1. How Does Aluminum in Soil Turn Hydrangea Flowers Blue?
If you grow hydrangeas in soil that does not contain aluminum, the flowers bloom purple instead of blue. When researchers at Nagoya University looked closely at the flowers, they found a mosaic of blue, purple, and red cells. The bluer the cell, the more aluminum ions it contained [1].
It is often said that "acidic soil makes blue flowers, and alkaline soil makes red flowers." In acidic soil, aluminum dissolves easily. This dissolved aluminum is carried to the flower’s sepals (the leaf-like parts under the petals) and binds with a pigment called anthocyanin [2]. However, soil type alone does not decide the color. The final color depends on a combination of the amount of aluminum, substances that help the pigment, and the acidity inside the cells [2]. In acidic soil, aluminum dissolves out easily. Normally, this aluminum is locked inside rocks and clay.
2. When Does Aluminum Locked in Rocks Start to Move Into the Soil?
Aluminum is the third most abundant element in the Earth's crust, after oxygen and silicon. It is the most abundant metal. It makes up about 8% of the Earth's crust by weight, or about 82 grams per kilogram [3].
Despite being so common, aluminum is rarely noticed in normal soil. This is because it is tightly bound with oxygen and trapped inside the minerals of rocks and clay. It only starts to move in environments that have become acidic [3]. Acid rain can be the trigger that releases this trapped metal. The next section looks at what happens in the soil when this rain falls.
3. How Does Soil Handle Acid Rain?
Normal rain is slightly acidic, with a pH of about 5.6, because carbon dioxide from the air dissolves in it. Acid rain, caused by sulfur dioxide and nitrogen oxides, usually has a pH of 4.2 to 4.4 [4].
Soil does not become acidic immediately when acid rain falls. A report from the Tochigi Prefectural Agricultural Experiment Station suggests that soil acidification happens in four stages. First, carbonates neutralize the acid. Second, calcium and other elements neutralize it. Third, acids are adsorbed and aluminum begins to dissolve. Finally, the weathering of rocks provides neutralization [5]. This means calcium is used up first. When calcium runs low, aluminum begins to dissolve into the soil.
In an experiment with artificial acid rain (1991–1993) described in the same report, results differed by soil type. In black soil found on forest floors, aluminum increased year by year. In gray lowland soil that was once a rice paddy, aluminum did not dissolve out [5]. Note that the most acidic water used in the experiment (pH 3.0) was stronger than actual rain. The next section asks what this moving aluminum does to trees.
4. How Does Moving Aluminum Harm Trees Such as Red Spruce?
A study of red spruce forests in the eastern United States (published in the journal *Nature* in 1995) showed a pathway where aluminum, released from mineral soil by acidic deposition, is transported to the forest floor soil. There, it reduces the calcium stored in the soil [7]. The US Environmental Protection Agency (EPA) explains that acid rain dissolves aluminum from the soil, which can harm both plants and animals. It also removes minerals and nutrients needed for tree growth from the soil [6]. The Tochigi report also states that excessive aluminum is considered the biggest factor hindering plant growth in acidic soil [5].
However, the exact mechanism of how aluminum stops root growth is not fully clear. So, what happens to trees if we put the lost calcium back into the soil?
5. What Happened When Calcium Was Added to the Hubbard Brook Forest?
In the Hubbard Brook Experimental Forest in New Hampshire, USA, soil calcium levels were found to have dropped significantly. In October 1999, researchers used a helicopter to spread 40.8 tons of the calcium-containing mineral wollastonite over a small watershed of about 12 hectares (about 30 acres). This amount was designed to slowly replace the calcium estimated to have been lost during the 20th century [8].
After analyzing 15 years of data, the watershed that received calcium showed a recovery in overall tree growth (biomass increase). Leaf area also increased, and growth was better than in the comparison watershed. The response varied by tree species, with sugar maple showing the strongest reaction. Researchers concluded that soil acidification caused by acidic deposition was linked to forest decline, and adding calcium could reverse it [8].
Furthermore, the ratio of calcium to aluminum in soil water (Ca/Al) is used as a measure of acidification stress. A ratio above 1 is used as a benchmark, and the treated watershed exceeded this level after treatment. It is thought that the balance between calcium shortage and aluminum is involved in why trees weaken [8]. However, these results are from forests in the northeastern USA that already had acidic soil. Not all forests may recover this way. Why does soil recovery lag behind the recovery of rivers and lakes?
6. Why Does Soil Recover More Slowly Than Rivers and Lakes?
In North America and Europe, emission regulations have reduced acidic deposition, and the water in rivers and lakes has slowly recovered. However, the calcium and magnesium lost from the soil remain significantly depleted. Therefore, it is pointed out that the effects of acidification may continue in forests for some time [8].
Water recovers first, and soil recovery follows later. Considering this, looking at what is inside the soil is essential for protecting forests. The story that started with flower colors led to invisible events inside the soil. Hydrangea colors are just one entry point to understanding this.
7. How Can You Test Soil Acidity Yourself With Purple Cabbage?
You can test the nature of soil using the juice from boiled purple cabbage. Mix soil with water in a cup, let it sit for a while, and add a few drops of the cabbage water to the cabbage juice. If it turns reddish, it is likely acidic; if it turns blue-green, it is likely alkaline. Note that this only gives a rough idea of acidity or alkalinity, not the amount of aluminum. After the experiment, wash your hands. Do this experiment with an adult.
You can also collect soil from different places in a park or garden and compare them using the same method. If you see hydrangeas, comparing the flower color with the nearby soil is one way to observe. However, flower color also changes due to variety and other factors, so treat this as fun observation rather than a definitive experiment. Official reports from the Tochigi Prefectural Agricultural Experiment Station and the US EPA’s explanations on acid rain are available online [5]. For safety, do the experiment together with an adult, and wash your hands afterward.