As with soil, there’s more to water when it comes to plants (and soil) than is evident at first glance. Elsewhere in this blog I’ve mentioned water secondarily and where relevant, but now it’s time to give the subject of water the attention it deserves, with its very own section! Some information mentioned elsewhere may be repeated in this section, but will be rephrased from the point of view of water and its interactions with soils and plants, rather than the other way around.
Plants, like all life, need water. And for the vast majority of plants that water is obtained via their roots. Water thus needs to be where the roots are, or roots need to be where the water is. And again for the vast majority of plants those roots tend to be in soil. Thus for water to be of use to most plants it needs to be in the soil where roots can access it.
All this is pretty obvious of course.
But how that water comes to be in the soil in the first place, and what happens after that, can be somewhat involved. And it’s that discussion that will underpin the topics in this new ‘Water’ section.
Water invariably enters soil via the surface. This entry could be from rain, surface runoff, overflowing creeks and rivers, us standing over with a watering can or hose, or an irrigation system. Even if we were to water with bore water we still must pump it up from underground first, just to reapply it to the surface. As growers of trees, we then expect that water to make its way through the soil to where the roots are.
Simple? Not really! For instance, how do we actually know that enough water has entered the soil? Too little or too much can have consequences. And has the water that entered then moved deeply enough for roots to access? Or has it moved too quickly down, draining out of reach before roots could uptake it? Or has it not moved much at all, and is collecting in the upper surface away from those roots?
And don’t forget that water is a solvent! As it travels through a soil profile it ‘collects’ various salts and other substances in that soil. If bore water, it would already have a good number of such compounds already dissolved in it, which would then enter the soil along with the water. Some, like sodium chloride (table salt) are harmful to plants, and some, like nitrates, are needed by plants. Some may deposit out into the upper soil, and some may travel deep down. This may be a good thing or a bad thing, depending on the compounds, their concentrations in the water, the soil they travel through, and how the water moves through that soil in the first place.
There are so many variables associated with water. These variables, along with the variables of soil composition and structure through which water travels, can make for a complex subject.
But to summarise very simply: how water enters soils, moves through soils, is retained by soils, drains from soils, as well as what it carries through soils, all ultimately determines how much water ever becomes available to plants. We will cover all of these, starting in the next post with the first variable, how water enters a soil, or its infiltration.