Last week’s column defined what an essential nutrient was and some ground rules. Today, will focus on a few more important concepts before discussing the essential nutrients themselves. These apply to cropland and a backyard garden or flower bed.
• Soils possess colloids, very small soil particles (clay) and humus, decomposed organic matter, that have a large surface area per unit volume. Due to their nature as colloids they possess a charge. Clays in our area possess a net negative charge. The size of the charge is a function of the type of clay. Humus possesses both positive and negative charges. In humus the extent of the positive charge is a function of soil pH. Which leads to the next point.
• The net negative charge of soil colloids allows them to attract and hold positive ions. It also helps the soil hold water against gravity. Colloids don’t just hold cations, positively charged ions, they can release them into the soil solution through cation exchange with other cations. This means soil colloids act like a savings account that nutrients can be withdrawn from. There is a set of rules that govern this exchange. The same holds true for negatively charged ions, anions, if any positive charge is present. Commonly soils in our area possess a much larger cation exchange relative to anion exchange.
• Other ions are present in the soil solution and on colloids – sodium, aluminum, strontium, cadmium, and so on. These cations compete for exchange sites on the colloids with nutrient cations. Some, like aluminum, are toxic to plants, others like sodium usually don’t harm plants unless excessive but do result in poor soil structure. Aluminum is a problem at low pH levels and sodium at pH readings above 7.
• Under acid soil conditions many of the exchange sites are unavailable to hold nutrient cations as they are occupied by hydrogen and sometime aluminum. The effective cation exchange is low compared to what the potential is. Liming will raise the pH and free up exchange sites.
• When large quantities of sodium are present, they can occupy many exchange sites and present problems. The easiest way to deal with this if economically feasible, is to apply gypsum, calcium sulfate, to the soil to displace the calcium.
• The ideal pH range for the seventeen essential nutrients is fortunately the same as the proper pH range for our common crops, 6.3 to 7.3. This is neutral to slight acidic. Other plants prefer more acid of more basic conditions.
• Cations, positively charged ions, are more stable in the soil environment, i.e. less like to leach below the root zone, since they can be held on soil colloids. Anions, negatively charge ions, typically are not held in the soil and subject to leaching. This has implications for fertilizer timing and application method.
• If you have a low cation exchange capacity, sandy soil for example, it is impractical to increase cation exchange by adding clay. The only practical way to increase it is through adding large quantities of organic matter such as plant resides or manures over a long period of time and minimize tillage. Next week the macronutrients.
Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.