Phosphorus Matters II: Keeping Phosphorus on Farms
Lupines. Photo: Carol Mitchell/Flickr
“Next to clean water, phosphorus will be one the inexorable limits to human occupancy on this planet” wrote Bill Mollison in Permaculture: A Designers’ Manual more than 20 years ago (1). It is that important that we design phosphorus recycling into our food systems. Phosphorus is an essential element for growing crops and no porridge, chocolate bar or cherry jam can be made without it.
Mobilizing phosphorus present in the soil
In many soils phosphorus is naturally present in sufficient amounts, however, it may be chemically locked up and not available for plants. Most of agricultural soils in Western Europe and North America are oversupplied with huge amounts of superphosphate fertilizers, which results in binding phosphorus up with other elements so it ends up unused in the soil. In consequence, the concentration of phosphorus may be as high as 750 ppm, while only 45 ppm is necessary for growing grains (2). To determine whether you have a sufficient level of phosphorus in your soil, the surest way is to make a soil test. If the amount of phosphorus seems to be okay, but your plants show signs of phosphorus deficiency (purplish leaves, stunted stems), you may need help from a specially skilled team of phosphorus extractors – fungi. Fungi are decay experts in soils. The enzymes that they secrete allow them to break up lignin, cellulose, chitin shells of insects and bones of animals, which are too difficult to digest for bacteria. A single teaspoon of a healthy soil may contain several meters of fungal hyphae, invisible to the naked eye (3).
The tips of certain species of fungi have an extremely significant function. The strong acids they produce allow them to literally dissolve rocks and extract phosphorus from them. These fungi can form a mutually beneficial relationship with plants roots and can transport phosphorus to these plants. They are called mycorrhizal fungi.
Mycorrhizal fungi can extend the surface area of tree roots by 700 to 1000 times (4). They can harvest phosphates at great distances, many meters down and away from the plant and they bring it back through the fungal net, which is called plasmic streaming. Phosphorus is brought to a tree in exchange for sugars created by plants, as fungi don’t have the chlorophyll and the ability to photosynthesize.
Seedlings of trees, shrubs and perennials can be inoculated with mycorrhizal fungi while you grow them in the nursery. Make sure you get the right kind of fungal spores for your plants. You can inoculate roots of existing trees and shrubs by digging holes in a root zone and applying spores of mycorrhizal fungi near the roots. Seeds of annuals and vegetables can be mixed with inoculum as well, however, plants from the cabbage family (Brasicaceae), beets and spinach do not form mycorrhizal associations at all. Instead of buying inoculum in a shop, you can also experiment with making your own mycorrhizal inoculum.
The optimum range for phosphorus uptake by plants is pH 6.0 – 7.5, and on either side of the pH scale phosphorus becomes immobile. A conventional approach would be to adjust pH by adding sulfur in alkaline soils or lime in acidic soils. It can be quite expensive on a larger scale. But suppose you would like to grow an acid soil loving plant, such as Northern highbush blueberry, then what? The optimum pH range for this tasty fruit is as little as 3.5 – 4.8 pH, and it can fail completely when pH is higher. Since phosphorus is immobile at this low pH level, how can this plant grow at all? Well, it can receive phosphorus through partnerships with certain species of mycorrhizal fungi which do well in acid soils and don’t mind low pH when extracting phosphorus.
Mycorrhiza and herbicides
Using herbicides when mycorrhizal fungi are present in the soil may bring unexpected consequences. Fungi can transport more than just nutrients, but also various pesticides. A study in China revealed that mycorrhizal fungi transported a toxic herbicide atrazine to the roots of maize, which was hosting it (5). A similar thing could have happened on this pasture in Australia. In the foreground: pasture with good management (compost, compost tea and no herbicide), background: after years of using pesticides, trees are left dead or dying (6).
You could also try to adjust the pH by increasing fungal or bacterial domination in topsoil. You can apply brown organic mulches, such as woodchips and shredded branches, to support fungi and to lower the pH. Or, apply fresh green mulches and aerated compost teas to support bacterial growth and raise the pH slightly above 7. The reason for this is that bacterial slime is alkaline and acids secreted by fungi are, well, acidic and they lower soil pH.
