Practices affecting Soil - Physical

Physical

The total global biomass, consisting of the weight of all life on Earth, is virtually the same as the anthropogenic mass that humans have created between 1900-2020 - around 1140Gt. The biomass consists of the dry weight of plants, animals, bacteria fungi protists, archaea and viruses too. That vast majority of weight created by humans comes in the form of concrete  550Gt and aggregates - 386, but not soil aggregates but what they call sand, clay and gravel in the building trade. The human-made mass is accumulating at 30Gt/yr.

The United Nations estimates that <5% of the Earth's land surface was urbanized in 2020. Their impact goes beyond their physical footprint due to associated infrastructure, transportation networks, and indirect land use changes. According to the Food and Agriculture Organization (FAO) of the United Nations, at the same time, about 38% of the world's land area is used for agriculture, including arable/cropland and pasture for livestock grazing. Approximately 12% of the world's land area was classified as arable land and the remaining portion about 26%, is categorized as permanent pastures.  We’ll come back to them soon.

We saw earlier that cities  can see from book ‘Hungry City’ how the structure of cities reflects food habits, and thereby farm systems.

Soil of the city is under threat. The most obvious threat is the sealing of soil with impermeable materials such as asphalt, bricks, and concrete. Worldwide, cities are growing fast, and as they grow they consume massive areas of fertile farmland and its underlying soil. The scale of urban expansion is enormous. Across Europe, for example, upward of 500 square kilometers of land are sealed by asphalt, bricks, and concrete each year. To put this into perspective, this is an area roughly half the size of the city of Berlin. And in Germany alone, around 27 hectares of land are sealed every day, which is roundly the size of 30 football pitches. The consequences of this sealing for soil are dramatic: it prevents plants from being able to grow and abruptly ends the many functions that soils perform, such as the recycling and storage of water or nutrients, and the exchange of gases between the land and atmosphere. Soil sealing effectively suffocates the soil.”

That soil under the tarmac and concrete must be living anaerobically.  Most will have been compacted during the building operations and much buried so they would also be waterlogged. When water does not have the ability to drain from the soil, fresh oxygen cannot be pulled in Some plants like anaerobic conditions (blueberries), and can grow in waterlogged conditions (willows), so we could expect to grow them on Brownfield sites - ie those after industry.

"The sealing of soils by impervious materials is, normally, detrimental to its ecological functions. Exchanges of energy, water and gases are restricted or hampered and an increasing pressure is being exerted on adjacent, non-sealed areas.  " (Scalenghe & Marsam 2009)

Concrete jungle

Concrete just about kills the soil. Concrete covering causes reduced water infiltration, so there is less groundwater recharge, but more runoff. Concrete absorbs and retains heat resulting in higher surface temperatures. If you live in a city you know it is hotter, and you look for an oasis of coolness in the park. Perhaps the biggest affect is the restricted gas exchange, particularly of oxygen which we have seen time and again provides the means for aerobic metabolism, particularly helping animals and fungi, which is much better than anaerobic living which has to be driven by bacteria which get their energy from elsewhere. In turn this leads to poorer nutrient cycling especially as there is a lack of organic matter coming into the soil. There is also the soil compaction which reduce the crucial pore spaces, restricts possible root growth and smash that all important soil structure. All of which leads to loss of biodiversity, as it reduces or eliminates the habitat available for insects and springtails, although it would be interesting to see how mites are affected. 

On the 38% land used for farming, soil compaction is becoming a greater issue, as the size of farm machines gets bigger. Now, heavier than dinosaurs, soil has to put up with 5 or 6 traverses on grassland and may be up to 10 on cultivated land. Tramlines take up about 3.5% of any field. 50% of all tractors are bought by India and China.

GOOD

It is possible to engineer simple improvements to relieve the compaction. Simple plastic matrices can be put under pavements to allow roots to follow water channels in less compacted soil, so encouraging tree growth  “When using a suspended pavement system in an urban context, where space is extremely limited, excavating deeper is often the only viable option for providing trees with the soil volumes they need. Once the soil is installed it’s compacted to below 85% Proctor. Over time it will settle somewhat, but since the structure – not the soil – bears the load of the pavement, it will remain within acceptable densities for root growth. This is turn helps both flood problems but also the city temperature

Water

Many of our soils cannot infiltrate water effectively. We are seeing a huge difference in farms that are being managed very conventionally with corn-soybean rotations compared to farms being managed regeneratively and using adaptive grazing. In these water infiltration, we test for how long it takes for one inch of water, to infiltrate into the soil. On the conventionally managed soybean fields, it always takes over an hour to infiltrate even 1inch of water. On regeneratively managed soils water infiltration for the first 1 inch of water ranged from 21 seconds to just under two minutes. These soils had from 4-8inch depth of aggregation What is the common denominator for better water infiltration? Depth of soil aggregation. How do we build soil aggregation? By following the principles of soil health we can build soil aggregation rather quickly and see our water cycle begin to function as it should. Another benefit of building soil aggregation is the mineral cycle will also begin to function as it should resulting in the need for less purchased fertility.

From Dig a little, learn a lot