6.1.5: Understanding Soil Maps at a Broad Global Level

Soil scientists have done an enormous amount of work in mapping the patterns of soil at a global level. The most current and detailed effort comes out of mapping work from the Food and Agriculture Organization of the United Nations, now an independent agency that is known as the International Soil Resource Information Centre (ISRIC), and is based on classifying a set of diagnostic types of topsoil layers that occur in different climates, landscape ages, and vegetation types. The details of this system⁵ are beyond the scope of this course, however, and to summarize the introduction to global soil fertility in this unit we present a simplified version of the United States Department of Agriculture (USDA) system that is still in wide use by soils practitioners in the United States. The USDA system lines up very well with the ISRIC system at this simplified level and allows understanding of the broad strokes of soil nutrient geography in the way we have presented it (Figure 3.8).

Figure 3.8: Simplified map of soil types in the world and associated characteristics, referred to the USDA soil classification system. Source: adapted by S. Vanek from the USDA Natural Resource Conservation Service (NRCS)

This simplified map is intended to serve as a resource for your other learning in the course on how food systems may respond to the opportunities and limitations of soils, and also summarizes the learning in this module about how soils result from an interaction of parent material, time, climate, vegetation, and other factors. For example, you’ll notice that just four very broad summarized types (See section 1 of the soils key, “Dominant global soils” in Fig. 3.8) cover the vast majority of the earth’s surface, and can be organized into a rough typology of precipitation from wet to dry, along with their age and vegetation types (e.g. tropical and subtropical forests; other forest types; grasslands, and desert vegetation). Soils formed by temperate grasslands have been hugely important in recent history because once humans developed steel plows that were sufficiently strong to til prairie soils, these Mollisols could be farmed and became the breadbaskets of the modern era (e.g. the U.S. and Canadian Great Plains, the Ukraine, the Argentinian pampas). There are also small pockets of soils globally that depend strongly on their original parent material. Andisols or volcanic ash soils are an excellent example of this: although their global extent is minuscule and even invisible on our map (Fig. 3.8) at this scale, they often occur in areas with high population densities such as Ecuador, Japan, and Rwanda. The high densities of population are not an accident but occur exactly because these soils have high fertility potential and have become extremely important in these local food systems. The simplified global soils map is also a way to spatially conceptualize a number of key limiting factors in soils that food producers must face: acidic, P-retaining soils in highly weathered tropical and subtropical soils, P retention in volcanic soils, and the risk of salinization of soil in dry climate soils.

In addition, it is worth noting that the broad swaths of soil of young to moderate age and with moderate to high fertility (light green in our map) may be the dominant type of soil in the world and also includes many areas that are critical in terms of the sustainability outcomes for human-natural systems in relation to soils. Because these tend to be “medium-everything” soils (medium age, medium fertility, medium depth, medium pH, medium moisture, etc.) they do not actively dissuade human systems from occupying them with high population densities or intensity of management and production, especially as the global population increases. However these soils are often easily degraded, and so sustainable methods are especially important to guarantee future food production.

Finding out information on soils using the soil order suffix in the name of the soil according to the USDA soil taxonomy system.

Soil taxonomy is an enormous classification system that can initially be confusing. But knowing the first level of classification can be very useful, just like knowing whether an animal is a whale or a beetle is extremely helpful compared to not knowing anything. To classify soils broadly as to their limitations and productive potential, we can use the soil orders of the USDA system (see the order names in parentheses, in Fig. 3.8).

The key below will help you to use the last few letters of a USDA soil name, along with the ISRIC world soil mapping resource to query what types of soil are present around the world or specifically in your capstone regions. The categories are the same as what is presented in Figure 3.3, and you can use the query function in the ISRIC world soil mapper to find out what USDA soil names are present in each area, and draw conclusions about the potential fertility and properties of the soils at a broad level.

First, see the ISRIC resource is at SoilGrids. This was also used in the formative assessment for Module 3.1.

In the ISRIC mapper you will need to click on layers icon in upper right and set the layer to “Soil Taxonomy: TAXOUSDA” and select the “All TAXOUSDA subclasses” -- when you query the map using a right click of the mouse, you’ll get a percent breakdown of the different soil orders at that location.

Key to USDA Soil Taxonomy System

Soil name ending	Meanings	Example
-Epts -Ents -Alfs	Entisols : soils of recent deposition, no soil development. Inceptisols: the beginning of soil formation – medium to high fertility soils Alfisols: broad class of medium age, medium to high fertility soils	Glossoboric hapludalfs Orthents
-Ols	Mollisols: prairie soils, high organic matter, generally neutral pH, fertile, deep	Dystric haplustolls
-Ids	Aridisols – dry region soils, generally high pH	Argids
-Ods	Spodosols – coniferous forest soils with acid needle litter leaching features	Orthods
-Ults -Oxes	Ultisols – warm region, old, leached soils Oxisols – oldest tropical soils formed only of weathering remnants, metal oxides	Udults
-Ands	Andisols- volcanic ash soils	Vitrands
-Erts	Vertisols – highly weathered limestone, with shrink-swell clays.	Uderts

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⁵ Nevertheless, you may peruse this impressive global resource and the soil horizon definitions at ISRIC.