Sixteen nutrients are essential for plant growth and reproduction. They are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, boron, manganese, copper, zinc, molybdenum, and chlorine. Nearly all plant nutrients are taken up in ionic forms from the soil solution as cations or as anions. Plants release bicarbonate and hydroxyl (OH–) anions or hydrogen cations from their roots in an effort to cause nutrient ions to be freed from sequestration on colloids and so forced into the soil solution where they can be picked up. Nitrogen is available in soil organic material but is unusable by plants until it is made available by that material’s decomposition by micro-organisms into cation or anion forms.