Waaay Beyond Just Organic
Soil Minerals and Soil Testing for Organic Gardeners


Compost, Manure, Humus, Organic Matter
and their relation to Soil Minerals and Trace Elements

Minerals and Manure

I had an interesting conversation with an organic grower a while back. This was a lady who had operated a successful organic goat dairy and CSA* for years.  I was doing my usual "missionary" work, proselytizing  about minerals, and she seemed interested.  When I mentioned how much simpler it was to bring in a few ounces or a few pounds of the minerals that were actually needed than to pile on tons of compost and manure, she became more interested.  She admitted, rather sheepishly, that her gardening operation was "addicted to chicken manure."  She told me that no matter how much home-made compost or cover crops they used, they just couldn't get the growth response and production they wanted without bringing in chicken manure from one of the commercial chicken farms in the area.

Not long before this conversation, I had noticed a "free manure" sign in front of a veterinary hospital that I drove past regularly.  This was a large-animal facility specializing in horses, and they had a stable behind the clinic, sort of an "in-patient" wing.  I thought about the drugs that were undoubtedly used in treating the equine patients, and the various cleaners and sterilants that would need to be used in a stable for sick horses, and I wondered how many "organic" gardeners were picking up this free manure and using it on their vegetables.

Also around this same time, I was helping out at very large garden that was growing food for the community food bank and I offered to do a soil test and recommendations for them.  The results from that soil test were a real eye-opener:  One area of about 5,000 square feet had the highest levels of Copper, Zinc, and Boron that I had ever seen.  I asked the garden manager just what had been used for fertilizer, and he told me a story about having had enough chicken manure delivered to cover the whole garden, but some volunteers had showed up when he wasn't there and spread the whole load on this one small area.  He said that he had tried to shovel it back into wheelbarrows and spread it around better, but this one area had gotten way more than intended.  Interestingly, although the Phosphorus and Potassium levels were high, they weren't exceptional.  It was the Cu, Zn, and B that were through the roof.

The fertilizer breakdowns of manure that I've looked at generally only give levels of Nitrogen, Phosphorus and Potassium, sometimes Sodium.  None of them tell me anything about any drugs, hormones, or disinfectants the farmer may be using, or what may have been added to the animal feed.  From my reading in the ecological literature, though, I have discovered that one of the biggest dangers in water pollution is the high level of dissolved pharmaceutical drugs found in the supposedly "treated" water flowing out of sewage treatment plants.  The sewage treatment plants are dealing with human waste, of course, but we all know that many if not most large livestock operations give their animals drugs to keep them "healthy" or at least alive in their crowded and unnatural living conditions.  Many dairies use the rGBH growth hormone for cattle, and most feedlots for fattening cattle use large amounts of antibiotics.**  Cottonseed meal is frequently fed to cattle as a cheap protein source, and cotton farmers are allowed to use all sorts of pesticides that are banned from use on food crops, supposedly because cotton is not a food crop.  It's also worth mentioning that most of the cotton, corn, and soy grown in the US these days is genetically modified, GMO, and most of that genetic modification is for herbicide resistance, so most of the feedstock has been sprayed with herbicides a number of times. None of the manure from these operations is treated with the care given to human waste; usually it is just piled up for a while until they can find someone to haul it away.  It could and probably does have anything in it.  And all of it, mind you, is accepted and used for "organic" gardening and farming.

That's the bad news.  There is some good news mixed in, sort of.  Going back to the chicken manure with the high levels of Copper, Zinc, and Boron, where did those come from?  It turns out that commercial livestock growers and feed manufacturers are well aware of the value of and need for minerals in the diet.  They have a lot invested in those animals and they want rapid and sustained growth.  I doubt that one could find a cow pasture in the country that doesn't have a mineral block sitting out, and those aren't just 'salt blocks", they have the whole spectrum of trace minerals in them and are specially formulated to supply the minerals usually lacking in various areas of the country.  The chicken farmers know about this too, and the pig farmers.  They know, or their feed manufacturers know, that Copper and Zinc added to the feed ration increases growth, and that Boron is essential for the utilization of Calcium.  Those minerals are in the feed or provided to the animals "free choice" as an economic necessity.  They must be added to the feed ration because the soil on which the grains and legumes and hay were grown didn't contain those minerals in the proper amounts or balance.

