Soil Chemistry: the art and science of healthy soils

When you pick up a handful of soil you can observe certain characteristics – maybe it’s sticky or moist, perhaps it’s sandy or dark. In our first blog, we looked at the top layer of soil health, its structure, which is easier to grapple with because we can see it with the naked eye. A soil’s chemistry is invisible. It has to do with the most basic atomic constituents that interact, react, cooperate and obstruct.

In agriculture, soil chemistry is often associated with fertilizer application. This field of study helps farmers understand the inner-workings of their soils (which nutrients exist in abundance, and which are lacking) so they can make educated decisions about supporting them. In conventional farming, soil chemistry often boils down to identifying which fertilizers can be added to maximise crop yields. But if you’re a regenerative agriculture soil nerd like me, it’s more about discovering what naturally available plant-soluble nutrients already exist, and how they can be unlocked.

Soil chemistry testing methods are many and varied. And some are more effective than others.

More traditional methods are performed by extracting micro and macro nutrients from the soil using a strong acid or base. There is the Ammonium acetate method, the Morgan, or Mehlich III method, to name just a few. In these processes, all the potentially available nutrients that could be drawn up by a plant throughout the entire length of the growing season are captured at once.

But the potentially available nutrients that get immediately captured only represent a fraction of the total mineral nutrients actually found in soils. In fact, the majority of nutrients remain bound up in rocks and sand particles and are so stable they do not react to the extracts used in these testing methods.

Essentially, these methods do not mimic real-life processes. In reality, plants are not so efficient at extracting nutrients from the soil and must employ microorganisms to do it for them. Generally, soil bacteria secrete alkaline solutions and soil fungi secrete acidic solutions that can dissolve the nutrients; these microbes then ‘trade’ their nutrients for the plant’s sugars, something that plants possess in abundance.

A plant’s natural acids and bases are far weaker than the traditional extractants used by the soil chemistry tests and nutrient extraction happens over the length of many months. By using either water or very weak acids and bases for nutrient extraction, newer soil testing methods like the Haney soil test provide a better picture of which nutrients might actually be available to plants. These methods also test for microbial activity and organic sources of nutrients to give a much more complex picture of the soil nutrient profile. This also reduces the number of assumptions an agronomist must make, meaning their fertilizer recommendations are more accurate and targeted, saving greatly on the amount of fertilizer applied.

Once you know which nutrients are available to your plants, now what? Now you need to consider if there are shortages of certain nutrients or an imbalance in soil pH that might make it harder for plants to use them. At this stage, agronomists are often employed to analyse the data from the soil tests, and to offer recommendations on how to adjust the soil chemistry to ideal values.

But what are ideal values?

There are several theories about the optimal values of soil nutrients levels. Some theories take into account the nutrients that will be removed from the soil through farming a crop minus the nutrients that are already present in the soil, and then recommend adding what is deficit in the form of soluble fertilizer.

Other theories, like the Albracht method, try to balance the nutrients present to (so-called) ideal levels, which are based on the balance of cations (positively charged nutrients – calcium, sodium, magnesium and potassium) to each other (see this article from the University of Georgia on cation exchange capacity and how it’s impacted by soil pH). The soil is then adjusted through the bulk addition of certain mineral fertilizers that can shift the balance of cations.

And some theories ignore soil chemistry altogether, focusing instead on plant-nutrient testing and fertilizing based entirely on what the plant’s tissue or sap analysis says is deficient. Plant sap testing can be a very simple way for farmers to assess the relative health of their crops. Brix is the indirect measure of plant sugars, which, if used well, can be a great tool for farmers to manage plant health.

In the end, the theory used to generate your fertilizer recommendation will depend on what your agronomist or soil scientist is most versed in. And this will vary greatly from one agronomist to the next.

So what is the right way to fertilize? The answer is, it depends. Detailed soil analysis and recommendations are only useful if the interventions are available geographically, financially, and practically in your circumstance.

There are, however, a few good principles that work across landscapes and contexts.

1. Building your own working knowledge of the way your soils react to fertilizers and keeping a long-term record is essential to improving the efficiency of your fertilizer applications in the long run.
2. If you decide you need the support of an agronomist or soil scientist, be sure they are independent. They should be interested in your success and your soil health, rather than motivated by selling you fertilizers.
3. Frequent testing and observation will give you a clear picture of the direction your soil is taking. When you choose a fertilizer, consider those that are less harmful. A lot of the cheap chemical fertilizers like those that are ammonium or potassium chloride based have a very high salt index and are extremely detrimental to soil biology. (Remember: soil microbes are what will actually help your plants absorb fertilizers more efficiently, so it’s counter-productive to kill them off!)
4. Lastly, consider that it’s fine to treat short term symptoms of deficiency, but you should be searching for the root of your soil fertility problem and actively tryings to solve it.

Soil chemistry is a complex subject on its own. Adding to the challenge is the fact that a soil’s physical characteristics and biology affect fertility as much as the available nutrients. Empowering yourself with knowledge is essential so that even if you do choose to work with a soil fertility consultant you can participate in creating a solution that is sustainable and sensible for you – rather than being at the mercy of external advice. Whatever road you take to creating healthy, working soils, it should be holistic and one that you can stick to for the long-run.