Phosphorus matters: soil erosion & contamination

Part One: Closing the Phosphorus Cycle

Phosphate mine on Nauru island.
Currently part of it is reforested.
Photo: Jon Harald Søby

It might sound ridiculous, but for every container of bananas, coffee, tea or cocoa imported, we should send back a shipment of a fluffy, earth-like smelling compost. Why is that? With each container of food we import nutrients taken up by plants from the soil. We import calcium, potassium, magnesium, boron, iron, zinc, molybdenum, copper and many others. One of the essential elements imported in food is phosphorus. For every ton of bananas we import 0.3 kg of phosphorus, for every ton of cocoa it’s 5 kg and for ton of coffee it’s 3.3 kg of phosphorus. Tea is a bit more complicated, because the amount of phosphorus depends on the origin of tea – for example in 1 ton of tea leaves harvested in Sri Lanka there are some 3.5 kg of phosphorus, while tea from South India contains 6.6 kg of phosphorus (1).

Each year some 13.5 million tons of bananas alone are exported around the world (2), containing 4,000,000 kg of elemental phosphorus up taken by the plants from tropical soils. And most of this phosphorus never comes back to the soil it was removed from. Yes, but can’t the farmers replace the nutrients lost using fertilizers? That’s what the fertilizers are used for, are they not? Sure they can. Farmers can buy a bag of ground phosphate rocks or guano (bird or bat droppings) or even a bag of artificial fertilizer such as superphosphate if they don’t farm organically. No problem. They can replace every kilogram of phosphorus taken from the soil by plants and sent overseas with their produce.

Phosphorus Molecules

So, why should we send compost back on ships? This would add extra cost to the imported food and make it much more expensive! We should start closing nutrients cycle soon, because the world reserves of phosphate rocks, which are used for the production of phosphate fertilizers, are declining. They can be depleted even this century (3).

The problem with the lack of phosphate fertilizers does not start, however, when all phosphate rock reserves are gone. It starts as soon as the demand for phosphate fertilizers exceeds the supply of phosphate rocks available for export, meaning: farmers living in countries that do not have a local source of phosphate rocks would like to buy phosphate fertilizers, but there are not enough bags for everyone. And this situation may appear within the next 10-20 years.

This short timeframe is based upon the assumption that the demand for phosphate fertilizers will continue to grow and that within 10-20 years US reserves of phosphate rocks available for mining will be considerably depleted and USA will have to rely on imported phosphorus. It is unclear whether the phosphate exporting countries will be able to respond adequately to keep up with the rising demand by opening new mines or increasing production in the existing ones, which otherwise could lead to lack of sufficient amount of phosphate fertilizers on the market. A 50% rise in the US imports would require 50% rise of present world phosphate rock exports. A similar situation may exist in countries other than USA, but it was not taken into consideration due to lack of sufficient data. Demand for phosphate fertilizers in the USA may drop, however, owing to fall of agricultural production caused by droughts, depletion of water resources or by other climate related events. This could slow down domestic production of phosphate rocks and conserve these resources for a longer period of time.

What plants need Phosphorus for?

White sweetclover. Photo: Kristian Peters

Phosphorus is one of the key mineral nutrients that are necessary for plants growth. Phosphorus stimulates root growth, flowers blooming and seed development. It is an essential component of DNA, RNA, cell membranes, sugars and carbohydrates (4). Without phosphorus plants just don’t grow and there is no substitute for it. Although in many soils there are large reserves of phosphorus, it is often present in the form that cannot be used by plants (such as insoluble calcium or aluminum phosphate salts). Some plants, however, like white or yellow sweet clover for example (5), can mobilize phosphate by secreting organic acids (when harvested they can be used as a green manure with high phosphorus content), but far more efficient for this job are mycorrhizal fungi and microbes that secrete enzymes, various acids and chelating agents that turn organic and inorganic phosphate into a solution that can be taken up by plants (6). Nevertheless, when the content of phosphorus in the soil is low, all that farmer can do is to bring in some kind of phosphate fertilizer.

How much phosphate rocks is available for export?

Worldwide approximately 30 millions tons of phosphate rocks are exported every year, mainly from Africa (62.8% in 2006) (7). It sounds like a lot, but it is less than is needed for the consumption of a single country – the USA – the largest consumer, producer and supplier of phosphate fertilizers in the world. In 2006 the USA consumed 32.6 millions tons of phosphate rocks (8). Fortunately, USA is currently almost self-sufficient in production of phosphate rocks. In 2007 US imports accounted only for 2.8 millions ton of phosphate rocks (8.6%) and 99% of it came from just one origin – Morocco.

