Most home gardeners are probably aware of the detrimental effects of over-watering. Quite apart from the desire to save water, there is a vague recognition that excessive watering is bad for the plants. It is common to find in garden literature for example, references to this or that plant that is “sensitive to over-watering”. By understanding this term more precisely however, you can irrigate more effectively, and manage the water at your disposal more efficiently.
Irrigation alters two parameters in the soil. Firstly, obviously and self-evidently, water is added to it; less obvious, but no less critical, is that the percentage of oxygen present in the soil is subsequently reduced. As the plants are dependent on both a ready supply of oxygen in the root zone, together with adequate moisture, it follows that correct irrigation practice, always takes account of both these factors. Over-watering therefore, could more accurately be termed, “lacking in air”, and sensitive plants better understood as those requiring high percentages of oxygen in the root zone. To be technical about it, one could describe them as being particularly adverse to anaerobic conditions.
It is no coincidence that plants that naturally grow in dry climates tend to suffer, decline, and rot, when the soil drains poorly or when they are over-watered. As these regions experience extended periods of aridity, the plants are adapted to soil conditions that are highly aerated. A few examples are Mediterranean species like Lavender, Rosemary, and Pomegranate, shrubs like Leucophyllum from Texas, and most clearly, the large range of Australian ornamentals, such as Melaleuca, Eucalyptus, Grevillea and Myoporum.
For such flora, the best way to ensure a satisfactory balance between air and moisture is to water deeply but infrequently. Water should be applied in sufficient quantities so that it percolates down to the roots in the subsoil, while the topsoil dries out between each watering. The method is not dissimilar to watering houseplants, where for the most part, it is recommended that the top 2-3 cm of the potting mixture dry out before watering.
Furthermore, just as houseplants are irrigated to the point where water drains out of the hole at the bottom of the pot, so the garden plants, counter-intuitively perhaps, should also be watered to a depth beyond the extent of the root zone. What appears at first sight to be wasting water is in fact essential for preventing a dangerous build-up of salts in the soil.
Obviously, young plants with very shallow roots do not need water 20cm beyond their capacity to take it up. Neither is their any sense in watering too deeply in very shallow soil. However with established trees and shrubs, irrigating to a depth of about a meter, is a good rule of thumb to go by. It is even appropriate for perennial hot-climate perennial lawns like Kikuyu or Bermuda grass.
For this purpose, use a pole or stick to measure the depth to which moisture has penetrated the soil, while bearing in mind that “over-watering” does not mean irrigating somewhat to excess, but rather failing to maintain a proper air/moisture balance, invariably caused by a frequent but shallow irrigation regime.
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