“the dirt crust broke and the dust formed. Every moving thing lifted the dust into the air: a walking man lifted a thin layer as high as his waist, and a wagon lifted the dust as high as the fence tops, and an automobile boiled a cloud behind it.” – The Grapes of Wrath, John Steinbeck
Soil fertility is an immensely important characteristic of our life sustaining Earth, yet if you are like me you may think little of it. I vaguely recall reading and hearing in passing such subjects as “loss of topsoil” and “soil degradation” in the past, but I never sought out any data on the topic. Maybe gardening and reading The Grapes of Wrath for the Slow Food book club has made me more interested. Certainly anyone that lived through the dust bowl period and was impacted by it knows how critical healthy soil is to ways of life we might otherwise take for granted.
When the term soil fertility is mentioned usually it is the amount and quality of the A horizon aka “topsoil” that is being referred to. This is because topsoil is the first layer of real soil and contains the most richly broken down organic matter and microorganisms. This means earth worms and plant roots love it! While plant roots do penetrate the layers of soil beneath topsoil, they don’t get nearly as much of their nutrition from this depth. Furthermore most seeds can’t be planted very deep anyway so – no topsoil no produce.
There are two natural ways topsoil is lost; wind erosion and water erosion. Drought can speed up the effects of wind and water erosion but doesn’t really destroy soil by itself. On the flip side soil is naturally produced all the time. Organic matter that hits the earth is automatically transformed into soil by microorganisms and other critters. This process has a capacity to produce soil at around 1 to 2 inches every 100 years.¹ Lose more than 1 to 2 inches every 100 years and you are slowly eating away Earth’s capacity to provide us with healthy topsoil.
Now toss man into the equation and we speed up these natural methods. How? The primary cause is tilling or plowing. Traditional farming practices state that you till the ground at the end of the growing season to remove the dead plants and prepare fresh ground for next springs planting. But in between fall and spring you have winter’s rains and winds. Soil exposed with no roots to keep it in place is much more likely to be washed and blown away. The impact of tilling, along with lack of crop rotation and a few years of drought was the primary cause of the American dust bowl of the 30s that forced 2.5 million Americans to migrate west. 200,000 “settled” in California were they where greeted by the derogatory phrase Okie, terrible pay, and regular harassment by law enforcement and towns people that wanted the “undesirables” out of their country.²
Besides our ability to speed up erosion, we also created some of our very own soil degradation issues. There are many ways to list out the types of man made soil degradation, but basically you have salination (caused by the leaching of salts to the surface in dryland that is irrigated to raise crops), chemical (we dump toxins in the soil and then can’t grow anything there), and raising or lowering of the water table to unhealthy levels by poor irrigation practices. Here’s a link to an interesting map on the global impact of man’s effect on soil degradation (unfortunately from 1990 ).
The good news is that farmers certainly recognize the impacts of soil erosion and degradation and more and more of them are using no till or organic methods that treat the land much more naturally. In 1982 the United States lost 3.06 billion tons of soil on crop land due to erosion. In 2007 our crop land soil losses were down to 1.73 billion tons a year.³ Pretty significant progress but there are still quite a few green and red dots on this map, each of which represents 100,000 tons of soil loss above the natural soil loss tolerance rate.