However, some nutrients are available for plants in low pH, while others are available in high pH. The pH of soil should vary from micro-site to micro-site and it is the role of a healthy soil biology to control it. If we leave it to applications of lime or sulfur, the whole biological system will be temporally determined by this input, and the quantity of micro-sites of varying pH will be limited. So, instead of applying minerals in order to mobilize phosphorus by a chemical reaction, you could stimulate growth of a vigorous soil food web that will ensure extraction of essential elements and support their continuous recycling.
Choosing phosphate fertilizer
Why it is necessary to change pH for some crops
Northern highbush blueberry grows happily only on acidic soils, because it prefers to consume nitrogen in the form of ammonium, rather than in the form of nitrates or nitrites (7). When pH is neutral or above, then a certain group of bacteria, called nitrite bacteria, starts to convert ammonium to nitrites. Since nitrates are not the favourite choice of menu for the blueberry, they do not absorb nitrogen and wilt. When pH is low, ammonium is plentiful, nitrite bacteria are out of work and blueberries can flourish.
There are many soils around the world that are naturally deficient in phosphorus, such as soils in the Amazon Basin, on Java or in Australia. Others have been damaged by inappropriate farming practices – bare soils were flushed by rains, which washed away phosphorus, they were depleted by overharvesting of crops and their natural soil food webs were destroyed, making it impossible for plants to feed on anything other than artificial fertilizers. While soil food webs can be restored, wherever there is not enough elemental phosphorus present, for any reason, it must be brought back by the farmer. The other option is to wait until mountain-generating processes raise the bottom of the sea, where phosphate fertilizers end up. When the new mountain ranges are formed, the rain will start to wash phosphorus out of the rocks, making it available for plants again. But this will take some time – around 10-15 million years….
For organic gardeners one of the main sources of phosphorus are ground phosphate rocks. Good quality phosphate rock fertilizer should be free of all contaminants such as fluorides, heavy metals or radioactive uranium. It can be applied directly on soil (100 kg or more per hectare) tied to organic matter, mulch, compost and compost teas, to enhance soil biology and enable feeding plants through the activities of bacteria, fungi and other microorganisms. Another way is to incorporate rock phosphate into compost with a fungal dominance, so that fungi will transform rocks into a soluble form, or preparing a special phospho-compost (8). Inoculating plants with mycorrhizal fungi improves greatly effectiveness of phosphate rock fertilizers.
It has been discovered in Costa Rica, that phosphate fertilizers can be applied on top of the mulch, rather than below it. This idea has been conceived to prevent phosphorus from being bound up in the acid tropical soil. And it worked. Yields of beans rose more than 3 times (9).
Clay washed out from between layers of phosphate rocks during mining can also be used as a fertilizer. Particles of this clay are surrounded by natural phosphates and it’s called a colloidal phosphate. Thanks to clay the phosphorus is more easily available for plants than in phosphate rocks. It can be used together with manure on compost piles or directly on soil – manure acids will dissolve phosphates, which in turn will stabilize the nitrogen in manure (10).
Superphosphate fertilizers are made from chemically treated phosphate rocks. They are not recommended for use as they are highly concentrated and reactive. When applied on the field they react with calcium, iron, magnesium and aluminium, creating within seconds compounds that make phosphorus unavailable for plants. They may react also with trace elements, locking them up and causing deficiencies of micronutrients. Superphosphates are water soluble and they can be easily washed away by rains before plants have a chance to assimilate them, which later may cause the eutrophication of lakes and rivers. Not to mention that high concentrations of phosphorus in fertilizers (above 10) are lethal to mycorrhizal fungi (11). Superphosphates, however, do have their advantage: they were purified and do not contain toxic elements such as uranium. There is a disadvantage, though. The waste product of the purification process is stored in slag heaps, that are sometimes unprotected and, since they contain uranium, they are radioactive. Fluorides leaching from these heaps may also cause groundwater pollution.
Another material that is rich in phosphorus is guano – bird or bat droppings. Bones of fish that are eaten by seabirds contain a lot of phosphates, and as a result seabird guano also contains a high level of phosphorus. Guano has accumulated over centuries on small islands on the Pacific Ocean or on the coast of Chile and Peru, where it was mined in such large quantities that its deposits are now severely depleted. In contrary to phosphate rocks, it is a renewable resource, however, only over a long period. Apart from phosphorus, guano also contains high levels of nitrogen and calcium. It can be fresh, semi-fossilized or fossilized, depending on the source.
