3.4.3. Chemicals In Soil

Soil chemistry is the study of chemicals, pH, and chemical reactions or energy exchanges in the soil. There are significant amounts of chemical reactions and energy exchanges going on, in most soils. In some clay soils it’s possible to get enough electricity from the soil to run an electric fence, which some farmers have done.  See Further Study for more information.

Soil chemistry is a vast subject, but there are a few simple aspects about it that you should know.

Soil pH

Soil pH, which determines how acidic or alkaline your soil is, is an aspect of soil chemistry that can affect how well your plants grow. Most plants like a pH close to neutral. If pH is very acid or alkaline, plants can’t access key nutrients. One exception is blueberries, which like very acid soils. 

Finding out the pH of your soil is simple. You can buy inexpensive pH test kits; they usually aren’t very accurate but if you get a couple of kits and repeat tests, you can get a ballpark. We recommend sending soil samples into your state Extension Services or equivalent government body, which usually will do pH and basic soil chemistry tests for free or very inexpensively. They have professional equipment which is more accurate than most home pH tests. In Florida, they charge $7. Well worth it. 

Especially if you have acreage, you may want to get a full mineral assay too. We’ve listed some of our favorite soil labs that offer much more extensive testing of a wide range of minerals, toxins and organisms if you want to go that route. This is rarely necessary unless you have commercial acreage or you have reason to believe there may be toxins in the soil (a garden near an older building that may have been painted with lead paint, for instance). Share what you know about your soil with the lab and ask them for their advice on what to test.

Below is a pH chart that gives an overview.

The pH Scale

The scale for pH is logarithmic. What that means is that we start at 7, which is neutral. Every number in either direction is 10X as acid or alkaline as the number before it. So coffee is 10X as acidic as milk, and Sulfuric Acid is 100,000 times as acid as milk, for instance. That’s why it can kill you. But note that gastric acid, 10,000 times as acid as milk, does just fine in your stomach. 

Every plant prefers a certain pH. The vast majority of plants can do fine in a wide range of pH, around neutral, between 6-7.5. Most plants prefer a more acid soil rather than alkaline but can take slightly alkaline soils and many can handle a pH down to 5. There is more nutrient availability in acidic soils than alkaline ones and it’s easier to correct acidic soils. 

A few plants really like extreme pH. If a plant likes really acid soil, like blueberries, you will need acidic soil if you want blueberries. They need soil between 4.3-5.5 to thrive - up to approximately 500 times more acid than neutral soils! 

Nutrients are affected by soil pH. If the soil is too acidic or alkaline, what happens is that it locks up minerals that the plant can then not access. This is important because many people will add minerals or think their plant is diseased when you really just need to correct the pH. 

It’s relatively easy to correct the pH, with living soils. Live, healthy soils tend to neutralize both too acidic and too alkaline soils very nicely!  There are some soils that may be too acidic or alkaline to correct with organic matter or it may not be practical to do so, but this does work in a wide variety of cases. Using conventional methods, if soil is very acidic, the most common correction is to add lime. If too alkaline, the most common remedy is to add sulfur. We recommend you try building living soils first. Another advantage living soils have is that they can convert locked up minerals to bioavailable forms again.  

Minerals

Below is a chart that illustrates which minerals get locked up and where on the pH scale that happens. You can see that neutral to slightly acid or alkaline allows access to all of them.

Minerals can be locked up in soils for other reasons than pH. You may remember from chemistry class that minerals can have different forms depending on a variety of factors. These forms can be locked up and not available for uptake by the plant, or they can be “bioavailable” meaning that the plant can access and use that nutrient.

There is phosphorus in every soil to some degree, for instance, but it isn’t always in a bioavailable form. Frustrating right? Even more so for the plant. 

Here’s another nice thing about living soils. The bacteria and fungi in living soils perform a function of unlocking minerals in the soil that are usually locked up and not bioavailable through various processes. There are plants that also do this - some plants have a gift of being able to unlock some locked up phosphorus, for instance, and that plant can then be chopped and dropped or composted as its processes have made that phosphorus bioavailable to other plants now. These are called dynamic accumulators or mineral accumulators. 

Note: The science of mineral accumulation in plants is not very developed. There is a focus on nitrogen and phosphorus and some pretty clear studies published on those topics. But the charts floating around about dynamic accumulators are not definitive or the final word. Studies are still being done. I’m sharing one of these charts as an example. We’ll get more into mineral accumulators in the next section. 

Examples:

As an aside, some plants are great at accumulating metals including heavy metals. This is good to know as they can be used to remediate areas exposed to toxins. They do need to be discarded as toxic waste once used in that way, but they have now removed the metals from soil.

Using plants or fungi to clean up toxic sites is called bio or phytoremediation. One plant that accumulates a lot of metals is the sunflower, which is used extensively in phytoremediation.

Recently, some companies have started using plants to mine nickel from old strip mining sites. It’s cheaper to extract the nickel from the plant than from the soil, and it helps remediate the site, while harvesting nickel that would otherwise be mined from somewhere else.

A very partial list of plants that are metal accumulators

Nitrogen, phosphorus and potassium, oh my!

These are the big three macronutrients that are in chemical fertilizers and are the main ones listed on organic fertilizers as well. Plants need these in varying quantities, but because we create diverse systems in permaculture, we tend to ensure there is plenty of each of them for the various needs. It’s also important to have trace minerals in the soil which can be achieved with various soil building techniques we’ll get into. 

Key things to know about each of these:

Nitrogen is the protein for plants. It could be seen as the backbone. It is needed in high volume in most plants. It feeds the leaves; nitrogen deficiency often shows up in yellow or light green, weak looking leaves. Fully formed, healthy looking dark leaves with a lot of new, healthy growth usually indicate sufficient nitrogen. As a note, too much nitrogen applied when a plant is flowering and fruiting can cause it to drop flowers and fruit - it tends to go into leaf growth as priority. 

Sources: nitrogen fixing plants, live soils, manure, green organic matter, food waste

Phosphorus is used in every aspect of plant growth including roots, stem, leaves and fruit. Deficiencies show themselves in reduced growth, disease, and reduced fruit yield.

Sources: manure, urine, live soils, shellfish, hair, bones

Potassium helps the plant to perform many internal functions, it is a communications link, so to speak. Deficiencies may include yellowing or browning of leaves, stunted growth, lowered resilience to stress.

Sources: granite, kelp, wood ashes, crab waste, other sea life

Note that all of these are found in manure (including human waste), and live soils. Most mineral soils have plenty of each of these chemicals in them, but not necessarily accessible to plants. Live soils help make them so.