What is the future for hydroponic fertilisers?

How are hydroponic fertilisers made and what reserves are there for their future availability?

Answer by Rick Donnan

Population predictions indicate that there will be a corresponding increase in the demand for food. In turn, because of a shrinking in good arable land, there will be an increasing demand for fertilisers to meet the food requirements.

Although, no doubt, there will continue to be an increase in the use of organic type fertilisers and an improvement in sustainable farming practices, the demand will continue to be mainly met by artificial inorganic fertilisers. So, my answer looks at how individual fertilisers are produced and what is the future for their supply.

In general, the reserves of fertilisers will last for a very long time, but they are limited, and cannot last forever. This is basically the same answer that is applied to the reserves of fossil fuels but the ‘peak’ for most mined fertilisers is still many years away. Fertilisers used for hydroponics consist of only a very small proportion of total fertiliser use. While this will continue to increase, it will remain minor compared to fertiliser use in the soil.

Potassium fertilisers
Potassium (chemical symbol K, from the Latin kalium) is mined as ‘potash’, which is usually a mixture of potassium and sodium chlorides. It sometimes includes significant amounts of potassium sulphate, which is a more valuable form. About 80% is mined as solids from deep mines. Another 6% is extracted by dissolving the product in underground solution mines, and the remainder by harvesting natural brines using solar evaporation.

There are potash reserves in a number of countries, but by far the highest are about one-third in Russia, and almost half in Canada (in Saskatchewan—Australians and Canadians might remember that several years ago Australian mining giant BHP attempted a hostile buy-out of this company and the sale was eventually blocked by the Canadian Government). Other countries with substantial deposits include Belarus, Brazil, Kazakhstan and Eritrea.

The various potassium fertilisers used in hydroponics, such as potassium nitrate, phosphate (MKP), and sulphate, are made by reacting potassium solution with the appropriate acid and then crystallising out the resulting salt.

Nitrogen fertilisers
Nitrogen (chemical symbol N) is produced by an industrial process, known as the Haber-Bosch process, which was developed in the early 1900s. This combines nitrogen from the air with hydrogen produced from natural gas to produce ammonia, from which other nitrogen products are then made. The natural gas also supplies the heat and energy needed for the process.

The process involves initially reacting natural gas, which is basically methane, with steam. This produces hydrogen and carbon dioxide, which also has the function of removing the oxygen from the air so it doesn’t interfere with the next step (equation: CH4 + 2 H2O > CO2 + 3 H2). Then the hydrogen and nitrogen are reacted to form ammonium gas (equation: N2 + 3 H2 > 2 NH3). This requires high temperature and pressure, and even at well over 400C (7520F) and 200 bar (2,200 psi), it needs the use of a catalyst so the reaction will proceed at an economical rate.

The ammonia can then be reacted to make a range of nitrogen-based fertilisers. The most common form is urea (for fertilising the soil, but not suitable for hydroponic use), which is formed by reacting the ammonia with the CO2 from the first stage. Nitric acid is made by reacting ammonia and oxygen, and in turn this becomes the base for all nitrate fertilisers.

Annual production of nitrogen fertilisers is over 450,000 tonnes, mainly urea, anhydrous ammonia and ammonium nitrate, consuming 3-5% of the world’s use of natural gas. Ammonia is also the base for many industrial chemicals, especially explosives. AMFO (ammonium nitrate/fuel oil) is the major explosive used in mining, hence why solid ammonium nitrate is basically banned for use in Australia unless you have a special security licence.

Nitric acid is used to make nitrate fertilisers, especially those used in hydroponics. These are calcium nitrate, potassium nitrate, and magnesium nitrate.

Nitrogen fertiliser reserves are currently directly tied to fossil fuel reserves, especially natural gas. As the atmosphere consists of 78% nitrogen, the nitrogen component of fertilisers is unlimited.

Sulphur fertilisers
Sulphur (chemical symbol S) is one of the most common elements in the Earth’s crust and occurs naturally in many parts of the world, especially in regions with volcanos. Known in the Bible as ‘brimstone’, it has been mined in elemental form since antiquity.

During the 20th century, mining was gradually replaced by sulphur produced as a by-product from oil refineries. Sulphur is a major contaminant of oil and natural gas and must be removed. The quantities produced from this now dominate the market and mining became uneconomic by the turn of the century.

The reserves of sulphur are therefore very high, because once the supply from oil and gas is depleted or too expensive, production can revert to mining.

To use sulphur as fertiliser and most other industrial uses it is first converted to sulphuric acid. This process involves burning the sulphur to provide sulphur dioxide (equation: S + O2 > SO2). To make sulphuric acid the sulphur dioxide must be converted to sulphur trioxide (equation: 2 SO2 + O2 > 2 SO3). Under normal conditions this is a very slow reaction, so to make it economically fast enough, the reaction takes place over a catalyst in what is known as the ‘contact’ process. The sulphur trioxide is then reacted with water to give sulphuric acid (equation: SO3 + H2O > H2 SO4).

(As an aside, the earlier process was the ‘chamber’ process, which took place in huge, lead-lined chambers. When I was a university student, I spent one long vacation working at a superphosphate works, which used this process. A scary place, because hot, strong acid flowed from the chamber along open lead gutters, which you had to step across: an OH&S nightmare.)

