Many gardeners want to know what they need to apply to “feed” their plantings: fertilizer or compost? In fact, you can use either one … but there is a difference.
Fertilizer is composed of concentrated minerals. Three macronutrients are almost always present: nitrogen, phosphorus and potassium. They form the famous N-P-K, the three figures that appear in large print on fertilizer bags and containers. But your fertilizer probably also includes calcium and perhaps sulfur and magnesium (the percentages will be indicated elsewhere on the label): these are the other three macronutrients, the ones plants need in fairly large quantities. Many fertilizers, especially organic fertilizers, also contain other minerals: micronutrients or trace elements such as boron, copper, cobalt, iron, manganese, molybdenum and zinc. These are nutrients plants need for their growth, but only in very small quantities.
The minerals present in fertilizers dissolve quite rapidly and can therefore be absorbed quickly by plant roots. On the other hand, fertilizers contribute nothing to the texture and overall quality of the soil.
Compost is a soil amendment resulting from the decomposition of organic matter: lawn clippings, leftover food, dead leaves, garden refuse, etc. It contains minerals that cover the entire range of macro- and micronutrients, including nitrogen, phosphorus, potassium, but only at very low percentages. Instead, what it mostly contains is organic matter. Added to soil, it makes it lighter and less compact, improving its tilth and texture. And it feeds and maintains the soil’s microorganisms, a vital part of any soil that many gardeners fail to take into consideration.
When added to soil, compost usually contains enough minerals to satisfy the needs of slow-growing plants, but if the original soil is poor, it may be necessary to add fertilizer to get optimum growth from fast-growing plants, such as vegetables.
Is Commercial Compost Really Compost?
Certain composts really are 100% compost: homemade compost, for example, or vermicompost (compost made from worm castings). However, most commercial composts, such as cow manure compost, shrimp compost and sheep manure compost, are not pure composts, but a mixture of peat moss and compost. Mushroom compost can be true compost… or it too may be mixed with peat moss.
Commercial composts do lighten the soil, but contain very few minerals. Often the label doesn’t tell you the percentage of ingredients, so it’s very hard, if not impossible, to tell if whether such compost blends are mostly compost (ideal) or mostly peat moss (not so good). Some contain very little true compost, only enough so they can legitimately call it compost. Normally, it is always necessary to use a fertilizer in conjunction with these highly diluted composts.
In Just a Few Words
I’ve sometimes heard the phrase: fertilizer nourishes plants, compost nourishes the soil. That may be a rather simplistic explanation, but it’s pretty close to reality.