Vegging nutrients are specifically used during vegetative growth. During this phase, the plant will require higher levels of nitrogen. Nitrogen is essential for the growth of stems and leaves, and so is an absolute requirement for the vegetative phase. To choose the correct amount of vegging nutrients, consult the manufacturer's directions and feed as directed, then watch how the plant responds. Know the signs of over or underfeeding, and adjust the amounts appropriately. Often a pH imbalance can resemble a nutritional deficiency, so always check the pH of your plants before adding or adjusting nutrients.