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Salinity and Orchid Seedling Growth Salinity is the total amount of salts dissolved in water and its concentration can be measured in terms of electrical conductivity; more salt, more conductivity; no salt, no conductivity. Conductivity can be read on a gauge, a salinity meter.An acceptable range of salinity of water used for irrigation of orchid seedlings after fertilizer is added is 2 to 4 millimhos per centimeter...or 1280 to 2560 parts per million or PPMs. The reading before fertilizer is added is relevant here only for calculation of fertilizer needs and a decision for possible use of a deionizing or reverse osmosis system. The reading after addition of fertilizer relates to the plant's needs. How much and what kind is going to determine the impact on the total salt reading, but irrigation water that reads 3.0 mmhos/cm (or decisiemens/meter) before the addition of fertilizer is considered to be a high salinity hazard. Electrical resistance is measured in units called ohms, so logically, electrical conductivity, being the opposite of resistance, is expressed in ohms spelled backward...or mhos. It figures. Recently, the unit of measurement was changed to decisiemens per meter, but only the name was changed. Values are as before. (Americans, the only people in the western world who still haven't adopted the metric system, are struggling with these units of measurement. Just wait until they have to cope with oil pressure in their cars which is measured in grams per square centimeter. Maybe idiot lights will make a little more sense then.) For our purposes, the degree of saltiness of irrigation water used on our orchid plants refers to a little more than just the salty taste of ordinary table salt. (You avoid excessive use of table salt because it retains water in the human body...and bloats it, right? Keep that point in mind as we talk about other kinds of saltiness; there are many.) There are two sources of the soluble salts that cause the problem of salinity in the culture of orchid seedlings in containers: One is the fertilizer applied to the plants and the second is the water used to irrigate the plants. (The amount added by organic media is negligible because we replace the media frequently for other reasons.) The salts which nourish the plants can be a mixed blessing. A little is good; a lot can be harmful; just like the relationship between humans and food. The issue is not what you put into the fertilizer bucket, but what ultimately is put on the plant's roots.
These salts include nitrates, phosphate, potassium, calcium,
iron, magnesium, aluminum and others. Any one of these, or a combination, in
excess can cause salinity problems wherein roots are damaged or growth is
stunted...or worse, but a common symptom is the appearance of a plant which
needs watering. And it probably does.
Salt damage causes symptoms similar to dehydration because
that is just what is happening. Water intake by the plant is reduced, not
because none is available, but because the osmotic pressure which drives
water intake by the plant is strongly influenced by the salinity of the
medium surrounding the roots. Normally, water passes across the membranes of
roots because a low pressure has been created by transpiration-evaporation
in the plant's leaves...leaving a low pressure area in the vascular system.
Water is literally sucked in. (How water got to the tops of 300-foot
sequoia trees baffled plant physiologists until this process was better
understood.)
High salinity of the rooting medium causes an attraction for
water that can not only equal that of the vascular system of the plant...and
stop water intake, but can actually exceed it and draw water out of
the roots. Herein is the reason why media salinity is of great importance to
the orchid grower.
You cannot safely add fertilizer salts to irrigation water without knowing its effect on the plants...and that of the salts already in the water when you added the fertilizer. It is important to the plants that you know the total amounts of the salts in the water being offered to them. Weight control specialists start new clients out on a weight control program by having them record every bit of food they eat during a day. We often feed ourselves and our plants too much to fill some imagined need...and the total intake of the wrong things can be astonishing. For example, it makes no sense to add magnesium to irrigation water that already has a level adequate for the plant's needs. This principle applies to other salts as well. I know I'm asking a lot in suggesting you get a better handle on what the plant needs and what you are giving it, but I'm also sure you are aware that instruction to "...put a teaspoon of fertilizer in a gallon of water and feed the plants with it" is simplistic at best. This, because of the wide variation of salts available from both fertilizers and irrigation waters. The issue is not what you put into the fertilizer bucket, but what ultimately is put on the plant's roots. OK, seedlings are more susceptible to salt damage than mature plants and serve the same warning function as the canaries in the coal mines of old. They are the first to die off when the environment gets bad. Other things held constant, if your seedlings grow poorly, a good place to look for the cause is the water you use...and the fertilizer you put in it. More so than for mature plant culture, a prerequisite for good orchid seedling culture is good water...or, if you haven't got good water, a treatment program to make it that way. It's not difficult; there are ways that are neither complex nor expensive. And although pH extremes are of concern to orchid seedling growers, high salinity and its attendant problems pose a far more serious challenge. We will discuss the issue of high pH in a later article. [From Orchid Seedling Care by Bob Gordon. Published by Laid-Back Publications, Running Springs, CA]
Bob Gordon
P.O. Box 243
Running Springs, CA 92382
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