To address the question regarding the consequences of applying large amounts of chemical fertilizers over several years, it's important to understand the impact of such agricultural practices on soil. The possible consequences listed in the options are:

1. Accumulation of toxic substances in soil
2. Erosion of surface soil layers
3. Faster plant growth every season
4. Increase in earthworm population

The correct answer is "Accumulation of toxic substances in soil." Here's a breakdown of each option:

• Accumulation of toxic substances in soil: Chemical fertilizers often contain elements like nitrogen, phosphorus, and potassium, along with trace amounts of potentially harmful substances like heavy metals. Continuous application can lead to the buildup of these toxic substances in the soil, leading to soil pollution. Over time, this can degrade soil health and affect plant growth.
• Erosion of surface soil layers: While soil erosion is a common problem in agriculture, it is more directly related to aspects like water runoff and wind, rather than the direct application of fertilizers. The erosion of soil layers may occur due to poor agricultural practices, but it's not a direct result of applying fertilizers.
• Faster plant growth every season: Initially, the use of chemical fertilizers may promote increased growth due to the high availability of nutrients. However, over time, the negative impacts of soil degradation and pollution usually outweigh this benefit, leading to poorer soil health and reduced yields.
• Increase in earthworm population: Earthworms prefer non-toxic, organic-rich soils. The accumulation of chemical toxins from fertilizers typically results in reduced organic matter and a decline in earthworm populations due to hostile soil conditions.

With these analyses, we conclude that the primary consequence of long-term application of chemical fertilizers leading to soil pollution is indeed the accumulation of toxic substances in soil.