirmed experimentally with modern tools. Because Boron does not appear in a charged form in the soil, it can only be stored in humus, and that again favours no-dig approach, with abundant and stable organic matter.