Before you fertilize your garden, you should determine what nutrients your garden needs. This depends on what plants you grow and the characteristics of your soil. Soils vary by type (sandy, silty, clay, loamy), percent of organic matter, pH, salt levels, and amounts of major and minor nutrients in the soil. The plants you choose to grow also have specific needs. Fertilizers should not be considered a fix for soil problems but as a supplement to an already healthy soil.
The first step is to know your soil: the soil type, pH, and nutrients present. This information can be obtained by having a sample of your soil tested in a soil testing lab. Soil tests may be requested through Utah State University and soil testing kits are available at your local Cooperative Extension office. The test results will give you an accurate indication of your soil type and pH, along with fertilizer recommendations.
Plants require many nutrients for good health, all of which are present in the soil to varying degrees. The three major plant nutrients are nitrogen, phosphorus, and potassium. Nitrogen is water-soluble and leaches through the soil quickly, so it needs to be applied more frequently than other nutrients. Phosphorus and potassium move through the soil much more slowly. Many existing landscapes may already have adequate amounts of phosphorus and potassium due to years of fertilizer application. Minor nutrients include iron, magnesium, manganese, calcium, zinc, sulfur, and boron. These nutrients are usually present in sufficient amounts but some may be unavailable for plant use due to high pH levels.
pH is the measure of a soil's acidity. Most garden plants prefer a pH of 6 - 7.5, though some plants prefer more acid conditions (lower pH). The pH of Utah soils is generally above 7, which is alkaline. Some plant species show signs of minor nutrient deficiencies in high pH soils. Iron chlorosis is the most common minor nutrient deficiency. The primary symptom is interveinal chlorosis, or yellow leaves with dark green veins. This can be treated by applying iron to the soil in the form of iron chelates, or by adding sulfur products, which temporarily lower soil pH and therefore make soil iron more available to plants. However, these are only temporary remedies, so the best solution to nutrient deficiencies, including iron, is to plant alkaline-tolerant plants.
Fertilizers may be classified as organic or inorganic (synthetic). Synthetic fertilizers are often less expensive, take effect more quickly and contain a higher percentage of nutrients. Organic fertilizers, on the other hand, release nutrients over a longer period of time and rarely burn plants since the percentage of nutrients is low. Organic fertilizers also improve soil structure and increase microbial activity. Soil amendments like compost and manure not only provide nutrients, but also build a healthy soil. An annual application of good quality compost may provide all the nutrients your plants need.
Fertilizers, other than some soil amendments sold in bulk, always list the nutrients they contain on their packaging. The major nutrients are listed in the following format: Nitrogen(N)-Phosphorus(P )-Potassium(K). For example, a 20-10-10 fertilizer contains 20% nitrogen, 10% phosphorus, and 10% potassium. Minor nutrients are also indicated, if present.
Fertilizers should be applied before sowing or transplanting. Bulky organic fertilizers such as compost or manure, should be applied well ahead of planting (fall or early spring), to allow time for initial decomposition. Nitrogen can be applied at or after planting time. Established beds should be fertilized in fall or spring before new growth occurs.