Salination of soil refers to the accumulation of salts in the soil, which can have negative effects on plant growth and agricultural productivity. Option 1, pesticides, is incorrect because it does not directly cause salination of soil. Pesticides are chemicals used to control pests and do not contribute to the accumulation of salts in the soil. Option 2, soil erosion, is also incorrect because it refers to the loss of topsoil due to natural factors like wind and water, and is not directly related to the accumulation of salts. Option 4, crop rotation, is also incorrect because it is a farming technique used to enhance soil fertility, and does not cause salination. The correct answer is option 3, excess irrigation. When soil is excessively irrigated, water evaporates, leaving behind salts that accumulate in the soil over time, leading to salination.