The depletion of world phosphate reserves will impact on the production of biofuels, including the potential wide-scale generation of diesel from algae. The world population has risen to its present number of 6.7 billion in consequence of cheap fertilizers, pesticides and energy sources, particularly oil. Almost all modern farming has been engineered to depend on phosphate fertilizers, and those made from natural gas, e.g. ammonium nitrate, and on oil to run tractors etc. and to distribute the final produce. Worldwide production of phosphate has now peaked (in the US the peak came in the late 1980's), which lends fears as to how much food the world will be able to grow in the future, against a rising number of mouths to feed . Consensus of analytical opinion is that we are close to the peak in world oil production too.
One proposed solution to the latter problem is to substitute oil-based fuels by biofuels, although this is not as straightforward as is often presented. In addition to the simple fact that growing fuel-crops must inevitably compete for limited arable land on which to grow food-crops, there are vital differences in the properties of biofuels, e.g. biodiesel and bioethanol, from conventional hydrocarbon fuels such as petrol and diesel, which will necessitate the adaptation of engine-designs to use them, for example in regard to viscosity at low temperatures, e.g. in planes flying in the frigidity of the troposphere. Raw ethanol needs to be burned in a specially adapted engine to recover more of its energy in terms of tank to wheels miles, otherwise it could deliver only about 70% of the "kick" of petrol, pound for pound.
In order to obviate the competition between fuel and food crops, it has been proposed to grow algae from which to make biodiesel. Some strains of algae can produce 50% of their weight of oil, which is transesterified into biodiesel in the same way that plant oils are. Compared to e.g. soy which might yield a tonne of diesel per hectare, or 8 tonnes from palm-oil, in excess of 100 tonnes (I shall assume 125 tonnes) per hectare is thought possible from algae, grown in ponds of equivalent area. Since the ponds can in principle be placed anywhere, there is no need to use arable land for them. Some algae grow well on salt-water too which avoids diverting increasingly precious freshwater from normal uses, as is the case for growing crops which require enormous quantities of freshwater.
The algae route sounds almost too good to be true. Having set-up these ponds, albeit on a large scale, i.e. they would need an area of 3,200 km^2 to produce 40 million tonnes of diesel, which is enough to match the UK's transportation demand for fuel if all vehicles were run on diesel-engines [the latter are more efficient in terms of tank to wheels miles by about 40% than petrol-fuelled spark-ignition engines], one could ideally leave them to absorb CO2 from the atmosphere (thus simultaneously solving another little problem) by photosynthesis, driven only by the flux of natural sunlight. The premise is basically true; however, for algae to grow, vital nutrients are also required, as a simple elemental analysis of dried algae will confirm. Phosphorus, though present in under 1% of that total mass, is one such vital ingredient, without which algal growth is negligible. I have used two different methods of calculation to estimate how much phosphate would be needed to grow enough algae, first to fuel the UK and then to fuel the world:
(1) I have taken as illustrative the analysis of dried Chlorella , which contains 895 mg of elemental phosphorus per 100 g of algae.
UK Case: To make 40 million tonnes of diesel would require 80 million tonnes of algae (assuming that 50% of it is oil and this can be converted 100% to diesel).
The amount of "phosphate" in the algae is 0.895 x (95/31) = 2.74 %. (MW PO4(3-) is 95, that of P = 31).
Hence that much algae would contain: 80 million x 0.0274 = 2.19 million tonnes of phosphate.
World Case: The world gets through 30 billion barrels of oil a year, of which 70% is used for transportation (assumed). Since 1 tonne of oil is contained in 7.3 barrels, this equals 30 x 10^9/7.3 = 4.1 x 10^9 tonnes and 70% of that = 2.88 x 10^9 tonnes of oil for transportation.
So this would need twice that mass of algae = 5.76 x 10^9 tonnes of it, containing:
5.76 x 10^9 x 0.0274 = 158 million tonnes of phosphate.
(2) To provide an independent estimate of these figures, I note that growth of this algae is efficient in a medium containing a concentration of 0.03 - 0.06% phosphorus; since I am not trying to be alarmist, I shall use the lower part of the range, i.e 0.03% P. "Ponds" for growing algae vary in depth from around 0.6 - 1.5 metres and so I shall assume a depth of 1 m for simplicity.
