The composition of fresh plant matter includes about 80%–95% water. The exact percentage of water depends on the plant species and the turgidity of the plant at the time of sampling, which is a result of the time of day, amount of available moisture in the soil, temperature, wind velocity, and other factors. Because of the variability in fresh weight, chemical analyses of plant matter are usually based on the more stable dry matter. Fresh plant material is dried at 70°C for 24-48 h. The dry matter remaining constitutes roughly 10%–20% of the initial fresh weight. Over 90% of the dry weight of most plant matter consists of three elements: carbon (C), oxygen (O), and hydrogen (H). Water supplies hydrogen and oxygen. Oxygen also comes from carbon dioxide from the atmosphere, as does carbon.