One thing that I was wondering: is it even necessary to form such a contraint as its really referring to an unlimited item. Hence there actually is no constraint to be formed?

As for the optimization formula (maximise profit), I got it like this: F = 210x1 + 250x2 + 160x3. That should represent the profit made per ton of fertilizer sold. This could be totally wrong, but I thought cost of materials per ton of fertilizer can be found by (using Type I fertilizer as an example) 0.1*1500$ + 0.1 * 600$ + 0.2 * 1200 + 0.6 * 100$ = $510. By substracting that and production costs ($110) from the selling price of $830, you'd make a profit of $210 per ton of type I fertilizer sold. Do the same for type II and type III fertilizers and you get $250 and $160 respectively. Is this correct?