Great Bend Tribune
Published March 19 through April 30
Part I
Things are at a bit of a standstill currently as Mother Nature can’t seem to determine the thermostat setting, the wind seems to want to blow and much of the western half of Kansas waits for rain. Weather permitting, wheat ground is receiving nitrogen and herbicides. Some producers are hesitant to spend more money on their wheat crop with the price of wheat and dry conditions. Agronomically and economically, it is just as, if not more important under these conditions it to provide adequate weed control and make available necessary nutrients than under good conditions. At least those two sources of plant stress can be minimized. This week, and in several succeeding weeks, this column will discuss the essential nutrients involved in plant growth.
An essential nutrient (element) is defined as required for the plant to complete its life cycle. In other words, necessary for the plant to germinate, grow, flower, and produce viable seed. There are seventeen elements required by all plants to complete their life cycle. They are divided into two groups – macronutrients and micronutrients. The difference between the two groups is the relative amount needed by the plant. Macronutrients are required in relatively larger amounts compare to micronutrients. That doesn’t mean they aren’t as critical to the plant, it simply needs small amounts of them. The macronutrients are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Micronutrients include iron, manganese, zinc, copper, nickel, boron, and molybdenum. The last essential element identified, nickel, was only determined fairly recently. While we know it is essential, we seldom see deficiencies and there really aren’t recommendation for it so this is the last time it will be mentioned. These elements are found naturally in the soil as the result of soil formation processes. Soils will vary in their native levels of these elements. Native levels are a function of the parent material the soil formed from and some may be brought in by atmospheric deposition, directly from the atmosphere, and as the result of deposition from flood waters.
Most of these nutrients are all taken into the plant as ions, they possess an electrical charge. Some are positive, termed cations, while others are negative, termed anions. One, nitrogen, is found as both an anion and a cation in the soil. The exceptions are carbon, hydrogen, and oxygen which are taken into the plant as water and carbon dioxide.
While a column is insufficient to provide great detail on the rules of nutrients and their management, one rule is critical to keep in mind – Liebig’s Law of the Minimum. The Law of the Minimum states: “the yield of a crop is limited by the deficiency of any one essential element even though all others are present in adequate amounts.” Often in agriculture, landscaping, or even as a homeowner, some nutrients are overapplied to compensate for poor growth and yield when the problem is really a deficiency of another nutrient. In plain English, the Law of the Minimum is stating that the weakest link determines the strength of the chain. If sixteen of the seventeen nutrients are adequate, the one that is deficient will limit growth and yield.
Part II
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 flowerbed.
- 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.
Part III
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.
Part IV
This week’s column starts tackling the macronutrients we fertilize for – Nitrogen (N), Phosphorus (P), and Potassium (K). These are also termed Primary Nutrients as they are needed in larger amounts. Nitrogen is found in the soil as nitrate, a negatively charged ion, and ammonium, a positively ion. Phosphorus is found as a negatively charged ion and Potassium as a cation. N is the only one found in both a positively and negatively charged for. These nutrients are found in the soil solution and organic matter. Ammonium and potassium as cations may also be found on cation exchange sites. Nitrate and phosphorus being negatively charged are not.
Nitrogen is used by the plant in a number of key ways. It is part of the chlorophyll molecule necessary for photosynthesis. N also serves as the backbone for amino acids and amino acids are necessary to make proteins, RNA, and DNA. The plant can take up wither form but nitrate is much more common in the soil. N as nitrate is easily lost through leaching and with good precipitation moves below the root zone. Soil N testing, along with sulfur must be done to a two-foot depth because of the ease of movement. N comes from the decomposition of organic matter and through fertilizer additions. The most common forms as anhydrous ammonia, urea, and ammonium nitrate. Manures may serve as an N source, however, you must test the manure for N level and P concentration. Manure supplies much more P than N so you will not be able to supply all you N needs with manure or you would over apply P. Liquid, solution, nitrogen is a combination of urea and ammonium nitrate. Another important source of is rhizobium bacteria species in plants like alfalfa, peanut, soybean, clovers, and locust trees. These bacteria form a symbiotic, mutually beneficial, relationship by invading plant roots. They can take atmospheric N and convert it to a useable form by the plants. In exchange, the plant supplies it with a home and nutrition.
