Magnesium

Role in Plants. A magnesium ion (Mg2+) is found at the central position of every chlorophyll molecule, making it an important part of the photosynthetic apparatus. Magnesium ions also serve as enzyme cofactors, notably in ATP metabolism. Some magnesium is also present in cell walls.

Hydroponic Source. Magnesium sulfate (MgSO4) is the most common magnesium source for hydroponics. As is true for calcium, the amount of naturally occurring magnesium in the feed water should be determined before deciding how much reagent to use.

Deficiency Symptoms. Because of the importance of magnesium in photosynthesis, a lack has a profoundly negative effect and is difficult to recover from when it happens. Chlorosis between the veins of older leaves is a good indicator, with perhaps the addition of a red or orange tint. Growth is strongly inhibited. Magnesium has a complex relationship with Ca2+, K+ and NH4+ ions, and an imbalance among them can lead to magnesium deficiency.

Sulfur

Role in Plants. Although only two amino acids contain sulfur (S), few proteins would function normally without them. Sulfur is found in plant hormones, and in molecules involved in chemical defense, odor and taste. Many organic molecules, intermediate compounds and proteins, contain or are modified with sulfate side-groups.

Hydroponic Source. Growers have a choice of several other nutrient ions to pair up with sulfur when designing nutrient solutions. Potassium (KSO4), magnesium (MgSO4) or ammonium [(NH4)2SO4] sulfates are all good options. Deficiency Symptoms. Sulfur deficiency is difficult to distinguish from nitrogen deficiency. One difference between the two is that a lack of sulfur tends to cause more of an overall yellowing of a plant, rather than a yellowing of the older leaves first.

Excess Symptoms. There is little evidence that plants are harmed by high concentrations of sulfate ion.