He is a USDA employee. They try to support this bogus argument by comparing particulate aluminium phosphate with aluminum bound in the crystal lattice of a mineral. As a geologist, I can assure you there is little comparison between particulate aluminum phosphate and rocks, which are aggregates of minerals. The alum promoters also claim adding aluminum in the form of aluminum phosphate to agricultural soils is not a problem because of how much aluminum is in the Earth's crust. This is another bogus argument because again there is no comparison.
Research demonstrates that aluminum is toxic to fin fish.
Other aquatic life such as oysters is very susceptible to aluminum toxicity. The potential exists for aluminum phosphate to dissolve in place and leach into ground water. The alum problem could be postponed for many years while tons of particulate aluminum phosphate accumulate in agricultural soils. Alternatively, particulate aluminum phosphate could erode into waterways for years, mix with other bottom sediments, and eventually be churned up, resuspended, and dissolved during dredging or a severe storm. The above disasterous possibilities are only a few of the environmental problems.