Of all the nutrients besides carbon, hydrogen, and oxygen, N is needed in the largest amount. An N deficient plant will exhibit symptoms on lower leaves first as N is robbed from older plant tissue and moved to developing structures such as leaves and seed. It exhibits as a V shape moving along the midrib-back to the base of the leaf. Deficiencies may be caused by several factors including inadequate N fertilizer, saturated soils, dry soils, and factors that inhibit root growth and function such as root diseases and nematodes. Extended periods of cloudy and or cool weather also cause deficiency symptoms.
Ammonium-N and urea and rapidly converted in the soil to nitrate which is easily leachable by bacteria. This can be inhibited by inhibitors that designed to keep urea as urea or ammonium as ammonium but only for a short period of time. Since N can be easily lost by several processes, it is normally advisable to not apply all your N fertilizer at once but to split the application over two applications or even spoon feed it through irrigation water. In developed agriculture, N is typically the yield limiting nutrient. Next week P and K.
Part V
This week’s column finishes the discussion of the macronutrients known as primary nutrients Phosphorus (P) and Potassium (K). First though, here’s hoping everyone is having a happy Easter Weekend. Second, the moisture situation continues to improve wheat and there is a chance for significant rain for most of the state over the past week and continuing into this week. The only down side is the need to get corn in the ground. And we are fortunate to have warm but not high temperatures. Now on to P and K.
First up is phosphorus, P, which serves two main functions in plants. In plants, and in animals, P equals energy primarily as ATP. The products of respiration, the oxidation of sugar for energy, is captured in energy bonds in ATP. ATP is also involved in photosynthesis so P helps in producing sugar, the product of photosynthesis. P is also involved in DNA and RNA so it important in the genetic code, DNA, and translating that code, RNA. Its concentration is higher in actively growing parts of the plant and structures like seeds. Plants take up the orthophosphate form which is an anion, negatively charged. P deficiencies occur in older tissue first and are common in seedlings and young plants even with adequate soil P. This is typically caused by cool and/or dry conditions and symptoms typically disappear when conditions improve. Deficiencies result in dark green plants, often leaf purpling, and can delay plant growth and maturity. The best organic source of P is manure which is typically much higher in P than nitrogen. Inorganic fertilizers start as rock phosphate, however, the rock isn’t very soluble and is treated with acid. Today, except in lawn and landscape applications, straight P is rare. P fertilizers commonly contain nitrogen and may be purchased in liquid or dry formulations. Some P can also be obtained with N and K and is typically used as starter fertilizer. In developing agriculture, P is the limiting nutrient, not N.
Potassium, K, is interesting in several respects, it isn’t a structural component of plants and it isn’t part of compounds like P but its role is equally important. It activates over 60 enzymes throughout the plant, it helps the plant maintain a narrow pH range, and it also helps with disease suppression and decreases lodging. And it regulates the opening and closing of the stomatal guard cells in the leaves, the end of the transpiration stream bringing water and nutrients up from the roots. These openings also allow in needed carbon dioxide and let out oxygen. K is found absorbed as a cation by plants. Since most K stays in the vegetative structure of the plant, harvesting only for grain or seed allows much of the K to return to the soil as residue. K deficiencies occur in older tissues first and often first appear as yellowing along the edge, margin, of the leaf that works its way inward until the leaf dies. Deficiencies also cause the plant stomata to respond more slowly to the environment further stressing the plant and preventing water conservation. While most soils have abundant K, little is available so K fertilization is becoming more common here. Manure is an excellent organic K source. Potassium chloride and K-Mag are the common potassium here but it may also be found in low analysis starter formulations.
Part VI
This week’s column discusses the macronutrients known as secondary nutrients: Sulfur (S), Calcium (Ca), and Magnesium (Mg). Calcium and magnesium are found as cations, positively charged ions in the soil, while sulfur is typically found as sulfate, an anion, negatively charged.
While are three are important, sulfur is especially critical to producers. Proteins are made up of amino acids and all plant proteins need sulfur containing amino acids. Sulfur deficiencies result in lower protein levels in grains such as wheat. Certain vitamins such as thiamine, necessary for human health, contain sulfur. It is important in chlorophyll formation and aids in seed production. Plants such as garlic, mustard, and onions produce sulfur containing volatile compounds giving them their characteristic odor and taste. Plants deficient in sulfur are often a pale green and unlike nitrogen (N) and potassium, the symptoms show up on younger leaves first. Poor, spindly growth may also occur. Sulfate-S is not held by soils and is subject to leaching like N and a soil test is to a depth of two feet like N-tests is needed. Sulfur is primarily found in organic matter. Deficiencies are becoming more common for several reasons. Fertilizers are purer now and no longer have sulfur as an impurity. Scrubbers have been placed on coal burning power plants and S has been removed from fuels so S no longer comes from acid rain here. And finally, crop yields are much greater and we are removing more sulfur with crops. Sulfur deficiencies are more pronounced in sandier soils low in organic matter. More and more producers are using sulfur fertilizers, especially to boost wheat protein levels, in alfalfa production, and oil seeds. The most common sources are ammonium sulfate, calcium sulfate (gypsum), thiosulfates, and K-Mag. Elemental sulfur is also used but more cumbersome to work with and a slow release source. Since these are readily available, it is possible to use S fertilizers as in-season rescue treatments if early enough.
