By Crystal Kim, Red Butte Garden Horticulturist

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. Utah State University’s Analytical Laboratory performs inexpensive soil test for home gardens. You can also contact your local County Extension office for more information. 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 such as iron, magnesium, manganese, calcium, zinc, sulfur, and boron, are essential for plant growth and development but are only needed in small amounts. These nutrients are usually present in sufficient amounts but some may be unavailable for plant use due to high soil pH levels.

Soil pH is the measure of how acid or alkaline a soil is. Most garden plants prefer a pH of 6 - 7.5, with 7 being neutral, though some plants prefer more acid conditions (lower pH). The pH of Utah soils is generally higher than 7, which is alkaline. Some plant species show signs of nutrient deficiencies in high pH soils. Iron chlorosis is the most common minor nutrient deficiency, but chlorosis is sometimes caused by another nutrient that is deficient. The primary symptom of iron chlorosis 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 and other micronutrients more available to plants. However, these are only temporary remedies. The best solution to reduce nutrient deficiencies, including iron, is to plant alkaline-tolerant plants. Learn more about Iron Chlorosis.

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 help 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 by weight. Minor nutrients are also indicated, if present.

Fertilizer may be applied before sowing or transplanting if needed. If using synthetic fertilizer, mix it into the soil before transplanting to prevent burning of roots. 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. Established beds should be fertilized in fall or spring before new growth occurs.

For specific fertilizer recommendations for your garden, refer to your soil test results. For general fertilizer recommendations as well as nutrient concentrations for various organic fertilizers, please review the USU Yard & Garden Extension Organic Fertilizer webpage.