Gardening can be like looking at an iceberg: we often focus on what is on top at the expense of what lies beneath.
One of the essential elements of any crop is getting the pH level of the soil right. Yet what is pH – and how can we use it to make sure our plants survive and thrive?
A revelation
This topic is like a trip down memory lane! As a child, I was following my dad on his travels in the country and I still remember the day we were in his home village. At a crossroads there were three older folks – hoes and picks on their shoulders, their faces aged and leathered by years of exposure to the weather – who were definitely farmers and plot owners.
They were talking in their dialect about crops, how hard the season had been, not enough rain, not enough hours in a day. Their chat and meeting was not casual. They were deliberately waiting for one person, who showed up not long after we arrived – this person was the agronomist!
This well-dressed gentleman, who had the look of someone who had spent most of his life on studies and research, was paying his annual visit to the local folks to provide support and direction. I’m not sure if the farmers were comprehending his erudite lingo, but I grasped two words that immediately grabbed my attention: acid and pH.
I recognised those words as I was studying chemistry at school. Yet at that point, I thought that pH was only something used for car batteries and swimming pools. I realised then that the topic had application in agriculture as well – and by the look of that meeting, it was a serious subject: what a revelation!
I suddenly put more effort into my chemistry studies, driven by a curiosity over what acids, bases and pH had to do with our food. To be frank, I do not regret a minute spent on learning the subject, as this knowledge has been useful during my many years of amateur gardening and as a self-appointed promoter of self-sufficient living off the land.
What is pH and why is it so important to understand how it affects our soil?
I often compare gardening to a floating iceberg. Everything in our garden that is above the soil – namely plants, weeds, greenery, vegetables, fruits, pests or weather – is what I call the tip of the iceberg, the part we can observe and gain some direction from its bearings. Then there’s the part that happens below the ground, pure chemistry and biology, that is like the submerged part of the iceberg, enormous and unfamiliar.
Yet, a good solid structure – or even better, a perfectly balanced soil composition – can be the difference between healthy plants with abundant crops and a waste of time and money. We often hear that a particular plant needs a slightly acidic soil to thrive, so if we understand what the pH of our soil is, how we measure it and how we can alter it, then we have added another tool to our toolbox.
In chemistry, pH is a scale that measures the acidity or alkalinity of a substance. If a substance is diluted inside a water solution, pH (or “power of hydrogen”) reveals the quantities of hydrogen ions (H+) available to bond with other elements.
For ease of international convenience, the scale goes from 1 to 14, where 1 is a strong acid, 7 is neutral and 14 is a strong alkaline or base.
- Any value above 7 – like for example 7.5 – is referred to as high pH
- Any value below 7 is considered to be a low pH
- Pure water is neutral with a pH of 7.0
Both extremely high and extremely low pH values are related to dangerous substances that are highly unstable and unsuitable for the life of most organisms on Earth, from the microscopic virus to the human being.
What is a good pH level for my crops?
Vegetable plants, fruit trees, herbs, flowers and other ornamental greenery usually grow in soils with a pH that ranges from 5 to 8 – but there are big differences when it comes down to each specific plant.
Asparagus, beans, cauliflower and garlic thrive in soils with a high pH, between 7.5 and 8.0.
Meanwhile, tomatoes, celery, carrots and pumpkin grow well in slightly acidic soil with pH of 6.5.
Soil with a lower pH of around 5.5 is more suitable for vegetables and fruits like blueberries, potatoes, kūmara and eggplants.
It is important to prepare and adapt plots and garden beds specifically for crops.
The “one soil type does it all” approach may suffice at the beginning, but eventually problems such as poor growth, pests and diseases will signal that that plant is not doing well in that soil. Often an incorrect soil pH means the plant is not able to absorb specific nutrients, and this can cause some imbalances for a correct and organic grow.
What other nutrients does the soil need?
The most important nutrients for plants are nitrogen, phosphorus and potassium – these represent the bulk for basic organic growth. To these we can add other elements like zinc, magnesium, calcium and sulphur, just to name a few, that support additional functions like fruiting, flowering, photosynthesis and immunity. All these nutrients are always available in the soil, but the question is: “Is the plant capable of absorbing these nutrients when it needed and in the right quantities?” Only if the pH is right!
Elements like nitrogen, phosphorus and potassium are more easily absorbed by roots if the soil pH ranges from 6 to 8 (lightly neutral); sulphur, calcium and magnesium between pH 5 and pH 8 (slightly alkaline); and iron, copper and zinc from pH 4 to pH 7 (slightly acidic).
This explains why plants like blueberries, bulk feeders of iron, copper and zinc, will excel in acidic soil but will merely survive, flowering but not setting fruits, in neutral soil.
Deficient soils
Nutrient deficiencies are not often noticed as we focus more on what we can observe and manage – plant diseases, poor cropping and premature bolting are often linked to or blamed on environmental factors such as frost, dry spell, pest infestations or crazy seasons, while the root cause (excuse the pun) is actually where the roots are, under the ground in a soil not capable of releasing the right nutrients.
Once I made the mistake of planting lettuce, basil and gherkin seedlings into a newly built garden bed filled 100 per cent with compost. At the beginning it looked very promising – dark and friable. But two months down the track the seedlings were still the same size as when they were first planted, and struggling to stay green. The soil was full of nutrients, but none were available to the plants to absorb as the soil pH was wrong.
Knowing and controlling the pH of your soil is a bonus – but enthusiasm for trying new crops, patience in learning from any mistakes, and accepting Mother Nature’s cycles are the best tools to enjoy gardening and feast on its fruits.
The PH scale
While most plants thrive in soil ranging from pH 5-8, some do better in more acidic soil, while others lean towards a more alkaline environment.
Franco’s gardening tips
These days, pH testing kits are extremely user-friendly and take all the complicated chemistry out of the equation.
Available from most nurseries or DIY centres, they can vary from the cheaper testing kits to the more expensive meters. Testing kits will require a sample of your soil to be diluted into water and mixed with a few drops or a sprinkle of reagent. Testing meters (analog or digital) are fuss-free as you insert their metal probe directly into the soil for an instant reading.
Once you know your soil’s pH, try to adapt to it by cultivating only plants that do well in that kind of soil. However, if your soils do not satisfy any application due to the pH being too high or too low, substances like lime or sulphur can help to create a balance.
Lime is a strong alkaline and it will balance acidic soils, while sulphur will take care of soils with a high pH.
Is the soil in your plot or raised garden bed pH 6.5?
Jackpot, no action required! You have a very forgiving soil that will support many varieties of the most popular fruits and vegetables. Chemistry dictates that most of the vital elements providing nutrients to the plants are readily available for the roots to absorb when the surrounding soils are pH 6.5.
Keep the soil moist!
Water is the vessel that transports nutrients from the roots to the rest of the plant, so make sure you prepare the garden bed with the nutrients at the right depth. Plants with tap roots will naturally dig down deep looking for a water source to overcome potential droughts. If the nutrients are concentrated close to the surface, tap roots will miss out.