Grass crops need little calcium compared to broadleaf plants. It is an important component of plant cell walls; is involved in cell elongation/division, membrane permeability, and enzyme activation. Even under fairly acid conditions, there is normally adequate Ca available. Limestone, calcium carbonate, is calcium source, however it is used primarily to correct acid soil conditions not fertilize for Ca.
Magnesium is critical in photosynthesis as it is the atom that captures the energy of the sun in the light reaction so it can be used to make sugar in the dark reaction. Mg is also involved in oil and protein synthesis, and enzyme activation. Deficiencies show up as interveinal yellowing on older leaves and stunted growth. Lack of magnesium in grasses grazed by cattle can result in grass tetany, staggers, which is prevented by Mg supplementation. Ca and Mg come from the materials forming the soil as they weather and some from the decomposition of plant residues. Severe Mg deficiencies aren’t common and can be corrected with magnesium sulfate (Epsom salts) or K-Mag. Next week finishes this series with the micronutrients.
Part VII
This week’s column finishes the discussion of essential plant nutrients with the micronutrients. These are necessary for the plant to complete its life cycle but are needed in very small amounts compared to the macronutrients. The positively charged micronutrients are Copper (Cu), Zinc (Zn), Iron (Fe), Manganese (Mn), and Nickel (Ni). The negatively charged anions are Boron (B), Molybdenum (Mo), and Chlorine (Cl). Nickel was the last essential element identified and other than knowing it is essential, we know very little about it and deficiencies are almost nonexistent. So what do these elements do that makes them essential?
While needed in very minute quantities, these elements perform vital plant functions. They serve as catalysts for enzymes which allow them to perform functions from photosynthesis to cell division. Catalysts are not consumed in the reaction and can be used over and over. Therefore lesser amounts are needed. For a few highlights of functions:
- Chlorine, Cl, aids in disease suppression in grass crops, especially small grains.
- Copper, Cu, enhances the flavor of fruits and vegetables. It also increases sugar content.
- Boron, B, is essential in seed and cell wall formation.
- Molybdenum, Mo, is the catalyst for the enzyme that converts nitrate to ammonium for the production of amino acids.
In contrast to the macronutrients, we do not have good soil tests for all the micronutrients with the ability to relate the amount in the soil to needed fertilizer and sufficiency in the plant. Plant tissue analysis is often necessary in combination with soil tests. Often by the time tissue analysis indicates a deficiency, it is too late to correct the problem for the current year.
Nutrient availability here is primarily dictated by soil pH with nutrients available in the proper amounts from slightly acid to slightly basic. At high pH, nutrients such as iron are tied up by phosphorus and unavailable. At low pH values, very acid soils, nutrients like manganese can be over available and severely damage the plant. This is a key difference between micro and macro nutrients. For micronutrients, the difference between sufficiency for plant growth and toxicity, too high a concentration, is quite small. For example, a corn crop of 150 bushels per acre requires 0.008 lb of molybdenum, 0.27 lb of zinc and 1.90 lb of iron. Contrast that with 150 lb of nitrogen. Producers really do not have to deal with toxic levels of macronutrients.
As yields increase and land is farmed for over a century, some of these nutrients are starting to become deficient in soils. Micronutrient fertilization is slowly gaining in use as producers continually seek to increase yields. Here, common micronutrient fertilizer applications include zinc on corn and chloride (Cl) on wheat. In certain areas, boron is starting to be applied on alfalfa. For corn, adding a small amount of zinc may be the difference between 200 and 220 bushel corn. Chloride on sensitive wheat varieties can add 20% or more to the yield.
However, care must be taken to insure what is actually needed and to apply it in the proper amount. Finally, there may be certain plants requiring other elements. These elements are common to all plants.