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Essential Nutrients for Plants: Part III
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This week’s column starts tackling the macronutrients. But first, last week saw significant rains for most of the state of Kansas. As of Thursday much of the area had received between 1.5 to well over 3 inches of precipitation with some areas of the state considerably more. Does this make the wheat crop? Of course not but it significantly improves wheat conditions which were starting to deteriorate. And this should be extremely beneficial for pasture and alfalfa conditions and help the corn crop to get off to a good start. If the weather stays seasonable, this moisture gets the wheat crop a long ways towards heading and bloom. Today, we will discuss the three nutrients needed in the largest amounts, Carbon, Hydrogen, and Oxygen – C, H, and O.
People often don’t think of these as nutrients since we don’t directly fertilize for them. However, they make up the majority of the plant and what we utilize, 95% by dry weight. The sources of these three nutrients are carbon dioxide and water. Carbon dioxide comes from the atmosphere and enters the plant through openings in the leaves termed stomata. As long as the concentration is greater in the atmosphere than in the leaves and the stomata are open, carbon dioxide moves into the leaves. It provides carbon and oxygen. Conditions that cause the stomata to close such as heat or moisture stress and 100% relative humidity shut the stomata and prevent movement into the plant. Carbon dioxide is used in the plant along with water in photosynthesis to make sugar, the conversion of radiant energy into useable chemical energy. From there it is converted in more complex sugars, sucrose and fructose. Some stays in the chloroplast to maintain the cell but most is transported through the phloem throughout the plant and serves several purposes.
Oxygen comes from both carbon dioxide and water. The water, with a few exceptions enters the roots at their growing points and is transported through the xylem, along with dissolved nutrients to the rest of the plant. The xylem ultimately ends at the stomata. This movement, transpiration, takes place as long as the stomata are open which was discussed in the preceding paragraph. Water molecules are split in photosynthesis and the oxygen is used in making sugar as is hydrogen. Oxygen is also necessary for the oxidation of sugar, respiration, providing necessary energy for living cells to maintain themselves and for cell replication/growth.
Hydrogen comes from water provided as described previously. It serves as a component of sugar produced by the plant. It also exists as the positive hydrogen ion which is acid and serves various functions. Initially, the C, H, and O are used to make sugar but then what?
• The sugar is used to maintain cells and for growth. It is linked together to make starch and can be used to create oils and fats in the plant. All energy sources.
• Sugars can be modified to become structural components to make cellulose, hemicellulose, lignin and other structural compounds. Simply the products of photosynthesis are used to build the plant, keep it alive, and allow it to complete its life cycle.
Next week: Nitrogen, Phosphorus, and Sulfur

Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.