The calcium content of different tree types


Hi there! My name is Baran, and I’m doing a project for my grade 12 chemistry class where I design my own lab experiment. I’m currently thinking about measuring the % purity of calcium carbonate in samples of wood ash obtained from different woods using back titration. My dependent variable (the effect I will measure) is going to be the % purity of calcium carbonate, but I’m a little unsure about my independent variable (the thing I will change, such as the species of wood used). I just have a couple of questions:

1. Do hardwoods have greater calcium content than softwoods, or is calcium content in trees only dependent on the amount of calcium found in soil?

2. In your opinion, what independent variable would be the best for this type of experiment? Since I need to be measuring the impact of changing one thing on the % CaCO3 in the wood ash, but I don’t know what exactly influences the calcium content in wood. For instance, online it says that wood ash “contains 25 to 50 percent calcium compounds”, so what factors would differentiate a 25% sample from a 50% sample? Since calcium is what makes wood ash alkaline, could some wood ashes be more alkaline than others?

Thanks so much for your help!


Thank you for contacting the Toronto Master Gardeners. This sounds like an interesting project ! Hopefully it will be helpful for you to know that as a rule, hardwood trees produce three times more ashes and five times more nutrients than softwood trees. The principal nutrients in wood ashes are calcium, potassium, phosphorous and magnesium, the last three in much smaller amounts than calcium.

Calcium is an important macronutrient for trees (meaning that it is used in fairly large amounts). Calcium plays a major role in developing cell walls. It is also critical for activating enzymes that carry out many plant functions, and it is important for fruit development (so it’s very important for fruit trees, especially apples and peaches). Calcium is held in the soil, specifically in the clay and humus particles in soil, because calcium is positively charged and clay and humus particles are negatively charged. Calcium is made available to plants in water, which is taken up by the roots and transported up the plant in the xylem. Once the calcium reaches the leaves, it stays there. It cannot move back down or to other parts of the plants.

The amount of calcium in the soil varies. It depends on the type of soil (how much clay and humus). Calcium is depleted in the soil as it is taken up by plants. It can be added to soil by applying lime or gypsum. The amount of calcium taken up by a tree varies. It depends on the type of tree (different trees have different calcium needs). It depends on the pH of the soil. The availability of calcium is reduced in strongly acidic soil. It depends on the availability of water in the soil to transport calcium within the tree. It depends on the presence and quantity of other nutrients in the soil, for example calcium uptake can be suppressed by the presence of excess potassium and magnesium.

So the amount of calcium in a tree depends on the amount of calcium in the soil + the availability of that calcium to be taken up by the tree. And the amount of calcium in wood ashes depends on the type of tree that was burned, and the part of the tree that was burned, since at the time of burning there will be varying amounts of calcium in different parts of the tree, depending on where the tree is in its lifecycle (the calcium level in a tree tends to increase with the age of the tree).

As you can see, there are many variables and dependencies that affect the calcium content in trees. However, while chemistry experiments are not my area of expertise, it seems to me that the fact that calcium content is higher in hardwood trees than in softwood trees could be a useful starting point for you.

Best of luck with your project !

Sept 4, 2022