Phosphate rocks mine in Togo.
Photo: Alexandra Pugachevskaya

However, the reserves of phosphate rocks in USA are limited. In 2007 there were only about 1,200 millions tons left (9). As soon as USA runs out of its phosphorus there will be a huge demand for the phosphate rocks. When might this happen? If the consumption in the USA continues to grow, the US domestic reserves could be gone in 25 years (10). At the current rate of production this could be in around 40 years. Most of the phosphate rocks in USA are mined in Florida and according to Stephen Jasinski from the U.S. Geological Survey “production in Florida could begin to drop in about 5 years or imports will be needed if the new mines are not opened (11).”

Demand for fertilizers is growing at the rate of 2.8% per year (12). It is expected to continue to grow, because fertilizers are needed to feed the increasing human population and to satisfy the need for biofuels. The acreage of industrial farms around the world which rely on artificial fertilizers may still increase in the years to come (e.g. in Russia, Brazil or even Madagascar) and in consequence the overall demand for phosphate fertilizers will rise. Certified organic farms can also use phosphate rocks (in unprocessed form), when phosphorus is deficient in the soil.

There are many countries like India, Australia, Poland and most of the Western European countries which are completely dependent on imports of phosphate rocks for fertilizing soils and growing food. And we import it mainly from Morocco as well. Without phosphate fertilizers yields of wheat, maize, tomatoes, strawberries, potatoes and many other crops will drop and eventually they could even fail. In Poland we have huge reserves of phosphate rocks. The problem is that the content of elemental phosphate in these rocks is low, they are located under villages, forests or farmlands or there is too much water in the mines to continue extraction.

However, if we manage to close the phosphorus cycle, there’s no need to worry about phosphate rock reserves. What we have mined so far can circulate from farm to table and back again, without depleting the soils. Let’s have a closer look where the phosphorus is leaking now.

Where does the phosphorus go?

In tropical climate phosphorus can be lost as soon as the farmer burns the rainforest to clear the site. Most tropical soils are poor in nutrients, and phosphorus is stored not in the soil, but in the vegetation. When rainforest is burnt phosphorus is left in the remaining ashes, but these ashes can be washed away by rains very quickly. There may be some old branches or unburnt leaves left on the ground and microbes can feed on them releasing phosphorus to the crops for some two years. But later on, when there are no more sources of phosphorus for the microbes to feed on and to release for plants, the land becomes infertile. And the farmer? If he cannot afford to buy commercial fertilizers he burns down another patch of the rainforest or he is forced to move to the city. There are more than 300 million slash-and-burn farmers worldwide, each one clearing about a hectare of forest a year (13).

On many farms, however, fertilizers are applied and farmers continue to grow crops. Some minimal amounts of phosphorus may leach from farm to groundwater, especially when artificial soluble fertilizer is used (such as superphosphate) (14). Most phosphorus losses occur through surface soil erosion, when soil is washed away by strong rain, or through harvesting of plants. Runoff of the nutrient rich water from the fields into the streams, lakes and oceans often causes explosion of the algae population and can lead to depletion of oxygen, seriously affecting aquatic animals and even coral reefs.

And what was the former one? Harvesting of plants? That’s right. With each apple, carrot, cucumber, coffee, cherry or watermelon a small bit of phosphorus is taken away from the soil. It can be eaten by the farmer and his family or loaded on truck and transported to the market. It can be also shipped overseas to the foreign supermarkets. So long nutrients! Have a good time in Italy or France! Please come back… one day.

Phosphate processing plant in
Soda Springs, USA, operated by Monsanto.
Source: The Center for Land Use Interpretation

Before food reaches the table many crops are processed and there are various residues left which contain phosphorus, e.g. orange peels or rice husks. They are either composted or sent to landfill. Then, finally, the consumer prepares a meal from the food that farmers harvested, and then leftovers with the precious phosphorus are thrown into the garbage or on the compost pile. The meal is eaten and out of the pizzas, spaghettis and apple pies only less than 1% of phosphorus is absorbed by our bodies (15) and remaining 99% is, in industrialized countries, flushed down the toilet. The content goes to a wastewater treatment plant. Treated biosolids from the treatment plants are reused as soil amendments or sent to the landfills. Part of the phosphorus from the wastewater treatment plant is discharged with treated water into the rivers or the sea.