Phosporus fertilisers
Phosphorus (chemical symbol P) is mostly found as various forms of phosphate rock in shallow sedimentary layers, and it is usually strip mined in open-cut mines. About 10% of phosphate rock is igneous, that is, of volcanic origin. The original form of phosphorus used as fertiliser was guano (bird droppings), but any substantial deposits were exhausted by the end of the 19th century.

While it is possible to crush the better quality rock and apply it directly to the soil, this is now not much used. The original and simplest form of improved phosphate fertiliser is superphosphate. This is made by reacting sulphuric acid with the phosphate rock to form a mixture of calcium sulphate (known as gypsum) with calcium phosphate, which is effectively a slow release fertiliser. ‘Super’ was developed pre-war, but its use boomed after the war from the late 1940s onwards. It did a great job of enriching P depleted soils, especially in Australia where many soils were already naturally low in P.

If the initial process is taken further, phosphoric acid is produced and separated from the calcium sulphate/phosphate solid mix. The phosphoric acid is concentrated then reacted with other products to make higher grades of P fertilisers. So, reaction with ammonia gives Mono Ammonium Phosphate (MAP) or Di Ammonium Phosphate (DAP), and reaction with potassium salts gives Mono Potassium Phosphate (MKP). MAP and MKP are the main P fertilisers used in hydroponics, but there are a wider range of P fertilisers.

Phosphorus cannot be produced artificially, and currently is basically only produced from phosphate rock. This is mined in a number of countries, with about 70% of global production in the USA, China and Morocco (the biggest exporter), and 82% is used for fertiliser. There is considerable variation between estimates of reserves. These range from an estimate that reserves will be almost depleted by the end of this century, to one predicting many hundreds of years.

Ignoring supply and demand issues, the cost of rock phosphate is certain to continue to rise for the following reasons: new mines will be deeper with thicker overburden, environments will be more challenging (underground, offshore), ore will be lower grade and hence more expensive to process.

Calcium fertilisers
Calcium (chemical symbol Ca) is mainly used as a hydroponic nutrient. In the soil, calcium is usually adequately available, so that it is rarely added as a nutrient to the soil, other than occasionally with fertigation of very sandy soil. Where it is occasionally and increasingly used, is as finely ground lime or dolomite to raise the pH of acid soils.

In the case of hydroponics, the addition of calcium is essential, and this is done by using calcium nitrate. Calcium nitrate is produced by treating limestone (calcium carbonate) with nitric acid (equation: CaCO3 + 2 HNO3 > Ca(NO3)2 + CO2 + H2O). It is then neutralised with ammonia.

The greenhouse fertiliser grade contains some ammonium nitrate and water within the crystal, so its actual formula is approximately 5 Ca(NO3)2 + NH4NO3 + 10 H2O, typical analysis 14.4% N as nitrate, 1.1% N as ammonium, and 19% as Ca. This is an area to take care if you are calculating fertiliser formulations. You need to use the analyses from your fertiliser bags in your calculations. It is particularly important in this case because many books quote figures for pure calcium nitrate, say, taken from a chemistry handbook, and these figures are quite wrong.

In terms of reserves, calcium nitrate is an extremely low proportion of the overall use of both limestone and nitric acid. The main use of limestone is for the production of cement to make concrete. There are huge quantities of limestone available around the world, so its mining becomes a matter of quality and cost of production.

Magnesium fertilisers
Magnesium (chemical symbol Mg) is similar to calcium in that it is usually adequately available in most soils. It occurs naturally as Epsom Salts (magnesium sulphate). The common manufacturing method is to react dolomite (a mixture of calcium and magnesium carbonates) with sulphuric acid. This gives a solution of calcium and magnesium sulphates from which most of the calcium sulphate can be removed, because it has much lower solubility than the magnesium sulphate—the low solubility of calcium sulphate and calcium phosphate is the reason why concentrated hydroponic solutions must be split into two parts, to separate the calcium ions from the sulphate and phosphate ions.

Magnesium salts can also be obtained from seawater. It is the next highest component of sea water after sodium and chloride (common salt). It can be separated from evaporated brine by precipitating it as insoluble magnesium hydroxide. After filtering and washing, this can be reacted with sulphuric acid to give magnesium sulphate, or with nitric acid to give magnesium nitrate.

Magnesium sulphate is also used as a ‘health’ product, following on from its history from the spa at Epsom, England. Because it can be obtained from seawater, it is only limited by the availability of sulphuric acid (and cost).

Global fertiliser prices
When agricultural food prices rise, fertiliser demand also rises as farmers increase production. The fertiliser supply is restricted by the long lead times to develop new mines and production plants. If the demand rises faster than the supply, there can be a huge rise in fertiliser prices, as happened in 2008. This was exacerbated by high fertiliser manufacturing costs, largely due to high oil prices kicking up the cost of inputs and freight. Consequently, fertiliser prices, especially phosphate, surged more than three-fold very quickly.

In only a year, demand fell just as quickly as a result of the global financial crisis and falling food prices, plus farmers substantially reduced their fertiliser usage because of its high price. The result was that a year later fertiliser prices plunged back to about where they had been. In the longer term, prices will obviously remain vulnerable to the influence of supply and demand, plus the price of oil and especially natural gas.

Much fertiliser is wasted and P and N especially cause worrying water pollution. Obviously, open hydroponic systems should be recirculated. Nitrate is easily leached from the soil and phosphate removed by erosion. Both are wasted as sewage, plus human, animal and food waste. The best way to reduce fertiliser usage is to recover and reuse this waste. Progress is being made, but there is a long way to go.

PH&G May 2015 / Issue 155