Soil: How Much Water Can it Absorb?

Following on from Colin’s piece:

In addition to its value in reducing CO2 levels by converting soil to organic matter, and using a “conservative estimate”, the top 12 inches of soil has the potential to:

“hold four times its weight in water [Thus an] additional 1.56% of soil organic matter could reasonably be expected to hold another 228 tons of water per hectare–about three rail tank cars full. This is equivalent to 2.28 cm (0.9 inches) of extra water in the soil, which would do a great deal to mitigate flooding and drought, and increase overall biomass production in most areas”.

Here is the full article by Peter Donovan, taken from the Soil Carbon Coalition website

NOTE: This calculation is offered to show some degree of correspondence between atmospheric levels and soil carbon. There are too many variables and complex feedback loops to make reliable predictions. Were atmospheric carbon dioxide to decline, the oceans would release significant amounts of carbon dioxide, buffering and delaying the decline.

In Allan Yeomans’s book Priority One: Together We Can Beat Global Warming (2005, 2007), he states that an additional 1.6% of the top 12 inches of the world’s cropland and grazing land soils turned into organic matter would bring atmospheric carbon dioxide concentrations below 300 ppm (if we also quit adding carbon to the atmosphere). This figure is based on removing 80 parts per million of atmospheric CO2.

Yeomans’s calculation converts carbon to soot (Priority One, chapter 2). Here is a somewhat more straightforward calculation. Note that the figures are for straight carbon, not carbon dioxide.

The atmosphere currently contains about 800 Gt (gigatons or billion metric tons) of carbon. The vast majority of this is in the form of carbon dioxide, which is currently about 383 parts per million (Yeomans used 380 ppm for his calculation). Each of these parts per million = 800/383 = 2.089 Gt C. So, to take out 80 ppm we are talking about removing 80 x 2.089 or 167 Gt (167,000,000,000 metric tons) of carbon from the atmosphere.

Soil density is usually between 1.2 and 1.4 on a dry basis. That is in relation to the density of water which is 1.0.

A hectare of soil (100 m x 100 m), 12 inches or 30.48 cm deep, has a volume of 3,048 cubic meters. At a soil density of 1.2, this foot-deep hectare of soil weighs 3,658 metric tons. One percent of this weighs 36.58 metric tons, and if this 1 percent is organic matter (58% carbon by weight), it contains 21.21 tons of carbon.

According to the World Resources Institute (World Resources 2005: The Wealth of the Poor–Managing Ecosystems to Fight Poverty, table 11, p. 216), there are 5,096,000,000 hectares of crop and grazing land worldwide. Dividing our 167 billion tons to remove by this figure, each hectare has to take, on average, 32.8 tons of carbon. Since each percent of organic matter in the top foot of soil contains 21.21 tons of carbon, this comes to roughly 1.56% as the amount of soil that we must convert to organic matter on the top foot of these lands, to lower atmospheric concentrations by 80 ppm. In other words, if the organic matter content is currently 1%, we need to raise it to 2.56%, if 3% we raise it to 4.56%, and so on.

For an example of what can be done, see Martha Holdridge’s Report from my farm, where she reports organic matter more than doubled in five years, from 4.1% to 8.3%. Note that the sampling in this case was only 2 inches deep. Though soil organic matter may accumulate fastest in the topmost layers of soil, grassland soils typically store significant quantities of carbon as much as two meters down.

For a table showing how much each country must increase its organic matter to remove its fossil fuel emissions since 1950, see or download http://biospheremedia.org/299ppm.pdf

Using a conservative estimate that organic matter can hold four times its weight in water, such an additional 1.56% of soil organic matter could reasonably be expected to hold another 228 tons of water per hectare–about three rail tank cars full. This is equivalent to 2.28 cm (0.9 inches) of extra water in the soil, which would do a great deal to mitigate flooding and drought, and increase overall biomass production in most areas.

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