TOEFL iBT Writing. In this integrated writing task, you will write a response to

Now listen to part of a lecture on this topic in a botany class.
Fertilizers improve crop productivity, but whenever they’re used excessively, they wash into lakes, streams, and oceans. This-is why we need to balance the benefits of fertilizers against their harmful long-term effects.
Nitrogen, a major ingredient of fertilizers, quickly washes out from the soil and into our water supply, where it’s converted to nitrates. The increasing nitrate content of our drinking water is a problem because nitrates can be poisonous to humans. Once taken into our bodies, nitrates are converted in a process that interferes with our blood’s capacity to carry oxygen, causing serious illness—and sometimes death.
Phosphorus can be equally harmful. While phosphorus promotes the growth of plants in soil, it also promotes their growth in water. If enough phosphates enter lakes and streams, they stimulate the growth of blue-green algae. Consequently, clear water becomes clouded with massive blooms of algae. When the algae die, the oxygen in the water is consumed, and this speeds up the natural aging of the lake. In just a few years, phosphates can turn a lake into a dead zone.
Another plant nutrient, potassium, is important for a plant’s production of carbohydrates. It makes vegetables look pretty and taste good, but excess potassium from fertilizers can damage the plant’s metabolism, as well as the metabolism of animals that eat the plant.
One final point I’d like to make is that different soils need nutrients in different amounts. A common mistake is using a 20-20-20 fertilizer without first doing a soil analysis to find out what the soil needs. Without doing a soil test, using a balanced fertilizer can be a waste of time and effort because it’s not likely to produce ideal conditions in every garden.
Summarize the points in the lecture, explaining how they cast doubt on points made in the reading.