UK Case: I worked-out previously  that producing 40 million tonnes of oil (assumed equal to the final amount of diesel, to simplify the illustration) would need a pond/tank area of 3,200 km^2. 3,200 km^2 = 320,000 ha and at a depth of 1m, this amounts to a volume of: 320,000 x (1 x 10^4 m^2/ha) x 1m = 3.2 x 10^9 m^3.
A concentration of 0.03 % P = 0.092% phosphate, and so each m^3 (1 m^3 weighs 1 tonne) of volume contains 0.092/100 = 9.2 x 10^-4 tonnes (920 grams) of phosphate. Therefore, we need:
3.2 x 10^9 x 9.2 x 10^-4 = 2.94 million tonnes of phosphate, which is in reasonable accord with the amount of phosphate taken-up by the algae (2.19 million tonnes), as deduced above.
World Case: The whole world needs 2.88 x 10^9 tonnes of oil, which would take an area of 2.88 x 10^9/125 t/ha = 2.30 x 10^7 ha of land to produce it.
2.3 x 10^7 ha x (10^4 m^2/ha) = 2.3 x 10^11 m^2 and at a pond depth of 1 m they would occupy a volume = 2.30 x 10^11 m^3. Assuming a density of 1 tonne = 1 m^3, and a concentration of PO4(3-) = 0.092%, we need:
2.30 x 10^11 x 0.092/100 = 2.13 x 10^8 tonnes of phosphate, i.e. 213 million tonnes.
This is also in reasonable accord with the figure deduced from the mass of algae accepting that not all of the P would be withdrawn from solution during the algal growth. Indeed, the ratio of algal phosphate to that present originally in the culture medium (i.e. 158/213) suggests that 74% of it is absorbed by the algae.
Now, world phosphate production amounts to around 140 million tonnes (noting that we need 213 million tonnes to grow all the algae), and food production is already being thought compromised by phosphate resource depletion. The US produces less than 40 million tonnes of phosphate annually, but would require enough to produce around 25% of the world's total algal diesel, in accord with its current "share" of world petroleum-based fuel, or 53 million tonnes of phosphate. Hence, for the US, security of fuel supply could not be met by algae-to-diesel production using even all its indigenous phosphate rock output, and imports (of phosphate) are still needed.
The world total of phosphate is reckoned at 8,000 million tonnes and that in the US at 2,850 million tonnes (by a Hubbert Linearization analysis). However, as is true of all resources, what matters is the rate at which they can be produced.
I remain optimistic over algal diesel, but clearly if it is to be implemented on a serious scale its phosphorus has to come from elsewhere than phosphate rock mineral. There are regions of the sea that are relatively high in phosphates and could in principle be concentrated to the desired amount to grow algae, especially as salinity is not necessarily a problem. Recycling phosphorus from manure and other kinds of plant and animal waste appears to be the only means to maintain agriculture at its present level, and certainly if its activities will be increased to include growing algae. In principle too, the phosphorus content of the algal-waste left after the oil-extraction process could be recycled into growing the next batch of algae. These are all likely to be energy-intensive processes, however, requiring "fuel" of some kind, in their own right.
It is salutary that there remains a competition between growing crops (algae) for fuel and those for food, even if not directly in terms of land, for the fertilizers that both depend upon. This illustrates for me the complex and interconnected nature of, indeed Nature, and that like any stressed chain, will ultimately converge its forces onto the weakest link in the "it takes energy to extract energy" sequence.
A Hubbert analysis of human population growth indicates that rather than rising to the putative "9 billion by 2050" scenario, it will instead peak around the year 2025 at 7.3 billion, and then fall . It is probably significant too that that population growth curve fits very closely both with that for world phosphate production and another for world oil production . It seems to me highly indicative that it is the decline in resources that will underpin our demise in numbers as is true of any species: from a colony of human beings growing on the Earth, to a colony of bacteria growing on agar nutrient in a Petri-dish.
 "Biofuels and the fertilizer problem,", By Tom Phillpott. http://www.energybulletin.net/print.php?id=40300
 "Chlorella" - Wikipedia.
 "Biofuel from algae - Salvation from Peak Oil? http://ergobalance.blogspot.com/2007/03/biofuel-from-algae-salvation-from-peak.html