Experienced growers know that it's virtually impossible to grow long-term, healthy houseplants without using
fertilizer. However, there can be a dark side to fertilizer that must be dealt with if your plants are truly to look their best.
Fertilizer is sometimes referred to as "fertilizer salts" because that's just what it is: soluble salts. According to the Unviersity of Texas Greenhouse Management Handbook, fertilizers are classifed by the amount of salt they contain. For reference, researchers have developed a "salt index," using sodium nitrate as the baseline, which has a salt index value of 100 (this was picked randomly). Other fertilizer ingredients have varying salt indexes, including potassium chloride (salt index of 116) and ammonium nitrate (salt index of 105). The nitrates are well known sources of nitrogen, which is one of the macroelements and most common fertilizer ingredients. All fertilizers contain salts, including pellets, powders, and liquid fertilizers.
Although proper use of fertilizer is critical to encouraging healthy plants, over time, they leave behind a salt residue in the soil that can be detrimental to your plants. If it's bad enough, this salt residue can be seen as a white, hazy film on the soil surface. If you use clay pots, it will appear as a white haze on the outside of the pot.
Fertilizers aren't the only source of salt build-up in your houseplants. There can also be salts present in your
potting medium as well as the
water you're using. Salt concentrations in water are measured as the electrical conductivity (EC) of the water. The greater the EC, the greater the salt concentration. EC readings exceeding 2.0 millimhos/cc signal an excessive level of sodium. Finally, most soil mediums (peat moss, pine bark fines, perlite) contain negligible amounts of salt, with the notable exception of coconut coir. Coconut coir that was harvested and processed from plants that lived near the ocean can contain very high levels of salt and should be leached before use.
Flushing Salts From Your Plants
All of this salt can have a very detrimental effect on your plants, especially as the salt level continually rises with additional applications of water and fertilizer. Very few plants have evolved to tolerate high levels of sodium (the mangrove is a notable exception, and some species are well known for their ability to "weep" salt from their leaves as they secrete excess salt). Signs of excess salt include wilting, fungal attack, and plant collapse.
Instead of waiting for your plants to suffer from salt build-up, it's a better idea to regularly "flush" your potted plants to clean out excess salt build-up. Flushing plants is not difficult and can make a world of difference in your plants' health. To flush plants, follow these simple steps:
- Stick to a schedule. It's best to flush monthly or every four to eight weeks, depending on how often you fertilize.
- Using tepid water, water the plant heavily until water drips from the drainage holes. Do not let the plant sit in its container during this procedure (it will be sitting in the same salty water you're trying to get ride of!).
- Let the plant drip for 20 to 30 minutes, then repeat the procedure with tepid water and let the plant drain again.
- Make sure to clean the salt residue from the plant's tray. Trays can have extremely high levels of salt concentration left by evaporation.
This general procedure should work for most houseplants, but some more sensitive species might need a third round of flushing or might need to be flushed more often. As always, watch your plants carefully and let them tell you what they need.
Source:
http://houseplants.about.com/od/growinghealthyhouseplants/a/Flushing-Houseplants-Flushing-Fertilizer-Salts-From-Potted-Plants.htm
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