Maximum Yield Australia/New Zealand September/October 2021 | Page 32

The challenge of hydroponic growing is nutrient and pH levels can and do change much more rapidly than in conventional or other growing media , so frequent testing is vital .”

The challenge of hydroponic growing is nutrient and pH levels can and do change much more rapidly than in conventional or other growing media , so frequent testing is vital .”

Magnesium ( Mg ) — Like several other nutrients , magnesium toxicity can be hard to identify and may appear as a calcium deficiency as it can block the calcium when magnesium is in large amounts . It can also be caused by too much potassium , nitrogen ( in the form of ammonium ), or show when the hydroponic solution ’ s pH is too low . Plants suffering from too much magnesium will often show symptoms in the younger leaves . They will first present interveinal chlorosis , be lighter green in colour and / or have a mottled appearance . The leaf margins may remain green but will curl upwards . Overall , plants will be smaller when they have too much magnesium . Sulfur ( S ) — Too much sulfur in plants appears very much like too much nitrogen . Sulfur deficiencies are not common and if toxic amounts of nutrients exist — and look like sulfur toxicity — it is most likely nitrogen toxicity , but sulfur toxicity can exist . Plants will be stunted and woodier than usual . Leaves will be thicker and discoloured at the margin . The middle of the plant will be a light green .
Micronutrient Toxicity Symptoms :
Boron ( B ) — Not often found in toxic amounts but will present as leaf tip chlorosis or leaf tip burn which will then turn downward . Symptoms will appear in older , lower leaves first . Dead spots on leaves will feel “ papery .” Boron toxicity appears as overall nutrient burn , so it is also hard to single out and identify . Chloride ( Cl ) — Plants with excessive chloride will appear burnt on new growth and leaves will begin to turn a bronze-like colour .
Copper ( Cu ) — Copper toxicity will appear as iron toxicity ( see below ). Plants will start to decay rapidly , beginning with severe leaf necrosis . Roots may appear unusually large in plants with too much copper . Iron ( Fe ) — Iron toxicities are very rare as usually there is not enough iron in plant mediums or hydro solutions . Upper , younger leaves will begin to spot or mottle and then die . Interveinal chlorosis will appear at or near the leaf petiole . Too much iron can prevent the absorption of phosphorus . Manganese ( Mn ) — Toxic amounts of manganese appear differently on new leaves compared to older ones . New growth will appear with dark orange to brown mottling and will be stunted . Older leaves will get spots on their margins and show interveinal chlorosis . Flower heads will be loose on plants with toxic amounts of manganese . Too much manganese can appear as either an iron or zinc deficiency and is easily misdiagnosed . Molybdenum ( Mb ) — Too much molybdenum will block a plant ’ s ability to absorb copper and iron , so molybdenum toxicity appears as copper or iron deficiency . Silicon ( Si ) — Unlike most other symptoms of nutrient toxicities , excess amounts of silicon will not appear as leaf symptoms . Plants suffering from too much silicon will have woody and weak stems and their flower heads will appear severely deformed . Zinc ( Zn ) — Zinc toxicity needs to be addressed quickly if identified as it will kill plants quickly . Too much zinc blocks a plant ’ s ability to uptake iron , and older leaves will rapidly turn yellow and die . New growth will be stunted . Middle-aged leaves will form reddish-brown spots . Overall , leaves will appear long and narrow and the plant may have an overall brown-green appearance .
Correcting Nutrient Toxicity
Once identified , nutrient toxicities in hydroponic systems need to be addressed quickly . Whereas nutrient deficiencies can be corrected slowly , there is no luxury of time when attempting to salvage a crop showing toxic amounts of nutrients . Once toxicity is discovered , the hydro system will need to be flushed . Nutrients can be reintroduced carefully afterwards .
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