How well do you know your garden soil? Does it drain well or stay wet for a couple of days after a significant rain? What is the pH? Does it have sufficient nutrients available for your vegetables to use to grow? We often overlook the importance of soil management when it comes to building our gardens; however, soil is a foundational medium when it comes to plant growth. Soil keeps plant roots anchored and provides plants with essential nutrients, water, and air.
Soil is composed of three basic mineral particles, of three different sizes; sand being the largest particle, silt intermediate, and clay the smallest. Sand, silt, and clay each have characteristics that are specific to each particle. For example, sand is a larger particle and does not “pack” as tightly which creates larger pore spaces; because of this, water easily passes through sandy soil. This is good for drainage; however, it is bad for water holding capacity and is why sandy soils tend to dry out more rapidly. Clay particles are the smallest particles which allows them to pack tight and hold water better; however, clay can hold water too well which leads to poorly drained soil. The percentage of each particle in a soil determines the soil’s texture as well as its physical properties. An ideal soil texture consists of equal parts of sand, silt, and clay; this type of soil is referred to as “loam.” Usually, one component predominates, producing a clay loam or a sandy loam.
There are many different resources to determine the texture of your soil. The simplest and most generic method is the feel test; rub a moist sample of soil between your fingers. Sand will feel gritty, silt smooth, and clay sticky. Another test is the ribbon test; with this test, squeeze a small, moistened ball of soil between your thumb and forefinger to create a ribbon. Sand or sandy soils won’t ribbon; loam, silt, silty clay loam, or clay loam soil ribbons less than 1 inch; sandy clay loam, silty clay loam, or clay loam ribbons 1 to 2 inches; and sandy clay, silty clay, or clay will ribbon more than 2 inches. There are other tests available such as the jar test, ball squeeze test, hydrometer method, pipette method; the feel, ribbon, jar, and ball squeeze tests can all be done at home, but the hydrometer and pipette methods are commonly done in a laboratory setting. I encourage you to try one of the at home texture methods; the NRCS has a nice flow diagram to help with determining your soil texture. Figure 1 below displays the USDA soil texture triangle that will also be of use when determining texture.
In addition to soil texture, it is important to know and understand the pH and nutrient availability of your soil. Soil pH and nutrient availability can be assessed through a soil sample. Our Horticulture educator Chris Enroth provides us with great information on how to properly collect a soil sample . Soil sampling is not something that needs to be done every year. Ideally, it would be done every 3 years. Soil pH is a measure of acidity or alkalinity with an ideal soil pH of 6.5; the pH ranges from 6.0 to 7.0 is ideal for most garden vegetables. See Figure 2 for pH specifics for different vegetable crops. Soil pH can be raised (more alkaline) with the addition of lime or lowered (more acidic) with the addition of sulfur. Raising and lowering your pH does take time; once lime or sulfur is applied, it can take a year or more to see any movement in pH.
The soil test also provides information about the level of nutrients available. Depending on the soil test you choose, Phosphorus, Potassium, Magnesium, and Calcium are the most common nutrients you will receive information about. Nitrogen is an essential nutrient; however, it can be very mobile within the soil with values changing constantly which is why it is not included in a basic fertility test. Nitrogen is also the nutrient that plants need in the largest quantity. When organic matter is broken down in the soil, nitrogen is available for plant use. The nitrogen from organic matter usually is not adequate to meet the demands of many vegetables, so additional nitrogen from fertilizer is needed. If the organic matter level is high or manure has been applied, the only added nitrogen fertilizer needed is for vegetables that benefit from high levels of nitrogen which includes sweet corn, cole crops (broccoli; cabbage; cauliflower; kale; brussel sprouts), and leafy green vegetables. It is recommended to apply one or two applications of nitrogen as a sidedressing after planting for these crops. For soils high in phosphorus (25 ppm or higher), no phosphorus fertilizer is suggested. Crop yield is not likely to be improved by applying additional phosphorus on high phosphorus soils, so there is no benefit from its use. Potassium levels 125 ppm and above are considered high and no additional fertilizer should be added. Other nutrients (calcium, magnesium, boron, etc.) are not known to be deficient in most Illinois soils, so it is not recommended to apply these nutrients; however, if issues arise within your garden, it is typically a good idea to test for micronutrient deficiencies.
Nutrients can be added to soil in many different forms with the most common forms being manure, compost, or commercial fertilizers. Manure and compost are a nice option as they also add organic matter to the soil. Applications of fertilizer should be made based off your soil test levels.
For more information or any questions about soil management, contact your local Extension office.