Or, Fossil Fertilizer part II.

There's a story sometimes told of strange fossil bones brought back by Thomas Jefferson - as well as others - from Big Bone Lick in Kentucky. At the time there was no knowledge of dinosaurs - the word was coined decades later in 1842 - and it was not generally believed that creatures could go extinct. Jefferson, an amateur naturalist like many of his era, had dug fossils of large creatures in his home area and so entertained the notion that they might still roam the west. The bones brought back from Big Bone Lick in Kentucky were Mastodon. But, Jefferson's personal inventory was lost when one of Jefferson's slaves pounded the fossil bones to dust to use as bone meal soil amendment, just as was done with all bones produced on the farm.

At that time it had newly been learned that phosphorus promotes growth in plants and animals, and that pulverized bones were a good source. In addition to the on-farm production of bone meal there was some available from knackers. Whale bones were also an important source during the whaling period. The story of "petermen" scouring Europe for nitrates was repeated for phosphates:

By 1815, England was importing so many bones for bone meal that people on the Continent started complaining:

"England is robbing all other countries of their fertility. Already in her eagerness for bones, she has turned up the battlefields of Leipsic, and Waterloo, and of Crimea; already from the catacombs of Sicily she has carried away skeletons of many successive generations. Annually she removes from the shores of other countries to her own the manorial equivalent of three million and a half of men... Like a vampire she hangs from the neck of Europe."

A few decades later in the mid-nineteenth century a better source was found - rocks. It was first mined in the UK but a few years later in 1881 a source was discovered in Florida. Florida now accounts for 75% of US phosphate rock and 25% of the world's use. Those rocks were formed as sedimentary deposits in shallow seas. Some of the phosphate precipitated out of sea water, some was excrement and bones of sea creatures, and some was sediment carried into the coastal seas by rivers. The phosphate deposits are laced with fossil bones of long extinct creatures and the general area where it is mined in Florida is called Bone Valley. In a sense rock phosphate is still bone meal . . . fossil bone meal. The producers claim that they have about 300 years worth in Florida alone, but that the rock formation that they are mining extends all along the Atlantic sea coast. There is apparently no end in sight. As large as the US deposits may be they are dwarfed by others in the world. Morocco’s phosphate reserves are estimated to be nearly six times as large though the mining industry is not yet well developed.

Almost no one uses rock phosphate directly any more for fertilizer. The problem is that the phosphate in it is less than 5% plant available. Hauling all that rock dust from Florida, or wherever, to wherever it is needed, and then spreading tons of it on fields, for such a small yield is too expensive. The use of bone meal, already a comparatively small source, has further declined due to concerns about the spread of diseases such as BSE. I think that this is a specious concern except where the bone meal is used as a mineral supplement for animals. Both plants and animals needs phosphates, but leave the bone meal for the plants to be safe.

To make more of the phosphate in the rock plant available it needs to be converted to phosphoric acid. This is done most often by reacting the rock with sulfuric acid which is itself produced by controlled burning of sulfur. The heat produced powers the rest of the process and often provides excess power sold into the grid. The sulfuric acid combines with calcium in the rock to form calcium sulfate (gypsum), which is normally a soil amendment as well as a building material, but that depends on trace elements it might contain such as radium.

No matter the source it is orthophosphoric acid — the phosphate form that is taken up by plants - that matters. The phosphates produced by reaction of rock phosphate with sulfuric acid produce orthophosphates and also contain polyphosphates - a chain of orthophosphates joined together which dechain on contact with soil water. The most common forms of phosphate fertilizers are further reacted with ammonia to produce monoammonium phosphate (MAP) and diammonium phosphate (DAP). MAP is used on calcareous soils since it lowers PH, and DAP is used on acidic soils to raise PH.

Those who whinge about the use of fertilizers still need to add phosphorus to their soils, just as they need to add nitrogen and all of the 16 primary, secondary and micronutrients. Unless nothing is taken from the land - no crops or livestock are harvested - that which is taken must be replaced or the soil degrades. Leave nothing but footprints, take nothing but photos, is advice useful only to tourists.

