This will be the next installment of what will be an ongoing series on essential plant nutrients.  Phosphorus is considered to be the second most important nutrient but is one of the more problematic ones when it comes to availability.


Phosphorus has a Wide Array of Biological Functions in Plants

It is part of the structural component of nucleic acids, RNA and DNA. It is also a part of phospholipid layer of cell membranes and lastly it takes part in energy reactions to power the cell.  Since the cell density is high in flowering sites more phosphorus is used at this part of the life cycle.


Shoot apical meristem showing very high cell density.


There is no Source of Soluble Phosphorus Fertilizer in Organic Agriculture

There are currently 3 main sources of phosphorus that are derived from natural sources that can be certified organic: bat guano, rock phosphate, and bone meal.  Bat guano breaks down fairly quickly but isn't considered to be soluble, it's also a very limited resource making it quite expensive.  Rock phosphate and bone meal are both principally calcium phosphate which is extremely insoluble making the nutrients unavailable to the plant until they can be broken down in the soil.

*Did You Know*

There once was a war fought over bat guano (manure).  Islands off the Pacific coast of South America were invaded by Spain in 1864 as at the time they were the worlds principle source of phosphate.  The islands belonged to Peru and they were joined by Chile, Bolivia, and Ecuador in retrieving them. 



It is a Limited Resource

While most people have heard of peak oil, peak phosphorus is less known.  It is mined as rock phosphate and the majority of production is converted into water soluble fertilizer.  The amount in the ground is finite and is expected to be depleted in 50-100 years.  Research is underway to better recycle phosphorus in use but the the technology is in its infancy.  Running out of phosphorus would have a huge impact on food security. 


phosphate mining in rocky valley


Soils can Contain Lots of Phosphorus and Plants can Still be Deficient

As discussed in the previous posts Nutrient Lockout for Dummies Part 1 and Part 2 phosphorus has problems with solubility in the presence of many metallic ions, so it can be present in significant amount while not being in the soluble form necessary for plants.  When a crop is fertilized at the beginning of a season only 10-20% of the phosphorus will be used by the plant before becoming locked up in the soil.  Like nitrogen there is also a phosphorus cycle involving soil microbes which breakdown the insoluble forms into ones which are absorbable by plants.


plant in soil outside


Phosphate Fertilizers Have a Significant Effect on pH

The 2 most common phosphate fertilizers are monoammonium phosphate and monopotassium phosphate.  These both have 2 extra hydrogens attached to them which means that they are acidic (the monoammonium much more so).  Adding another ammonium group to the molecule will result in diammonium phosphate, and because there's less hydrogens the pH will be more neutral.  While it is possible to have di and tripotassium phosphate and triammonium phosphate, we don't see these in the fertilizer industry.  These additional groups added would result in salts in the alkaline pH range. 


Phosphorus Can be a Pollutant

As discussed in the previous post 8 Facts about Nitrogen in Plant Nutrition fertilizer can be washed out of the soil and wind up in water bodies in a process called eutrophication.  While the end result is much the same as nitrogen in that more algae growth means oxygen is used up suffocating the ecosystem.  How the nutrients act is quite different however. Nitrogen will eventually return to the atmosphere while phosphorus will persist in the environment.  Nitrogen will quickly be washed into a water body whereas phosphorus will slowly be washed out of the soil over time.


As you can see, while phosphorus is very important to life there are several issues which complicate its usage as a plant nutrient.  Hopefully in the coming decades technology can better manage this vital resource.