Because their nitrogen is water soluble, all liquid fertilizers are quick-acting. Their nitrogen is available to plants almost immediately, and therefore tends to be in strong doses. Because it is taken up by plants promptly its effect lasts only a couple of weeks. To sustain consistent levels of available nitrogen, liquid fertilizers must be used several times over the growing season. For this reason liquid products are less convenient and efficient for use as main meals for plants over your whole yard. They are, however, very useful for specific situations where the rapid delivery of nitrogen and other nutrients to plants is urgent.

While you can pour liquid fertilizers directly onto the soil as a drench, they are best used as foliar, or foliage, sprays. Leaf tissues can absorb the nutrients directly and quickly and the micro-life in the soil avoids irritation from the acidic effect of the quickly released nitrogen. Use liquid fertilizers during the growing season to supplement the granular main meal to boost the energy of plants that produce fruit and flowers steadily over the whole season such as tomatoes and annual flowers such as petunias and impatiens. Use very dilute solutions of liquid fertilizer when you water houseplants to maintain their vigor.