Seems to me (as stored for six years) it would not be ANFO.
Concerning HD (High Density) vs. Low Density, it is the latter that is used by explosive manufacturers and mining, and the former for agriculture. There is a pretty good discussion of it here from both the agriculture and explosive side:
Ammonium nitrate was the first solid nitrogen (N) fertilizer produced on a large scale, but its popularity has declined in recent years. It’s been a common N source because it contains both nitrate and ammonium, and it has a relatively high nutrient content.
Production
Large-scale production of ammonium nitrate began in the 1940s when it was used for munitions during wartime. After the end of World War II, ammonium nitrate became available as a commercial fertilizer. The production of ammonium nitrate is relatively simple: Ammonia gas is reacted with nitric acid to form a concentrated solution and considerable heat.
Prilled fertilizer forms when a drop of concentrated ammonium nitrate solution (95 percent to 99 percent) falls from a tower and solidifies. Low-density prills are more porous than high-density prills and are preferred for industrial use, while high-density prills are used as fertilizer. Manufacturers produce granular ammonium nitrate by repeatedly spraying the concentrated solution onto small granules in a rotating drum.
Since ammonium nitrate is hygroscopic and therefore readily attracts moisture from air, it’s commonly stored in air-conditioned warehouses or in sealed bags. Manufacturers typically coat the solid fertilizer with an anti-caking compound to prevent sticking and clumping.
Small quantities of carbonate minerals are sometimes added prior to solidifying, which eliminates ammonium nitrate’s explosive properties. These additives lower the N concentration and are sparingly soluble, making the modified product less suitable for application through an irrigation system (fertigation).
Agricultural use
Ammonium nitrate is a popular fertilizer since it provides half of the N in the nitrate form and half in the ammonium form. The nitrate form moves readily with soil water to the roots, where it’s immediately available for plant uptake. The ammonium fraction is taken up by roots or gradually converted to nitrate by soil microorganisms. Many vegetable growers prefer an immediately available nitrate source of plant nutrition and use ammonium nitrate. Animal farmers like it for pasture and hay fertilization since it’s less susceptible to volatilization losses than urea-based fertilizers when left on the soil surface.
Ammonium nitrate is commonly mixed with other fertilizers, but these mixtures can’t be stored for long periods because of a tendency to absorb moisture from the air. The very high solubility of ammonium nitrate makes it well suited for making solutions for fertigation or foliar sprays.
Management practices
Easy handling and high nutrient content make ammonium nitrate a popular N fertilizer It’s also very soluble in the soil, and the nitrate portion can move beyond the root zone under wet conditions. Nitrate can also be converted to nitrous oxide gas in very wet conditions through the process of denitrification. The ammonium portion isn’t subject to considerable loss until it’s oxidized to nitrate.
Concerns over illegal use of this fertilizer for explosives have caused strict government regulation in many parts of the world. Restrictions on sales and transportation have caused some fertilizer dealers to discontinue handling this material.
Non-agricultural uses
A low-density form of prilled ammonium nitrate is widely used as an explosive in the mining industry and on construction sites. Manufacturers intentionally make it porous to allow rapid adsorption of fuel oil (termed “ANFO”).
Concerning "regulation in many parts of the world," this is so, you cannot buy it without being either a manufacture (for making products) or a farmer/agriculture (peeps used to be able to for gardening). Purchases leave a paper trail. The military is another matter.
If the warehouse contained a N Prill HD form of AN, than this would account for its long storage as it is coated. It would account for its end use more in line with agriculture, although it could be later mixed in the form of ANFO.
The other thing is that today, as a conglomerate, the explosive industry is pretty much owned and run out of Australia. There are other manufactures who buy from them for a specialized end use as licensed producers. Very controlled. There really is not much in the way of movement of explosive products (and AN) that happens without large oversight (again, military excluded).
As for ANFO, it is used in mining for the simple reason that it is cheap and has massive breaking power forces in rock. It still (when put in a hole) requires a booster, an emulsion explosive or other that is put under and sometimes over top (depends) of the ANFO, with an initiator (shock-tube and cap or cap alone if electrical tied to the booster and not the ANFO). Other blasting uses don't include ANFO for the reason that it is harder to control the blast resulting in unintended fracturing of geology and flyrock.
Uncannily close, osit.