Not all phosphate rocks are used for production of fertilizers. Around 5% are used as animal feed supplements and another 5% for industrial applications, e.g. for the manufacture of detergents. Some of us (like the author) are allergic to phosphates in soaps or washing powders and are a living proof that we do not need to use them at all. There are plenty of natural soaps and washing powders without phosphates we can buy or we can make our own.

Phosphate is used also for production of glyphosate, a herbicide which is known under a trade name Roundup. The manufacturer of Roundup, Monsanto, owns even a whole phosphate mine and rock processing plant in Idaho, USA. Luckily, organic gardeners don’t have to spray any of these. A much better idea would be to use the remaining phosphate rock reserves to restore degraded lands, rather than to produce herbicides or detergents.

Closing the nutrients cycle

Ideally the same amount of nutrients that left the farm should come back to it. To achieve this goal we should compost or ferment all residues from farms, food processing plants and households and make them available for farmers. And yes, we need to compost urine and feces as well. There are many types of compost toilets, including the simplest sawdust toilet to the commercial types with electric fans. If handled properly they don’t smell badly and the final product of the compost toilet is just a plain ordinary compost. It can be collected in the city in special containers, standing along the curb near the containers for recycling glass and plastics. Joseph Jenkins’ “Humanure Handbook” is a great resource on the subject.

All organic waste can be collected as a part of a municipality recycling program and leftovers from the kitchen can be picked up weekly from the separate curbside container. For backyard gardeners and farmers who eat their own food there are many methods of composting to choose from – buckets, triangle cages, compost tumblers, worm composting, loose heaps or classic wooden containers. There are even composters which can be kept directly in the kitchen without any suspicious smells.

It seems also a good idea to extract carbon and hydrogen from the food residues in the form of biogas which is primarily methane (CH4). It can be used for cooking, heating, electricity generation or for powering vehicles. The exciting thing about biogas is that we don’t waste any of the minerals from the organic matter – carbon is taken by plants from the air in the form of carbon dioxide and hydrogen comes from water. After fermentation process in a biodigester the organic matter is still perfectly useful as a fertilizer.

If the resources of phosphate rocks become depleted this organic waste recycling program will be crucial for farmers. They will be able to buy or receive finished compost according to the amount of food they sold. It may sound absurd, but the content of phosphorus or other nutrients in crops may eventually be counted in the future, so that we can determine how much compost the farmer should receive. Ideally local food should be involved in this scheme to minimize transport needs. And what about the food from overseas farms like coffee or tea? Well, things get much more complicated here. Theoretically, we could exchange nutrients in the form of food, so that for every kilogram of coffee would send back wheat or barley with the equal content of phosphorus. What farmers can do now is to bring compost from the cities, where people eat imported food. The other option is sending compost back. Hmm… Wouldn’t it be just perfect to have a village scale economy where all nutrients would circulate without cars, trucks, cargo ships and complex municipality programs?

Growing food security

Trees in bloom in the Hunza
Valley. Photo: bongo vongo

In places like the Hunza Valley (currently northern Pakistan) and many others around the world, people have grown food in one place for hundreds of years without depleting the soil. As Rob Hopkins writes in his Transition Handbook about the Hunza Valley:

Here was a society which lived within its limits and had evolved a dazzlingly sophisticated yet simple way of doing so. All the waste, including human waste, was carefully composted and returned to the land. The terraces which had been built into the mountainsides over centuries were irrigated through a network of channels that brought mineral-rich water from the glacier above down to the fields with astonishing precision.

Apricot trees were everywhere, as well as cherry, apple, almond and other fruit and nut trees. Around and beneath the trees grew potatoes, barley, wheat and other vegetables. The fields were orderly but not regimented. Plants grew in small blocks, rather than in huge monocultures. Being on the side of a mountain, I invariably had to walk up and down hills a great deal, and soon began to feel some of the fitness for which the people of Hunza are famed. The paths were lined with dry stone walls, and were designed for people and animals, not for cars.

People always seemed to have time to stop and talk to each other and spend time with the children who ran barefoot and dusty through the fields. Apricots were harvested and spread out to dry on the rooftops of the houses, a dazzling sight in the bright mountain sun. Buildings were built from locally-made mud bricks, warm in the winter and cool in the summer. And there was always the majestic splendour of the mountains towering above. Hunza is quite simply the most beautiful, tranquil, happy and abundant place I have ever visited, before or since (16).

Rakaposhi mountain near the
town of Gilgit, Hunza Valley.
Photo: bongo vongo