Garden Soil Information
How do plants move nutrients from soil into their roots?
For starters, it’s important to remember what soil is. About 95 to 98 percent of the soil beneath our feet is basically tiny chunks of rock. These chunks of rock contain many different chemical elements, but the vast majority of the weight of the rock is made up of oxygen (which plants need but don’t get from the soil) along with aluminum and silicon (neither of which plants need, though some plants do accumulate silicon). While some of the nutrients plants need are present in rock, those nutrients are present at higher concentrations in the organic matter (pieces of formerly living things, like plant stems and insect bodies) that makes up the other 2 to 5 percent of the weight of soil.
Unfortunately, none of the nutrients present in either rock or organic matter are directly available to plants. Jethro Tull (1674-1741), an early agricultural researcher, thought that plant roots had tiny mouths that they used to “eat” soil. This turned out not to be true, and plants have no way to take actual soil (rock or organic matter) into their tissue. Instead, they have to wait for nutrients to be released from the rock or organic matter in very small, chemically simple forms.
In the case of nitrogen, for example, nitrogen atoms that in a living animal are built into protein molecules containing hundreds or thousands of atoms must be broken off of these molecules (usually by the feeding of bacteria or fungi) such that they are present in the soil in very small, simple forms – most often the molecules NH4+ (ammonium) or NO3- (nitrate). With minor exceptions, these are the only forms in which plants can take up nitrogen. The story is pretty much the same for each of the other elements plants get from soil. That is, big molecules in rock or organic matter must get broken down by physical, chemical, or biological processes so that just the right small molecules or even atoms are present in the soil. For some elements like phosphorus, there’s only a very narrow range of soil conditions under which this happens, so these elements are ones that are most likely to be in short supply for plants.
Unfortunately (again!) having the right small nutrient molecules present in the soil isn’t enough. Just as a soccer ball must be moved by a soccer player to make a goal, a significant amount of water must be present in soil to carry nutrient molecules to (and then into) roots. Plants drink their nutrients – they don’t eat them!
Even if nutrients are present in the right forms with plenty of water, we still haven’t covered exactly how those nutrients get into plants. At one level, that’s really complicated – it involves elaborate protein “machines” built into the walls of the cells that make up plant roots. It also involves some complex chemistry and physics.
At another level, the way plants take up nutrients is simple, resembling the workings of a common children’s toy. Many small children I know have boxes with different shapes cut into the sides. A box like this comes with a set of blocks whose shapes match the shapes cut in the box. The protein “machines” that transport nutrients through the surface of a root are like the specially shaped holes in the box, and the nutrients are like the blocks – all the child (or plant) has to do is find the right block (or molecule) to go through the right hole.
One difference between children and plants is that while a child might struggle for a minute to find the right hole for one block, plants can move thousands or even millions of nutrient atoms or molecules through a single transporter in seconds (they also have millions of transporters). Another difference I’ve already mentioned is that while a child uses its fingers to bring the block to the hole and push it through, plants rely on water to carry nutrient atoms or molecules up to and through nutrient transporters.
The analogy to the child’s toy may make it sound like plants don’t make mistakes (most toy boxes will only allow each block through a single hole), but some nutrient transporters are actually not all that specific. A few, like the one that moves the essential element zinc, also unintentionally pick up elements like cadmium, which is quite toxic to humans. This is one of many reasons to keep lead, cadmium, and other metals out of our gardens and farm fields.