UAN 28% Storage — Liquid Nitrogen Fertilizer Tank Specification

A Panhandle Co-Op at 5 AM in April

A co-op south of Amarillo loads UAN 28% from a 12,000-gallon HDPE tank every spring side-dress window. The draw schedule is brutal: fifteen trucks between 4 AM and 9 AM, then another ten between 3 PM and 7 PM, for twenty-two straight days. The tank site has three vertical HDPE tanks manifolded together, each rated 1.4 ASTM specific gravity, with 2-inch bulkhead bottoms and top-mount fill risers. The EPDM gaskets were replaced three seasons ago; the 316SS bolts haven't been touched since commissioning in 2018.

That spec is exactly right for UAN 28% service. UAN is among the gentlest chemistries that get stored in bulk liquid fertilizer tanks - less corrosive than 10-34-0, far less aggressive than anhydrous ammonia. But the volume, the thermal cycling in Panhandle weather, and the salt-out risk at the bottom of the tank make tank specification something you get right once or you fight for a decade. This pillar covers the tank system, the salt-out physics, Y-drop and side-dress application logistics, and the Panhandle/Plains agronomy that drives volume.

The Chemistry - 28-0-0 Liquid, 50/50 AN+Urea

UAN 28% is a water solution of ammonium nitrate (NH₄NO₃) and urea (CO(NH₂)₂) dissolved in water at roughly 40% AN, 30% urea, 30% water by weight. The product label 28% refers to total nitrogen content. The blend is engineered to carry as much N as possible while keeping the salt-out temperature low enough for year-round outdoor storage in most climates.

• Salt-out temperature: 1°F (-17°C) for UAN 28 - meaning the solution stays clear down to that temperature before ammonium nitrate begins to crystallize out.
• Specific gravity: 1.28 at 60°F.
• Weight per gallon: 10.66 lb at 60°F.
• Nitrogen forms: 25% ammoniacal, 25% nitrate, 50% amide (urea). This balance is why UAN acts like neither straight urea nor straight AN in soil chemistry.

Compared with UAN 32 (32% N, salt-out 32°F), UAN 28 trades N density for cold-weather storage tolerance. Northern-tier Plains co-ops and Canadian prairie operations typically carry UAN 28 for winter fill; southern-tier Panhandle, Oklahoma, and Texas co-ops more often carry UAN 32 and blend down when needed.

MOC Stack - Field-Proven for UAN Service

No OEM publishes a dedicated UAN line on their resistance chart - it's handled under urea solution for the operational system-of-construction and under ammonium nitrate for concentration bands. The synthesis that matches field practice:

• Resin: HDLPE or XLPE at 1.35-1.5 ASTM specific gravity. Linear PE is fine for indoor/covered installations; XLPE is preferred for outdoor in UV-intensive climates (Panhandle, Oklahoma, Kansas, Nebraska) for stress-crack resistance on long UV exposure. Consult manufacturer chart for your specific tank size.
• Specific Gravity: 1.35 ASTM works for most UAN tanks; 1.5 ASTM preferred for tall vertical tanks (above 12 ft sidewall).
• Fittings: Polypropylene or PVC at bulkheads. Glass-filled nylon threaded fittings work short-term but loosen under thermal cycling - avoid.
• Gaskets: EPDM. This is one of the few chemistries where EPDM outperforms Viton; Viton swells slightly in urea service over years.
• Bolts: 316SS. UAN has mild chloride content (trace) but the nitrate-dominant chemistry keeps 316 pitting below threshold at normal storage temperatures.

The Snyder chart published rating for urea solution (50%) specifies HDLPE/XLPE, 1.35 ASTM SG, PP or PVC fittings, EPDM gaskets, 316SS bolts - that's the closest direct OEM guidance for UAN service. Enduraplas/Equistar rates saturated ammonium nitrate as Satisfactory in HDPE at both 70°F and 140°F, which covers the AN fraction of UAN.

Application Windows - Side-Dress, Pre-Plant, Y-Drop, Fertigation

UAN 28 moves in four application windows across the growing season:

• Fall/winter fill: co-op and retail tank stations take delivery from the pipeline or terminal system (CHS, Koch Fertilizer, Nutrien, CF Industries, Yara). UAN 28 is preferred over UAN 32 for northern-tier winter storage because of the lower salt-out threshold.
• Pre-plant (March-April): broadcast or banded application ahead of corn, small grains, or cotton planting. Applied via floaters, toolbars with knife injection, or broadcast boom.
• Side-dress (May-June): the dominant UAN application - split-applied N in corn at V4-V8 growth stages. Applied via coulter-injected toolbars or Y-drop hoses that deposit UAN in the root zone between corn rows without foliar contact.
• Fertigation: metered into center-pivot irrigation water for in-season N delivery. Requires a separate injection pump, check valve, and backflow prevention - UAN's low vapor pressure makes it one of the safer fertilizers to fertigate.