(Meanwhile, us humans are wandering around the middle aisles of the grocery store filling our shopping carts up with food that doesn't have nearly the mineral content of what the factory farm animals are fed, and we are wondering why we are always craving something. One could say that the grocery business doesn't make money on satisfying hunger.)

Whatever the motivations, most manure from commercial livestock operations contains trace minerals, and it may be those minerals that caused the lady who I wrote about at the beginning of this article to have a garden that was "addicted" to chicken manure. Zinc, Copper, and Boron are growth stimulants for plants, too.  Unfortunately, that manure may also be loaded with drugs, antibiotics, hormones, and other chemicals such as dewormers, disinfectants, pesticides, herbicides, and GMOs.  And we just don't know.  Even if the minerals in the manure are minerals we want and need in our gardens, we generally have no way of knowing how much of which mineral is in the manure.

Note, please, that I have no problem with using good quality well-composted manure for gardening.  If you raise livestock on healthy organic feed, or have access to manure from someone who does, count yourself very fortunate.  But don't count on it to supply the correct balance of minerals that your particular soil needs, until or unless the feed for those animals is grown on mineral balanced soil.

*CSA stands for Community Supported Agriculture, in which the grower sells "shares" of the crop to local people.  CSA members get a share of whatever crops are in season, usually on a weekly basis, throughout the growing season.

**The feedlots, and confinement dairies, must give the cattle antibiotics because they are fattening them on grain, seeds, and soy, which are unnatural foods for cattle.  Cattle are naturally grazers, eating grass and herbs and occasionally leaves from shrubs and trees.  They have a huge population of microbes (mainly protozoans) in their stomachs to break down the cellulose in these plants and turn them into food.  These microbes are not happy with grains, so the feedlot operators give antibiotics to kill off the normal microbes, and also give the cattle bicarbonate of soda to deal with the resulting acid indigestion.  Large feedlots buy baking soda by the carload and antibiotics by the ton.  Another unfortunate aspect of this unnatural feeding is that it changes the fatty acid profile of the milk and meat so that the fats cause atherosclerosis and heart disease and other problems in people who consume them and in the cattle.  Grass fed beef and dairy do not cause health problems, they confer health on those who consume them.  Much more on grass fed beef and dairy at

Compost and Minerals or
Why Does My Garden Need a Soil Test?

By Agricola
March 23, 2008

We all know that a fertile soil grows better crops, just as we all know that nutritious food grows a healthier body, and the same minerals that make the soil fertile are the minerals that make food more nutritious.  The lack of essential minerals in the soil will have the same sort of  detrimental effect on crops that the lack of minerals in our diet has on our health.  The analogy goes even further:  It is largely the presence of healthy soil microorganisms that make the minerals available to the plant, and it is largely the same sort of microorganisms in our digestive systems that make the minerals in our food available to our bodies.  Neither the plants nor our bodies can do much with simple ground-up rocks; the minerals first need to be changed into a form that can be absorbed.  That is what a healthy, biologically active soil does for the plants, and what a healthy population of probiotic organisms does in one's digestive system.

Now we begin to get into something interesting and controversial:  The organic and biologique (Euro-speak for organic) gardening movements are all about creating that healthy population of soil organisms by increasing the organic matter content of the soil.  Great effort is put into making biologically active compost and applying it to the garden and croplands, but little or no effort is put into supplying the microorganisms in that compost with minerals.  Organic matter is vitally important to a fertile soil, but it is only one-third of the whole equation, or, one could say, one leg of a three legged stool that supports the whole food chain for life on Earth.  Those three legs are biology (the living and formerly living parts of the soil), minerals, and energy (as in energy flow like a current of water or electricity).  In most cases, if one gets the minerals right, the biology and energy flow will fall into line automatically; one cannot prevent life from growing in an environment that has all of the essential minerals it needs; the soil and plant life will find those spots and thrive there.  This essay will address the biology and mineral aspects; we'll save the energy leg of the stool for another time.

