Photo by Melody

Phosphorus, a key element in all plant (and animal) life

By Darius Van d'Rhys (dariusJanuary 14, 2009
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Phosphorus, the “P” in NPK, is one of the essential nutrients for plant growth. It is a macronutrient, meaning it’s one of the major nutrients plants need, while secondary nutrients like calcium, magnesium and sulfur are needed in smaller amounts. All nutrients are necessary, to work together in balance for optimum plant health, growth, and production of flower, fruit, and seed. Phosphorus, much like potassium, plays an important role in photosynthesis, and energy movement within the plant tissues.

Gardening picture

All life forms need phosphorus, and we humans get phosphorus (along with calcium) to build our bones and teeth from plants. Phosphorus affects all functions of plant growth and is required for the formation of carbohydrates/sugars. The work of phosphorus begins early in the development of plant tissues, as younger plants have a greater requirement for phosphorus. The plants need it to produce DNA and RNA along with proteins and enzymes for growth. Inadequate levels of phosphorus result in weak, immature-appearing or stunted plants, while the leaves on older plants sometimes appear a dark bluish-green.

Where does phosphorus come from?
"Phoshate is generally found in rock formations and ocean sediments as phosphate salts.” Phosphate salts released from weathered rocks usually dissolve in water and will be absorbed by plants.[1] The quantities of P in agricultural soils are generally small, although there is some from weathered rocks (like the mineral apatite, which is basically an insoluble tri-calcium phosphate[2]) found dissolved in soil water. With soils low in P, and plants requiring higher amounts, we often need to add P either in fertilizers, or by adding organic material. Actually, most P in soils is found in organic matter like dead and decaying plant and animal material, and microbial life.

 

 

 Image Image
 Phosphate Red Rock Phosphate

 

Healthy plant roots exude substances (organic acids) that breaks down calcium-phosphate compounds into a soluble form they can use. The mycorrhizal fungi growing in and around the plant roots need P themselves for growth and metabolic function. They release enzymes that contribute heavily to the breakdown of calcium-phosphorus compounds into soluble forms, both for themselves, and for the plants. For information about the relationship of calcium-phosphorus compounds in more detail: Plants need calcium, too

Plants take up phosphates only from the soil via their roots, and not through foliar applications. The better the soil tilth, nutritional balance and microbial activity, the greater the root growth, thus better/higher P uptake. There is seldom a toxic effect of too much phosphorus in the soil since soil solution levels are generally low. However, too much P fertilizer can bind with other nutrient elements and remain largely insoluble. Acidic soils have more available P than alkaline soils because the acid compounds help break down the phosphorus. In alkaline soils high in calcium, soluble P tends to bind with the calcium and become insoluble.

Other uses
I only mention this because ultimately it will affect the availability of phosphorus fertilizers. “Phosphorus compounds are widely used in explosives, nerve agents, friction matches, fireworks, pesticides, toothpaste and detergents. About 50 percent of the global phosphorus reserves are in the Arab nations. In 2007, at the current rate of consumption, the supply of phosphorus was estimated to run out in 345 years. However, scientists are now claiming that a peak usage will occur in 30 years and that ‘At current rates, reserves will be depleted in the next 50 to 100 years.’ "[3]

In the phosphorus cycle, the phosphorus which plants take up is recyclable in food consumption, green manure, decaying animals (including us), and run-off into our rivers and oceans, which ultimately becomes sediment (and then rocks) for millions of years, until weathering begins the cycle again. I can only assume that the P used in other commercial applications is eventually recycled also.

A soil test will tell you if you need P, and because plants use it up rather quickly, you probably do need it. Relatively soluble forms of P fertilizers are products like superten, triple superphosphates and DAP (di-ammonium phosphate) or their derivatives. Less soluble forms are hard rock phosphates.[2] Continuously adding animal manures and green (plant) manures to your soil also keeps the microbes busy breaking down materials to release phosphorus the plants need.

 

 Image
 Organic matter

 

Endnotes
[1] http://www.lenntech.com/phosphorus-cycle.htm
[2] http://www.ballance.co.nz/Education/Fertilisers_in_the_soil/Phosphorous_in_the_soil.asp?PageID=4064&ID=9&CatID=2056&Level=1
[3] http://en.wikipedia.org/wiki/Phosphorus

Photo Credits

Hands with Fertilizer (Thumbnail), iStockPhoto # 3340606, Used by Permission
Phosphorus, Creative Commons Attribution-ShareAlike 3.0 License.
Red Rock Phosphate, iStockPhoto # 5156004, © Jeffrey Heyden-Kaye, Used by Permission
Soil in Hand, iStockPhoto #498015, © Clayton Hansen, Used by Permission

 

Click for other Soil-Related Articles:

Plants need calcium, too 

Microbes in Soil 

Brix? Good soil!

Biochar: Carbon 

Weeds!!!

Understanding Soil Testing 

Rock Dust…DUH!!

Nitrogen 

Potassium   


  About Darius Van d'Rhys  
Darius Van d'RhysI have a 'growing my own food' obsession that comes from my overlapping interests in cooking, nutrition and gardening. I am also a "teacher", a writer, a builder… and a craftsperson and... and… and many other things, LOL. In fact, I guess I am a generalist, and a Seeker. I live in the southern Appalachian Mountains on a hillside with a creek in front, and drive a 15 year old truck I lovingly call “My Farmer’s Ferrari.” Editor's note: Darius passed away on March 19, 2014. Her readers will miss her greatly and we are thankful for her legacy of wonderful articles.

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Discussion about this article:
SubjectTopic StarterRepliesViewsLast Post
dry calcium bachngolf 4 9 Jan 7, 2010 9:23 PM
How did you guess my NY's resolution? Elphaba 2 18 Jan 14, 2009 8:36 PM
Nice phicks 1 14 Jan 14, 2009 7:19 PM
Thanks, Darius AlohaHoya 1 15 Jan 14, 2009 7:19 PM
Very much appreciated! mrs_colla 2 19 Jan 14, 2009 7:18 PM
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