Because, the passage itself describes about the nature of greenhouse effects. The greenhouse gases are absorbed by the soil. In order to protect the sensitive plants, we are going now for the modern greenhouse method i.e. artificial method. Transparent material which is nothing but like a sheet which covers the house is contructed to oppose the greenhouse gases to protect the plants. As the heat passes, some amount of heat won't escapes to atmosphere. This results in excess heat inside and leads the plants to die. Scientists say it's better to be the natural method of greenhouse effects.