If you’ve been following along with me you should well know by now my interest in growing high quality nutrient dense foods. Unfortunately food like that is a rare exception, both in the grocery store and at the farmers market. Before we can begin to discuss what that really means and how to achieve it, there are a number of ideas that all gardeners are taught that need some amending. I must first bring up the problem of pH.
Simply put, pH measures the concentration of hydrogen ions, where a low pH is called acidic and a high pH is called alkaline. A quick search on the internet for pH gave me a random University Extension Office document that says “Soil pH is important because it influences several soil factors affecting plant growth”. It goes on to inform the reader that it can do many bad things like preventing bacterial release of nitrogen, it can cause nutrient leaching or restricts availability of nutrients to plants, creates heavy metal toxicity and makes clay soil hard to cultivate. It goes on to say that “a pH determination will tell whether your soil will produce good plant growth or whether it will need to be treated to adjust the pH level.” Most importantly it gives you a solution to this insidious problem: add lime if your pH is below 6.5 and sulfur if it’s higher than 7. And if your one of those lucky fellows who’s soil is in the sweet spot of 6.5 to 7.0 then you don’t have to do anything at all. Sounds like pH is very important and according to this we certainly couldn’t have good plant growth without it.
That is the conventional approach to pH and anyone that knows me well can picture me cringing right now. The issue I have with this is that pH is treated like it has meaning unto itself. The soil is considered secondary to the pH. If this recommendation were followed up by your typical NPK soil test lab and their garden soil correction plan of applying composted chicken manure, it’s quite clear that attention is only indirectly given to plant nutrition. Following these recommendations is a shot in the dark. If you’re lucky your soil conditions may actually improve until you do the same thing again next year, but for many gardeners it can do precious little to improve “good plant growth” and can actually make things worse.
First we need to talk a little about what pH is. It stands for “power of Hydrogen”. pH is a scale ranging from extreme acidity starting from a score of 1, to extreme alkalinity and a maximum score of 14, with 7 being the happy middle, the point of perfect balance. Water (H2O) separates easily into two ions with one of hydrogen (H+) and the other called hydroxyl (OH-). When these ions are in a one-to-one ratio you have a balanced pH of 7. If there are more hydroxyl ions (OH-) than hydrogen (H+) then the pH will be higher than 7, and if there are more hydrogen (H+) ions then the pH will be lower than 7. Thus, pH is a measure of hydrogen (H+) ions in solution.
This is however too simple to be useful. Hydrogen ions only cause soil acidity as measured by a standard test when they are in water, think of the water based test kits you see in the stores. In the soil, most hydrogen ions
are loosely attached to the soil particles along with many other positively charged ions called cations. These cations are there because of attraction to the negative charged particles, called anions. Anyways, these soil born hydrogen ions may not even contribute to soil pH unless the soil conditions change. Another interesting point is that as plants and microorganisms develop they will pull the plant nutrient ions, the cations, away from the soil particles and swap them out with additional hydrogen ions, thus causing the pH to drop during the growing season.
Materials you might consider adding to the soil to raise the pH are things considered to be alkaline such as calcium lime. However, this is relative to what might already be in the soil. For example calcium applied to a high magnesium soil may actually lower the soil pH. The same goes for acids, it’s relative. The application of an acidic substance to the soil can actually further increase your pH, the wrong direction. Further, your soil could be desperately short in calcium, an essential plant nutrient, and have a high pH. In that case following the standard pH amendment procedure of not applying calcium lime would keep your soil in a very poor state and the healthiest weed growth yet.
Lastly, those that worry about pH will also state that too low a pH can release too much nutrients causing heavy metal toxicity or too high a pH can cause the locking down of the nutrients so they’re not available to the plants. This may be true in a sterile laboratory, but in a biologically active soil it’s quite different. Biological compounds interact differently than non-biologic compounds. For example, a high pH soil can be induced to release nutrients when organic humates, molasses and biochelates are used. One more thing, if you measure your soil pH on a weekly basis you will see it change throughout the year, this is at least partially due to increasing or decreasing amounts of microorganism activity.
Thus your soil pH score is the result of complex chemical, physical and biological interactions and doesn’t give any clues about what might actually be in your soil. Taking actions based only on this number is the surest way to limit the nutrient potential of your vegetables and induce plant stress. A pH reading however isn’t completely useless, in part II to this article I will elaborate further on this topic and how it can be a useful tool to growing great foods.