Minerals Whence and Whither

Summer 2017

By Will Bonsall

Minerals are the part of “organic” that isn’t organic. That is, all of the elemental components of organic matter – carbon, hydrogen, oxygen and nitrogen – are in the air we breathe, always in adequate supply; all we have to do is create a soil community that sequesters those elements in stable-but-ever-changing humus.

Minerals, on the other hand, are the solid bits that remain after all the organic stuff is oxidized, as in burned or decomposed. Thoroughly burn a bale of hay, and the mineral residue – the ash – can be measured in spoonsful, but while minuscule in quantity, the minerals in that ash nevertheless are essential for a healthy life – for your life, for that of your crop plants and for that of the soil.

Given that  “dirt” consists mostly of minerals, why should you worry whether it will provide enough minerals to meet the needs of plants? Any kind of dirt – sand, clay, gravel – may prop up the plants, but the food part must be in water-soluble form to be useful.

Since all minerals originally welled up from the bowels of the earth, it is reasonable to assume that rocks derived from “weathering” of igneous rocks (such as granite) should be sufficient to meet plant needs, except that lots of stuff got moved around and changed during that process of soil deposition. Grinding, etching and washing sorted those minerals in diverse ways, giving us sand, silt, clay, etc. In addition to that physical segregation, many of the soluble minerals were removed altogether from the reach of crop plants as they were carried to the ocean or leached deeper in the soil.

The part of the earth that remains after all this sorting is silica, the primary ingredient in the earth’s crust and one of the least soluble. Aside from the physical support function already mentioned, silica is a trace mineral important for bones, teeth, skin, eyes, glands, organs and collagen, whereas in plants such as grasses, silica forms a microscopic sheath, or armor, helping protect against disease and insect attacks. Unfortunately, just as you cannot obtain dietary silica by chewing on bottles, neither can plants (with some notable exceptions, discussed later). Furthermore most of these essential minerals are or were originally locked up in silicate compounds. (I used to work in the mining business, which is how I happen to think this way.) Just as a smelter must grind ores finely and add solvents to extract the desired minerals, so do geological forces grind and dissolve soil particles, releasing minerals for plants. However, many minerals remain attached to and locked in those soil particles.

The major soil mineral nutrients – potassium, calcium, magnesium, phosphorus – are abundant and ubiquitous in most of the earth’s rocks, so how can they be deficient in plants? In addition to those minerals still waiting to be unlocked from the original soil particles, many minerals were unlocked but are now re-locked in insoluble precipitates.

The problem of mineral availability is aggravated by the fact that we’re exporting them as fast as we’re importing them by the two major forms of erosion: leaching and the marketplace. Yes, the marketplace is a one-way street that thwarts our every effort to create a cyclical ecosystem. And thanks to the modern flush toilet, those minerals are all going “away.” This fact is all-important in my mind, because if importing “organic soil amendments” is based on mining (albeit in someone else’s backyard), then I question what we mean by the word “organic.”

Since my own growing is mainly for home use, I avoid many of the paths leading to and from the marketplace, but what about all those folks trying to make a living from organic farming? Are they doomed to be miners, and is there nothing they can do to be more sustainable? While they cannot stem the outflow of minerals via the marketplace, they can use what’s already there.

Remember what I said about minerals contained in soil particles. While grinding those particles finer may be impractical, some plants can in fact “eat the rocks,” etching the particle surfaces and “organicizing” those minerals; that is, rendering them soluble and thus available to our less resourceful crop plants. For example, buckwheat has an appetite for apatite (calcium phosphate) in the soil and incorporates it into its plant parts and eventually into the soil community. Primitive equisetum (horsetail) and some sedges and ferns seem to dissolve (or at least concentrate) silica and make it available to the biosphere. Soil lichens penetrate rock particles and accelerate their breakdown and release of mineral goodies within. Of course these things don’t lend themselves to crop rotations (except buckwheat), but when we find these organisms growing in “waste areas,” we can harvest them to add their mineral wealth to our compost piles.

The real concern is for those minerals – including trace elements such as boron, fluorine, iodine, selenium, etc. – that simply aren’t there in the first place. Perhaps they got sorted out in the process of soil formation, or they got carried away by leaching or other processes. Unless you’re working with pure sand, those minerals likely are down there, only too deep for our shallow-rooted crop plants to access. In most of Maine they’re where the glacier dropped them, and that’s where they’ll stay unless something reaches down to appropriate them.

Enter the big old hardwood forest trees whose deep-delving roots can pump those “virgin” minerals up and add them to the biosphere (the world of the living) in the form of leaves, twigs and other plant parts. By incorporating great masses of these ultimate soil-building materials into your cropland – say, as compost or mulch – you can assure that those vital building blocks will find their way to the surface, onto your plate, and into your body.

That’s the key: By creating high humus levels comparable to natural soils – which manure-based organic farming fails to do – you can guarantee that those minerals, including elusive ones such as boron and iodine, will remain in the topsoil, endlessly bouncing from one life form to another. Even when released by death and decay, their soluble forms are jealously retained by the intricate mesh of humus, waiting there until they’re called for by any of the myriad organisms, including your food crop. Even if you find it necessary to import minerals to replace those that have fled to the marketplace, take every precaution that once added they stay there, cycling in the endless dance of life.

Aside from adding minerals as plant nutrients, we’re often advised to add some amendment, typically lime, to raise the pH, which in doing so may help release some other locked nutrient. Is that the only way to do it? I often refer to this mindset as “cake-mix farming,” where you add some of this and that ingredient and stir, and out comes a crop that you send off to market, as if the soil is merely the mixing bowl where these ingredients are assembled from elsewhere. That’s not my idea of organic, especially when we can often get much better and longer-lasting results from something we do as opposed to something we add. For example, if the soil is poorly drained and anaerobic, you may add tons of lime per acre to counter acidity, and the effect will be minimal and temporary (aside for producing an excess of calcium, especially in proportion to potassium and magnesium). Digging and ditching may get a similar result without importing any stuff.

Of course acid soil can be caused by things other than poor drainage. For example, in rainy climates alkaline minerals may be leached away, leaving behind aluminum and iron, which are acid-forming. (Conversely, soils in arid regions tend to be alkaline.). Or perhaps acidity is due to the chemical composition of the original rocks. In either case the remedy is the same: Increase levels of humus, which tends to act as a soil buffer, effectively reducing either extreme of pH.

Sometimes the problem is too little acidity, as with blueberries. You can add some elemental sulfur to sour the soil, but you can also add heavy mulches of pine needles or softwood chips. In addition to the many benefits of adding any mulch, the pH effect will last longer, plus the mulch material may come from your own neighborhood, and incidentally is harder to use for other non-acidic applications.

Always remember that humus, regardless of its source, ultimately has a buffering effect on the soil, causing both extremes of acid and alkaline to tend toward slightly acidic – just what most of our crop plants prefer. A soil test can help determine whether building up humus will buffer the pH enough to grow a particular crop.

I began my working career in the mining business and I have no desire to return to it. Recycling is ever so much more fun.

About the author: Will Bonsall lives in Industry, Maine, where he directs Scatterseed Project, a seed-saving enterprise. He is also the author of “Will Bonsall’s Essential Guide to Radical Self-Reliant Gardening” (Chelsea Green, 2015). You can contact Will at [email protected].

Scroll to Top