Zinc Sulphate Monohydrate — ZnSO4·H2O Zinc Fertiliser Technical Data

Zinc sulphate monohydrate is the most widely used soluble zinc source in agriculture. It supplies zinc in the sulphate form, immediately available for root uptake or foliar absorption, alongside a useful quantity of sulphate sulphur. Like magnesium sulphate, it exists in more than one hydration state, and the monohydrate is preferred wherever material is shipped in bulk, blended dry or stored for long periods, because it carries markedly more zinc per tonne than the heptahydrate and behaves far better physically.

Substance identity

Specification

Hydrate comparison, from formula weights:

The monohydrate carries about 1.6 times the zinc per tonne of the heptahydrate. Two offers quoted per tonne of product are therefore not comparable; convert both to cost per unit of contained zinc.

Note also that total zinc and water-soluble zinc are different declarations. Some zinc sources (oxides, oxysulphates, frits) declare a total zinc figure of which only part is water-soluble and immediately available. Zinc sulphate’s advantage is that these two numbers essentially coincide. When comparing zinc products, compare water-soluble zinc, not total zinc.

Properties

Solubility. Zinc sulphate is readily soluble in water in both hydrates. Aqueous solutions are acidic because the hydrated zinc ion hydrolyses. The monohydrate dissolves somewhat more slowly than the heptahydrate at equal particle size, and fine grades dissolve faster than coarse ones — the same surface-area relationship that applies to magnesium sulphate.

Physical behaviour. The monohydrate is comparatively stable in storage, free-flowing when dry, and considerably less prone to caking and moisture pickup than the heptahydrate. It is this stability, combined with the higher zinc concentration, that makes it the standard form for inclusion in dry bulk blends and compound fertilisers.

Thermal behaviour. Loses its water of crystallisation on heating to give the anhydrous salt; decomposes at high temperature to zinc oxide with evolution of sulphur oxides.

Chemical compatibility. Zinc is precipitated by alkalinity. In concentrated solution, zinc sulphate is incompatible with strongly alkaline materials and with concentrated phosphate sources, with which it can form sparingly soluble zinc phosphates.

Applications

The agronomic role of zinc. Zinc is required by a large number of plant enzymes — including carbonic anhydrase, alcohol dehydrogenase, Cu/Zn superoxide dismutase and several classes of DNA- and RNA-handling enzymes — and by the structural “zinc finger” motifs of many regulatory proteins. Agronomically the most consequential single role is in auxin metabolism: zinc is needed for the synthesis of tryptophan, the precursor of indole-3-acetic acid. When zinc runs short, auxin supply falls, cell elongation stalls, and the plant produces the characteristic architecture of zinc deficiency.

Deficiency symptoms. Zinc is relatively immobile in the plant, so symptoms appear on young, upper leaves and growing points — the mirror image of the magnesium pattern. Typical expressions:

• shortened internodes and clustered leaves at the shoot tip (“rosetting”)
• reduced leaf size (“little leaf”)
• interveinal chlorosis, often with a distinctive bleached or bronze cast, on the newest fully expanded leaves
• in maize, whitish interveinal banding on young leaves (“white bud”)
• in rice, brown lesions and stunting following transplanting into flooded soil
• in citrus, characteristic mottled interveinal chlorosis with reduced fruit size
• delayed maturity and poor grain or fruit set even where the canopy looks acceptable

Because zinc affects reproductive development, a “hidden hunger” yield penalty can occur without dramatic visual symptoms, which is why tissue and soil testing matter more for zinc than for nutrients with unmistakable visual signatures.

Why high-pH calcareous soils are the classic problem soils. Zinc availability is governed by the solubility of zinc in the soil solution, and that solubility is strongly pH-dependent: as a rule of thumb, the activity of Zn²⁺ in solution falls by roughly two orders of magnitude for each unit increase in soil pH. On calcareous soils several mechanisms compound this:

• carbonate minerals buffer pH in the range where zinc solubility is lowest, and zinc adsorbs onto and coprecipitates with calcium carbonate surfaces;
• high bicarbonate concentration in soil solution and in irrigation water interferes with zinc uptake and translocation within the plant;
• these soils are frequently low in organic matter, and organic matter supplies the soluble complexing ligands that keep zinc mobile;
• heavy phosphate fertilisation antagonises zinc uptake, an interaction that is more pronounced where zinc is already marginal;
• land levelling and topsoil removal expose subsoil that is both more calcareous and lower in zinc.

