By definition, fertilizer is any material, organic or inorganic, that supplies plant life with additional nutrients and minerals. Commercially produced fertilizer must have a rating based on nitrogen, phosphorus and potassium proportions, and should indicate filler ingredients. Because your yard soil has specific nutritional needs, randomly choosing any fertilizer could mean under- or over-feeding your soil. The most effective method of finding the best-rated fertilizer is to conduct a soil test. Soil test reports will identify the proportions of nutrients your soil needs in order to support plant life.
Soil Test Results
A soil test is a process that collects a sample of soil from different locations in your garden or yard. Private soil testing labs and university extension departments offer soil testing for a fee. After testing the soil for mineral content such as phosphorus, nitrogen, potassium, calcium, magnesium and sulfur, the soil pH, absorption and soil permeability, the lab will generate a report. The report, usually posted online, should include crop-specific information, a legend with technical terms and index values, with fertilizer rates based on plant type. Gardening centers can advise you based on test results which fertilizer will produce optimal results.
Commercial fertilizer by law must be printed with proportions that roughly represent the nitrogen, phosphorus and potassium content. They should also have a breakdown of filler ingredients. Filler ingredients like feed, sand, compost, or manure aid in the ease of spreading and plant absorption.
Organic fertilizers contain ingredients rich in essential plant nutrients. As the ingredients decompose, nutrients are slowly released into the soil and a rich layer of nutrient-rich compost material is left in the soil. Slow release through decomposition ensures nutrients don't leach from the soil into groundwater. Ingredients such as bat guano, rich in nitrogen and phosphorus, blood meal, rich in chelated iron and essential amino acids and seed meals, are ingredients commonly used in organic fertilizer.
Inorganic fertilizer tends to be quick release, heavy in nitrogen, phosphorus and potassium and weak in filler ingredients such as calcium, sulfur and magnesium, which plants need for greater absorption, permeability and aeration. Unlike organic fertilizers that decompose over time, leaving a fertile layer of compost and microbes, inorganic fertilizers dissolve quickly and initially cause rapid plant growth before leveling off. During rainy periods, fertilizer can leach into groundwater.
According to Steve Solomon, author of "Gardening When It Counts," the highest grade of fertilizer is one you make yourself. He suggests a basic recipe: 4 parts seed meal, 1/2 part finely ground agricultural lime (pulverized limestone), 1/4 part gypsum (hydrated calcium sulfate), 1/2 part dolomitic lime (limestone partially replaced over time by dolomite), 1 part bone meal or high-phosphate bat guano and 1/2 to 1 part kelp (seaweed) meal. Mix uniformly in parts. If you can't find gypsum, double the lime.
Many of the ingredients can be found online, in smaller packaging at garden centers or at feed stores. For low feeding plants, Solomon recommends 1/4 inch layer of steer manure or finished compost and 4 quarts of organic fertilizer mix per 100 square foot area. Up the fertilizer to 6 quarts for medium feeding plants, and for heavy feeding plants, add an extra 1/4 inch of steer manure.