Phosphates can also be found in mud from ponds, in freshwater mussels, in fish waste, in algae or in recent volcanic ash. Many plants, such as comfrey, lupine, sweetclovers, nettle or vetches accumulate phosphorus and they can be used as green manure. Note, however, that they don’t produce phosphorus in the way that nitrogen is fixed from the air by legumes. Rather, they just extract phosphorus from one place and you can put it somewhere else, leaving the source with less phosphorus.
Building your own phosphate factory
Bat house on a tree
If you would like to collect phosphorus from your local area, the exciting way to do this is to establish a small bat colony. If there are bats living in your neighbourhood, especially in buildings, you can build a bat house for them. Bats will come to rest there and… they will leave their droppings underneath. You can place a container under the bat house and collect their guano. The additional benefit is that insectivorous bats consume large amounts of moths, mosquitoes, flies, grasshoppers and crickets among many others. They are high-class specialists in insect control – in just one hour a single brown bat can catch 1200 mosquitoes. In fact, they are so effective in eating mosquitoes that in India an establishment of bat colonies around Calcutta was considered as a way of dealing with excessive mosquitoes numbers (12).
If bats are not your kind of animal, you may consider another type of a phosphate factory – a pigeon house. Pigeons mostly eat seeds, and these are usually rich in phosphorus. Their manure is rich in nitrogen as well, so it could be very useful on farms, and some people in the Middle East still keep them. If you are wondering how the permaculture principle of "every element should serve many functions" could be applied with regards to pigeons, there is one interesting thing that some breeds of pigeons can do: they can carry letters. Harry Potter fans may feel a little disappointed and prefer owls for sending letters, but the advantage of pigeons is that they can do it for real.
The adapted ones
Proteoid roots of Acorn
Banksia. Source: Annals of Botany
A small group of plants, which includes lupines and macadamia trees, has developed a unique strategy to adapt to phosphorus-deficient soils. Instead of forming mycorrhizal associations, they create densely clustered roots that enhance phosphorus uptake. These roots received a scientific name of proteoid roots, after the Proteaceae plant family. Despite their unimpressive name, proteoid roots of white lupine have an extraordinary ability: they excrete citrate and in this way increase availability of phosphorus in the root zone (13). Well, why not call them power roots instead? Or, phosphorus-I’m-coming-to-get-you roots? They deserve a better name.
The intriguing thing about proteoid roots is that plants do not form them when phosphate fertilizers are applied. To the surprise of a farmer,
Macadamia nuts on a tree
macadamia trees can show signs of phosphorus deficiency even though a significant amount of phosphate fertilizer was added. When phosphorus is present in soil, even in small quantities, these plants grow well by themselves. And, when there really isn’t enough phosphorus, then compost and mulch can be used, instead of phosphate fertilizers (14).
Protecting phosphorus from being washed away
Phosphorus loss occurs especially on bare, sandy soils, where you have little trees and get heavy rains. While natural systems such as forests can lose 0.1 kg of phosphorus per hectare per year, bare crop systems can lose even 100 kg of phosphorus per hectare in one year (15). In heavy soils or loams loss is generally very small. Most phosphorus in the environment is in the insoluble form and unlike nitrogen, which can be dissolved in water, it is washed away with soil particles or organic matter.
Lupines in New Zealand. Photo: Anita 363/Flickr
Soil eroded after storms carried to the sea by Betsiboka river in Madagascar.
Photo: Earth Observatory
Since this is known, protecting phosphorus is easy. A good soil structure can be created by adding organic matter and compost. Soil biology can be further improved by brewing compost teas. Together with compost they will add an army of nutrient recyclers to the soil: active bacteria, fungi, flagellates, amoebas, ciliates and beneficial nematodes. These microorganisms will retain phosphorus in their bodies and the functioning of a whole healthy soil food web will allow recycling it. It is also worth mentioning that certain species of bacteria can also dissolve phosphate rocks and they help in converting phosphorus into forms that are edible for plants (16). A no-dig system can be introduced to prevent erosion and protect soil life, and trees can be planted on at least 30% of land. And it takes mulch, mulch and mulch to protect soil from rain.
