Is Soil Fertility Decreasing?

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Robert Pavlis

Is Soil Fertility Decreasing? Are agricultural soils less nutritious today than they were 50 years ago? I think most people believe these statements to be true. The idea has certainly been promoted a lot in the last 20 years. Our food is less nutritious than it used to be and the main reason is that soils are being depleted of nutrients.

There is also a big movement to remineralize our poor soils. Hopefully, adding things like rock dust will bring it back to historical levels. The organic movement is also very big on solving these kinds of soil issues. Before we can understand the benefits of such actions it is important to determine how big of a problem we really have. How depleted are our soils? Which nutrients are actually missing?

Is soil fertility decreasing?
Is soil fertility decreasing?

Is Soil Fertility Decreasing?

Plants grow by absorbing nutrients from soil. When we remove the food from the fields we also remove nutrients. It seems very logical to conclude that soils must be losing nutrients.

But there is another process at play. Soil was originally created by the decomposition of rocks. As rocks break down they release nutrients into the soil. With the exception of nitrogen, rocks are the source of the nutrients our plants need. Keep in mind that the term rocks includes things like silt and sand, which are just small rocks.

Growing Great Tomaotes, by Robert Pavlis

Nutrients are also added back to soil through fertilization, increasing soil fertility.

The microbes in the soil help break down the rock and make the nutrients available to plants. The organic matter in soil, including all life forms act like storage systems for holding nutrients. The whole process is very complex and we don’t really understand very much of what goes on.

We can measure nutrient levels in soil on a macro scale and look at water soluble nutrients but that does not show us the total amount of nutrients in soil. There are too many factors at play to really know what the nutrient levels are in soil. One of the best ways to measure soil fertility is to look at plant growth and measure nutrients in plants.

Is Food Less Nutritious?

In the late 1990’s several studies were done that compared the nutrient levels of today’s food to that of 50 years ago. They found that nutrient levels were lower today. People quickly concluded that this was proof of a decline in soil fertility. Modern agriculture had used up the nutrients in our soil and now our food supply was suffering.

If you go back and review the studies, you quickly realize that the researchers never concluded that our soil was depleted. They reported on the data and gave several possible explanations. The popular press and special interest groups cherry picked the data that supported their cause and started reporting that our food was much less nutritious and that our soils were depleted.

A popular quote for a British study is that “copper levels dropped 80%” and this is quickly extrapolated in headlines to “fruits and vegetables are 80% less nutritious”. Only copper had such high numbers in the study. The reality is that the lower copper levels are probably due to the fact that we stopped using toxic copper pesticides. No one bothered to ask if the higher copper levels 50 years ago actually harmed us.

Here is what one of the most commonly referenced studies actually said.

As a group, the 43 foods show apparent, statistically reliable declines (R < 1) for 6 nutrients (protein, Ca, P, Fe, riboflavin and ascorbic acid), but no statistically reliable changes for 7 other nutrients. We suggest that any real declines are generally most easily explained by changes in cultivated varieties between 1950 and 1999, in which there may be trade-offs between yield and nutrient content.” (ref 3)

Soil Science for Gardeners book by Robert Pavlis

Note: In order to see the “statistically reliable declines” they had to group 43 different types of food into one group. As individual foods they did not see a decline.

There are many issues with this kind of historical review. The sampling techniques and testing procedures changed a lot during the 50 year period. The varieties grown have changed. Due to globalization, food is now grown in different places and different soils can produce food with very large differences in nutritional values.

Robin J.Marles (ref 1) has done an extensive review of these studies and concluded that:

  • Mineral nutrient composition of vegetables, fruits and grains is not declining.
  • Allegations of decline due to agricultural soil mineral depletion are unfounded.
  • Changes are within natural variation ranges and are not nutritionally significant.
  • Eating the recommended daily servings provides adequate nutrition.

The varieties grown today may be slightly less nutritious, some have suggested a factor of 10-15%, but the difference is not significant given our access to food, at least in developed countries.

Food is not less nutritious today.

Are Soils Less Nutritious?

The examination of nutrients in food over time does not support the idea that soils are less nutrient rich today compared to the past.

Another way to look at historical data is to compare the ratios of calcium, magnesium and potassium. These macronutrients are very important for plant growth, and exist in different amounts in soils. They are also commonly added as fertilizer if levels get too low.

