Was Rachel Carson the first person to discover the environmental impact of fertilizers? If not, why was she the first one to talk about it publicly?

It would be useful to go back to the history of pesticides where we can find evidence about first pesticide Sulfur which was also used long time before Carsen even existed. Sulfur can be a cause of acid rains as seen in the precautions paragraph in the article about Sulfur. Having now knowledge about how Sulfur could cause harm to the environment, it could be useful to hop onto the article concerning acid rains. The first sentence in the history paragraph of the article connected to acid rains goes as follows: "The corrosive effect of polluted, acidic city air on limestone and marble was noted in the 17th century by John Evelyn, who remarked upon the poor condition of the Arundel marbles." This proves that people had knowledge about how pesticides could cause harm to the environment even before Carsen. However, I am unaware if Evelyn was the first to note such effect.

Concerning the second question, Carsen might have been the first person to publicly analyse the effects of DDT's on the environment but to be considered is that DDT is only one of the modern day fertilizers and there have existed several other pesticides and fertilizers in the past that have caused harm to the environment and thats effects have already been analysed and released to public.

Rachel Carson changed our world. Her book, Silent Spring (1962), raised public awareness of how, in mid-20th century, pesticides companies was destroying America (the people and the environment), and by extension, all of the natural world.

She wasn't the first to "discover" this, her long List of Principal Sources in Silent Spring (43 pages in my Penguin edition) clearly attributed the many reports of government departments, academic papers and individuals who helped her. But it was her explanation -- one of the best naturalist books I've ever read -- which truly showed her fear for our ecological system, the need for moderation in maximising profits once government policies are hijacked by large companies, and most of all, the evocative power of her words in describing the impact of modern science (man-made chemicals) on our environment:

There was once a town in the heart of America where all life seemed to live in harmony with its surroundings. The town lay in the midst of a checkerboard of prosperous farms, with fields of grain and hillsides of orchards where, in spring, white clouds of bloom drifted above the green fields. In autumn, oak and maple and birch set up a blaze of color that flamed and flickered across a backdrop of pines . Then foxes barked in the hills and deer silently cross ed the fields, half hidden in the mists of the fall mornings. ...

Along the roads , laurel, viburnum and alder, great ferns and wildflowers delighted the traveler’s eye through much of the year. Even in winter the roadsides were places of beauty, where countless birds came to feed on the berries and on the seed heads of the dried weeds rising above the snow. The countryside was, in fact, famous for the abundance and variety of its bird life, and when the flood of migrants was pouring through in spring and fall people traveled from great distances to observe them. ...

Then a strange blight crept over the area and everything began to change. Some evil spell had settled on the community: mysterious maladies swept the flocks of chickens; the cattle and sheep sickened and died. Everywhere was a shadow of death. ...

Source: Silent Spring (1962), the opening lines of first three paragraphs of Chapter 1, A Fable for Tomorrow.

(I know I've gone beyond the real question on Rachel Carson, but she fought very hard, against scientists who acted as lobbyists who were employed by pesticides companies, and against mainstream news organisations which publicly ridiculed her, to raise public awareness of scienctific discoveries misused. I think it's worth our time and effort (mine anyway) to at least understand and remember her historical role in environmentalism, naturalism and science in society.)