How to Choose Fertilizer Bags

Why fertilizers cake, the humidity at which urea, DAP and nitrates start absorbing water, and which bag construction each one needs.

A fertilizer bag has one enemy that a cement or grain bag does not: the product pulls water out of the air. Granules that leave the plant free-flowing arrive as a single hard block if the bag lets humid air reach them, and a farmer who has to break a bag open with a hammer does not buy that brand again. Choosing a fertilizer bag is therefore mostly a question of how much moisture protection the particular product needs, and then of making sure the seams and the handling do not undo it.

Why fertilizer cakes

Every fertilizer salt has a critical relative humidity. Below it, the granule stays dry. Above it, the granule takes up water from the air, its surface dissolves, and when conditions change the dissolved salt recrystallises as bridges between neighbouring granules. Pressure from the bags stacked above pushes the granules together and makes the bridges stronger. That is caking. The lower a product's critical humidity, the more of the year the air around it is wet enough to start the process, and the better the bag has to be.

A mixture absorbs water at a lower humidity than any of its ingredients would alone, sometimes far lower, so an NPK blend containing urea and a nitrate needs more protection than either straight product. Ask the fertilizer maker for the figure for the actual grade rather than assuming it from the main ingredient.

The four constructions

Our PP laminated fertilizer bag is the third type: woven polypropylene with 15-25 GSM of PP lamination, 80-100 GSM in total, with a moisture vapour transmission rate below 5 grams per square metre per day. It is made in 50 x 85 cm and 55 x 90 cm for 25 and 50 kg fills, or to a custom size, with an open or heat-sealed mouth and a sewn or block bottom. The same coated construction, in a food-grade form, is used for our PP laminated animal feed bag and PP laminated sugar bag, which face the same moisture problem with a different product. Converters who make their own sacks start from the PP laminated woven fabric roll, which carries the same barrier at 80-130 GSM.

The seam is where moisture gets in

A coating stops vapour passing through the fabric. It does nothing about the row of needle holes along a stitched bottom and a stitched mouth. In a humid store those holes are the path, and caking starts as a hard lump just inside each seam. There are three ways to close the path. Seal the mouth with heat instead of thread, which a coated face allows. Put a liner inside and seal or tie the liner, so the stitching only holds the outer sack. Or keep the stitch and fold the fabric over twice before sewing so the holes do not line up. Which one is possible depends on the filling line, so decide the closure together with the bag.

Sunlight, stacking and slip

Sunlight

Fertilizer is stored outdoors at ports, railheads and dealers. Order the sacks UV stabilized for the months and the region of exposure.

Stack height

Pressure drives caking and loads the bottom tier. Fabric strength and stack height should be set together.

Slip

Coated sacks are smooth and slide on each other. Anti-slip treatment keeps a pallet or truck load in place; our anti-slip fabric is rated at a bag-on-bag friction coefficient of 0.55 or more.

Chemical contact

Polypropylene resists fertilizer salts. Sulphur and some agrochemicals weaken UV stabilizers, so declare them.

Hooks

A hook hole in a coated sack is a moisture leak. Where bags are hooked, a liner is the safer barrier.

Stacks kept outside are commonly sheeted as well, with covers cut from UV stabilized PP woven fabric. On the first of those, our guide to UV stabilization explains how to state outdoor exposure as a figure that can be tested, and on slip, the anti-slip PP woven fabric page gives the friction rating and the widths it is made in.

Bag weight and markings are often fixed by law

Fertilizer is a regulated product in most countries, and the rules reach the bag. The fill weight may be fixed: India's subsidised urea, for example, is sold in 45 kg bags rather than 50. The markings are usually prescribed too: grade, nutrient content, net weight, maker and batch, in set positions and sizes. Some countries also name the sack standard the bag must meet. In India that standard is covered by a compulsory certification order, which our guide to the BIS quality control order explains. Print space and bag dimensions should be designed around the required markings first and the branding second; our fertilizer bag takes up to six colours.

A short selection path

Take the product's critical humidity and compare it with the humidity of the worst month in the worst store on the route. If the store is always drier, an uncoated sack may serve. If it is sometimes wetter, use a coated sack and deal with the seams. If the product is a nitrate or a blend, if the goods cross the sea, or if they will be held through a monsoon, use a coated sack with a liner. Then add UV stabilization for outdoor time, anti-slip for palletised or trucked loads, and check the fill weight and markings against the destination's rules before artwork is approved.