Good quality water is becoming scarcer and more expensive. So naturally, greenhouse growers are looking for ways to reduce their water bills.
More and more new products are coming to the market claiming to save water. However, growing media is still the most important factor in how successful a grower is when watering. A proper growing media actually helps growers save water.
One way to save water is to make the growing media capture more of the water you are supplying. In this space, I’ll discuss how various growing media materials hold and release the captured water and how those water-holding and release characteristics affect your watering practices and, in turn, plant growth.
Sales brochures on growing media often present the water-holding characteristic of a media as that media’s capacity to hold water so many times (e.g. eight to nine times) its weight. But you fill pots by volume. So you want to know how much water that media holds, say, in a gallon pot.
The best growing media companies give their media’s container capacity value, which tells the maximum amount of water a media can hold in a particular container size. However, the container capacity value still doesn’t show how the held water is released and how much of that water is actually available for plants.
So first, I’d like to discuss some basic principles of plant physiology and soil physics regarding water. Then, I will apply those principles to help you water most efficiently. This knowledge will help you understand plant responses to diverse growing media materials.
Plants take up water from growing media in a way that’s similar to how we remove water from a soaked sponge. When squeezing the sponge, we apply only slight pressure to remove water that can come off easily. We apply more pressure and squeeze harder to remove more water and so on. At some point, even with a lot of pressure, the sponge doesn’t release any more water, even though it still has some water in it.
In a similar way, a plant extracts water from growing media by generating suction pressure. As water content in the growing media declines, the plant generates more and more suction pressure to extract water. Generation of more and more pressure, of course, requires energy that plants could have spent elsewhere – to expand stems, leaves or flowers.
At some point, the plant cannot generate enough pressure to extract water from the growing media. Then the plant wilts, even though there is still some water in the growing media.
Pressure On Growing Media
Scientists measured pressures at which water is easily available or unavailable to plants in growing media. These are the suction pressures that plants have to exert to get water from growing media. This pressure is expressed here in “kPa” (or kiloPascal, a unit that is similar to “psi,” or pounds per square inch; 1 psi = 7 kPa).
Immediately after you water a growing media to saturation, water in that media is basically at zero pressure, and hardly any suction pressure is needed to extract that water. Soon, after drainage from the pot stops, the pressure needed to extract water in the growing media is about 0.3 to 1.0 kPa. Water held at such pressure is easily available to plants.
Water in a growing media that can come out until you exert 5 kPa is easily available to plants, meaning there won’t be much setback to plant growth in this pressure range. When plants are growing vigorously, water in growing media is generally at about 3.5 kPa.
When water in a growing media reaches a point where you have to exert a pressure beyond 5 kPa – and up to 10 kPa – water is still available to plants. But plants have to spend more energy to get that water.
When water in a growing media reaches a point where you have to exert pressures beyond 10 kPa, most plants start experiencing water stress. At these pressures, a growing media may still look or feel damp, and plants may not be wilting. But water therein is not easily available to plants, and growth reductions are already occurring.
Normally, a watering should have occurred by the time water in growing media reaches 7 kPa pressure. But, if growing media keeps drying beyond 10 kPa before re-watering occurs, plants slow their growth expansion. As a result, plants would have short stems and produce fewer and smaller flowers. Conversely, without water stress, you can get long stem roses, for instance.
There are definitive differences between field soil and soilless growing media in how they each supply water to plants. The amount of water available when a plant is exerting 5 to 10 kPa to draw water shows the buffer capacity of a growing media. This capacity ensures protection in case of a danger – like if someone forgets to water, if a heat wave occurs or during shipping to stores. Compared to water in soil, water in soilless media is held loosely. Soilless media release more water at lower pressures, so it is easier for plants to take up water in soilless media. This is a big benefit for plants growing in soilless media, as they need to exert only a little energy to extract water and use the saved energy for growth expansion.
But, there is a cost to the grower who now has to pay greater attention to the watering practices. Because there is only a limited volume of growing media in a pot, a plant can deplete water in that limited volume rapidly.
Growth Before Wilting
A wilting plant is a symptom we associate with water shortage. That’s because we can see that symptom. But well before a plant wilts, many plant physiological processes are affected and plant growth is damaged when water shortage starts to occur.
For a plant, water is not available equally over any pressure range, even over the so-called easily-available water range of 1 to 5 kPa. As water available to a plant starts to decline, it starts to conserve water by shutting its stomata through which water transpires (and cools the plant). But carbon dioxide used in photosynthesis enters the plant through the same stomata. Shut stomata means reduced photosynthesis, therefore reduced plant growth.
Although there are differences between plant species as to how much growth is reduced, growth reduction occurs even in cactus when there is shortage of water. Unhindered growth of a plant, therefore, depends on keeping a plant’s stomata open and its transpiration uninterrupted.
Transpiration rate of a plant, however, is not the same all the time. On a cloudy, cool, humid, still day, plants hardly transpire. On a sunny, hot, dry, windy day, transpiration can exceed the water available from growing media.
Your task is to ensure that water available to a plant synchronizes with the transpiration demand of that plant at any given time. And this means the growing media at any given time should have enough water available at least pressures, so the plant can exert the least effort to obtain water and reach its full potential growth.
Such a condition should exist for a long time so you don’t have to water often. And how can you achieve that condition? You’ll learn that in part two of this series.