Author Topic: Introduction to Plant Nutrients  (Read 2096 times)


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Introduction to Plant Nutrients
« on: April 27, 2011, 01:53:09 PM »

Plant Nutrients Macronutrients are essential elements required in large quantities by all higher plants and include nitrogen, phosphorus, potassium, calcium and magnesium. Micronutrients, including elements such as iron, manganese, copper, zinc, molybdenum and boron, are also called trace elements, because a plant only needs these nutrients in small quantities. Micronutrients are found in numerous plant hormones and enzymes and in compost, and are vital to plant health.

Nitrogen All grass requires nitrogen to build plant proteins (such as chlorophyll) and to grow. Nitrogen is available in two forms: quick-release water soluble  and slow-release water insoluble.

Water-soluble nitrogen is immediately available to the plant and causes a rapid growth spurt. It can be taken up directly by the plant, bypassing the natural digestive process of the plant/soil system, which provides nitrogen through the action of soil organisms. It is also used up quickly and is easily washed into ground and surface water, causing nitrogen contamination as well as excessive aquatic plant growth and algae blooms. Soluble nitrogen sources include urea, ammonium nitrate, ammonium sulfate and ammonium phosphate as well as some organic fertilizers such as fish emulsion. They tend to be less expensive per pound than slow-release fertilizers.

Slow-release fertilizers include natural organics, as well as synthetic or coated nitrogen materials. The release of nitrogen in organic materials is due to the activity of soil biology, such as microbial decomposition. Since the activity of microorganisms is dependent on soil temperature and moisture, nitrogen availability in slow-release fertilizers will vary with the time of the year and weather, but can continue for up to a year.

NOFA organic standards for nitrogen amendments prefer slow-release organic compounds such as compost, organic fertilizers, compost teas and lawn clippings. The standards prohibit synthetically derived nitrogen sources such as nitrates, urea and ammonia, as well as Chilean nitrate, leather meal and sewage sludge.

Under organic soil management, much of the nitrogen is provided in the form of complex organic compounds that soil organisms break down and deliver to the plant. If soluble inorganic nitrogen fertilizer (such as sodium nitrate, ammonium nitrate, calcium nitrate or ammonium
phosphate) is used exclusively, it will suppress the soil?s natural biological activity and sacrifice the benefits of a healthy soil foodweb. When you use soluble nitrogen fertilizers, the grass experiences both feast and famine. When nitrogen is plentiful, leaf growth increases and root growth decreases. When nitrogen is depleted, leaf growth decreases and root growth increases. When the next round of soluble nitrogen fertilizer is applied, there is a sudden burst of leaf growth. Leaf blades will be more succulent and contain a higher concentration of free amino acids, which attract grass-eating insects and make the grass more susceptible to disease organisms. Water use also increases and the grass wilts more easily under hot, dry conditions.

With soluble nitrogen fertilizers, it is difficult to regulate the amount of nitrogen delivered to the plant. By contrast, when organic fertilizer is added to an intact soil foodweb, nitrogen is released in a more uniform, naturally regulated way. An additional benefit of organic fertilizer is that little of its nitrogen is lost through leaching, volatilization or denitrification.

Organic lawn and turf care applies organic fertilizers to an intact, healthy soil foodweb to deliver a sustained and regulated supply of nitrogen.

Phosphorus Phosphorus is an essential part of a plant?s genetic material and a large component of its energy transport mechanism. 
Through the overuse of fertilizers, it is also the cause of degraded water quality in most of our inland rural and suburban communities. If clippings are left on a lawn, very little (if any) phosphorus is required on an annual basis.

Most soils contain adequate levels of phosphorus; however, if the soil pH is lower than 6 or greater than 8, phosphorus will be tightly bound to soil particles, making it all but inaccessible to plants. 
Furthermore, when soluble phosphate is applied to biologically degraded soil, what the plant cannot use immediately is quickly fixed to soil particles and becomes unavailable. On the other hand, when a less soluble form of phosphate (such as phosphate rock) is applied to biologically active soil, the soil organisms slowly release the phosphate over a long period of time. As a result, phosphate need only be applied every five years, or even less frequently for optimum grass growth. Follow soil test recommendations for phosphorus fertilization.

Compost also contains a small amount of organically bound phosphorus (less than one percent), but the rate of application usually makes up for the low concentration. This phosphorus is released slowly in synchrony with the growth of the grass. Unless phosphorus tests at ?medium? or below, use organic fertilizers that contain less phosphorus than nitrogen. Depending on the starting materials, many composts are also high in phosphorus, so be aware of the nitrogen/phosphorus/potassium test results for your compost.

Potassium, Calcium and Magnesium Potassium is found in a plant?s fluids and is primarily involved in maintenance of its turgidity, or hydraulic pressure, and in the transport of materials throughout its vascular system. Adequate potassium levels make the grass more resistant to wilting and also increases winter hardiness.

The form of potassium available to plants occurs in nature as a positively charged element, or cation (designated K+). Mineral soil may contain large amounts of potassium, but it is usually in an insoluble and unavailable form. Available K+ is highly soluble and is easily leached out of welldrained soils unless the soil contains negatively charged particles that hold cations. Stable humus created by soil organisms is an ideal carrier of potassium. Humus has many negatively charged sites that hold available potassium for plant use. 
Building stable humus in the soil requires regular application of organic matter.

The availability of potassium and the pH of the soil are closely linked. Humus found in a soil with a neutral pH, which is optimum for turf, can hold large amounts of potassium. Biological activity around the roots lowers the pH, which releases the potassium and makes it available to the plant. Calcium is an essential component of cell membranes; it helps to maintain the structure of cell walls. It is also a component of messenger molecules that allow the plant to respond appropriately to environmental cues and challenges.

Magnesium is the core element in chlorophyll, the green pigment in plants. In chlorophyll, the central magnesium ion is bonded to a large organic molecule composed of carbon, hydrogen, oxygen and nitrogen. 
This molecule is responsible for converting the energy of sunlight into chemical energy through the process of photosynthesis. In photosynthesis, the energy absorbed by chlorophyll transforms carbon dioxide and water into carbohydrates and oxygen. This is one of the most important molecules for life on earth.

Like potassium, calcium and magnesium are cations, and their accessible form is dissolved in the water found in the pore spaces in soil. Calcium and magnesium compete with potassium for binding sites on negatively charged particles in the soil. Excess potassium will displace magnesium and may cause a magnesium deficiency. A good ratio of calcium, magnesium and potassium is 70:11:3, but some deviation is not harmful. An excess of magnesium may reduce stress tolerance, increase disease susceptibility and attract insect pests.

It is important to determine what the ratio of potassium, calcium and magnesium is with a soil test so that the correct kind and amount of soil amendment can be applied. There are many forms of calcium and magnesium amendments: calcium sulfate (gypsum), calcium oxide (burned or quick lime), calcium carbonate (crushed limestone), tricalcium phosphate, dicalcium phosphate, calcium nitrate (nitric acid), calcium/magnesium carbonate (dolomitic lime), magnesium sulfate (Epsom salts), potassium/magnesium sulfate and magnesium oxide. The kind and amount of amendment will depend on the results of the soil test, including the pH. If the pH does not have to be adjusted, a neutral amendment like calcium sulfate, magnesium sulfate or potassium/magnesium sulfate can be used.

The release of calcium and magnesium after liming is more effective and better regulated when applied to a biologically active soil. Regular applications of organic material will also increase the ability of the soil to retain and exchange cations.