Soil Mineral Depletion - Can a healthy diet be sufficient?


Martin Hum

Year of publication: 

You eat a nutritious diet and avoid anti-nutrients, so you should be getting all the minerals you need to stay healthy. Or are you? Martin Hum digs up the dirt on why the nutrient content of our food is on the decline.

"We know more about the movement of celestial bodies than we do about the soil underfoot". These words of Leonardo da Vinci are almost as true today as they were 500 years ago. To most people, soil is just dirty, annoying stuff that sticks to our shoes. But without it, the land surface of our planet would be barren rock, virtually devoid of life. Soil is the prime source of the minerals on which every living cell depends for its structure and function. Yet, all over the world, minerals are disappearing from agricultural soils at an alarming rate. It’s time governments and the agricultural industry woke up to the problem.

To understand the story of soil minerals, we have to go back 12,000 years to the end of the last ice age. As the glaciers retreated, they left behind the ground-down remains of rocks as clays and moraines, which were quickly colonised by plants and animals. These early soils were enormously rich in minerals and their depth and fertility increased as organic matter became incorporated in them through the cycles of growth and decay. Leaching by rainfall and scouring by rivers has undoubtedly washed into the sea a proportion of the minerals originally present in ancient soils.

To a large extent minerals have been, and continue to be, recycled in natural ecosystems – taken up by plants, consumed by animals and then returned to the soil through their wastes and decomposition of their bodies. Traditional farming techniques maintained soil fertility through crop rotation and leaving land to “lie fallow”. Food was consumed close to where it was grown, human and animal manure was spread back onto the fields and, even until the middle of the 20th century, farmers lived by the doctrine “look after the soil and everything else will look after itself”.

But things have gone badly wrong. In 1992, the official report of the Rio Earth Summit concluded “there is deep concern over continuing major declines in the mineral values in farm and range soils throughout the world”. This statement was based on data showing that over the last 100 years, average mineral levels in agricultural soils had fallen worldwide – by 72 per cent in Europe, 76 per cent in Asia and 85 per cent in North America. What has caused this staggering decline?

Most of the blame lies with artificial chemical fertilisers. In the mid-nineteenth century, German chemist Justus von Liebig conducted experiments in which he analysed the ash and gases that remained after plants were burnt, in order to understand which elements were essential for their growth. His primitive equipment only identified three – nitrogen, phosphorus and potassium – known by their chemical symbols as NPK. Although von Liebig later acknowledged that many other minerals are present in plants, his experiments laid the foundation for a lucrative agrochemical industry, selling NPK fertilisers to farmers, with the promise of miraculously increased yields. NPK fertilisers have certainly revolutionised agriculture, but at the cost of a tragic degradation in the quality of the soil.

We now know that, under ideal circumstances, plants absorb 70 to 80 different minerals from the soil, while the number returned to it by plants grown with commercial fertilisers can be counted on the fingers of one hand. Every crop that is cut or animal that is sent to market marks a further depletion in the mineral status of the soil on which it was raised. Organic wastes that in former times would have been composted and returned to the land are nowadays mostly consigned to landfill sites or incineration.

We know more about the movement of celestial bodies than we do about the soil underfoot.There are other, unseen ways in which the move to chemical farming prevents crops from taking up even the sparse amounts of trace minerals left in the soil. Soil contains myriads of bacteria, fungi, plant and animal life, in a state of constant interaction and balance. Every one of these organisms needs dozens of different minerals to survive and play its part in the ecosystem. Some bacteria have a vital role in converting soil minerals into chemical forms that plants can use. NPK fertilisers gradually change the soil pH towards acidic conditions in which these bacteria cannot survive. To combat soil acidification, which reduces crop yields over time, farmers often spread lime on the land along with fertilisers. This adds back magnesium and calcium and raises the soil pH, but it also converts manganese and some other trace minerals into chemical forms that plants are unable to absorb.


Pesticides and herbicides also reduce the uptake of trace minerals by plants. One of the most important relationships plants have with their soil-dwelling companions is a mutually beneficial one with certain kinds of fungi. This kind of symbiosis is called mycorrhiza (from the Greek words for mushroom and root). The tiny hairs on plant roots are invaded by the thread-like mycelia of the fungi, which can form networks covering several acres. The fungus obtains carbohydrates from the plant root, at the same time supplying the plant with nutrients it draws from the soil. This gives plants access to a vastly greater mineral extraction system than is possible by their roots alone. But plants are also susceptible to fungal diseases, which reduce crop yields, and the use of chemical fungicide sprays is widespread. These destroy mycorrhizal fungi and so further reduce the ability of plants to absorb soil minerals. Insecticides can also reduce trace mineral uptake by inactivating choline-containing enzymes in plants, essential for the absorption of manganese and other minerals.

