This document discusses soil fertility and fertilizers. It defines a fertilizer as a material added to soil that supplies essential nutrients for plant growth. Fertilizers are important for building up nutrient levels in soil and replenishing nutrients lost through processes like leaching. The document also explains how soil pH influences the availability of different nutrients and lists the optimal pH range for many common plants. It notes the pH levels of Illinois soils and provides a table showing the amount of limestone needed to adjust soil pH depending on its type and desired pH level.