Fertilizers, both liquid and granular, provide essential nutrients to plant growth. Generally, the key nutrients are nitrogen (N), phosphorus (P) and potassium (K), often referred to by their elemental symbols N-P-K. Different fertilizers will provide a different quantity of each nutrient, which can be matched to the nutrients lacking in your soil and necessary for your plants. Granular and liquid fertilizers, however, impact soil and plants at different rates and thus meet the needs of different situations.
Types of Liquid Fertilizer: Compost Tea
Compost tea is the simplest liquid fertilizer to make at home, as it is made with the organic material in your compost pile, which is then brewed in water for several days or weeks in a closed bucket. The resulting liquid is a highly concentrated nutrient tea which can be diluted and sprayed on your plants. The amounts of nutrients vary depending on the plants that are in the compost and the amount of time it is brewed for.
Types of Liquid Fertilizer: Fish and Seaweed Fertilizer
Fish and seaweed fertilizer is another good natural source of nitrogen and phosphorus. It is very similar to compost tea except that it is made with inedible sea products, such as fish that are too oily or bony to eat, or kelp. Fish and seaweed fertilizer is an excellent source of nitrogen, which is difficult to add into your soil without nitrogen-fixing plants or synthetic fertilizers.
Liquid fertilizers are used to add an immediate source of nutrients to growing plants, as they can be absorbed much more efficiently by a plant's roots and leaves than solid granular fertilizers can be. This type of fertilizer is good for an immediate gain, but not for an extended release, meaning that it works best for seedlings and plants that are absorbing a large amount of nutrients in a relatively short time frame.
Granular fertilizers are made either organically out of plants and animals, or are synthetically manufactured out of chemicals. They can be manufactured organically out of a mixture of composts, seaweeds, manure and other natural deposits, or they can be made with synthetic compounds. The mixture is then dried out and formed into pellets which can be tilled into the soil before you sow your plants or mixed into the soil directly where you are planning to transplant.
Types of Granular Fertilizers: Organic
Organic granular fertilizers are more variable than synthetic fertilizers in the weights of specific nutrients that they contain, which means that you don't have quite as much control over your soil inputs with organic fertilizers as you do with synthetic ones. However, organic fertilizers are more stable and stay in the soil much more readily than synthetic fertilizers, meaning that you will not have to use as much fertilizer later, and you have less risk of nitrogen run-off contaminating your water supply.
Types of Granular Fertilizers: Synthetic
Synthetic granular fertilizers are chemically manufactured, often out of fossil fuel products. They are a more direct source of nitrogen than traditional crop rotation systems, which use symbiotic bacteria on the roots of leguminous vegetables to fix nitrogen. However, synthetic fertilizers are easily soluble in rain and irrigation water, which leach the unabsorbed nutrients out of the soil instead of letting them build up in the soil.
Granular fertilizers have a much slower release rate than liquid fertilizers, and so are better for more established plants that will take nutrients up gradually, and don't need a large supply of fresh nutrients to promote growth. They can be added to the soil at the beginning and/or end of the growing season, but rarely need to be applied more than once a year since they take several months to break down completely.