The combined effect of soil mineral depletion and the reduced availability of those minerals that remain is that most of the food we eat is mineral deficient. In 1927, Dr RA McCance started a study of the nutrient content of common foods at King’s College, University of London. Together with his colleague, Dr EM Widdowson, he published the results in 1940. The analyses have been repeated at regular intervals since, providing a unique picture of the way the composition of our food has changed. The table below summarises the reductions in the average mineral content of 27 vegetables and 17 fruits, between 1940 and 1991. The latest edition of McCance and Widdowson’s work was published in 2002, but no updated comparison of the figures has yet been published. However, it is likely that mineral values have continued to fall.

Reduction in average mineral content of fruit and vegetables between 1940 and 1991

Mineral                                            Vegetables                                     Fruit
Sodium                                            -49%                                               -29%
Potassium                                      -16%                                               -19%
Magnesium                                    -24%                                                -16%
Calcium                                           -46%                                               -16%
Iron                                                   -27%                                               -24%
Copper                                            -76%                                               -20%
Zinc                                                  -59%                                               -27%

What can you do to ensure you get the minerals you need?


  1. One obvious place to look is the sea – after all, that is where many minerals lost from the soil eventually end up! Sea vegetables are particularly high in minerals. For instance, dulse seaweed contains 75 times as much iron as spring greens. Shellfish also contain good amounts of minerals, especially zinc.
  2. Eat organic. Organic foods generally have higher levels of minerals than those grown with chemicals. But don’t rely on supermarket shelves – try to find a grower who has been using organic methods for at least 10 years. The longer soil has been farmed organically, the higher its mineral content is likely to be.
  3. Take supplements – but don’t fall into the trap of thinking all brands are the same. You get what you pay for and the cheaper multi-mineral supplements often contain low amounts and chemical forms that the body can’t absorb well. Look for supplements that have minerals in a chelated form, or as orotates, citrates or gluconates. Alternatively, colloidal minerals are a good bet.
  4. Grow your own. If your garden hasn’t been used for growing vegetables or if you’ve been doing so organically, the soil is likely to be much richer in minerals than agricultural land – especially if your house is more than 50 years old. You won’t be able to provide all your food this way, but what you grow will be far superior nutritionally to anything you buy at the supermarket.
  5. Campaign. Get in touch with a group such as the Food Commission or the Soil Association to get further information and find out what you can do to raise awareness of the problem of nutrient depletion. Write to your MP and your local supermarket, and talk to anyone who will listen!

A new study published earlier this year shows that, as might be expected, mineral levels in animal products reflect the picture in plant foods. Comparing levels measured in 2002 with those present in 1940, the iron content of milk was found to be 62 per cent less, calcium and magnesium in Parmesan cheese had each fallen by 70 per cent and copper in dairy produce had plummeted by a remarkable 90 per cent.

The Government is putting resources into improving health by encouraging people to eat a healthy diet, including five portions of fruit and vegetables a day. But you scarcely hear a word about the problem of soil mineral depletion. Food seems to be considered as something quite separate from its source and means of production. Maintaining the health of the nation’s soil and striking the right balance between agricultural yields and nutritional quality are not high on any political agenda. But this is not rocket science. The foundation of human health is the quality of the food we eat, which relies ultimately on the vitality of the soil on which it is raised.

Vitamins, enzymes, amino acids and a host of other biologically active substances are essential for our bodies to function properly. They virtually all include minerals as an integral part of their chemical structure. Dr Linus Pauling, twice Nobel Prize winner, said “You can trace every sickness, every disease and every ailment to a mineral deficiency”. Minerals are needed for the proper formation of blood and bone, the maintenance of healthy nerve function, the regulation of the heartbeat and for reproduction and foetal development. They are essential to the processes of growth, healing and energy release. And it is not just the presence of minerals in the body that is important – they must be in the correct ratios to each other. The level of each mineral has an effect, directly or indirectly, on every other, so if one is out of kilter the whole system is affected.

Minerals are an essential part of our natural diet and a lack of them may in part account for our increasing susceptibility to the “diseases of civilisation” – such as heart disease (magnesium), cancer (selenium), diabetes (chromium) and mental illnesses (zinc). Every one of us should take care to get the minerals we need, for the good of our health.


Food Commission . 020 7837 2250
Soil Association . 0117 314 5000

The Institute for Optimum Nutrition is an independent educational charity.
Registered company number 2724405, registered charity number 1013084