When the level of these three nutrients is examined (ref 2) we find that, over time, the ratio in plants has remained the same. This suggests that these nutrients are either being depleted proportionally, which is very unlikely or that they are NOT being depleted. These nutrients are routinely measured in agricultural soils and these results suggest that soil fertility is not declining.

Plants Don’t Use Many Nutrients

How can we grow food year after year and not run out of nutrients? An important fact is that plants don’t use a lot of nutrients. van Helmont did an interesting experiment in 1684, where he showed that you could grow a 164 pound tree from a couple of ounces of soil.

The amount of minerals in food is very small compared to the amount in soil. But how small?

Consider this calculation. Soil contains 1-5% iron – let’s use an average of 2% which is 400,000 mg/sqft, in the plow layer. A carrot weighs 50 g and contains 0.3 mg of iron. So that 1 sq ft of soil has enough iron to grow 1.3 million carrots. That is a lot more than 50 years of farming.

Plants Don’t Grow Without Nutrients

An important concept about plant growth is commonly over looked. Plants, like animals are programmed by genetics. Their genetics determines which nutrients they need in order to grow and function correctly. A significant decrease in any of the required nutrients results in poor growth, sickness or death.

Agricultural plants are growing bigger and producing more than ever before. This fact alone indicates that they are getting the nutrients they need. The nutrient levels in soil may be going down, but they have not reached a critical point where it becomes important to remineralize the soil above what is currently being done with fertilizers.

What Does This Mean For The Gardener?

If agricultural soils are not losing minerals to any significant extent we can be sure that our landscapes are also not suffering. Provided that some organic matter is returned to the soil each year there is no need to fertilize our gardens.

Vegetable gardens are a little different, because we do remove food from them each year. But most backyard gardens are not as extensively harvested as farms and a little compost or manure will provide plenty of nutrients for your needs.

Gardeners are fertilizing way too much. It is not needed for the garden, and it hurts both the pocket book and the environment. I grow 2-3,000 different kinds of plants. I don’t fertilize when I plant and I never fertilize after planting – the plants are growing fine after 14 years. I do mulch with wood chips which adds some nutrients and I leave dead plant material in the garden to feed future plants.

Fertilizers are not required in a landscape garden.

References:

1) Mineral Nutrient Composition of Vegetables, Fruits and Grains; https://www.sciencedirect.com/science/article/pii/S0889157516302113?via%3Dihub

2) Are Depleted Soils Causing a Reduction in the Mineral Content of Food Crops?; http://soils.wisc.edu/facstaff/barak/poster_gallery/minneapolis2000a/

3) Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999; https://www.ncbi.nlm.nih.gov/pubmed/15637215

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

21 thoughts on “Is Soil Fertility Decreasing?”

  1. Robert, thanks for such detailed information in the comments. I would just like to supplement it with an interesting article on soil fertility. I do not understand all the information written there. But overall very cool. Especially about tracking soil fertility with EOSDA Crop Monitoring.
    https://eos.com/blog/soil-fertility/

    Reply
  2. So why do farmers add nutrients like phosphorus and nitrogen to the soil every year if plants don’t take up significant nutrients? Doesn’t the fact that farmers need to fertilize show that plants do take up significant amounts of nutrients?

    Reply
    • Good question. The goal of farmers is to maximize profits – and I say that as a good thing – they need to make a living. So if they can add $100 of extra fertilizer to produce $110 in profits they will do that. That does not mean plants make use of that $100 of fertilizer. Most of it can be lost and still produce a profit with the small amount that is used.

      Do farmers need to fertilize? To make a living and be competitive they do. But what happens if they stop farming and stop fertilizing. The ground is soon covered with plants – healthy plants that don’t need added fertilizer.

      Fertilizer is not needed to grow plants. It is needed to produce a profit and produce enough food for the large population we have.

      This link will show some very interesting charts to illustrate the increase in yields since we started using synthetic fertilizer. https://ourworldindata.org/crop-yields

      Reply
  3. Where does the soil go? Well down the Amazon, and the Mississippi, and blown from the desertifying edges of the Sahara, Gobi and countless other deserts. And where does it land? Well 75% of the earths surface are covered with water, so your idea that we can just farm the places where the topsoil falls will at the very least require a lot of innovative equipment
    I personally believe that soil is more than a number of chemicals into which we stuff seeds to harvest energy, making calories we can ingest. This is a great big beautiful thing we have here- we and the bugs and weeds and bacteria and every kind of slime and creepy crawly that we share the planet with.
    If we turn the whole thing into a big sterile chemistry experiment, we have no way of knowing that we can survive that development.