The side-dress window is the peak-demand event for UAN tank systems. A mid-size grain co-op serving 40,000 corn acres can draw 800,000 gallons of UAN 28 across three weeks. Tank storage is sized to this peak plus a 30-40% buffer.

Y-Drop and Coulter Injection - Why UAN Won the In-Season Market

Twenty years ago, side-dress N meant anhydrous ammonia injected at 6-8 inches depth with a shank toolbar, or dry urea broadcast. Today the dominant side-dress tool is the Y-drop: a pair of hoses per row that trail behind a boom spray rig and deposit UAN at the base of the corn stalk, between the rows, without foliar burn. UAN works because:

• It's liquid, so metering is precise per row.
• The salt-out risk doesn't matter during May-June application weather.
• Half the N is in the nitrate form - immediately plant-available - while the urea half volatilizes into ammonium over 7-14 days, extending the N release curve.
• Foliar burn risk is low with proper drop-hose placement; contrast with anhydrous which can't be applied in-season without risk of plant damage.

Tank and pump specification at the retailer that fills Y-drop applicators matters: operators want fast fill, accurate metering, and zero salt-out residue contaminating the applicator pump. A well-spec'd UAN tank with EPDM-gasketed fittings, a bottom drain for flush access, and a vent big enough to handle rapid-fill air displacement (2-inch minimum on tanks up to 2,500 gal; 4-inch on larger) is the standard.

Panhandle and Great Plains Volume - Why UAN Is the Regional Chemistry

The Texas Panhandle, Oklahoma Panhandle, western Kansas, eastern Colorado, and the central Nebraska irrigation districts run on UAN. Three reasons:

• Corn-centric rotation: continuous corn and corn-soybean systems need 150-200 lb N per acre split across pre-plant and side-dress. UAN is the workhorse for both.
• Irrigation infrastructure: center-pivot coverage across the Ogallala aquifer region makes fertigation economical. UAN fertigation is the cheapest N delivery mode per pound when pivots are already running.
• Pipeline access: the Magellan, NuStar, and Enterprise NH3/NH4NO3 pipeline network plus barge access on the Arkansas River keeps UAN terminal prices competitive with dry urea after shipping.

A typical retail ag-supply co-op in the region maintains 80,000-250,000 gallons of UAN tank storage across multiple polyethylene vessels. The Panhandle-style tank farm layout is three to six 10,000-15,000 gallon verticals on concrete pads with secondary containment sized at 110% of largest tank volume, manifolded to a central fill/draw system.

Winter Storage and Heat-Trace Design

UAN 28 salt-out at 1°F means the tank stays liquid through a normal Panhandle winter in most years - but polar vortex intrusions happen, and the February 2021 Texas cold event dropped Amarillo to -13°F for four days. That event salted out UAN tanks across the region, and several operations suffered pump damage on thaw-out. Prevention design:

• Tank-blanket heater: 500-2000W electric panel mounted to the tank sidewall, thermostat-controlled at 10°F above salt-out. Draws only during cold snaps; minimal parasitic load.
• Heat-traced suction piping: bottom-drain line and pump suction get self-regulating heat tape at 5-10 W/ft; plumbing is the first thing to freeze and the last thing to check.
• Insulation jacket: fiberglass blanket or polyurethane spray-foam jacket over the tank sidewall; reduces heater duty cycle dramatically.

For UAN 32 service in the same region, all three are mandatory, not optional.

Secondary Containment and Environmental Compliance

UAN is regulated under state fertilizer licensing (each state has a fertilizer control act) plus federal SPCC if total on-site oil storage exceeds threshold (fertilizer itself doesn't trigger SPCC, but co-op diesel and used oil may). Typical compliance stack for a retail UAN tank farm:

• Secondary containment: concrete or geomembrane-lined berm at 110% of largest tank volume. Many states require 150%. Check your state fertilizer regulator.
• Spill plan: written site-specific Fertilizer Emergency Response Plan per state rules. Most co-ops file this at initial licensing and update every 3-5 years.
• Fill-point protection: bollards, hose racks, and a flat concrete pad under the fill connection to catch drips.
• Groundwater monitoring: some states require sentinel wells within 500 ft of the tank farm for nitrate monitoring; most do not.

