A seed, some dirt, some light, some water and a lot of love — these are the traditional ingredients. However, it’s not so easy to grow a plant and according to Prof. Murray Brian McBride, crop and soil sciences, not all soil is created equally.

On Sep. 3, in a seminar entitled “Soil Testing for Copper and Other Trace Metals: The Challenge of Estimating Bioavailability and Toxicity,” McBride addressed a crowd of botanists and chemists in Emerson 135, proposing a new strategy in soil analysis.

“The question is, what total soil concentration of toxic metals is tolerable?” McBride asked.

Soil is a mosaic mixture of various elements and compounds, including trace metal elements, inorganic salts and organic compounds. Within any soil sample, the relative concentrations of these chemicals directly affect plant health.

Plants require certain nutrients for healthy growth. Known as essential elements, these nutrients both include organic compounds and trace metals, such as zinc, nickel, manganese and copper. The overall fitness of plant organisms depends on the presence and concentrations of these trace metals.

Due to this trend in plant health, McBride believes an efficient soil test is an essential tool for evaluating possible crop production. However, he recognized that scientists have not yet found an ideal method.

“You’d think we’d have this all worked out because we’ve been doing these tests for 60 years, but we haven’t been able to find good results,” McBride admitted.

In general, according to McBride, soil tests aim to analyze two primary traits in the soil-plant relationship. First, soil tests measure “bioavailability,” which is the amount of nutrients that a plant successfully accumulates. In addition, soil tests analyze “toxicity,” a general term that describes the consequences of unhealthy soil.

As McBride described, two factors affect bioavailability and toxicity: capacity and intensity. “Capacity” is the concentrations of the trace metals in the “labile metal pool.” When the concentration of one or more of the trace metals is too low, a plant fails to accumulate enough molecules, resulting in stunted growth and misshapen appendages. When the concentration of one or more of the trace metals is too high, the soil acts like a toxin, poisoning the plant and stunting growth.

“Intensity” is the transport kinetics that exist between the root system and the soil; intensity is determined by the solubility of soil compounds and the structure of root systems. Soluble trace metals readily diffuse into plants, increasing the plant’s bioavailability while increasing the risk of metal toxicity. At the same time, specific root structure may possess superior carrier molecules, which enhance the uptake of trace metals. These diffusive structures both increase the bioavailability of nutrients and the risk of metal toxicity.

Traditional soil tests analyze soil composition to predict potential plant growth, but as McBride explained, this strategy fails to study plant structure.

“It’s asking a lot of a soil test to ask what will be up in the leaves from the soil, but that’s what we’ve been asking,” McBride said. “I’m not sure we have one yet, but the ideal soil test should be sensitive to both capacity and intensity.

McBride researches the relationship between plant structure and metal bonding, using spectroscopy and other techniques to study the bonding of metal ions and organic structures.

“To determine how much zinc or cadmium is going to be taken up by a plant — forget it. You’ll never do that with a soil test,” McBride suggested. “It requires plant tissue tests, but soil tests may provide a quick analysis.”

In addition, McBride recognized that many other factors affect the relationship between plants and soil, including pH, clay content, bacterial colonies and fungal populations.