Farmers can pull another ace out of their sleeves – charcoal! It is an ancient soil amendment, tried and tested for thousands of years by Indian tribes in the Amazon. They used it with pieces of pottery to create Terra Preta, the black soil, which is still fertile today, an exceptional thing in this region of the world. The porous structure of charcoal provides a great habitat for microbes, it persists in the soil for a very long time and it retains nutrients, including phosphorus (17). Charcoal (or biochar) can be made not only from wood, but also from agricultural residues, such as rice husks (18).
Roots of vetiver grass 6 months after planting.
Photo: The Vetiver Network International
To slow down run-off in the mountainous areas, crops can be grown between rows of trees planted on contour, in an alley cropping system. These hedgerows can be planted with nitrogen-fixing trees, or other fast growing species. Prunings from the hedgerows can provide much needed mulch for crops.
Instead of trees, vetiver grass can also be planted on contour. Its roots grow 3-4 meters deep and it can reduce erosion by as much as 90% and recharge ground water (19). Over the years, on steep slopes, natural terraces will form behind the hedge, as soil will accumulate there. A vetiver grass system is easy to establish and requires little maintenance. It can also be used for stabilizing road embankments, river banks, preventing landslides and for wastewater purification.
Some say that free market is the most efficient way of allocating scarce resources. This may be true. If you are a farmer from Europe then letting the invisible hand of the market allocate the remaining reserves of phosphate rocks could be no problem for you. Let the most competitive ones win! However, if you own half an acre of land somewhere in Sub-Saharan Africa, your soil is poor in nutrients, yields are low and you hardly make ends meet, then you can easily notice a simple thing – with free market rules, scarce resources don’t go to those who need them most. They go to those who can pay most.
In 2008 some 82 million people were added to our planet. The largest part of this population growth took place in the South: in Asia, Africa and in South America. All these young people, a population four times larger than the population of New York, will need food, water, clothes and a place to live. They will need land where crops will be grown for them. And to grow these crops many nutrients are essential. One of them is phosphorus. Since the reserves of phosphate rocks are scarce who will get it?
Bill Mollison again:
Of all the elements of critical importance to plants, phosphorus is the least commonly found, and sources are rarely available locally. Of all the phosphate fertilizers used, Europe and North America consume 75% (and get least return from this input because of overuse, over-irrigation, and poor soil economy). If we really wanted to reduce world famine, the redirection of these surplus phosphates to the poor soils of Africa and India (or any other food-deficient area), would do it. Forget about miracle plants; we need global ethics for all such essential resources (20).
Field of rice in Bihar, India. Photo: yumievriwan/Flickr
It is possible to calculate a fair share of the remaining phosphate rocks for each country, depending on the soil’s condition and number of population. And that’s exactly what should be done. A global agreement is necessary for sharing the last phosphate rock reserves in a common sense way.
Planting rice in Madagascar.
Photo: Gail Johnson/Flickr
Our current industrial agricultural system and the global economy that supports it are inherently unsustainable. Extracting a limited resource, such as phosphorus, and sending it to landfills or dumping it in the ocean doesn’t make much sense. Sooner or later reserves of phosphate rocks will become depleted, then what? There is some back up in the form of deposits on the continental shelves and on seamounts in the Atlantic and Pacific Oceans (21), but the cost of mining it can be very high and even if industrial farmers were able to buy them, what about farmers from Botswana? What about farmers from Madagascar or India? What will be the cost of food, when the price of fertilizers goes up? Recycling phosphorus is just common sense and it seems inevitable, if we wish to continue living on Earth. It means that the exchange of our entire food supply and waste management systems is inevitable as well. Honorable presidents, distinguished prime ministers, sooner or later we will have to do it.
Why wait till the industrial food supply system collapses from lack of phosphate fertilizers or because they are too expensive to buy? Farming the way nature does provides not only healthy soils and good yields, but also nutritious food, flavoursome food. A juicy tomato with its characteristic, charming smell, instead of a watery, tasteless, red ’something’. Our economy can be more local, so that it will be possible to easily recycle nutrients, and as a result people will be more connected. These changes can be for better, not for worse.
