Soil acidity (by Rebecca Haschek)
Rebecca Haschek, from Diamond Creek, has recently been undertaking post graduate studies in Agriculture and, in her article below, shares some thoughts on soil acidity from her soil science class. She has tried to keep the chemistry to a minimum here to make it accessible to all.
What is soil acidity and why is it an issue?
Soils become more acidic over time because of various natural processes. The way we manage our soils can speed up or slow down these processes, having an impact on the health of our soil, the availability of nutrients and (indirectly) plant growth. In agricultural settings, these processes are proactively managed, however, as home garden producers, we don’t always have the knowledge needed to address soil acidification in our own gardens.
Soil acidity is actually a measure of the acidity of the water within soil, rather than any solid materials. Within the soil water, there are a number of hydrogen ions (H+) and hydroxide ions (OH–) and it is the relative abundance of these which determine pH, the measure of acidity. The pH can be anywhere from 0-14; a lower pH (<7) means that the soil water has more hydrogen ions (H+) and is more acidic; a pH of 7 means that the soil water has the same number of H+ and OH– ions and is neutral; and a higher pH (>7) means that the soil water has more OH– ions and is alkaline. An ideal soil pH for plant growth is usually in the range of 6-7, i.e. slightly acidic to neutral.
Why is the acidification of soils undesirable? In extremely acidic soils (pH of 4.5 or less), aluminium is released into soil water, stunting root growth and severely limiting crop yields. High acidity also allows chemical processes to tie up nutrients, making them unavailable to plants. For a plant showing yellowing from a lack of nutrients in high acidity, adding more nutrients (fertilising) is not going to help long term; rather, restoring the pH closer to the correct range will allow the soil to become more fertile again.
The main ways that soils acidify are through organic acid accumulation combined with removal of plant material, and through the over-use of fertiliser combined with nitrate leaching. Let’s look at these in more detail.
Soil acidity caused by removal of plant material
Organic acids are accumulated in plants through photosynthesis, where plants use carbon dioxide (CO2) to make new plant material, including organic acids. The plant can then exchange the acid’s hydrogen ion (H+) for a nutrient from the soil such as potassium (K+) or calcium (Ca2+). This exchange decreases the acidity of the plant (or increases its alkalinity) and increases the acidity of the soil. When plant material is harvested, the corresponding alkalinity of the plant is removed, and the acidification of the soil system cannot be reversed.
Bringing in organic material to replace what has been harvested appears an easy solution to this type of acidification. However, the addition of organic matter further contributes to acidity rather than neutralising it because, as this new organic matter breaks down, it contributes its own organic acids into the soil increasing acidity further. Please note that this does not mean we should stop adding organic matter to soil. To the contrary – the addition of organic matter increases soil health, and increases the soil’s ability to buffer pH changes; however, it does not ameliorate soil acidification.
Soil acidity caused by nitrate leaching
Nitrate leaching refers to the loss of nitrate ions (NO3–) through soil water moving deep into the soil. Nitrate is one of two forms of nitrogen taken up by plants for their growth (the other being ammonium, NH4+). Rainfall or irrigation allows nitrate ions (NO3–) to move easily through soil, and they are lost into the subsoil, ground water or offsite water flows. In agricultural settings, large amounts of leaching can cause environmental damage in the form of algal blooms. For a home gardener, where the scale is much smaller and environmental damage less likely, leaching concerns are around the loss of a nutrient and the subsequent acidification of the soil.
Note that, when nitrogen is added to soil in the form of ammonium-based fertiliser, such as urea or animal manures, ammonium is converted into nitrate through a series of reactions which release hydrogen ions (H+) into the soil water, increasing the acidity.
When nitrates are lost through leaching, as with the removal of plant material, the chemical process to reverse this acidity is no longer able to take place. An effective way to minimise nitrate leaching is to fertilise as closely as possible to the plant requirements to reduce the amount of excess nitrate in the soil – slow-release fertilisers are a good way to do this.
How to amend soil acidity
Soil acidity can be amended through the application of crushed limestone (CaCO3), which is alkaline when dissolved. This restores the balance of hydrogen ions (H+) and hydroxide ions (OH–) in the soil water and allows chemical and biological processes to fix toxins and release nutrients in the soil water. In agriculture, acidity is actively managed by the regular application of lime (at rates determined by agricultural activity, soil type and how much crop is removed).
In home gardens, the amount of lime needed to address soil acidity is harder to determine as it depends a number of factors including the clay content of the soil and the amount of organic matter in the soil. For example, clay particles can hold onto hydrogen ions in acidic conditions and, as lime is added, these hydrogen ions are released into the soil water, acting as a pH buffer. In this case, more lime would be needed to lower acidity. In sandy soils, less lime is needed as it does not have the buffering capacity of clay.
Whatever the soil type, adding too much lime will result in an alkaline soil and, if pH is increased too far, chemical processes will reduce nutrient availability and stunt plant growth, just like in highly acidic conditions. Soil pH test kits are a good way to ensure that your soil pH is in the ideal growing range and are relatively inexpensive to purchase.
* * * * *
In summary, soil acidity can be complex, with multiple factors involved. Being aware of acidification processes and taking small measures to keep soil acidity in an optimal range will help ensure our plants are able to get the most out of the soil and keep it in good shape for the future.
Thanks for your very informative article. As you say, soil pH is not as ‘simple’ a thing as it seems, and buffering capacity is very important. Leaf, Root & Fruit, who are based in Kyneton, have just published an article entitled The impact of compost on garden soil: a case study, which reports on an experiment that they have been conducting. They conclude that the best approach for backyard growers is just to use a lot of compost, which not only buffers pH but also makes many other nutrients readily available to plants and does not require close monitoring.