What kills comfrey?

Found this formula from House Of Strauss in the UK, web site has been gone for quite some time now.

Spread an ounce ( 7 Grams ) of ammonium sulfate on the cut surface of the roots in the ground. In about 4 weeks, the chemical becomes sulphate of ammonia. Can be used to kill swarms of small comfrey plants.

Stir 1 pound ( 455 Grams ) of the white crystals into a gallon ( 4.5 Liters ) of water and apply with a rosed watering can over a 10 foot ( 3 Meter ) area. In 8 weeks, any crop can be sown or planted after it.