The 2019 Nebraska State Fire Marshal updated rules for liquid fertilizer storage add requirements for tank labeling, emergency contact signage, and fire department pre-plans - worth reviewing if operating in Nebraska or across state lines.

System-of-Construction Table (Snyder Industries)

This is the exact specification Snyder Industries publishes for this chemistry. Every column is required — changing any of them voids the service rating.

Concentration-Band Compatibility (Enduraplas / Equistar Data)

Polyethylene chemical resistance by concentration and service temperature. Satisfactory (S) = long-term service. Limited (O) = occasional only. Unsatisfactory (U) = do not use.

Frequently Asked Questions

What's the real difference between UAN 28 and UAN 32?

Four-point nitrogen-density difference and a 31°F difference in salt-out temperature. UAN 28 is 28% total N with salt-out at 1°F; UAN 32 is 32% total N with salt-out at 32°F. UAN 28 is the winter-storage choice for the northern Plains and Midwest; UAN 32 is the higher-density choice for Texas and Oklahoma where outdoor winter temps rarely threaten salt-out. Per gallon you pay less freight on UAN 32 because you're moving more N per gallon; per acre applied, the applicator has to run at different meter settings to deliver the same N rate.

Can I store UAN 28 in a used water tank?

Physically yes if the tank is HDLPE or XLPE rated at 1.35 ASTM SG or higher. What needs changing: verify gaskets are EPDM (most water tanks ship with EPDM, so this is often already correct); verify bolts are 316SS; confirm fittings are PP or PVC not glass-filled nylon; install a vent sized for rapid fill/draw; add a bottom drain for salt-out cleanout access. Run a pH check and visible-particle inspection after first fill to confirm no residual contamination from water service.

Why not use Viton gaskets for UAN - Viton works for most corrosive chemistries?

Viton (FKM) swells slightly in urea service over 2-5 years of continuous exposure. The swelling causes rebound loss and eventual gasket failure - the exact failure mode Viton prevents in formic or HCl service. EPDM is the correct gasket spec for UAN, urea, urea-ammonium-nitrate blends, and most agricultural fertilizer solutions. This is published in Snyder's urea-solution MOC guidance and consistent with field experience across the Plains retail network.

Does UAN 28 freeze, or does it just salt out?

Both happen, but salt-out happens first. At 1°F the ammonium nitrate begins to crystallize from solution - you get a slushy layer at the bottom of the tank while the upper solution stays clear. If the temperature continues to drop into the negative teens, the water fraction also begins to freeze, locking the AN crystals into a solid cake. Thaw-out from the frozen state is much slower than from the salt-out state; plan heating capacity accordingly.

What's the fire-code classification for UAN?

UAN solutions below 32% N are classified as non-hazardous for transport under DOT rules (no UN number for UAN below a threshold). Storage is regulated as a liquid fertilizer under state rules, not as a hazardous material. This contrasts with solid ammonium nitrate, which is heavily regulated under DHS Chemical Facility Anti-Terrorism Standards and ATF explosive-precursor rules. Dissolved in water at UAN concentrations, the explosive-precursor concern does not apply.

Can I fertigate UAN through a center pivot?

Yes - UAN is the preferred fertigation fertilizer for center pivots. Typical setup: separate chemigation injection pump (positive-displacement or peristaltic), dual check valves, vacuum breaker on the main line between the pivot well and the injection point, and an interlock that stops injection if the pivot drive stops. Most states require a chemigation permit plus annual re-certification. Nebraska, Kansas, and Texas all have specific chemigation rules - check your state rules and local NRCS cost-share programs that sometimes fund the required backflow prevention.

Source Citations

• Snyder Industries — Chemical Resistance Recommendations (current edition)
• Enduraplas / Equistar Technical Tip — Chemical Resistance of Polyethylene (12-page reference)

Related Chemistries in the Agricultural Nitrogen Cluster

Related chemistries in the agricultural-nitrogen cluster (urea + ammonium + UAN + phosphate-nitrogen fertilizer):

• UAN-32 — Higher-concentration variant
• Ammonium Nitrate (AN) — Nitrate-N fertilizer
• Urea Solution — Urea-only liquid fertilizer

Related Hub Pillars

For broader chemistry context, see the OneSource Plastics high-traffic chemical-compatibility hub pillars:

• Sulfuric Acid (H₂SO₄)
• Sodium Hydroxide (NaOH / caustic soda)
• Ferric Chloride (FeCl₃)