Nitrogen, phosphorus and potassium are the nutrients emphasized in traditional chemical soil analysis tests. Soil characteristics such as acidity and alkalinity (pH) and levels of trace minerals are sometimes included in the assessment. Biological soil tests assess the populations of beneficial bacteria, fungi, protozoa, earthworms and other microorganisms. "Creatures living in the soil are critical to soil quality," according to the National Resources Conservation Service."
Soil Chemical Tests
Chemical soil tests provide recommendations for fertilizers and soil amendments. Soils are depleted by natural processes such as weather and plant growth. Plant growth stimulation is supported by adding nitrogen, phosphorus and potassium fertilizers (N-P-K). They do not feed the soil's nutritional needs. The soil's nutritional needs are assessed by analyzing the biological content for the presence of bacteria, fungi, earthworms, protozoa and other small organisms.
Chemical analysis of soil also assesses soil structure. Soil structure determines whether plant roots can grow and expand, whether water leaches away and whether there is adequate organic matter content. Organic matter content in soil helps it absorb and hold water effectively, as well as provide nutrient content. Soil is most often determined to be clay-like, sandy and porous, or balanced.
Biological Soil Tests
Alternative soil tests analyze biological activity. Soil has billions of microorganisms in small amounts, each one providing a necessary function for healthy soil and plant life. Dr. Elaine R. Ingham of the Natural Resources Conservation Service says the living organisms of the soil food web "range in size from the tiniest one-celled bacteria, algae, fungi, and protozoa, to the more complex nematodes and micro-arthropods, to the visible earthworms, insects, small vertebrates, and plants."
Organic Matter Tests
The University of Massachusetts Soil and Plant Tissue Testing Lab offers a standard compost test that assesses nutrients, heavy metals, organic matter, aluminum and pH levels. Compost is used as organic fertilizer, soil amendment and also as compost tea foliar spray. Tests to determine its overall biological vitality help the home gardener to adjust compost contents as needed.
Soil tests and Organic Gardening
Maintaining soil vitality and fertility is essential to organic gardening and farming. Soil health determines the nutritional value of the food grown on it. Organic soil has been shown to increase the flavinoid level in tomatoes by 79 percent, in a study at the University of California at Davis. Organic farmers test soil to determine if additional organic soil amendments such as compost are needed. Biological activity in soil is increased by adding compost.