In simple terms, plants need fertilizer. This is especially true for plants grown in containers or grown hydroponically.
With the proper and timely application of minerals deemed essential for plant growth and reproduction (commonly referred to as nutrients), a plant can achieve its full potential.
However, with every feeding administered there is the likely possibility that not all of the provided nutrients will find their way to a plant’s roots.
Inorganic and synthetic fertilizer sources commonly marketed to growers often exist in the form of mineral salt ions. Because of this, nutrients not taken into the plant can remain in the soil in salt form and slowly build up over time.
Excessive application rates, imbalanced nutrient ratios and rigorous feeding schedules can lead growers down a potentially dangerous path because salt buildup can eventually cause unwanted problems, such as nutrient lockout.
It can even kill plants. Growers might find it beneficial, if not imperative, to flush or leach the rooting medium to clear out any unwanted fertilizer salt buildup, giving plants a clean slate.
Why Do I Need to Flush My Plants of Excess Nutrients?
Fertilizer salt, though similar to table salt, is not the kind added to food to enhance its flavor. In general, a salt is simply an inorganic mineral that can be dissolved in water.
When raw ingredients used to make inorganic and synthetic fertilizers are added to water, they become soluble salt often referred to as fertilizer salt.
The reason such mineral salts are popular as fertilizer is because elements in this form are the exact type of ions easily taken in by plant roots.
This is what makes synthetics so fast-acting and reliable. Some common inorganic fertilizer salts include ammonium nitrate, potassium sulfate, magnesium sulfate (a.k.a. Epsom salt) and calcium chloride—just to name a few.
These products are the end result of minerals being mined from the earth and manufactured into mineral salt and fertilizers. The mineral salt is then mixed with water and used to feed plants.
What is not absorbed by plants stays in the rooting medium. Once the water evaporates, the soluble salt will remain intact. This can be seen by looking under the cap of an often-used synthetic fertilizer bottle.
Eventually the salt will build up on the rim of the bottle and around the cap. This is similar to what can occur in soil. The more often plants are fertilized, the more salt will accumulate and raise the level of dissolved salt (salinity) within the rooting medium.
Soluble salt can also come from tap water if used as a water source, and from some types of potting soil or media. Poor drainage and under-watering can also contribute to salt buildup.
As stated earlier, plants can readily use mineral nutrients that are in the form of soluble mineral salt ions.
The roots of a plant naturally contain different levels of mineral ions called root salt that help create a stable, natural flow of water and nutrients into the plant’s vascular system.
If the amount of fertilizer salt added to the rooting medium is more than what the plant needs and can use, the plant will be affected.
As the salt accumulates, it can start to disrupt the flow of water and elemental nutrients into the root. If salt levels reach the point of excess, they can actually begin to draw water out of the plant and back into the soil.
When to Flush Your Plants
Some signs of high salt concentration include a browning of the leaf tips, reduced growth (especially new growth), the aborting of lower leaves, dead root tips and wilting.
Rooting media that have high soluble salt content will also have a high pH level. As the pH of a rooting substrate rises, the result will be a change in the overall availability of certain nutrients, and sometimes it can even cause an alteration in the ionic form of some nutrients, changing them into unusable forms.
In these cases, the plant might show visual signs of a mineral deficiency—mainly of micronutrients—but this can be misleading.
Though the apparent deficiency might be real, adding more fertilizer would exasperate the situation, leading to more plant injury.
Address any possible fertilizer salt buildup in the rooting zone and eradicate it before applying any more fertilizer.
Remove as much of the salt from the rooting medium as physically possible using a technique called flushing or leaching.
Allowing a relatively large amount of water to flow freely through the container in a small amount of time will once again dissolve the fertilizer salt, pulling or leaching it along as water travels through and out of the medium.
When leaching or flushing the root zone, use about two times as much water as the volume of the container.
The amount of water needed for a proper flush and how often it should be done depends on a couple factors: the type of grow medium being used and the frequency/amount of fertilization applied.
Mediums such as perlite and expanded clay will require less water to successfully flush out excess salt than a soilless growing medium containing coco coir or a rockwool slab would.
Deciding when to flush is a little harder to determine. For plants that are on a heavy feeding schedule (fertilized at least once a week), it is a good idea to preemptively flush the growing medium once a month or so before any unwanted damage caused by excess salt remaining in the root zone occurs.
The average houseplant fertilized monthly or bi-monthly could use a good flush every six to eight months.
How to Flush Your Plants of Excessive Fertilizers
When flushing, use a water source free of ions and with a TDS (total dissolved solids) quantity of zero. Most local tap water has a level of around 200 to 300 ppm (parts per million) of TDS. RO (reverse osmosis) water is free of soluble salt ions, has a TDS of zero and is a good choice to use for flushing.
If the medium being flushed has an extremely high concentration of excess salt, there are products on the market called clearing solutions that can be added to the flushing water to aid in removal.
Do not let the container sit in its tray during the flushing process. It will only end up re-absorbing the salt intended for removal. Repeat the process of flushing and leaching as many times as required.
To make sure salt is being removed, use an EC/ppm meter to check the TDS level of the water before flushing through the rooting medium.
When using quality, ion-free RO water, the initial reading should show zero TDS present. Flush the container plant with the water/solution and capture the runoff drainage in a bucket.
Using the EC/ppm meter, check the drainage solution. It will most likely have risen, sometimes as much as 500 to 600 ppm of TDS. Be sure to clean off any salt accumulation that remains in the tray and on the sides of the container from evaporation.
After flushing, allow as much of the water to drain from the rooting medium as possible and return the plant to its tray.
After a nutrient flush, a plant can show signs of improvement within a day or two and the grower will be able to resume a normal fertilization schedule with the next watering.
Preventing Excess Salt Accumulation in the Garden
Preventing, or at least minimizing, excess salt accumulation in a garden will help growers avoid potential problems along the way. Pay close attention to the feeding or fertilizing schedule and how it will affect the growing plant and root zone.
Many products are so highly concentrated and often imbalanced with regards to nutrient ratios that it is easy to unknowingly create excess levels of certain elements within the root zone.
Applying too much fertilizer too often will only result in wasted nutrients, imbalances in the rooting media and overall lower yields.
By carefully researching the particular plant being grown, one can more effectively choose or formulate a nutrient solution that will supply the plant with just enough of each specific nutrient needed for a normal growth cycle.
A colleague once told me that a grower needs to take care when planning a fertility program to ensure sufficiency and not excess.
This includes keeping application rates and the frequency of feedings at levels more in balance with the nutrient status and needs of the plant at a given time, which will help limit remaining mineral salt ions within the medium after an application.
When it comes to regular watering, always supply enough so the water—preferably RO water—drains out the bottom of the container and discard the runoff immediately.
Choosing a grow medium with good drainage will help minimize fertilizer salt buildup as long as the runoff is always discarded.
Summing It Up
When growing plants in containers, some form of a regular fertilization schedule is a must. While inorganic/synthetic fertilizers can supply plants with a fast-acting, reliable nutrient source, overly or improperly applying them can lead to excess salt buildup and the accompanying problems it can bring.
By incorporating a nutrient flushing or leaching phase into a regular fertility program, and by paying careful attention to the amount and frequency of feedings, a grower can maintain a more balanced environment within the root zone and ensure a healthier, more productive plant.