If we manage to close the phosphorus cycle in our countries soon enough, we will have plenty of phosphate rocks left. We will be able to use them for restoring degraded lands, for planting trees, and greening our planet once again.
A big thank you goes to Geoff Lawton who provided many of the ideas and practical solutions that are presented in this article. Geoff recorded his thoughts and comments while teaching abroad and he sent them to me as audio files. His insights are a backbone of this work.
- B. Mollison, Permaculture: A Designers’ Manual, 2004, p. 192.
- J. Lowenfells, W. Lewis, Teaming with Microbes, 2006, p. 53.
- Ibid, p. 61.
- Honglin Huanga, Shuzhen Zhanga, Xiao-quan Shana, Bao-Dong Chena, Yong-Guan Zhua and J. Nigel B. Bellb, Effect of arbuscular mycorrhizal fungus (Glomus caledonium) on the accumulation and metabolism of atrazine in maize (Zea mays L.) and atrazine dissipation in soil.
- See also: K. Lewis, B. McCarthy, Nontarget tree mortality after tree-of-heaven (Ailanthus altissima) injection with imazapyr, Northern Journal of Applied Forestry, 25(2):66-72, 2008. In this study a herbicide imazapyr was injected to Tree-of-heaven (Ailanthus altissima), which in some regions is an invasive tree. The results showed that imazapyr injections not only killed all injected tree-of-heaven, but also 17.5% of neighboring (within 3 m) noninjected tree-of-heaven and eight other tree species 62 weeks after treatment. The possible ways of transmission of the herbicide were root grafts, mutually shared mycorrhizal fungi, root exudation and absorption, and/or leaf senescence.
- Methodology: Integrated Production of Highbush Blueberry, edited by Danuta Krzewinska, 2005, p. 7.
- See: chapter 9 “Ways of improving the agronomic effectiveness of phosphate rocks” in: F. Zapata and R.N. Roy, Use of Phosphate Rocks for Sustainable Agriculture, FAO 2004. Available at: http://www.fao.org/docrep/007/y5053e/y5053e00.htm#Contents
- R. Bunch, Five Fertility Principles, The Overstory #20, http://www.agroforestry.net/overstory/overstory20.html, accessed on 16.01.2009.
- P. Sullivan, Alternative Soil Amendments, ATTRA, http://attra.ncat.org/attra-pub/altsoilamend.html, accessed on 13.01.2009.
- J. Lowenfells, W. Lewis, op. cit., p. 151.
- Bats, The Ecologist, http://www.theecologist.org/pages/archive_detail.asp?content_id=352, accessed on 15.01.2009. In some parts of the world bats may carry viruses that are dangerous to humans. Before building a bat house in your backyard, please make sure there are no health concerns.
- J. F. Johnson, D. L. Allan and C. P. Vance, Phosphorus Stress-Induced Proteoid Roots Show Altered Metabolism in Lupinus albus, Plant Physiology, Vol. 104, Issue 2, p. 657-665.
- A. L. Shigo, Troubles in the Rhizosphere, The Overstory #70, http://www.agroforestry.net/overstory/overstory70.html, accessed on 13.01.2009. See also: G. Porter, R. Yost and M. Nagao, The Application Of Macadamia Nut Husk And Shell Mulch To Mature Macadamia Integrifolia To Improve Yields, Increase Nutrient Utilization, And Reduce Soil P Levels.
- B. Mollison, op. cit.
- R. Ivanova, D. Bojinova, K. Nedialkova, Rock Phosphate Solubilization by Soil Bacteria, Journal of the University of Chemical Technology and Metallurgy, 41, 3, 2006, 297-302.
- Soil Fertility Management and Soil Biogeochemistry, Cornell University, http://www.css.cornell.edu/faculty/lehmann/research/biochar/biocharmain.html, accessed on 16.01.2009.
- S. M. Haefele, Black Soil – Green Rice, Rice Today, April-June 2007, p. 26-27.
- Soil erosion, The Vetiver Network International, http://www.vetiver.org/g/soil_erosion.htm, accessed on 16.01.2009.
- B. Mollison, op. cit.
- S. M. Jasinski, Phosphate Rock, Mineral Commodity Summaries, January 2008, p. 124,
(available at: minerals.usgs.gov/minerals/pubs/commodity/phosphate_rock/).
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