Herbicides; positive - they make it easy (sometimes) to kill unwanted plants. negative, they can be poisonous in a way that leaches into ground water, and can be a less than healthy substance to introduce into a healthy ecosystem. They also tend to be overused.
Pesticides; positive - they can help people deal with pest problems in their gardens and landscape. negatives - they often kill more than the bugs that are targeted, killing "good" bugs as well, and are also likely to be unhealthy for people to be in contact with.
Fertilizers; positive - they help replace nutrients that get taken from the soil by continuous farming. negative - some nutrients again, drift from the soil, and can affect ground water, and surrounding bodies of water. There lakes in Ohio that have developed toxic algae from fertilizer run-off, and are so poisonous now, that small pets have been known to die after falling in. Read up on Grand Lake St. Mary's in northwest Ohio and you can see just how bad it can get.
