Nitrogen is the one plant nutrient that must be added for optimum plant growth anywhere in the world. It is needed in large quantities and is in chronic short supply. All plants and microorganisms compete for the available nitrogen because it is a constituent of protein, the basic substance of all living matter. In its most common mineral form, nitrate, nitrogen is highly soluble and mobile, so it is easily washed from the soil by rain or irrigation. In nature, nitrogen is recycled endlessly through plant and animal tissues. Free nitrate is absorbed and built into living matter, which dies, decomposes, and becomes nitrate again.
Nitrogen is also the only plant nutrient that can be used to regulate the form that plant growth will take, rather than just keeping a plant healthy. Nitrogen stimulates shoot growth in plants. If a high level of nitrogen is present in the soil, the internal resources of plants are directed to make vigorous shoot growth. This means that other types of growth—such as flower, fruit, and root growth—slow down. If you are raising lettuce, this is highly desirable; lettuce that grows rapidly is succulent and sweet. But if you are raising tomatoes, the plant might go on making vigorous leafy growth well into the summer, only beginning to make blossoms and fruit as cool weather approaches.
Fertilizers without nitrogen or with only a little nitrogen are sold to promote flowering and fruiting. However, it isn’t the presence of phosphorus and potassium that promotes flowering, it is the absence of nitrogen.
Nitrate is leached from soil or absorbed by plants within weeks of application. It needs to be applied every month or so during the growing season, or applied in a long-lasting form, such as slow-release or organic.