Aluminum toxicity

Introduction

Aluminum is the most abundant metal in the earth’s crust. Of course, it isn’t in its metal form that we know it as, such as aluminum foil or pots and pans. It is in its ionic form throughout the soil. Aluminum ions can be poisonous to plants, and due to the abundance of aluminum in the soil, there is risk of aluminum toxicity especially in places of low soil pH. Fortunately, most of the time aluminum is in forms that don’t bother plant growth. To explore aluminum and how it interacts with plants, we will begin with a bit of soil chemistry.

How does soil pH affect aluminum?

The most common symptom of aluminum toxicity is the inhibition of root growth. In fact, root growth can be slowed or stopped within one hour of exposure to aluminum! Aluminum is found in various forms in the soil depending on the soil’s pH. pH is a measure of the concentration of hydrogen (H⁺) ions in a mixture and is measured on a scale of 1 to 14. The higher the hydrogen concentration, the lower the pH and the more acidic the mixture is. 1 is extremely acidic (like battery acid), while 6.5-7.5 is considered neutral (like clean water), and 14 is extremely basic or alkaline (like bleach or drain cleaner). Alkaline soils have a high concentration of hydroxide ions (hydrogen and oxygen combined, OH^-).

In neutral and alkaline soils, aluminum combines with other molecules in the soil to create nonphytotoxic forms. Phytotoxic is a fancy word meaning poisonous to plants, therefore nonphytotoxic means not poisonous to plants. In soils with a pH lower than 5.5, however, aluminum takes on what is generally considered its phytotoxic form, wrote simply as Al³⁺.

The ”3+” of Al³⁺ indicates that it is a trivalent cation. Let’s break that term down! Tri = three. Valent refers to the positively charged protons or negatively charged electrons that float around on the outermost part of an ion, or the valent ring. Cation is the term used for a positively charged ion, versus anion, which is a negatively charged ion. So what does all that mean for you, reader? Al³⁺ is an ion with a strong positive charge.

How does aluminium impact the plant?

Aluminum, which we will call Al³⁺ for the rest of this article, affects plants almost exclusively at the root apex. This refers to the entire root tip. This includes the root cap which protects the meristem. The meristem is where all the actual growth and cell division occurs and is at the front of the elongation zone. Al³⁺ has been found to barely impact the mature parts of the root system.

Al³⁺ can cause a substantial amount of damage at low levels. The primary cause of trouble is that Al³⁺ can displace other cations like magnesium (Mg²⁺) or calcium (Ca²⁺). This reduces Mg²⁺ and Ca²⁺ uptake, which are both used to control growth and activate enzymes in the plant, crucial for plant development.  Alternatively, the strong positive charge of Al³⁺ can bind up anions that the plant needs, like phosphate (PO₄^3-). This makes those anions unavailable for plant needs, and in the case of phosphate, causes phosphorus deficiency in the plant.

Some other problems Al³⁺ can cause include blocking nutrient transport channels and slowing down cell division.

How do plants protect themselves from Al³⁺?

Al³⁺ can do quite a bit of damage in acidic soils. Approximately 40% of all arable land (land capable of agricultural production) is acidic or at risk of acidity. Some soils are naturally acidic based on the vegetation they evolved under or the bedrock it formed from. This is especially seen in tropical and temperate places, meaning many developing countries suffer from aluminum toxicity and correspondingly low crop production despite otherwise fertile soils.

Soil can also be acidified. This can happen from human actions like applying ammonium or sulfur fertilizers, or through natural processes like decaying plant matter. With so much acidic soil around the world, plants have both naturally and with human influence developed methods to protect themselves from Al³⁺.

The protective mechanisms plants have can be separated into two categories: exclusion or tolerance. The most popular theory of exclusion is the plant’s ability to release organic acids, like citric and malate acids. These chelate Al³⁺, which means that the acids bind to Al³⁺ and turn it into a nonphytotoxic form.

Tolerance mechanisms are less understood. Generally, plants that have evolved with acid soils can tolerate Al³⁺, and it’s believed to be gene expression induced by the presence of Al³⁺. In fact, in some rare cases, the presence of Al³⁺ stimulates nutrient uptake!

So, what is the next step?  

Many people would simply think “stop acidifying the soil” is the solution to managing aluminum toxicity. And yes, that would be a great solution! However, artificially increasing the pH of soil is often cost prohibitive, unsustainable, and can be environmentally harmful. Many of the processes that acidify soil are either natural or are crucial for adequate agricultural production.

There are two relatively affordable and easy solutions that have been shown to prevent crop loss due to aluminum toxicity. One is to continue breeding tolerant crop genotypes. Much work has already been done with wheat varieties, especially since many cereals and grasses are tolerant of aluminum. Another is the addition of other minerals to the soil to balance nutrition.

There is still work to be done to improve plant growth in the presence of aluminum, but with greater understanding of plant and soil biochemistry, science is well on its way to mitigating these issues.  Every day, better farming practices are implemented, access to better plant stock increases, and production on acidic yet fertile soils worldwide improve.

For more interesting articles diving into the fascinating sphere of plant science, keep checking this space!

By Stacy Berry | 13 September 2022

Stacy Berry was born and raised on a farm in Alberta, Canada. She graduated in 2016 from the University of Alberta with a Bachelor of Science in Agriculture, major in Crop Science. Stacy has mostly worked in municipal government (vegetation management), and primary agriculture production (including horses, cows, and bees). She currently works for Nutrien Ag Services and is excited to be writing for Lab Associates! She is an animal lover who enjoys reading good books, eating good food, travelling, nature, the outdoors, and spending time with friends. 

References

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