Through the weathering of rocks to soil, essential minerals such as phosphorus, calcium and magnesium are released by lichens, algae, fungi and bacteria. Nutrients are also added to the soil by other inputs from the abiotic environment, as well as by the decomposition of fallen leaves (“litter fall”) and plant and animal bodies. These nutrients can then be taken up from the soil by plant roots. The amount of nutrient uptake by plants is dependent upon the quantity of root hairs in the soil. Since tropical soils are typically shallow, many of the root hairs are close to the surface, and are particularly numerous in nutrient-poor soils. Microbes are vital to plant-soil interactions since they affect the availability of nutrients from the soil. (See G10, Role of fungi; also G5, Nutrient cycling).