We have to feed plants so that they can feed us. That conclusion was arrived at by virtually every early agricultural civilization be it in China, Mesopotamia, Europe or the Americas. Their key observation was that crops grow better in fields where animals leave their droppings. Yields can also be increased with the use of human 鈥渘ight soil,鈥� as the ancient Chinese discovered. The Greeks and Romans also spread manure on fields but why this works would not be revealed until the 19th聽century when German chemist Justus von Liebig demonstrated that plants require nitrogen, phosphorus and potassium for growth. These nutrients are sourced from soil with the result that the soil eventually gets depleted. Manure can replenish them, acting as a 鈥渇ertilizer,鈥� a word that derives from the Latin 鈥渇ertilis鈥� for 鈥渇ruitful鈥� or 鈥減roductive.鈥�
Liebig went on to show that plant growth is limited by the essential nutrient in shortest supply and came up with his famous analogy in which he compared nutrients to the staves of a wooden barrel. Clearly, the amount of water the barrel can hold is determined by the height of the shortest stave. Adding more nitrogen to soil, for example, cannot compensate for a phosphorus or potassium deficiency.
Not all of a crop鈥檚 needs can be met by chemicals in the soil. In a classic experiment in the 17th聽century, Belgian physician Jan Baptist van Helmont planted a willow tree in a pot and weighed the amount of soil used. After the tree had grown for five years, he weighed it as well as the soil in the pot. He found that the tree gained much more weight than the soil had lost. All he had added during the growing period was water and he therefore concluded that this must have been the source of the added weight. He was wrong, although not totally. Water does furnish hydrogen and some of the oxygen a plant needs to make its tissues but most of the increase in weight van Helmont noted comes from carbon dioxide absorbed from the air. That was not understood until another hundred years had passed and the mystery of 鈥減hotosynthesis鈥� was unravelled.
There is no shortage of carbon dioxide in the air so that is never the limiting nutrient. However, supplying sufficient phosphorus, potassium and nitrogen can be a problem. By the 19th聽century, the amount of manure available to meet the fertilizer needs of the growing acreage devoted to food crops was insufficient. Fortunately, by that time chemists had developed methods to analyze substances for the presence of a number of common elements. Bones were found to be rich in phosphorus and ground bones were spread on fields. An even more important discovery was that seabird droppings, known as 鈥済uano,鈥� widely found on islands off the coast of South America are rich in phosphorus and nitrogen. By the middle 1800s, huge amounts of guano were being shipped to Europe.
Then in 1842, Sir John Bennet Lawes in England discovered that treating insoluble phosphate rock, mostly fluoroapatite [Ca5(PO4)3F], with sulfuric acid produces 鈥渟uperphosphate鈥� [Ca(H2PO4)2] which is water soluble and is an excellent source of phosphorus for crops. Potash mines produced ample amounts of potassium chloride but supplying nitrogen was a problem especially when supplies of guano could no longer meet demand. Curiously, about 80% of air is nitrogen, but it turns out that plants cannot use nitrogen from the air, they require a soluble form from the soil.
That problem was solved in1909 by German chemist Fritz Haber who discovered that under high pressure and temperature and with the aid of an iron catalyst, nitrogen from the air can combine with hydrogen to yield ammonia. This in turn can react with carbon dioxide to form urea, the most widely used nitrogen fertilizer in the world. Along with potassium from potash and phosphorus from phosphate rock, urea fueled the 鈥淕reen Revolution鈥� that improved crop yields dramatically and saved millions of lives from starvation.
While we cannot feed the growing population without fertilizers, there are some issues with their production. The Haber process requires the input of energy to achieve the high temperatures and pressures needed. Then there is the question of the source of hydrogen which is usually produced by reacting steam with natural gas, which is mostly methane. This is not an environmentally friendly reaction since it also releases carbon dioxide, a greenhouse gas.
It is possible to make 鈥済reen ammonia鈥� without using natural gas as a source of hydrogen. Water can be split into hydrogen and oxygen by passing electricity through it, but to be 鈥済reen鈥� that electricity has to come from solar energy, wind turbines or waterpower, not from burning fossil fuels. The 鈥済reen鈥� hydrogen is then combined with nitrogen from the air to yield ammonia that is converted into urea.
The Iran War has introduced another problem. Qatar is the world鈥檚 leading producer of urea and the Persian states also supply a great deal of the natural gas needed to produce hydrogen. Closure of the Strait of Hormuz means less urea being shipped to the West which in turn means a rise in the cost of food.
It is not only the fertilizer industry that is hit by the war. Pesticides and plastics are also produced from petroleum, as of course are gasoline and heating oil. Any process that requires an energy input is affected and will probably be for a long time. That includes condoms. Malaysia鈥檚 Kerex Company is the world鈥檚 largest condom producer churning out some 5 billion a year. About 85% of these are made from natural latex but converting this into rubber involves heating with sulfur in a process called 鈥渧ulcanization鈥� that requires a lot of energy. The condoms not made from latex use polyisoprene or polyurethane that are produced from components in petroleum. Condom prices will go up which may lead to less use, which in turn leads to more mouths to feed and more demand for oil and fertilizer and probably more wars.
