Remember to Flush: Removing Excessive Salt Build-Up
In simple terms, plants need fertilizer. This is especially true for those being grown in containers or hydroponically. With the proper and timely application of the 14 mineral elements deemed essential for plant growth and reproduction (commonly referred to as nutrients), a plant can grow wonderfully and achieve its full potential. However, with every feeding administered, there is the likely possibility that not all of the nutrients will find their way into the plant roots.
Inorganic and synthetic fertilizer sources commonly marketed to growers often exist in the form of mineral salt ions. As a result, the nutrients that are not taken into the plant via the roots 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 a grower down a potentially dangerous path. As the fertilizer salts build up over time, they can eventually cause unwanted problems such as nutrient lockout or, at its worst, even plant death. Growers may find it beneficial if not imperative to flush or leach the rooting medium to clear out any unwanted fertilizer salt build-up, giving the plants a clean slate to grow.
The Trouble with Salt
The first thing that comes to most of our minds when we see the word salt is probably sodium chloride or table salt. But fertilizer salts, though similar to table salt, are not the kind added to food to enhance its flavor. Without getting too technical, a salt is simply an inorganic mineral that can be dissolved in water.
When the raw ingredients used to make inorganic and synthetic fertilizers are added to water they become soluble salts often referred to as fertilizer salts. The reason such mineral salts are popular as fertilizer is because elements in this form are the exact type of ions that are 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 (aka Epson 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 salts and fertilizers. The mineral salts are then mixed with water and used to feed plants. What is not absorbed by the plant stays in the rooting medium and once the water evaporates the soluble salts will remain intact.
This can be seen by looking under the cap of an often-used synthetic fertilizer bottle. Eventually, the salt will build upon the rim of the bottle and around the cap. This is similar to what can occur in the soil. The more often the plants are fertilized, the more the salt will accumulate, raising the level of dissolved salts (the salinity) within the rooting medium. Soluble salts can also come from tap water if used as a water source and from some types of potting soil or medium. Poor drainage and under watering can also contribute to salt build-up.
How Plants Use Salt
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 salts that help create a stable, natural flow of water and nutrients into the plant’s vascular system. If the amount of fertilizer salts added to the rooting medium is more than what the plant needs and can use, the plant will be affected. As the salts accumulate, they can start to disrupt the flow of water and elemental nutrients entering into the root, and if salt levels reach the point of excess they can actually begin to draw water out of the plant and back into the soil.
Signs of High Salt Concentration
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 when at excessive levels, wilting.
Rooting mediums 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 which may lead to nutrient lockout. In these types of cases, the plant may show visual signs of a mineral deficiency, mainly micro-nutrients, but this can be misleading. Though the apparent deficiency may be real, adding more fertilizer would only exasperate the situation leading to more plant injury. The best thing to do is to first address any possible fertilizer salt build-up in the rooting zone and eradicate it before applying any more fertilizer.
How to Solve Excessive Salt Build-Up
The way to deal with any problems associated with excess fertilizer salt build-up is to remove as much of the salts from the rooting medium as physically possible. This is done by 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 salts, pulling or leaching them along as it travels through and out of the medium.
When leaching or flushing the root zone the general guide to follow is to use around 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 growing medium being used and the frequency/amount of fertilization applied. Mediums such as perlite and expanded clay, substrates that do not hold on to water and nutrients well, will require less water to successfully flush out excess salts than a soilless growing medium containing coco coir or a rockwool slab would.
When to flush is a little harder to determine. For plants that are on what I call a heavy feeding schedule (fertilized at least once a week), it may be a good idea to preemptively flush the growing medium maybe once a month or so, before any unwanted damage caused by excess salts remaining in the root zone can occur. The average house plant that is fertilized monthly or bi-monthly could use a good flush every six to eight months.
Check Your Water
When it does come time to flush or leach the growing medium make sure to use a water source that is free of ions and has a TDS (total dissolved solids) quantity of zero. The average local tap water has a level of around 200-300 ppm (parts per million) of TDS. RO (reversed 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 also products on the market called clearing solutions that can be added to the flushing water to aid in their removal.
Heavily watering (remember 2x the volume of the container) the rooting medium with the flush water or solution and allowing all of it to drain from the holes in the bottom of the container will help remove the fertilizer salts. Do not let the container sit in its tray during this process. It will only end up re-absorbing the salts that were intended for removal. Repeat the process of flushing and leaching as many times as is needed.
How to Make Sure the Salts Are Removed
Here is a way to check to make sure that salts are being removed. Using an EC (electric conductivity) /ppm meter, 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 500-600 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 that will and return the plant to its tray. After a nutrient flush, a plant can show positive visual 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 Salt Build-Up
For a grower, however, the main focus in all of this should be less on dealing with salt build-up and more on prevention. Preventing or at least minimizing the amount of excess salt accumulation will help the grower avoid potential problems along the way.
The first aspect to pay close attention to is the feeding or fertilizing schedule and how it will affect the growing plant and rooting zone. Many of the fertilizer products on the market today are so highly-concentrated and often imbalanced with regards to nutrient ratios that it is easy for a grower to unknowingly create excess levels of certain elements within the root zone. Applying too much fertilizer too often will only result in a waste of nutrients, imbalances in the rooting medium caused by the fertilizer salt build-up, 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 in order to continue on a normal growth cycle.
A colleague of mine 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 that are more in balance with the nutrient status and needs of the plant at a given time which, in turn, will help limit the amount of remaining mineral salt ions within the medium after an application.
When growing plants in containers, whether it’s flowers or fruits or just foliage, 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 build-up and the accompanying problems that 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 more healthy and productive plant.