Garden Soil Information
What do plants take out of the soil?
A single tomato seed weighs less than 1 gram (3/100 of an ounce), but the tomato plant it grows into can weigh more than 20 kg (45 lbs). Where does all that additional weight come from? For thousands of years, even as humans came to depend almost entirely on agriculture for food, no one had any idea or even thought to ask the question.
Then, in the early 1600s, an early Flemish researcher named Jan Baptiste van Helmont experimented with growing plants in pots of carefully weighed soil. In one trial, he grew a tree from a seedling to a weight of 86 kg (189 lbs) in 90 kg (200 lbs) of soil, watering the tree with rainwater. Carefully re-weighing the soil, he found only 57 g (2 oz) missing. While van Helmont’s conclusion – that the material making up the tree had come from the water – was not exactly correct, he had made a very important discovery, which was that almost none of it had come from the soil. (In case you’re confused, I should note that most of the weight of a living plant is water, but if you put the plant in an oven and force the water out, very little of the dry material that’s left came from water).
Four hundred years after van Helmont, we know that most of a plant’s tissue is literally built from air, through the process of photosynthesis (photo = light, synthesis = making). That is, plants use light energy from the sun to accumulate, or “fix,” gaseous carbon dioxide, which they build it into sugars. At the same time, we have also learned that plants do get at least tiny amounts of a number of different elements from the soil, and without these elements the plants are unable to grow or reproduce.
In case you’re curious, here’s the list of 14 elements plants typically get from the soil, together with the symbols chemists use for them: nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni). We say “typically,” because while plants do almost always get these nutrients from the soil, it’s possible to make a liquid solution that contains these elements and grow plants in that. Growing plants in water is called “hydroponics” (hydro = “water”, “ponos” = work), and because it allowed researchers to create solutions missing a particular element, hydroponics was critical in helping to figure out which nutrients plants needed and which they could live without.
Now, none of the elements mentioned above are really abundant in plants, but they are listed above in order of decreasing abundance in dried plant tissue. This means that an average plant contains a relatively large amount of nitrogen (if you took 100 kg or 100 lbs of dried plant tissue, about 1.5 kg or 1.5 lbs would be nitrogen) and an absolutely tiny amount of nickel – so little nickel that a giant redwood tree weighing more than 450,000 kg (1 million lbs) would contain only a few grams (much less than one ounce) of the element.
As in every living thing, the relative abundance of each chemical element in a plant reflects what the plant does with it. Nitrogen is relatively abundant because proteins contain nitrogen, and every plant cell contains many different proteins performing many different functions all the time. Nickel, in contrast, is rare because plants only use nickel to do a few things, and only at certain points in their lives. You could perhaps say that this means nickel is less essential to plants than nitrogen, but plants do die (or at least fail to reproduce) if they don’t have nickel, so even the rare elements are important in their way.