HorticultureKeymasterSeptember 22, 2016 at 11:55 amPost count: 98
Cannabis plants grown outdoors will typically have lower nutrient status in their leaves compared to plants grown indoors. This has been shown to be a result of exosmosis, or the leaching of mineral nutrients from foliage by rainfall and fog. All plants grown outdoors will lose some mineral nutrients from their tissues to rainfall and heavy dew just by osmosis from inside of the leaf.
This is the opposite of what happens when we foliar feed cannabis crops, and with large plants it can be pronounced enough that outdoor growers need to correct for this nutrient loss via exosmosis. Studies of exosmosis in plants date back over 40 years;
Some plants exude and emit liquid water through pores called hydathodes in a process called guttation driven by water pressure inside the plant. Stomata that are open on foliage are a direct conduit of exosmosis when plants are soaked by rain. The waxy cuticle covering leaves and shoots is also slightly permeable to water so certain mineral ions can escape through this boundary when rainfall is heavy. Young leaves and shoot tips have higher concentrations of nutrients in their tissues, and have thinner cuticular waxes, so these parts of plants are more vulnerable to exosmosis.
Another study is shown here, although only the abstract is available:
These studies showed how rainfall will leach SO4, Ca, Mg and K from leaves of alder trees. However they also show that organic molecules like amino acids, vitamins, carbohydrates, phenolics and hormones will be drawn out of plants when exosmosis can happen. Large outdoor plants can change the nutrient content and “flavour” the soil in the area they are growing by just leaching their internal reserves from leaves onto the ground they are growing on.
In large outdoor crops, this can lead to nutrient cycling where fields of plants reabsorb the minerals lost from their leaves, and plants modify the soil near them with phenolics, amino acids and other organic molecules also lost through exosmosis. This changes the microbial populations in the soil, soil fertility and soil forming processes.
Stress conditions can increase this nutrient loss through exosmosis; prolonged darkness, drought, pollutants (ozone) and acid rain all make the foliage and shoots of plants become more permeable to water and solute loss. Acid rain can increase nutrient loss by exosmosis by 2 – 10 times.
Astute cannabis growers know to watch for nutrient deficiencies at all stages of growth, but the depletion of minerals by exosmosis usually occurs when outdoor plants become large, and their new shoots are growing long distances from roots. Foliar feeding the plant with a complete nutrient solution around 700 ppm and surfactant is the fastest way to restore imbalances from exosmosis.
A regular foliar feeding program will benefit all cannabis crops, but outdoor growers should consider using this method until the start of flowering. Regular use of a food grade surfactant (eg polysorbate-20) along with a dilute nutrient solution and seaweed extract to foliarly feed outdoor crops will correct for any nutrient deficiencies throughout the plants, and foliar feeding with surfactant and seaweed extract will repel any insect or fungal pathogens from visiting the crop.
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