Preventing and Correcting Nutrient Toxicity in Hyd...

粟英才 2021-09-12

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One of the many benefits of growing in hydroponic systems is the grower controls all the inputs. This means the quality and volume of nutrients the grower wants will go into the system. The downside is there is no help from Mother Nature. It is then totally on the grower to make sure they don’t get too much of a good thing and the hydroponic system becomes a toxic soup of too many nutrients for your plants. Like so many well-meaning, young goldfish owners who overfeed their pet to death, it’s possible to feed your plants in a hydroponic system to death as well. There is a lot of available information about nutrient deficiencies in crop production, but not as much information is available about toxicities. To correct or prevent nutrient toxicity in plants, we must first know what it is and how to identify it.

Identifying Nutrient Toxicity

Not all plants present symptoms of toxicity and not all show it in the same ways. There are, however, many similarities and some assumptions can be made to at least indicate there is a toxicity issue. It is, though, incredibly important to be able to distinguish between deficiency and toxicity symptoms. When one or more nutrients are present in a hydroponic system in abundance, it can impede the proper absorption and utilization of other elements even if they too are present in sufficient amounts. The nutrient or nutrients in excess will block the other nutrients. The cure for deficiencies will almost certainly cause further harm in plants experiencing toxicities, so positive identification is the first step to addressing any toxicity issues in your hydroponic system. Let’s check out some of the most common toxicities and what they look like in hydroponic crops.

Macronutrient Toxicity Symptoms:

Nitrogen (N) — This is often the nutrient most likely to be found in toxic amounts, especially for newer hydroponic growers. Nitrogen toxicity can present in several possible ways. Leaves may be lighter green in color than normal, especially older, lower leaves. The plant stalks may be shorter than normal and slenderer than expected. Flower heads may not fully form or may not form at all, and root systems may be exceedingly dense. In extreme cases of N toxicity, stems may become stiff and brittle.

Phosphorus (P) — Plants with excessive amounts of phosphorus can be difficult to diagnose as there are not always specific symptoms to identify. Phosphorus toxicity is somewhat rare and generally will present as copper (Cu) or zinc (Zn) deficiencies as too much P will impede the absorption of copper and zinc. Some symptoms may include stunted and abnormally green plants and/or purple pigmentation on the undersides of leaves.

Potassium (K) — Potassium toxicity will generally present as an overall weaker plant. Growth will be stunted and leaves will be smaller than normal. Chlorosis may be noted at the leaf tips and veins and/or edges of the leaves may be purple. These symptoms generally appear on lower, older leaves first. Like phosphorus toxicity, potassium toxicity is not usually seen. Too much K can interfere with the uptake of several secondary macro- and micronutrients. This then can appear as deficiencies in nutrients such as magnesium, manganese, iron, and zinc.

plant with nutrient burn

Secondary Macronutrient Toxicity Symptoms:

Calcium (Ca) — Toxicity with calcium can be difficult to identify. Too much of it will block potassium and manganese absorption so it is easier to spot calcium toxicity as a deficiency in the other two nutrients. Plants with too much Ca will often be severely stunted. Leaves may be more crinkled or form a rosette. Older leaves may have interveinal white spots near their margins and may curl downwards. Chlorosis and then necrosis (cellular or tissue death) will appear at leaf margins too. The growth pattern of the whole plant will be more horizontal overall.

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 cause 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 color 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 discolored at the margin. The middle of the plant will be a light green.

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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 color.

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 (Mo) — 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.

hydroponic system

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 a toxicity is discovered, the hydro system will need to be flushed. Nutrients can be reintroduced carefully afterwards.

All water will need to be drained from the system. Only pure, preferably distilled, or deionized water should be reintroduced. If these are not available, then at least filtered or reverse-osmosis supplied water should be used. Plants should be carefully monitored during this process and it may need to be done more than once to draw out all the excess nutrients that are stored in the plants’ systems. There are many commercially available flushing agents that can be used in addition to pure water, but at the very least, pure water must be used. “Pure” in this context means neutral as measured by a pH meter having a reading of 7.0 /-.

Most growers who catch toxic nutrient buildup in time and perform a timely flush will be able to salvage their crops. Within a week, plants should begin to show signs of recovering if they are able to. A modified nutrient regimen can then be reintroduced. Feeding can resume after this period, starting with rates of one-quarter to one-third of the normally prescribed amount. Each week, the amount can be increased so that by the third or fourth week, normal levels of nutrients may be resumed, though of course carefully monitored to prevent toxic buildup again.

Preventing Nutrient Toxicity

Like so many other things in life, the best way to deal with a problem is not to have it in the first place; nutrient toxicity is no exception. Monitor, monitor, and monitor again. Hydroponic growers should be taking daily readings of pH, EC, and/or PPM. These are simple procedures any grower can competently learn how to do. 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. Visual monitoring is essential too. Growers should become familiar with the nuances of their crop’s physical characteristics to be able to quickly determine if there is something that’s not right.

If there are concerns that any of your hydro crops may be suffering from a nutrient toxicity, do not hesitate to seek advice and act.

Bring a picture or sample to your local hydroponic or garden center. Samples can also be submitted to your local cooperative extension office or state laboratories for testing. These tests are routine and costs are relatively low, especially compared to the cost of an entire lost crop. With careful monitoring and following of dosing instructions, most incidences of nutrient toxicity in hydroponic systems can be avoided.