Plant Nutrition: Needs and Deficiencies

The details: 
Like all living things, plants must obtain certain elements from their environment in order to sustain their biological functions necessary for survival. Of all the nutrients used by plants, 16 of them are considered ‘essential’ in the sense that plants cannot manufacture them themselves, but rather, must obtain them from their surrounding environment. These essential nutrients are as follows, listed in order or importance:

The structural nutrients:(1) carbon, (2) hydrogen, and (3) oxygen.

The primary macro-nutrients: (4) nitrogen, (5) phosphorus, and (6) potassium.

The secondary macro-nutrients: 
(7) sulfur, (8) calcium, and (9) magnesium

The micro-nutrients: 
(10) iron, (11) zinc, (12) manganese, (13) copper, (14) boron, (15) chlorine, (16) molybdenum, and (17) nickel.

Carbon, hydrogen, and oxygen: 
These nutrients are absorbed by a plant from its environment as carbon dioxide, oxygen, and water. These elements are so prevalent in the environment and so easily absorbed by plants that they never need to be supplemented as a fertilizer; hence, fertilizers never carry these elements as their major ingredients. Plants use carbon, hydrogen and oxygen for a whole host of biochemical functions, but one of the major functions of these three elements is the manufacture of cellulose sugar, the main chemical component of wood which allows plants to hold themselves up and grow tall. Carbon, hydrogen, and oxygen are thus accordingly labeled ‘the structural nutrients’.

Nitrogen: 
Nitrogen, like carbon, hydrogen, and oxygen, is extensively used for a whole host of biochemical functions. Nitrogen is found in all DNA, RNA, protein, enzymatic reactions, chlorophyll, etc. Nitrogen differs from the structural nutrients on two major points: it isn’t used as heavily for plant structure (wood) and isn’t readily attainable from the environment. For this reason, nitrogen is often the single greatest ingredient in most commercial fertilizers. More than any other element, nitrogen stimulates plants to grow vegetatively. Nitrogen is what gives leaves and lawns their characteristically rich green color. Nitrogen deficiency in plants is noticeable by stunted growth and yellowing leaves starting at the bottom of the plant and moving gradually upward as nitrogen deficiency persists.

Phosphorus:
 Phosphorus, like nitrogen, is related with a whole host of biochemicals within plants. Phosphorus is unique in that it helps to stimulate the growth of roots.For this reason, phosphorus is essential in helping new plants establish in the soil where they have been planted or transplanted. Phosphorus also aids in and stimulates the growth of reproductive parts in a plant. Phosphorus will help flowers bloom and help fruit and vegetable plants to produce. Phosphorus is so good at stimulating reproductive growth, in fact, that heavier amounts of phosphorus will actually hasten on reproductive growth, such that plants will produce flowers and fruits sooner than otherwise would have been the case. Phosphorus deficiency results in purpling of foliage starting with the lower leaves and creeping up the plant as deficiency persists.

(You can see an excellent example of phosphorus deficiency in grape vines here.)

Potassium: 
Potassium differs from both nitrogen and phosphorus in that it is less commonly a component of biochemicals manufactured for day-to-day functions within plants. One of potassium’s greatest functions within a plant has to do with water relations – potassium is often responsible for transferring water here and there within a plant by way of osmosis. In the same sense, potassium aids in the transport of photosynthetic products or other biochemicals throughout a plant including defensive chemistry. A plant with adequate supply of potassium has higher drought tolerance and greater disease resistance. Potassium deficiency results in yellowing and sometimes spotting of lower leaves and moving up the plant as deficiency persists. Potassium deficiency also creates high susceptibilities for disease and insect predation.

Sulfur: Sulfur is necessary for the manufacture of sulfur containing amino acids which in turn are essential in the manufacture of proteins. It is a sulfur-sulfur bond that gives protein its folding ability and thus its enzymatic or catalytic function. Deficiency is marked by yellowing leaves, thinned stems, and spindly growth all starting at the top of the plant and working their way down with persisting deficiency.

Calcium:Calcium is the major contributor to plant cell wall strength; calcium deficiency results in weak cell walls. Calcium is also important in the water relations and translocation of biochemicals in a plant, albeit less involved than potassium. Deficiency symptoms include yellowing and/or deformation of plant anatomy starting at the most actively growing portions of the plant such as stem and leaf tips.

 

Magnesium:
 Magnesium, like calcium, is also important in the water relations of a plant. Magnesium is also involved in energy transfer reactions within plants, thus magnesium is essential for healthy and continued respiration in plants. Deficiency is noticeable by interveinal chlorosis (only the veins remain green) starting in lower leaves and moving upward with persisting deficiency. The picture at right is an excellent example of interveinal chlorosis in a crabapple tree.

Iron: 
Iron functions as an electron transferring element in plants and is therefore heavily involved in photosynthesis. Iron is also involved in enzymatic activity as a co-factor. Deficiency is observed as yellowing of leaves starting at the periphery and moving inward with continuing deficiency. Extreme deficiency results in bleached leaves.

Zinc: 
Zinc is also involved in many enzymatic activities but its precise function with proteins is still poorly understood. Zinc deficiencies have several varied characteristics: shortened stem segments; bushy clustering; narrow, thickened leaves; malformed fruit; malformed leaves where one part of the leaf grows while other parts do not. These symptoms occur at the top of the plant and work their way downward.

Manganese: 
Manganese has been found to be a part of enzyme activation and some photosynthetic reactions. Like iron, manganese can also function as an electron transferring element. A manganese deficiency creates interveinal chlorosis in upper leaves.

Boron: 
Boron is primarily associated with strengthening of plant cell walls. Boron deficiency is marked by twisted leaves, cracked and thickened stems, and lumpy or uneven fruit; all these symptoms occur in the upper parts of plants first.

Chlorine: 
Chlorine is involved in osmotic relations within plants. A plant deficient in chlorine will wilt and have chlorotic leaves starting at the periphery.

Molybdenum: 
Molybdenum is important in a few nitrogen manipulating reactions, particularly with nitrogen-fixing plants. Molybdenum deficiency mirrors symptoms of iron deficiency.

Nickel: 
Nickel has also been found to help with nitrogen fixation. Some plant scientists still debate over whether nickel should be considered an essential nutrient to plants or not.

Making sure that your plants have an adequate supply of these nutrients will significantly contribute to your garden’s health. Just like you and I, plants will much more easily withstand the trials of their environment when they are well nourished. When nutrient deficiency symptoms start to appear in a plant that usually means the plant has been undernourished for some time. For this reason, it is always a good idea to regularly work compost into your soil, or to fertilize regularly, to ensure that your plants are always getting an adequate supply of the nutrients they need.

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