After the identification of three of the main macronutrients that plants need to grow – nitrogen, phosphorus, and potassium (N-P-K) – and the development of manufactured nitrogen fertilizer in the 1910s, agricultural soils began receiving large doses of N-P-K but little else. Early agronomists overlooked the importance of biological activity provided by the underground ecosystem of soil microbes, earthworms, and Mycorrhizal fungi. Chemical fertilizers such as N-P-K depress this activity, increasing plants' vulnerability to pests and diseases. They might also diminish crops' nutritional quality, which has declined substantially since the widespread adoption of chemical fertilizers. Some researchers attribute this decline to the condition of the soil; others cite the tendency of modern plant breeding to select for characteristics such as yield rather than nutritional quality.