As noted, life will find those places that have all of the mineral nutrients needed for growth and reproduction, but on the other hand, one can have a highly organic soil and still not have healthy crops if the minerals are missing or out of balance.  Any grower who has tried growing plants in pure, sterile organic matter, such as unfertilized potting mix, will know that doesn't work too well.  The plants need more than just water, air, light, and an organic medium to send their roots through:  They need mineral nutrients too.  Standard chemical fertilizers supply three of those nutrients:  Nitrogen, Phosphorus, and Potassium, the familiar NPK listed on the fertilizer package as, for instance 5-10-10.  In addition, air supplies the nutrients Oxygen and Carbon (from Carbon dioxide) while water (H 2 O) supplies Hydrogen and Oxygen.  With these six nutrient elements, Nitrogen, Phosphorus, Potassium, Carbon, Hydrogen, and Oxygen, just about any plant can be grown, but it won't necessarily be a healthy plant and it surely won't make very nutritious food.

We noted above that soil microorganisms are necessary to make soil minerals available to plants, so how do the plants manage to grow when fed only a chemical mixture of N, P, K, and water?  Commercial chemical fertilizers are made with highly soluble salts of NPK that the plants are able to absorb through their roots and use much as a person unable to eat can get nutrients from an intravenous IV drip.  Plants are simpler in their nutrient requirements than higher animals, and able to use elements in simpler forms, so it is easier to grow large and healthy-looking plants on an IV drip than it is to keep humans healthy on one.  In nature these simple forms of soluble salts are seldom plentiful in the soil.

Moving to the next step in organic-matter based fertility, what happens when a plant is grown in pure, rich compost, without any mineral soil?  Often it will do well, or at least appear to do well, growing large and quickly, but it also may be susceptible to fungal diseases and blights, and the food grown will often lack flavor.  Compost, of course, does contain some minerals, the amount and range of minerals depending on the source of the compost.  Leaf compost will contain the minerals that the leaves contained, compost made from garden waste will have the minerals that the garden crops absorbed while growing, compost made from animal manure will contain the minerals that were in the feed the animals ate, perhaps including grain imported from other areas with different soil minerals, and often including mineral supplements that the animals were fed.  Chicken manure is known as the manure with the most fertilizing power, and that is largely due to the very high grain content that the chickens are fed, as well as the insects that the chickens eat whenever available.  However, chicken manure is also usually a good source of Calcium, Boron, Copper, and Zinc; not because the grains in the chicken feed are high in those elements but because the chicken feed is fortified with those elements.  Similarly, cattle and horses are generally given a "salt block" both in the pasture and in the barn or paddock and many times are given a powdered mineral mix "free choice" at their feeding stations.  Hog feed is also fortified with minerals.  Interestingly, the salt blocks given to horses and cattle are different in different areas of the USA, so that whatever minerals are usually missing in the local pastures can be supplied.  The veterinarians, veterinary researchers, and farmers are all well aware that the animals need far more minerals than are normally found in their hay or grain rations alone.

Think of how strange this is:  Even the smallest farmers make sure that their cows and horses have a salt block, even the person with only a small flock of laying hens in the yard supplements the birds' diet with at least oyster shell grit to make the eggshells strong, yet they give little or no thought to mineralizing the pastures where their animals graze, the hay and grain fields where the feed is grown, or their gardens where they raise the food to feed their families or to sell.  Nor do they give much thought to how many of these essential minerals they or their families are getting.

The claim is often made that "organically grown food has more minerals", but seldom is it backed up with factual evidence, so let's take a look at that claim.  Obviously if the minerals are not in the soil, they cannot be in the crops grown.  What quantity of minerals are available from compost, for instance?

Completely dry plant matter consists mostly of compounds made from the air elements Carbon, Hydrogen, Oxygen, and Nitrogen.  The Nitrogen originally comes from the air, but is made available to the plants by soil microorganisms, or today by synthesized Nitrogen fertilizers.  If this dry plant matter is burned, perhaps 95% of it will return to the air as some combination of these four elements.  The remaining 4 or 5% is unburnable ash, and that is where the soil minerals reside.  The minerals in that ash will naturally vary depending on the species of plant and the soil in which it is grown.  The ash from wheat straw will be high in Silica, an essential nutrient but not one in short supply in our croplands.  Silica is the most abundant of all Earth elements.  The ash from the wheat kernels themselves will be much richer in essential minerals, as the plants concentrate them there to feed the seeds for the next generation.  If one starts with 100 lbs of fresh compost, which will likely be around 75% moisture, and then dries it to leave 25 lbs of dry organic matter, and then burns that to ash, one will end up with about 1 1/4 lbs of mineral ash total, perhaps a double handful.  One can see that there is really not a whole lot of minerals in that 100 lbs of compost, and we haven't looked yet at just which minerals are to be found in that ash.

