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?
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.
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)
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.
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