Flooded rice soils are a second classic deficiency setting, where reduction chemistry, sulphide formation and bicarbonate accumulation depress zinc availability even on soils that test adequately when drained.

Delivery routes.
– Soil application — broadcast, banded or included in a blend or compound. Banding near the root zone reduces the quantity needed on high-pH soils, where broadcast zinc is rapidly fixed.
– Fertigation — dosed into irrigation water; requires a fine grade and attention to solution compatibility.
– Foliar spray — the fastest correction of an established deficiency and the most efficient route on strongly calcareous soils, where soil-applied zinc has low recovery. Sprays must be repeated because zinc redistributes poorly within the plant.
– Seed treatment / seedling dip — used in some rice and cereal systems.

Other uses. Zinc sulphate monohydrate is also used outside crop nutrition, including as a zinc source in animal feed premixes and as a raw material in industrial applications; those grades are specified against different purity and contaminant criteria than fertiliser grade.

Handling and storage

Store dry, sealed, under cover, off the floor, out of direct sunlight, and clear of foodstuffs and feed unless the grade is specifically qualified for that use.

Segregate from alkaline materials. Wear gloves, eye protection and, where dust is generated, respiratory protection. Zinc sulphate is irritating to skin, eyes and the respiratory tract, and is harmful to aquatic organisms — this is the handling point that most deserves emphasis. Contain spillages, collect them dry where possible, and keep washings out of drains, ditches and watercourses. Consult the current safety data sheet before use.

For tank mixing: add to circulating water, keep away from concentrated phosphate and strongly alkaline stock solutions, jar-test unfamiliar combinations, and filter stock solutions feeding drip systems.

Notes

• Compare zinc products on water-soluble zinc per tonne, not on product tonnage or total zinc.

• Zinc has a comparatively narrow window between deficiency and excess. Over-application, particularly repeated over years, accumulates in soil and can become phytotoxic, so rates should follow soil and tissue analysis rather than routine insurance dosing.

• On strongly calcareous soils, expect low recovery from broadcast soil-applied zinc; banding or foliar application is generally the more efficient route.
• Heavy-metal limits for fertiliser-grade material differ between jurisdictions; confirm that the certificate of analysis meets the destination market’s requirements.
• Zinc sulphate is a plant nutrient product; it is not a plant protection product, and any non-nutritional claim would require separate registration.

---

Frequently asked questions

What is the practical difference between zinc sulphate monohydrate and heptahydrate, and which one should I buy?

Both are the same salt; the difference is water of crystallisation. The monohydrate (ZnSO4·H2O) is 36.4% zinc by molecular weight; the heptahydrate (ZnSO4·7H2O, CAS 7446-20-0) is about 22.7%. Per tonne of product the monohydrate therefore delivers substantially more zinc, so freight, packaging and handling cost per unit of nutrient are lower — usually the deciding factor on long shipments. The heptahydrate dissolves faster in cold water and is often preferred where a spray tank has to be filled quickly. The monohydrate is less hygroscopic, stores and blends better with dry NPK, and is the usual choice for granular blends and soil application. Decide on handling and dissolution needs first, then compare landed cost per kilogram of zinc.

Why do commercial grades quote around 33-36% Zn when the formula suggests 36.4%?

Pure ZnSO4·H2O is 36.4% zinc by molecular weight, so that figure is a ceiling, not a target. Real material carries some free moisture, some insoluble or unconverted residue from the feedstock, and often a fraction of higher hydrate — no commercial lot is a perfect monohydrate. Producers therefore guarantee a minimum below the theoretical value, and the level depends on grade and process. Two further things move the number: whether the guarantee covers total zinc or water-soluble zinc, and how moisture was determined. A guaranteed minimum of 33% with a typical value near 35% is a different commercial proposition from a hard 33% ceiling, so ask for both the guaranteed minimum and the typical assay.

