In Greenhouse Grower’s last issue, we discussed concepts of physiology of plant water uptake. In this article, we’ll discuss physics concepts pertaining to growing media’s capacity to store and release water. Then, we’ll apply the concepts to improve your watering practices.
Pores In Growing Media
Different growing media have different capacities to store and supply water to plants. These differences are due to pore spaces in growing media.
Water obviously requires a space to stay. In a growing media, water is held in pore spaces. When growing media materials are mixed, pores form between their particles. Large particles form large pores and small particles form small pores. However, in a mixture of large and small particles, small particles occupy and fill the large pores between large particles.
Water in large pores drains first and fast. Water comes out of such pores with little pressure. Small pores, on the other hand, hold water firmly. Thus, as the size of pore space decreases, water is held more tightly. An increasing pressure is needed to pull water from smaller and smaller pores. So, if a growing media has very small pores, although it holds more water, less of that water is easily available to plants (for example, water in clay).
Although the pore spaces between particles influence how much water is held by a growing media, there are other influencing factors. Pores inside a particle also influence the water held. For instance, we can test sand and peat with the same sized particles, which would form the same pore sizes between their particles. But peat holds more water because peat particles are open and have pores even within them.
Some growing media components like peat and coir have an exceptionally high level of internal pores that are also open, interconnected and accessible to water. That’s one reason these components have high water-holding capacity. If some of these internal pores are very narrow, water therein requires more pressure to come out.
In perlite and vermiculite, some internal pores are disconnected and closed. In expanded clay pellets, all internal pores are closed. Such internal pores are not effective in holding water.
Water is also held by solid particles as a coating or film on their surfaces. The affinity with which such water is held differs between materials used as growing media. If a material’s affinity for water is stronger, more pressure is required to extract such water.
All these factors influence how a growing media releases water to the plants under different suction pressures.
Curves Of Growing Media
How a growing media releases water at different pressures can be determined and drawn as a curve. For this, a saturated media is gradually de-saturated by applying either pressure or suction. As the media dries, its water content is determined. Such determined water release curve is unique property of that growing media. From this curve, one can draw conclusions on the water release abilities of that growing media and then apply proper watering practices.
Water release curves of some often-used growing media materials are presented in the chart at right. Look at the curve for peat. Soon after watering and free drainage, peat would have about 80 percent water. If you multiply this percentage by the pot size, say a gallon pot, you get 0.8 gallons or about 100 ounces as the amount of water in a gallon pot. By 5 kilopascals (kPa) pressure, the water amount is about 0.4 gallons or 50 ounces. So, 50 ounces of water is easily available to a plant in a gallon pot. If you don’t water even until 10 kPa pressure, about 10 more ounces of water is still available to that plant.
Now, look at the rockwool curve. Immediately after watering, rockwool holds as much water as peat. But rockwool releases almost all the water with very little pressure. Virtually no water is retained by 5 kPa pressure and, thus, rockwool has no water buffer capacity. Water stress can set in on plants growing in rockwool if a watering is not scheduled ahead of 3 kPa pressure.
In rockwool and other fibrous materials, water is held in large pores at contact points between the particles. Major portions of such water drain simply by the pressure induced from gravity. Major portions are also lost to air – often before plants can hardly use it. Because little water is retained in such materials for later use by plants, these materials require frequent watering.
Now, look at the curve for sand. Even if we apply a lot of water, sand can’t hold as much water as peat. And much of that water is also released at very low suction pressures. So, for sand too, you have to come back and water frequently. Also, because of the small pores in it, sand doesn’t release all the water and retains a certain amount of water, even with increasing suction pressure.
Hydrating Growing Media
One can grow in many materials but how one waters these materials should differ. As we saw, materials with substantially different water release curves require substantially different watering schedules.
You can schedule efficient watering electronically. There are sensors that can be placed in the growing media to know what pressure water is at held there. Then, you can automate watering – for instance, initiate watering at 7 kPa and terminate at 1 kPa.
You can also schedule efficient watering manually if you base it on the calculations from the water release curve of your growing media. Suppose your hydrangea growing in a gallon pot requires (including for evaporation) 12 ounces of water per day. As we’ve seen in the examples already shared, plant useful water reservoir size is about 60 ounces in peat and about 6 ounces in sand. So, in peat, you would have five days interval before you re-water, whereas in sand you would have to water twice a day.
On the other hand, instead of fast, vigorous growth, if you can accept a slow growth rate, you can supply less water to your plants. In fact, you can use water as a tool to control plant growth and size. But, be careful not to lose plant and flower quality in the process. Water-stressed plants would be short with fewer branches and fewer flowers. However, good marketing might call the stressed plants hardened or toned!
You also have to consider the shelf life of plants at retail stores because of the watering practices there, which can be infrequent and erratic. If water available to plants is less than their transpiration demand, plants stress. They appear dull and gray. They also shed leaves, abort flowers and, of course, wilt permanently in response to the water stress. If this happens, you would lose sales.
As you can see, an efficient balance of obtaining the desired plant growth and saving water is required. But now that you have armed yourself with the data on capacities of various growing media materials to store and release water, you have a better ability to decide which option is best in your situation to obtain the desired plant growth and save water.