    Reply
  4. Robert you are wrong. Fertility is diminishing by every metric. Individual ions are not fertility, especially when they are in the form of salts that rapidly leach from the land and destroy soil microbial life. Top soil or humus is fertility. Its the literal measure of soil food web productivity. Yes, alternatively you can Continue and IV drip of fossil fuel generated salts and pesticides to fields of monoculture, but its unsustainable and making us sick.

    Reply
    • From wiki, the definition of soil fertility: “Soil fertility refers to the ability of soil to sustain agricultural plant growth”. Plants use individual ions – they can’t use humus. So fertility is the availability of nutrient ions.

      If you think fertility is decreasing, show me the proof.

      Reply
  5. A decline in nutrients per mass of food might be the result of selection of food for large size. An apple is largely a sphere. As the diameter of the sphere increases so too does the surface area but at a slower rate. If the nutrients are concentrated in the skin we would be getting fewer nutrients per bite. Apples have grown enormously large as compared to those 100 years ago.

    Reply
    • Too many ifs in there. Each cell in a plant needs a certain level of nutrients in order to make the cells. Nutrients are not concentrated in the skin, although maybe it has higher levels of some chemicals. A bigger apple = more or larger cells = more nutrients.

      Reply
  6. What is Topsoil Erosion?

    Topsoil is more than just dirt, it is a living thing made up of countless microorganisms [3-5] The Earth is covered with an average of three feet of topsoil, the layer of dirt that provides the nutrients for most of the planet’s land vegetation, and is critical for producing food from agriculture. [5] Good topsoil maximizes crop yield while protecting the crop from degradation. The quantity of soil eroded is determined by multiple factors; including weather (wind and rain), the surface the soil (including the steepness on slope the soil is on), and the cover on the soil surface (i.e. plants or trees). [4] Soil erodes and then replenishes naturally at a rate of only an inch or two per several hundred years. [5] A problem only arises when soil erodes at an accelerated rate and cannot be replenished quickly enough. Accelerated erosion occurs when the energy from rain or wind contacts bare soil, detaching bits from the surface. [4]

    Accelerated Topsoil Erosion?

    Industrial farming practices such as tilling and use of agrochemicals, as well as deforestation to produce farmland have dramatically increased the rate of soil erosion. Around the world, topsoil is vanishing much faster than it forms According to David Montgomery, a geologist at the University of Washington, its clear that we’re losing more and more topsoil every day. “The estimate is that we are now losing about 1 percent of our topsoil every year to erosion, most of this caused by agriculture.” [5] The United States is losing soil at a rate 10 times faster than the soil replenishment rate while China and India are losing soil 30 to 40 times faster. [6] With soil erosion rates so much higher than the replenishment rate, it is no wonder that the world is quickly running out of fertile topsoil.
    Negative Effects of Topsoil Erosion

    The most visible effect of topsoil topsoil erosion is the decreasing crop yields. Soil erosion makes it more difficult for the soil to store water and support plant growth. Erosion can cause yield reductions of 30 to 90% in some root-restrictive shallow lands of West Africa. [7] Nationally, It is estimated that the total annual cost of erosion from agriculture in the USA is about US$44 billion per year. [8] Given that freshwater is becoming a scarce and valuable resource in our world, it is will become increasingly important that we use as little water as possible to grow crops. [9]

    Soil erosion also contributes to environmental damage (see Fig. 1). In addition to promoting water loss, it results in loss of nutrients, soil organic matter and soil biota, harming forests, rangeland and natural ecosystems. [2] Erosion increases the amount of dust carried by wind, which not only acts as an abrasive and air pollutant but also carries about 20 human infectious disease organisms, including anthrax and tuberculosis. [2] FINALLY, ABOUT 60 PERCENT OF SOIL THAT IS WASHED AWAY ENDS UP IN RIVERS, STREAMS AND LAKES, MAKING WATERWAYS MORE PRONE TO FLOODING AND TO CONTAMINATION FROM SOIL’S FERTILIZERS AND PESTICIDES. [2]
    http://large.stanford.edu/courses/2015/ph240/verso2/
    https://www.scientificamerican.com/article/only-60-years-of-farming-left-if-soil-degradation-continues/

    Reply
  7. Excellent article, Robert! This is such a persistent myth and people feel very strong about it – almost as an article faith. It’s especially prevalent in the YouTube gardening community, where rock dust, sea salt, and other “remineralization” fertilizers are constantly pushed.

