A common mistake among novice gardeners and would-be farmers is the assumption that all garden fertilizer products are created equal. For a number of reasons this is incorrect, and using the wrong or inappropriate product for a particular soil can thwart even the most committed green thumb. There are, in fact, many different kinds of garden fertilizer, each designed for different kinds of soil. Choosing the correct kind can mean the difference between stunted greens and a luscious, rewarding crop.
Perhaps the most important step in the growing process generally occurs before the first spadeful of soil is ever turned. Testing the pH-balance, nitrogen content, and other characteristics of a garden's soil are crucial to determining which garden fertilizer, and even which varieties of plants to use. Either naturally or due to man-made influence, soil may be particularly acidic or alkaline, and the right fertilizer can help neutralize any imbalance, and make it more hospitable to plants.
There are numerous procedures for testing soil, but a basic strategy is to dig up several small trowels full of earth, from various locations throughout the garden, and use a quality soil-testing kit to determine its overall state. Beware the strip-type soil testers, as they are notoriously unreliable. Worthwhile testing materials can generally be purchased at garden centers and nurseries, but, depending on the area, local universities and government agricultural extensions may offer free kits. Different kits test different soil characteristics, so local expert advice should be sought when testing for the first time, to know what to look for.
Once the acidity, elemental content, saturation capacity, and other factors are known, the proper garden fertilizer can be purchased. The appropriate choice should be rich in any areas the garden soil has shown to be deficient. Nitrogen, phosphorous, and potassium are the most critical substances for plant growth, but other elements — including sulfur and magnesium — also play a large role in a successful garden.
The proportion of the big three — nitrogen, phosphorous, and potassium — contained in a particular bag of garden fertilizer is typically indicated by a series of numbers, for instance '20-10-10.' This formula means that the bag contains 20% nitrogen, 10% phosphorous and 10% potassium. The remaining 60% of the fertilizer is not actively beneficial to plants, and is referred to as ballast.
While virtually all garden fertilizer products feature large amounts of the big three, secondary elements like sulfur — along with others, such as iron and zinc — are also included. Known as micronutrients due to their trace, but still important presence, these may or may not be present. Consulting the results of the soil test usually informs which of these elements are needed and, in turn, which garden fertilizer to select.