All of the nutrients can be found in small quantities and low availability in manures, sludges and other urban wastes, but this won't support the level of production needed in the world. Perhaps worse in these energy conscious times, it takes such large quantities of manure that moving that much volume of material is often difficult to justify. When a grower is faced with a choice of applying 65 pounds of DAP per acre or 12,000 pounds of manure to get 30 pounds actual phosphate per acre he usually chooses the DAP to save a lot of fuel, machinery time and labor.

When the nutrient needs of the whole world are considered there just isn't that much manure available. Though there is too much manure in some places there's not enough to go around - assuming anyone could afford the energy to haul it. The only way it is affordable is in the production of premium crops for wealthy patrons. It won't feed the world. One commenter, Vaclav Smil, notes that:

“In 1900 the virtually fertilizer-free agriculture was able to sustain 1.625 billion people by a combination of extensive cultivation and organic farming on the total of about 850 million hectares of land,” . . . “The same combination of agronomic practices extended to today’s 1.5 billion hectares of cropland would feed about 2.9 billion people (or 3.2 billion after adding food from grazing and fisheries).

“This means that without nitrogen fertilizers no more than 53% of today’s population could be fed at a generally adequate per capita level of 1900 diets. If we were to provide today’s average per capita food supply with the 1900 level of agricultural productivity, we could feed only about 2.4 billion people, which is about 40 percent of today’s total.”

Though he was specifically talking about nitrogen the same is true for phosphorus. But there are problems with both nitrogen and phosphorus. Nitrogen leaches away or evaporates after soil bacteria break down plant available forms to gasses. Phosphorous can be immobilized in the soil, become fixed (changed to unavailable forms) and, in time, form highly insoluble compounds. Fixation in soils may result in only a fraction (10 to 15%) of the phosphorus applied in fertilizers and manures being used by plants in the year of application. To compensate phosphorous is sometimes applied at a higher rate than raw numbers would indicate was needed, which leads to a build up in the soil for fields in long service, and leaching of phosphates into ground and surface waters. This is true no matter what the source of nutrients. Manure is as prone to this problem as DAP.

There's no work around for phosphorous as some claim for nitrogen. Legumes and their nodulating bacteria can't suck it out of the air. Indeed, phosphorus is necessary to legume growth, so a shortage affects nitrogen too. It's a bit tricky to recognize phosphorus deficiency since the plants look OK. They will be stunted and thin but still deep green and so only really noticeable to the trained eye or by comparison to plants grown in fertile soil. They can even be very deep green and in extreme cases tinged by purple. Less often there is the sickly yellowish cast that most can recognize as nutrient deficiency, but it can be misinterpreted as nitrogen shortage.

There are things we can do to get more benefit from less phosphorus. Keeping soil PH closer to neutral, especially in acidic soils, improves soil chemistry making more phosphorus available to plants. Soil tilth - its texture and organic matter content - is also important for improved soil chemistry. Reduced tillage improves the soil environment for mycorrhizal fungi which are critical to transport of relatively immobile phosphorus, bartering it for plant sugars much as bacteria barter nitrogen. Fungi also barter nitrogen, but less so than phosphorus. And, they increase organic matter and so tilth. Yeah fungi! (don't get me started on fungi). Balanced fertility - the proper timing and amounts of all 16 soil nutrients - achieves more with less.

And we need to do more with less. Smil fretted about feeding a world of 6 billion but there will soon be half again as many. To feed 9 billion people it will take enlightened agronomic practices that make effective use of fertilizer. All of the knowledge and skill we have, rather than some ideologically motivated subset, must be applied to the problem. We can't afford to skimp and we can't afford waste.

Update:

Also see:

Phosphorus Transport and Dirt Glue for earlier discussions of phosphorus in soils.