Liquid fertilizers are any "fertilizers wholly or partially in solution that can be handled as a liquid," according to the Fertilizer Manual put forth by the United Nations Industrial Development Organization, International Fertilizer Development Center. In some cases, this refers to fertilizers sold in dry form, but which are meant to be suspended in water before being applied. Their chief advantage over dry fertilizer is the ability to measure and apply them accurately.
Kelp, fish and other aquatic life emulsions are common liquid organic fertilizers, valued for their high nutrient and microbiotic activity. Their macronutrient count tends to fluctuate, but is generally very low. Organic sprays based on animal manure or urine are higher in macronutrients and are more often used in the agricultural industry for crops. Organic liquid fertilizers are considered slow-release, nonburning fertilizers.
While compost is widely regarded as one of the most useful and nutrient-diverse amendments you can provide for your plants, its liquid counterpart, compost tea, is just as useful. Compost tea has been used as a foliar feed (for the plants that benefit from it), a supplemental fertilizer, and part of integrated pest management programs to reduce or suppress plant diseases.
Some of the first chemical liquid fertilizers were nitrogen-based, and to this day consist of ammonia, ammonium nitrate and urea. Phosphorus liquid fertilizers are primarily phosphoric acid, while potassium fertilizers are a solution of potassium chloride. Single-macronutrient fertilizers are made to address a single deficiency in the nutrients that soil provides. Because they are so specialized, they may need to be specially formulated to mix with other macronutrient fertilizer sprays.
It is far more common to find chemical liquid fertilizers with more than one macronutrient in their guaranteed analysis. Like their dry fertilizer counterparts, these liquid fertilizers are specially formulated for a specific plant, a specific application time, or both. Applying these types of fertilizers without knowing the nutrient content of the soil can lead to a waste of fertilizer, nutrient toxicity in the soil and pollution.
Quick-release fertilizers can burn the foliage of a plant, even if the fertilizer is in liquid form. Always apply per package directions, and err on the side of too little rather than too much. Insufficient fertilizer won't hurt your plants, while excess fertilizer will. You can reapply fertilizer later if a soil test shows low nutrient counts in the soil.