Macronutrients. An element such as potassium, nitrogen or calcium, essential in large quantities for plant growth is termed macronutrient. They are generally present in plant tissues in concentration of at least 10 mol Kg^-1 of dry matter. Examples are Carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, calcium, magnesium, sulphur, silicon and iron. Micronutrients or trace elements, are needed in very small amounts (less than 10 mmol Kg^-1 of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. Beneficial nutrients. They are required by higher plants in large quantities. Examples are sodium, silicon, cobalt and selenium. Toxic element. Any mineral ion that reduces the dry weight of tissues by about 10% is called a toxic element. Example – excess of manganese causes the development of brown spots in plants. Essential elements. Such elements without which the physiological functions of plants are affected are called essential elements. These include calcium, hydrogen, oxygen etc.