Water movement: Several Forms of Attraction
When you think of how water moves through your medium, you may automatically think of the downward movement due to the gravity. Gravitational pull is very important, but there are other forces at play: matric and osmotic potential. When you apply water to the point of saturation all soil pores are filled with water. At this point, the dominant force is gravitational. In the coarse pores water is held loosely and can move easily in the direction of gravity. Once gravity has removed excess water from your medium, you have reached a point called "field capacity". At this point the other forces come into play.
Matric potential is a result of the attraction between water and soil particles. Capillary action is a good example of the presence of matrix potential. If you take a thin straw and put it in a glass of water, the water in the straw will be slightly higher than the surrounding water in the glass (the thinner the straw, the higher the water). Pores in soil or soilless mediums are much finer than the diameter of even the finest straw so these pores have a gravity defying ability to move water (up to several meters if the medium is compacted). In fact, some hydroponic systems use a wick system that supplies water by taking advantage of capillary action.
Osmotic potential relates to the attractive force between salts and water, and the fact that water will move from a less concentrated solution to a more concentrated one. This force also has the ability to move water against gravity through soil. It is also an important concept in terms of the way plants take up water; when the concentration of solutes within the plant is greater than that of the surrounding soil water, water will flow into the plant. However, when salt concentration reaches a certain level plants will not be able to draw water from the salty media.
The topic of soil water flow is quite complex. Many resources are available online for those interested in a more detailed scientific explanation.
Plants grow well when the soil moisture content is around 70% of the field capacity. At this point, the force that holds water is equivalent to a force of pulling 100-150cm water column. The macro-pores are filled with air and most micro-pores are filled with water. Roots can take water easily form the medium and respire normally.
Some water in your medium will not be available to your plants. Once your soil begins to dry out, it becomes more difficult for your plants to draw water from the media because of the incredible power of adhesion, the ability of water to "stick" to your media. The fine pores of medium also hold water strongly and the water is less available to plants. If the force that holds water reaches around15 bars (the force equivalent to pulling 150m water column), the plants will not be able to take water from the medium and will probably have reached the permanent wilting point. At this point there is still water in the medium but it is not available to plants. Even air-dried soil contains water; this water is referred to as hygroscopic water because the attraction to the particle is so strong that the only way to remove it from the soil is by drying it in an oven above 100°C.
To check the moisture status you can take a handful of medium from the container. If you can feel moisture, but no water can be squeeze out, you have not over-watered the plants. If the medium is dry and you do not feel moisture, you need to water. A great way to water your plants is to apply water from the bottom of container (by setting the pot in water, or finding a similar solution) so that water can be drawn up by the medium by capillary action. In this way the macro-pores will not be blocked and air exchange between the medium and the ambient air can take place easily.
Check your water before watering
When watering your media, it is important to check the water temperature to ensure that there isn’t a large difference between the temperature of the water and the medium (aim for ±5°C). Otherwise thermal shock will make plants unhappy. For example, sudden drop in temperature can induce iron deficiency in species sensitive to iron deficiency. For indoor plants, use water that has been stored at room temperature for at least one day. For outdoor plants, water plants in evening or in the morning. Do not water plants with cold tap water in the middle of a hot summer day.
Secondly, check water quality if the water is not rain water or fresh surface water from a known catchment. The electrical conductivity (EC, an indicator of soluble salt content) should not be over 1.5mS/cm. Ground water tends to have high EC. If EC is high (say over 0.5mS/cm), it is advisable to check the pH of the water. For most plant species it is acceptable if the pH is below 7.2. If water pH is high (say up to 8.3), this indicates that a considerable amount of bicarbonates are present. Iron deficiency-sensitive plants are likely to show iron deficiency symptom if water with a high pH is used for a long period of time. To resolve issues with high pH, you can neutralize the water with an acid (such as nitric or sulfuric acid). Be careful to select the right products to use. Some products designed to reduce pH are composed of phosphoric acid; if using this product, you have to adjust feeding program to account for the additional phosphorous. Vinegar can also be used. However high amount should be avoided since high level of organic acids can be toxic to roots (low level usually stimulates root growth, but this is beyond the scope of this article).
Plant roots can require as much as three times more oxygen (by volume) than water, so understanding water movement and the forces that drive water movement are keys to the urban gardener's success. That said, the best way to increase long-term yields from your garden is to experiment with your unique system. Experiment with different percentage of mix ingredients to supercharge your medium and provide the best possible environment for root growth. So the next time you tip your watering can or hear your pumps come on, make sure you are thinking about how important water management can be to your indoor (or outdoor) jungle.