    Reply
  8. Thx for your article. One Q:
    How about erosion as a cause of soil depletion? Especially in hilly Italy you can see quite clearly that every year a lot of soil ends up in streets, streams, rivers and finally the sea. That’s one of the main reasons for the need to change to a no tilling no artificial fertilizer approach like agroforestry- permaculture- food forests. The soil food web can recover and within years productivity is up to sustainable levels of productivity. matching the old approach but without the inputs Otherwise there will be no arrable land after 50 harvests. This soil depletion must have an effect on fertility and nutrients as well. What do you think?

    Reply
    • Erosion is the movement of soil from one place to another. The soil and nutrients are still there – somewhere. The post is not talking about loss of soil, but loss of fertility.

      No-till, as you say, bring life back into the soil and increases productivity – that just shows that the soils had fertility before the no-till was started.

      Reply
      • Maybe I was not clear enough. I am talking about top soil and loss of it each year. The upper layers of soil have the most fertility. If these are removed year after year then the fertility will be lost

        Reply
        • This is true on a very local scale, but not on a global scale. It is only lost in the place where you remove the top soil. The area where you or the environment put the top soil still has the fertility.

          Reply
          • Unfortunately you are wrong. Loss of topsoil due to various factors is a global phenomenon.

            “A third of the planet’s land is severely degraded and fertile soil is being lost at the rate of 24bn tonnes a year, according to a new United Nations-backed study that calls for a shift away from destructively intensive agriculture.

            The Global Land Outlook is billed as the most comprehensive study of its type, mapping the interlinked impacts of urbanisation, climate change, erosion and forest loss. But the biggest factor is the expansion of industrial farming.

            Heavy tilling, multiple harvests and abundant use of agrochemicals have increased yields at the expense of long-term sustainability. In the past 20 years, agricultural production has increased threefold and the amount of irrigated land has doubled, notes a paper in the outlook by the Joint Research Centre (JRC) of the European commission. Over time, however, this diminishes fertility and can lead to abandonment of land and ultimately desertification.

            The JRC noted that decreasing productivity can be observed on 20% of the world’s cropland, 16% of forest land, 19% of grassland, and 27% of rangeland.”

            https://www.theguardian.com/environment/2017/sep/12/third-of-earths-soil-acutely-degraded-due-to-agriculture-study

          • The Guardian is not a reliable source of information. If there is a decrease in productivity on 20% of croplands, why is it that crop production is up?

            Where does the soil lost due to erosion go?

          • Erosion and loss of topsoil is a huge global issue. In the US Great Plains erosion rates are typically 6x or more than the rate of soil formation, which is why farmers have lost so much topsoil over the past century, and have to use so much fertilizer compared to historical practices. The main issue is loss of organic matter, and the principle cause is soil tillage (and lack of soil cover in winter) which accelerates decomposition & loss of organic matter. The fix for this is pretty simple and straightforward: no till, cover crops, and crop rotation. All can be applied to gardens as well. Geomorphologist David Montegomery’s recent book “growing a revolution” based on years of research in agricultural areas around the world does a good job of documenting these patterns and provides many good references on the foundational studies. Plants do remove a significant amount of nutrients from the soil. If you do a soils test it is easy to see how this works. Without addition of manure or compost or fertilizer or nutrients from mulches or cover crops you will see losses of major plant nutrients like nitrogen, and phosphorus quite quickly from annual crops. So I agree that blanket statements about nutrient losses are often misleading, and claims about nutrient losses in food are suspect, but farmers and gardeners should be very careful in working to maintain organic matter and nutrient rich soils if they want to sustain productive gardens. This does not necessarily mean we need to use fertilizers, but we need to at least look at soil fertility if plants start looking unhealthy.

          • It is true that crops remove nutrients, but most agricultural systems are replaced, at least the major nutrients. There is also a loss of organic matter which is a significant problem, both with the soil and with CO2 levels in the air.

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