Houseplants, because they are kept in relatively small containers, have less access to nutrients in the soil than most garden plants. This only makes fertilizer more crucial in their development. But just as there are many types of houseplants with different characteristics, there are many different kinds of fertilizers to provide for their different needs.
Nitrogenous Fertilizer types
Houseplants with lots of foliage and lush greenery rely on fertilizers with high nitrogen concentrations to keep them green and growing. Sodium nitrate is a solid or powdery compound that normally doesn't need to be blended or diluted with other materials before being introduced to houseplant soil, but it can cause that soil to clump. Ammonium sulphate is a white, saltlike, water-soluble fertilizer that is retained by soil, meaning that one dose can stay within a houseplant's soil for months or even years despite repeated waterings.
Phosphate Fertilizer Types
Phosphorous is used principally by plants with minimal foliage and minor root systems that support large flowers, such as orchids and show roses. This chemical aids the root system in gripping and digging through, and provides some of the key components for flower pigmentation. Rock phosphate is a cheap and naturally occurring fertilizer, but it must be ground finely before being hand-sifted through the houseplant's soil. Bone meal is a good phosphorous fertilizer that provides the soil with a little nitrogen as well. It can come as solid compressed bricks or powder, and is very slow acting, meaning it should be incorporated into a plant's soil for long-term plant food, while more fast-acting phosphorous fertilizers are used in the meantime.
Potassium Fertilizer Types
Potassium is necessary for strong stalks and stems; it also helps the plant fight against bacterial and fungal infections by promoting the growth of a thick outer coat. There are only two kinds of potassium fertilizer: potassium chloride and potassium sulphate. Potassium chloride is largely made from potash (plant ashes). Because it is highly soluble in water, it can often be washed out of houseplant soil accidentally. That's why potassium sulphate might be used instead, which is essentially potash treated with magnesium sulphate to help keep it from being leached out of the soil.