That depends on the fertilizer. Plants need four elements that they have trouble getting out of the air or water: nitrogen, magnesium, phosphorous, and sulfur. Some plants work together with bacteria who can get the nitrogen from the air, but even these plants need the magnesium, phosphorous, and sulfur. Of these, phosphorous is usually what limits plant growth - plants need lots of it, and it is less common than the other two.

So, if you're growing plants that don't make friends with bacteria, then you need to put nitrogen in your fertilizer, but otherwise, you mainly need the phosphorous.

It so happens that volcanoes put out lots of these elements (except for nitrogen). This is why volcanic soil is so good at growing things, and why people tend to crowd the slopes of dangerous volcanoes to find good farmland. For these reasons, volcanic ash is a common thing to put into fertilizer, since nature has already provided all of the ingredients in one place.

Fertilizers are any material that enhances the growth of plants, so there are a variety of them and each has different ingredients. However, most fertilizers contain a few of the same ingredients, or macro-nutrients. These are nitrogen, which promotes leaf growth, phosphorus, which helps roots grow, and potassium, which helps make plants disease resistant and grow more fruit or flowers.

Additionally they may contain calcium, magnesium, and sulfur and micro nutrients, including copper, iron, manganese, zinc, and other rare minerals.

If you are curious about a specific fertilizer, you can always check on the bag for what the ingredients are or do a quick google search for the brand. Hope that helps!

There are different types of fertilizer. Some only have one ingredient (a compound made of several chemical elements), and others have more (several compounds). A typical single-ingredient fertilizer has a compound made of nitrogen, oxygen, and hydrogen. The elements in a typical multi-ingredient fertilizer include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.

Some fertilizers also have copper, molybdenum, zinc, and other metals in very small amounts. All of the elements are present in the fertilizer as part of a larger molecule, not as elements with no electric charge. Different fertilizers are used on different plants because different plants need different nutrients, but generally, nitrogen, phosphorus, and potassium support growth of the plant by providing building blocks and facilitating movements of water in the plant.