Fertilizer adds nutrients and texture to soil that needs to support trees, vegetables, herbs, shrubs, and flowers. The different types of fertilizer depend on whether your soil is acidic, alkaline, sandy, clay, rocky, weak, or rich. Fertilizer can be divided into organic and chemical types, those that help fruit versus leaves, and those that feed specific plants or improve the overall quality of soil.

Two different types of fertilizer are inorganic and organic. Examples of inorganic fertilizer are chemical additives that are designed for plants to directly absorb, such as nitrogen, phosphorus, and potassium. These three essential elemental nutrients should naturally occur in healthy soil, but some plants require more of them. Organic fertilizer, like bat guano, compost, peat moss, wood ash, and manure, are general soil amendments. They don't burn or harm plants and have long-term positive effects on the soil without damaging ground water, but have lower concentrations of nutrients.

Most inorganic, concentrated fertilizer is rated based on the percentage of nitrogen, phosphorous, and potassium, with a rubric called NPK. The amount of nitrogen will encourage growth of stems and leaves by promoting protein and chlorophyll. More flowers, larger fruits, and healthier roots and tubers will result from added phosphorus. It also helps plants resist certain diseases. Finally, potassium from potash, thickens stems and leaves by fostering protein development. This means that vegetables will prefer a different NPK ratio than would roses or an orange tree.

Different plants will require different pH levels. If you need to nudge your soil's pH toward the acidic side, you could use inorganic fertilizer such as aluminum sulfate or ammonium sulfate. Lime changes soil chemistry to make it more alkaline. Sometimes blood meal or other organic matter can also affect acid levels in the soil.

Organic fertilizers have more diversity, so it might be harder to decide which ones are best. In general, though, these kinds of fertilizers cannot burn plants, get into ground water, affect surrounding growth, and don't need to be applied on strict watering schedules as do NPK amendments. One additive, like freshly cut grass, might have multiple benefits. Vegetation material called mulch, such as hay, peat moss, leaves, grass, bark, wood chips, seed hulls, and corn husks all help to aerate the soil, insulate the ground against temperature change, and add needed nutrients.

Commonly known fertilizers, like cow manure and compost, are perfect choices for a garden that has many different plants in the same bed. Lesser known options, like bat guano, seaweed, poultry manure, and bone meal, easily mix into existing soil to improve it over a period of years. They are all rich in vitamins and minerals even if they are slightly more expensive. Gradually releasing their nutrients allows plants' roots to pick and choose which ones benefit them best over their entire growth cycle.