Understanding Salinity & Osmosis for a Sustainable Garden

The physical law known as osmosis affects plants and animals alike, and it is important to understand how it works, so your garden can benefit, rather than be harmed by its universal effects. You will also be healthier if you pay attention to salinity and osmosis in your own body.

The principle of osmosis states that where there are two saline solutions separated by a semi-permeable membrane, the solution with the lower salinity will migrate across the membrane to the saltier solution until the two solutions are equal in salinity. I recommend you re-read the foregoing statement and really understand what it means, because it is VERY important.

In humans, animals, and plants mineral salts are essential to life. They are taken into the bloodstream of the body by mouth, or the sap of the plant through the roots, and are then used to build the cell structure and maintain the health of the living organism.

In plants, so long as the salt solution in the plant is stronger than the solution outside of the plant, available water will continue coming into the plant. A plant is more than 80% water by weight, and so plants need water constantly. In hot weather it’s especially important, since as much as 95% of the water that enters a plant is used to perform transpiration – like human sweating – to keep the plant cool.

Most everyone understands that if water is withheld from a plant it will quickly wilt and die. What most folks don’t understand, however, is that even if ample water is available to the plant roots, if it is as salty as the solution inside the plant, the plant CAN NOT absorb any of that water. And if the water being applied is saltier than the water inside the plant, water will LEAVE the plant.

How can something like that happen? Who would be so foolish as to water their plants with salty water? Actually it can happen fairly easily, and it does happen more than people realize. Let me mention two ways that are probably the most common, so that you can avoid having it happen to you.

First, many people apply 2-3″ of manure to their growing beds, in the desire to fertilize the plants and improve the soil structure. The problem is that many times manure – especially feed-lot manure – is quite salty, containing from ½ to 2% each of nitrogen, phosphorus, and potassium, in addition to other elements, even including such things as sodium chloride or common table salt because of salt-licks provided to the animals.

Applying 2+ inches of manure to a 30′-long soil-bed requires 200-300# of manure, and can add several POUNDS of these various salts to your soil. The salinity this creates will often pull water out of your plants, “burning” and even killing them. By contrast, the Mittleider method of feeding your garden adds about 7 ounces of salts to the soil in a 30′ bed a few times over the course of the growing season.

The second way saline water can get into your garden is if you use water that has had some kind of salt added upstream from your garden, or from a well with saline water. This almost happened to me just this morning.

I was watering the Armenia Project’s model garden I’m using to demonstrate and teach the Mittleider Method in Ashtarak, a small city near the Armenian Capital city of Yerevan. The water comes in a small canal, and it was clear when I began, but as I started to water a bed of eggplant, the water suddenly became very cloudy and dirty. Luckily, I noticed what was happening and stopped watering immediately.

If you ever find yourself with saline water in your plants’ root zone, you should flush the salts out as quickly and completely as possible. This requires heavy watering several times with clean water. Sometimes it’s fairly easy, and sometimes it’s difficult or even impossible to accomplish before your plants have died.

As with most everything in life, prevention is much better than cure, so avoid the conditions that can lead to a salinity problem, and you’ll help assure yourself of a sustainable garden with healthy, fast-growing plants.

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