Most growers when they talk about
flushing are actually describing something called leaching. Also, a lot of
folks think it’s just something to do a week or so before harvest, rather than
periodically throughout the lifecycle. So what is leaching? Well, it’s really all
about reducing the concentration of dissolved minerals around your plants’
roots and in the growing media. Maybe you’ve increased your nutrient solution
strength too quickly and your plants aren’t responding well. Perhaps you’ve
even noticed some tip burn? Not good. Example: you’re growing hydroponically,
let’s say in a soilless mix: coco coir and perlite perhaps? And you want to
feed your plants a nutrient solution at 1,000 ppm, that’s 2.0 millisiemens per
centimeter, anyway you test the runoff and– hey, look at that– it’s all the way up
to 3.0 millisiemens, or 1500 PPMs. No wonder your plants look a little
frazzled, huh? So, what can you do? Well, you can panic and top feed your plants pure
water ;that’ll certainly lower the concentration quickly around the root
zone, but it also risks creating a form of osmotic stress: plants struggling to
adapt to sudden changes in solute concentrations around the roots. It’s
preferable to apply a dilute nutrient solution, say 500 or 750 PPM, that’s 1.0
or 1.5 millisiemens, and water through, testing the runoff periodically until
it’s around your target 1,000 ppm. Note the symmetry between my corrected, more dilute nutrient solution and the original leachate reading.
Why do mineral-based nutrients tend to accumulate in the root zone? I’m so glad
you asked that! Remember, plants transpire to keep
themselves cool; they uptake lots of water, but actually hold on to very
little. Most of it simply passes through the plant and eventually back into the
air through tiny pores in their leaves as water vapor. There’s also a
considerable amount of water lost to evaporation. Now I don’t you two worry
unduly about transpiration; it’s natural and necessary, and well, it’s just what
plants do, but in the process of sucking up all this cooling water, some nutrients
inevitably get left behind. In other words, plants uptake more water
than nutrients relative to their ratio in the nutrient solution. This is why
hydroponic growers who use recirculating systems top up their reservoirs with
water or half-strength nutrients every day or two. If they’ve neglected to do
this their plants would end up feeding on an ever concentrating nutrient
solution as time went on, eventually leading to toxicity problems, and plant
tissue damage. Leaching then starts with testing the leachate or runoff. If it’s
over 300 PPMs higher than your input nutrient solution, you top feed with a
milder nutrient until the runoff is where you want it. Like I said before, if
you’re 300 PPMs over your input, leach with a solution that’s 300 PPMs under.
Some growing systems are more prone to salt build-up than others. If you top feed
your plants with drippers or manually with a
watering can, then you’re probably going to be okay, so long as you water
regularly, and achieve 20 to 30% runoff each time. Passive wicking
systems and flood and drain, where plants are fed from the bottom and the
capillary action of the roots draws up the nutrient solution are more prone to
salt build-up. As well as periodic leaching, there’s also the more widely known,
pre-harvest flash. This is what most folks mean when they’re talking about
flushing; it’s typically performed by growers cultivating plants which produce
the majority of all of their harvest in one it at the end. Yeah, okay, like most
determinate tomato varieties, for example. Usually, two weeks before the harvest
growers start to lower their nutrient concentrations. Just don’t go straight
into it with pure water; first go half strength, and eventually say in the last
week, to quart strength. The idea is to encourage the plant to use up any
residual nutrients within itself so that only pure plant flavors remain. Reverse
osmosis water mixed with the flushing agent is the most aggressive flush and
should be reserved for the final few days. The final flush is a balancing act:
start it out too early, and you deny your plants important nutrients where they
arguably need it the most and it could hit your yield; too late and you risk
leaving excess residual mineral stored in plant tissue. Enzyme-based products
and compost tea can be a good prelude to a pre-harvest flush, as they help to
break down any dead root material and allow nutrients from these to be fed
back to the plant. Reducing the nutrient concentration signals to the plant that
the end is near, just like when you stop offering seconds to your in-laws at
holiday dinners, and can speed up the ripening process. Your plants’ leaves,
especially older lower leaves, will begin to turn pale and may even turn reddish
brown as mobile elements are drawn from them into the newer growth, buds, flowers,
and fruits. This is a sign that your final pre-harvest flush is working. Also,
when we start to starve the plant like this, we effectively force it to draw
sugars from its larger older leaves into the flowers and fruits, themselves.
Tobacco growers, for instance, feed with plenty of water prior to harvest to
reduce nitrate content in the leaves, helping to improve flavor, and create a
better burn. Growers in NFT and DWC systems have the easiest time flushing
because there’s really not any growing media to leach of nutrients. You simply
change out the reservoir and you’re done. Coco coir growers may find it hard to
flush out calcium and magnesium, as this media tends to hold on to these elements
for dear life. Using a coco and perlite mix makes it easier to leach effectively.
Hydroton expanded clay balls are relatively easy to leach, as they don’t
really hold on to nutrient solution. However, if they do undergo a drying out
period, the nutrient salts can dry on the outer surface and become insoluble. You
see these often on the surface of the container where they can dry out under
light and air movement. Grodan stonewool is another easy pre-harvest flusher. It has
zero CEC so it easily lets go of residual nutrients. Mother Earth grow
stones are another great choice for the same reason; peat based potting mixes are
harder to flush. Organic soil growers shouldn’t worry about flushing at all.
Simply build up the initial soil recipe, you can check out mine here, use the
liquid organic feet or top dress additional nutrients as and when
required, and then switch to water a few weeks before harvest. To sum up:
periodic leaching and pre-harvest flushing should be done gradually and
carefully. It’s really something quite specific to high-value, consumable crops.
You really won’t find commercial cucumber hydroponic growers talking
about this. However, the moral of this story is always be aware of the nutrient
concentrations in your root zone, as well as in your reservoir. I hope you found
this helpful! Please don’t forget to subscribe, and if you use Instagram and
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drill. This is Everest, fully fattened, yet tasting exceptionally clean! Bye-bye.