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The Secrets of Foliar Spraying

Your tomato plants look limp and sickly.  Their lower leaves have turned a nasty yellow between the veins.  You need to do something quickly.  Searching the web, you discover your tomatoes have magnesium deficiency.  Under the bathroom sink, you find an old bag of Epsoms Salts and an empty spray bottle.  Dissolving a tablespoon of the salts in a couple of pints of warm water, you spray the leaves of the tomato plants all over.  A couple of days later, the plants are bright green and healthy again. 

From this example, it looks like foliar spraying could be the magic bullet we are all looking for.  Within one hour, according to the scientists, a plant can transports minerals from its leaves all the way down to its roots.  Compared to root feeding, this looks like the fast track.  However, foliar spraying is not an alternative to good growing methods.  It is best seen as a powerful addition that has its own secrets for success.

Mineral Deficiency Spraying
Spraying for mineral deficiencies can be particularly effective: magnesium for tomatoes, zinc for grapes, boron for many vegetables; the list is long and complex.  Plants signal their need for help by exhibiting distress in leaf, bud and flower.  As the plant’s ‘primary care person’, your task is to diagnose the problem and provide corrective procedures.  Mineral spraying acts rather like an injection; it gets the medicine into the plant’s system as quickly and efficiently as possible. 

The main stumbling block is our limited diagnostic skills.  Each species of plant has both general and specific mineral needs.  When these minerals are missing from the soil or hydroponic solution, a range of confusing symptoms appear.  We may not discover the specific reason quickly enough to prevent plant collapse.  Even when we do, that plant will take time to recover and may never reach optimum productivity. 

Spraying for mineral deficiencies is emergency medicine -- fast and efficient.  To be successful, we need to know which element is missing and have the cure ready to hand.  This is not always possible, so, in general, it is better to think in terms of prevention rather than cure.  We do not wait until sick to take vitamins (a contraction of ‘vital minerals’).  Just so, rather than spraying when a deficiency appears, put in place a program of foliar fertilization to increase plant health and resilience.  If deficiency spraying is specific first aid, foliar fertilization is preventative health care.

Foliar Fertilization
We all have had the basic course in fertilization: plants need NPK – nitrogen, phosphorous and potassium.  This is like saying humans need carbohydrates, fats and protein.  It tells us the basics but certainly does not say how to eat well.  We need a balanced diet with nourishing foods -- and plants are similar.  They prefer nutrients in which the complex chemicals are bound organically.  Rather than a dose of chemical nitrates, plants thrive best on organic products that provide not only the NPK but also a range of trace elements.

Vegetation evolved in the oceans, bathed in a solution containing every imaginable mineral.  Seaweed takes food directly from seawater.  Land plants, like their marine ancestors, can take in nourishment through the pores or stomata on their leaf surfaces.  Stomata are tiny mouths that breathe in CO2 and exhale water and oxygen.  They also transport nutrients up to ten times more efficiently that root systems.  Foliar feeding bolsters the nutrients available to each plant, like a regular dose of vitamins and supplements. 

Most vegetation requires a minimum of 16, but probably more like 50 essential minerals and trace elements.  Is it just coincidence that some of the best providers of these elements come from the ocean?  Fish products are high in organic nitrogen; kelp is a wonderful source of minerals, particularly potassium, while algae has a range of trace elements and hormones beneficial for cellular development.  Research suggests that natural sea salt contains a vast range of trace elements.  When sprayed in a very diluted form, sea minerals provide most elements needed to prevent deficiencies. 

Foliar fertilization is fast becoming an essential addition to standard cultivation techniques.  For many growers who have grown up with chemicals, it is a small step to organic fertilization – the NPK is just packaged differently.  However, there is another, less well-known aspect to plant cultivation based on biology rather than chemistry -- the realm of the microbes.

Spraying with Compost Tea
When plants evolved on land, they formed an alliance with the microbial life in the soil and air.  Certain species of bacteria and fungi became the chefs that prepared the plant’s food, the medics that helped them fight disease.  Plants like to dine on biologically predigested nutrients; it is easier for them to assimilate.  Healthy plants have a strong immune system that includes a ‘bio-film’ of microbial life on the roots, stems and leaves.  To make use of these biological principles to feed and protect our plants, we can spray with compost tea. 

Compost tea is “brewed” by aerating a mixture of water, compost (sometimes humus or worm castings), and organic nutrients such as molasses, kelp, fish emulsion, and yucca.  This produces a nutrient-rich solution containing vast colonies of beneficial bacteria and fungi.  The microbes digest the nutrients into organic compounds that can be easily taken in by the plant.  These same microbes colonize the surface of the leaves to help fight off disease.

When you spray with compost tea, you envelope the plant with living organisms -- and you enhance the web of life of which the plant is a part.  The results can be astounding: large, mineral rich vegetation with clear glossy leaves, decreased disease, and even lessened insect attacks.  Plants treated with foliar fertilization and especially compost tea have higher “Brix” levels – a measure of the carbohydrates and mineral density in the sap.  High Brix is said to make the plants less attractive to pests and more resilient to stress.  If they are vegetables, they even taste better!

Compost tea, unlike mineral sprays and foliar fertilization, cannot be over-applied and does not burn leaves.  The microbe-rich droplets drip off the leaves to improve soil and growing solutions.  Those same microbes can clean up toxic chemicals and turn them into nutrients.  For growers who regularly use compost tea, there is nothing better.  The main drawback is that brewed compost tea is not always available and, being alive, has a limited shelf life.  If you brew your own compost tea, it needs to have the best ingredients and proven test results.

Whether you apply a mineral solution to deficient plants, have a regular foliar fertilization program or go the distance with compost tea, foliar spraying benefits your plant quickly and profoundly.  Find that old spray bottle; hook up your hose-end sprayer; invest in a commercial spray pack.  Once you see the results, you will never neglect this method of plant care again.

Tips on Spraying
Below are guidelines for foliar spraying:

  1. When mixing up your formulation, whether mineral, organic fertilization or compost tea, use non-chlorinated, well oxygenated water.  Bubble air through chlorinated water or leave it to off-gas overnight.  You can try using seltzer in your foliar spray to give plants an added CO2 boost.
  2. Make sure mineral ingredients are dissolved and the solution is very dilute.  Chemicals in high concentration tend to ‘burn’ foliage and leave a salt residue.  Compost teas need to be diluted 10–1. 
  3. Add a natural surfactant or wetting agent to help the solution flow over and stick to foliage.  Yucca is a natural surfactant and is often a component of compost teas.  Use true organic soaps such as Dr Bronners, Tom’s, or Pangea.  The great majority or other soaps contain detergents that do not break down easily.
  4. Young transplants prefer a more alkaline solution (pH 7.0) while older growth like a somewhat more acid (pH 6.2) spray.  Use baking soda to raise pH and apple cider vinegar to lower the pH of your spray.
  5. Spray with a fine sprayer for foliar fertilization and a coarser, low pressure sprayer for compost tea.  The microbes in compost tea need large protective water droplets.  Apply in the early morning or evening when the stomata are open.  Do not spray if the temperature is over 80F or in the bright sun.  Harsh ultraviolet rays can kill microbes in compost tea.
  6. Cover at least 70% of the foliage, paying particular attention to the underneath of the leaf surfaces. 
Apply foliar fertilization or sprayed compost tea every two to three weeks during the growing season.