Magnesium Deficiency in Crops: Symptoms, Causes, and Correction

What magnesium deficiency looks like

The classic sign of magnesium deficiency is interveinal chlorosis on the older, lower leaves: the tissue between the veins turns yellow while the veins themselves stay green. Because magnesium is the central atom of the chlorophyll molecule, tissue that runs short of it cannot hold its green colour (Penn State). Symptoms often begin at the leaf margin and near the tip, then spread inward between the veins.

Magnesium is highly mobile inside the plant. When supply runs low, the crop withdraws magnesium from mature leaves and moves it to the new growth at the top, which is why the oldest leaves show damage first and the youngest stay green longest. If the shortfall is severe or withdrawal is slow, symptoms can eventually climb into the mid and upper canopy (Penn State).

As deficiency worsens, the yellow interveinal areas can take on a reddish, bronze, or violet tint and finally turn brown and necrotic; badly affected leaves become thin and brittle and may drop early. On tomatoes and other high-tunnel crops the lower leaves often purple before defoliating (SDSU Extension). Similar interveinal patterns appear across corn, small grains, potatoes, tomatoes, tree fruit, and many vegetables.

Why it happens: sandy soils, leaching, and potassium antagonism

Magnesium deficiency is largely a soil-chemistry problem, and three conditions drive most cases.

Sandy soils and leaching. Magnesium is held on the soil's cation-exchange sites, and light, sandy, low-CEC soils simply hold less of it. Because the magnesium ion is weakly held and water-soluble, heavy rain or over-irrigation flushes it below the root zone. Deficiency is therefore most common on sandy soils that are easily leached (SDSU Extension), and on acidic soils where exchangeable magnesium is already low.

Potassium (and cation) antagonism. Magnesium, potassium, calcium, and ammonium all compete for the same uptake sites on the root. When one cation is present in excess, it can block magnesium uptake even where soil magnesium looks adequate on paper. Excessive potassium in particular can induce magnesium deficiency, as can fertigation with high rates of ammonium (SDSU Extension). As a working rule, Michigan State University advises that magnesium should make up a greater share of the soil's cations than potassium does (%Mg > %K).

Absolute levels beat ratios. The old idea of chasing an ideal calcium-to-magnesium ratio has not held up: varying the Ca:Mg ratio between 2 and 8 has shown no effect on yield when both nutrients are adequate. What matters is the actual amount of plant-available magnesium — below about 35 ppm, or under 3% of base saturation, is considered low (Michigan State University).

Confirm before you treat

Several disorders mimic magnesium deficiency. Potassium and manganese shortages, natural leaf senescence, and even some herbicide or salinity injury can all yellow older or interveinal tissue, so symptoms alone are not proof. Confirm the diagnosis with a soil test for exchangeable magnesium and, where possible, a plant-tissue analysis before spending money on inputs.

This matters for cost as much as accuracy: extension guidance is explicit that you should not apply magnesium products such as Epsom salt unless you actually see — and have confirmed — a deficiency (SDSU Extension). Blanket applications waste product, and pushing magnesium too hard can in turn interfere with potassium and calcium uptake. Test first, then treat only what the test flags.

Correcting a visible deficiency: foliar magnesium sulphate

When a growing crop shows a confirmed deficiency and needs magnesium quickly, a foliar spray of magnesium sulphate heptahydrate — Epsom salt — is the standard rescue. It is fully and rapidly water-soluble, so it dissolves cleanly in the tank and is taken up through the leaf within days, bypassing the soil chemistry that caused the problem.

For field crops, Michigan State University Extension gives a foliar rate of 10 to 20 lbs of Epsom salt in 30 gallons of water per acre (roughly 4.5–9 kg in about 114 L), repeated if symptoms persist. In greenhouse and high-tunnel systems, magnesium sulphate is also run through drip lines at about 60 ppm, or applied as a 1–2 lb per 100 gallon drench (SDSU Extension). Peer-reviewed field work supports foliar magnesium too: a 2025 study on densely planted sweet corn found that foliar magnesium sprays improved yield and grain carbohydrate content.

Two practical cautions. First, concentrated sprays can scorch sensitive foliage, so many programmes use a more dilute solution (on the order of 2%, i.e. roughly 20 g/L) and apply in the early morning or late afternoon to limit leaf burn — confirm the safe concentration and rate for your crop and water quality with a local agronomist or soil-test authority. Second, foliar feeding is a fast fix, not a cure for a depleted soil: it buys the current crop time, while rebuilding soil magnesium is a separate, soil-applied job. RunziChem supplies the heptahydrate (Epsom-salt) grade used for these sprays — see magnesium sulphate.

Building soil magnesium: kieserite, dolomitic lime, and blends

Correcting the soil so next season's crop does not relapse calls for a soil-applied source, and the right one depends on soil pH and the other nutrients you need.

• Magnesium sulphate monohydrate (kieserite-type) carries roughly 15–18% magnesium (Michigan State cites about 18%) plus sulphur — far more magnesium per tonne than the heptahydrate — and its granular, slower-dissolving form suits broadcast soil application and dry NPK blends. On sandy, leaching soils its gradual release over the season is an advantage.
• Dolomitic limestone (about 11% Mg) is the go-to magnesium source when the soil is also acidic and needs liming, because it corrects magnesium and raises pH at the same time. If soil pH is already adequate, do not use it; reach for a sulphate source instead.
• Potassium-magnesium sulphate (K-Mag / Sul-Po-Mag, ~11% Mg with ~22% K2O and ~22% S) supplies magnesium, sulphur, and potassium without changing pH, and magnesium oxide (~55% Mg) is very concentrated but slow to react.

For soils that need magnesium but not lime, Michigan State University suggests broadcasting 50 to 100 lbs of actual Mg per acre, or banding 10 to 20 lbs of actual Mg per acre at planting. The heptahydrate-versus-monohydrate trade-off — solubility versus magnesium density and freight — is covered in our Epsom salt vs kieserite comparison.

Epsom salt vs kieserite: matching the form to the job

The choice between the two magnesium sulphate grades comes down to the job:

• Spraying leaves or running through irrigation, or need a fast rescue? Use the heptahydrate (Epsom salt).
• Building or maintaining soil magnesium, or blending into granular NPK? Use the monohydrate (kieserite-type) for its higher magnesium density and slow release.
• Soil also acidic and short on pH? Consider dolomitic lime instead.
• Want magnesium plus potassium and sulphur without raising pH? Consider K-Mag.

Whatever the form, set the actual rate and timing from a current soil or tissue test and local agronomic advice — the figures above are typical ranges, not a prescription for any one field.