Of course, there are many beneficial plant nutrients to be found in the 98 3/4 lbs of compost that we are not measuring as ash, such as humus and fulvic and humic acids, ammonia and nitrate Nitrogen, natural growth stimulants, beneficial fungi and bacteria, perhaps earthworms and arthropods, but we are talking here about the actual mineral content of the compost; there are thousands of books and articles written about that organic portion but very, very few about soil minerals.

Let's look at another factor in using compost or organic matter as a mineral source:  How much would we need to use to add significant amounts of needed minerals to the soil?  This gets a little difficult to quantify, but going back to research done by Davidson and LeClerc in the 1930s, we find that the amount of Potassium found in ash from commercial vegetables was around 7%, the amount of Calcium averaged about 2% (they also measured 95%+ moisture content and 20% ash from dry matter, which leaves only 1% total ash, but let's be generous and stick with that 1 1/4 lbs we came up with above).

1 1/4 lbs= 566 grams
566 grams x 2%=   11.3 grams Calcium per 100 lbs compost
566 grams x 7%=   39.6 grams Potassium per 100 lbs compost

Even a sandy loam requires at least 2,000 lbs of Calcium per acre for best growth.  What if we measured the minerals and found that we needed to add 1,000 lbs of Calcium?  How much compost would that take, at 11 grams per 100 lbs?  I'll spare you the arithmetic:  It would take about 4,000,000 lbs:  Four million pounds of that 75% moisture content compost per acre to add 1,000 lbs of Calcium.  Wait, it gets worse:  While we were adding that 1,000 lbs of Calcium we were also adding almost 4,000 lbs of Potassium, far too much.  Well balanced soils need about 1/7th as much Potassium as Calcium, so this soil would call for about 280 lbs of  Potassium per acre; we would be adding over 3,700 lbs too much, assuming that we were crazy enough to try adding four million pounds of compost anyway.

Putting that in terms a backyard gardener could relate to, one would need 90,000 lbs of compost per 1,000 square feet of garden just to bring the Calcium level up to par.

It's easy to see from the example above that although compost might be a reasonable source for Potassium, if we knew the Potassium needs of that soil in the first place, it isn't going to work for most of the other minerals.  Just to add 140 lbs per acre of Potassium would require 80 tons of this particular compost per acre.

Note that we haven't even considered the other fifteen or so other essential plant minerals, nor the other thirty or so essential minerals needed by humans and animals.  As essential and marvelous as compost and organic matter are, we are not going to be able to depend on them to provide a balanced supply of minerals to the soil.

The only way to know what the mineral content of the soil is, is to measure it.  That's what a soil test does.  Further, the only practical way to add the minerals that are needed, and to bring them into balance, is to add them in the mineral form, not as some minuscule portion of the organic matter.  Agricultural "sweet" lime, which is simply ground up limestone, is about 40% pure Calcium.  To add 1,000 lbs of Calcium using sweet lime would require 2,500 lbs of sweet lime per acre.  That is do-able. That is 56 lbs of sweet lime for a 20' x 50' garden.  To add 140 lbs of Potassium one would only need to use 280 lbs of naturally mined sulfate of potash per acre, or 6 lbs per 1000 square feet.

Your garden also probably needs a few ounces of Copper, Zinc, and Boron.  It may or may not need Phosphorus or Magnesium or Sulfur.  How are you going to know?  By getting an inexpensive laboratory soil test, and either learning how to interpret it yourself (not hard to do) or paying someone who does know how a few dollars to interpret it and make recommendations for your particular soil.  Then, for the first time, you can quit guessing and know exactly what your soil needs to grow the most flavorful, healthy, mineral and nutrient rich crops you have ever had.

The Ideal Soil:
A Handbook for the New Agriculture

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Teaches all you need to know to become your own soil minerals expert!

(Yes, really)

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