There is an old saying that you are what you eat. This is as true of plants as it is of humans. Without the proper nutrients, plants will not grow and produce as expected. For plants, proper nutrition comes from the nutrients in the soil or from commercial fertilizer.
Water soluble fertilizers are designed to mix with water and applied directly to the plant with a hose or sprayer. This type of fertilizer is ideal for foliar feeding but may also be applied to roots. By applying the mixed fertilizer to the leaves, plants are able to utilize the nutrients right away. Nutrients are absorbed through the leaves and go directly to plant growth. Run off penetrates the soil and provides nutrients that are absorbed by the roots.
Granular fertilizer is applied to the soil and must be mixed into the existing soil to prevent burning the young roots of growing plants. This formula breakdowns over a period of time when exposed to rainfall or regular watering. It may be applied prior to planting and during the growing season. Contact with leaves will cause plant damage.
Each of these fertilizer types comes in a premixed formula that consists of three main elements: nitrogen, phosphorus and potassium, sometimes referred to as potash. The numbers indicated on the bag, such as 10-10-10, represent the percentage of each element in the formula. To determine which formula is right for you, you need a basic understanding of the effect each element has on plant growth.
Nitrogen works to promote lush green leaves and rapid growth. When applied at the right time, nitrogen will give your plants a boost in growth, but it is best applied early in the growing season, well before the onset of blooming and fruit. If too much nitrogen is applied close to blooming time, you may sacrifice blooms and end up with lush vegetation that fails to set fruit. A lack of nitrogen will result in yellowing or curling of older leaves and stunted growth.
Phosphorus promotes vigorous blooms and healthy plant cells. Apply a fertilizer high in phosphorus just prior to blooming to encourage profuse blooms that are brimming with color. Plants suffering from a phosphorus deficiency may show signs of purpling along the veins.
Potassium is needed to build a strong root system to support the plant's growth and to maintain health. This is especially important for large plants that require vigorous root systems for support. A lack of potassium will result in stunted growth and general poor health.