Iron (Fe)

Marijuana Horticulture

by Jorge Cervantes

Iron is available in a soluble chelated form that is immediately available for absorption by the roots. Deficiency indoors is common in alkaline soils.

Iron is fundamental to the enzyme systems and to transport electrons during photosynthesis, respiration, and chlorophyll production. Iron permits plants to use the energy provided by sugar. A catalyst for chlorophyll production, iron is necessary for nitrate and sulfate reduction and assimilation. Iron colors the earth from brown to red, according to concentration. Plants have a difficult time absorbing iron. Acidic soils normally contain adequate iron for cannabis growth.

Iron deficiencies are common when the pH is above 6.5 and uncommon when the pH is below 6.5. Symptoms may appear during rapid growth or stressful times and disappear by themselves. Young leaves are unable to draw iron from older leaves, even though it is present in the soil,. The first symptoms appear on the smaller leaves as veins remain green and areas between the veins turn yellow. Interveinal chlorosis starts at the opposite end of the leaf tip: the apex of the leaves attached by the petiole. Leaf edges can turn upward as the deficiency progresses. Leaves fall off in severe cases. Iron deficiency is sometimes traced to an excess of copper.

Treat deficiency by lowering the soil pH to 6.5 or less. Avoid fertilizers that contain excessive amounts of manganese, zinc, and copper, which inhibit iron uptake. High levels of phosphorus compete with the uptake of iron. Improve the drainage; excessively wet soil holds little oxygen to spur iron uptake. Damaged or rotten roots also lower iron uptake. Increase root zone temperature. Apply chelated iron in liquid form to root zone. Chelates are decomposed by light and must be thoroughly mixed with the growing medium to be effective. Exposing the nutrient solution to light causes depleted iron. Sterilizing the nutrient solution with UV light causes iron to precipitate. Leaves should green up in four to five days. Complete, balanced, hydroponic nutrients contain iron, and deficiencies are seldom a problem. Organic sources of iron, as well as chelates, include cow, horse, and chicken manure. Use only well-rotted manures to avoid burning plants.

Excess of iron is rare. High levels of iron do not damage cannabis, but it can interfere with phosphorus uptake. An excess of iron causes the leaves to turn bronze accompanied by small, dark brown leaf spots. If iron chelate is over applied, it will kill the plant in a few days. Treat excess iron by leching plants heavily.

The following group of micronutrients is seldom found deficient. Avoid deficiencies by using a high-quality hydroponic fertilizer that contains chelated micronutrients.