How do I compare two offers on a like-for-like basis?

Convert everything to cost per kilogram of zinc before looking at anything else: divide the delivered price per tonne by the zinc fraction. A tonne at 35% Zn carries about 350 kg of zinc, one at 33% about 330 kg — a difference a headline price per tonne hides. Then adjust for what changes effective value: water-soluble versus total zinc, water-insoluble matter, moisture, packaging, and freight, which is charged on mass rather than on nutrient. Give credit for the sulphate sulphur, roughly 17-18% S by stoichiometry, if your programme needs sulphur anyway. Finally compare particle size distribution if the material goes into a dry blend, because size mismatch between components causes segregation in transport.

What should I check on the certificate of analysis besides the zinc figure?

Water-soluble zinc, not only total zinc. Water-insoluble matter, since that residue is what ends up in a spray tank filter. Moisture, with the drying method and temperature stated — drying too hot strips crystal water and inflates the apparent zinc content, so figures produced by different methods are not comparable. Free acid, which affects solution pH, corrosivity and bag life. Chloride and iron if equipment corrosion or solution colour matter to you. And heavy metals: cadmium, lead, arsenic, mercury. That last point is not a formality, because zinc ores are naturally cadmium-bearing and some zinc sulphate is made from secondary feedstocks such as galvanising residues. Ask which analytical method was used and whether values are guaranteed minima or typical.

What should zinc sulphate not be mixed with?

Three groups cause most of the trouble. First, calcium sources: combining a sulphate with calcium nitrate or calcium chloride in a concentrated stock tank precipitates calcium sulphate, which coats screens and blocks drip emitters. Keep calcium and sulphate in separate stock tanks and let them meet only in dilute form in the main line. Second, concentrated phosphates — MAP, DAP, phosphoric acid, polyphosphate liquids — which precipitate zinc phosphate and remove the zinc you paid for. Third, strongly alkaline materials: lime, copper-lime mixtures, or high-bicarbonate water, because zinc drops out as hydroxide and carbonate as pH rises toward neutral and above. Always run a jar test with your own water, at your intended concentration and order of addition, before committing a full tank.

How should it be stored, and why does it cake?

Store under cover in sealed moisture-barrier bags, on pallets clear of floors and walls, out of direct sun and away from humidity swings. The monohydrate is not the stable hydrate at ordinary ambient temperature once free water is present: absorbed moisture converts surface crystals to higher hydrates, and the recrystallised bridges between particles are what turn a bag into a solid block. Caked material is still chemically sound, but it has to be broken and screened before it will meter or dissolve properly. Keep it away from alkaline stock such as lime, cement and ammonia, and away from feed and food. The dust is acidic, irritating to eyes and airways, and corrosive to bare steel over time; follow the safety data sheet.

When is zinc sulphate monohydrate the wrong choice?

Several situations. If soil or tissue testing shows zinc is already adequate, more buys nothing: zinc is not consumed like nitrogen, it accumulates, elevated levels are phytotoxic and can antagonise iron and copper uptake, and some jurisdictions cap cumulative zinc loading. On high-pH calcareous soils, broadcast sulphate zinc is fixed quickly into poorly available forms, so banding, foliar application or a chelated zinc source is usually the better tool. In drip systems fed by hard, alkaline water, a chelate avoids the precipitation and clogging risk described above. In concentrated liquid formulations containing phosphate, a sulphate is simply the wrong chemistry. And fertiliser grade is not a substitute for feed or pharmaceutical grade — those applications require material specified and certified for that use. Zinc is also harmful to aquatic organisms, so runoff and washwater need controlling.

Related substances

• If chlorosis is showing on the older leaves rather than the youngest, magnesium and not zinc is the likelier cause – the magnesium sulphate page covers that diagnosis.
• Phosphate and zinc interfere with each other both in the soil and in a concentrated stock tank, so the MAP page is worth reading before the two are applied in the same operation.
• The alkaline zone around a dissolving DAP granule is exactly where applied zinc becomes least available, which makes the placement of the two products a joint decision.