News from the Edible Demo Garden

April 2024: The Importance of Soil Testing

The soil in the Edible Demo Garden gets a lot of attention. Compost and organic amendments are added regularly. Moisture levels are monitored, and rainwater is supplemented with drip irrigation and hand watering. Mulch and cover crops are used to protect the soil and tilling is kept at a minimum. The goal of these efforts is to maintain the structure and composition of the soil so that the growing plants have access to the air, water, and nourishment they need.

However, soil can look and feel healthy, and still be lacking nutrients essential for optimum plant growth. Plants suffering from malnutrition look unhealthy and show symptoms of deficiency. Growth may be stunted, leaves may be chlorotic (yellowed) or distorted, and shoots appear short and thin. Too little or too much of any one nutrient can cause problems. The best way to check for a lack of nutrients in the soil is to test a sample.

 

How are the nutrient components of soil tested?

Inexpensive do-it-yourself soil test kits are readily available in garden centers, and they can be a convenient way to determine soil pH and levels of the primary macronutrients: nitrogen, phosphorus, and potassium. More comprehensive tests which cover secondary macronutrients and micronutrients, can be performed by commercial soil testing laboratories.

Any soil test should start with a carefully collected soil sample. Use a stainless-steel trowel or shovel to obtain a sample at least six to eight inches below the soil surface. Avoid wet soil, if possible, the sample needs to be dry for testing. Taking samples from several places in a garden bed and mixing them can be useful in measuring overall chemical properties. However, samples from different garden areas (vegetable, perennial, lawn, etc.) should be kept separate. Soil testing laboratories generally provide specific instructions on sample collection.

 

Why does soil pH matter?

Nutritional elements in the soil need to be in a soluble chemical form to be absorbed by plant roots. There must be sufficient moisture in the soil to allow the roots to take up and transport the nutrients. The temperature of the soil also needs to fall within a certain range for uptake to occur. Finally, the solubility of the nutritional elements and the activity of the microorganisms responsible for the chemical transformation of nutrients are directly influenced by soil pH.  An essential nutrient can be present in the soil, but not in a form the plant can use. Each nutrient has an ideal pH range for plant availability. At pH levels that are too acidic or alkaline, some nutrients can become insoluble and not accessible to plant roots. Phosphorus, calcium, and magnesium are elements that become less accessible as soil increases in acidity, while the availability of iron, zinc, and manganese is reduced in more alkaline soils. The ideal pH for most plants is between 5.5 and 7.5.

 

What nutrients are commonly lacking in soil?

The nutrient most commonly lacking in garden soils is nitrogen. Most plants absorb nitrogen in the form of ammonium or nitrate. These forms readily dissolve in water and leach away. Excessive irrigation and heavy rains can lead to nitrogen deficiency in the soil. Phosphorus, potassium, calcium, magnesium, and sulfur are the other macronutrients that may be deficient, depending on drainage and pH levels. A lack of iron, zinc, or manganese, all considered micronutrients, results in interveinal chlorosis, a condition in which the leaf veins remain green while the rest of the leaf turns yellow. The uptake of these micronutrients is also affected by pH.

 

For more information on assessing soil health, click here.

For hands-on practice in soil testing come to the workshop co-sponsored by the Edible Demo Garden and the Edibles Guild. Click here to register.