Net Weight, Moisture and Fines: Why a 25 kg Bag Plants Less Than You Paid For

The delivery note said 40 bags at 25 kg, one tonne. The buyer’s own check on the loading dock put the average at 24.1 kg, and two bags at 23.4 kg. That is 36 kg short across the pallet, about 3.6 percent, which on a 14,000 dollar order is 500 dollars. Nobody had weighed a bag on arrival before, so nobody knew whether this was new or had been happening for three years.

Net weight is the least glamorous specification in the polymer trade and the one most likely to be quietly short. This article covers where the missing mass goes, what moisture and fines actually cost in field terms, and a receiving checklist you can run with three inexpensive instruments.

Where the Missing 900 g Goes

Three things account for almost all short weight. The first is filling tolerance: a volumetric filler set to 25 kg will produce bags between 24.2 and 25.4 kg, and a supplier who sets the target at the low end of the tolerance saves 1 to 2 percent across a year. The second is moisture, which is mass that you paid for but which does nothing for absorption. The third is packaging, where the bag and liner weight, typically 180 to 240 g, may or may not be included in the declared net figure.

None of these is necessarily fraud. A filler running at 2 percent under target is within most commercial tolerances, and a moisture content of 9 percent is within many product specs. But the cumulative effect on a large order is real, and the only defence is to weigh and test on arrival, in a documented way, so that a trend is visible. Buyers who weigh every pallet for a year typically find a 1.5 to 4 percent average shortfall, and recover most of it in the next negotiation.

Moisture: 8 Percent Is Normal, 14 Percent Is a Discount You Paid For

Dry polyacrylate is hygroscopic. At 8 percent moisture it flows well and absorbs at its rated capacity. At 14 percent the granules start to feel cool and slightly tacky, they bridge in hoppers, and the effective absorption per kilogram drops by 6 to 8 percent because the polymer is already partly swollen. At 18 percent, which happens after a humid sea freight with a failed liner, the material clumps and may develop mould in the bag.

The field consequence is direct. If you dose 6 kg per hectare from a bag at 14 percent moisture, you are applying the equivalent of 5.5 kg of dry polymer, and the difference shows up as thinner establishment on a sandy site. You can measure moisture with a cheap grain moisture meter calibrated against an oven test, or with a 100 g sample dried at 105 degrees for four hours. A kitchen scale and a domestic oven will get you within 1 percent, which is enough to have the conversation.

Fines Below 0.3 mm

A well-screened agricultural granule contains under 5 percent fines. Material that has been handled roughly in a pneumatic transfer, or blended from a wide screen cut, can contain 15 to 25 percent. Fines cost you twice: they blow away during broadcasting, and they block the metering system on any mechanical applicator. On a windy site, 20 percent fines can mean 10 percent of the delivered mass never reaches the ground.

The test takes five minutes with a 0.3 mm sieve and a 100 g sample. Shake for two minutes, weigh the passing fraction, and record it. Anything over 8 percent justifies a claim, and anything over 15 percent justifies rejection. If you are buying a 0.5 to 1 mm cut for nursery use, also check the top end: more than 5 percent above 2 mm is a sign the supplier is blending rather than screening.

A Receiving Checklist With Three Instruments

You need a platform scale accurate to 10 g, a 0.3 mm sieve, and a moisture meter or an oven. On arrival, weigh five bags per pallet and record the mean and the range. Take a 100 g composite sample, sieve it, and record the fines. Take a second 100 g sample, dry it, and record the moisture. Then write one line on the delivery note: pallet 3, mean 24.1 kg, fines 11 percent, moisture 13.2 percent, accepted under protest.

That sentence is worth more than any clause in the contract, because it creates a record at the moment of delivery and it puts the supplier on notice. Suppliers who are confident in their process will welcome it; the ones who do not will offer a discount, which tells you what the shortfall was worth. Once you have a baseline, it also lets you compare grades honestly, since a 25 kg bag at 6 percent moisture genuinely contains more polymer than the same bag at 14 percent. For reference figures on moisture and screening, see the data published with the SAP powder product range.