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What Makes A Soil?
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Last week’s column asserted that soil is the foundation of our agricultural industry and a scarce resource. This week let’s start to examine why. First, 70% of the surface of the earth is covered with water. Of the remaining 30%, only 11% is considered arable, suitable for farming, or approximately 3.3% of the total of the earth’s surface. Of that total, well over half has been degraded to some extent.  Often that degradation is due to soil erosion (loss of the soil) by air or water. Why is that a big deal? A large part of the answer is found in how soils form. There isn’t enough room for great detail but for at least an appreciation of the process.
The process of soil formation is summed up in one simple equation: soil f (PM x C x B x LS x T) where the formation of soil is a function of:
· PM – Parent Material is what the soil is formed from. Initially soils formed from igneous, sedimentary and metamorphic rock and the mineral composition of the rock determined how fast the primary minerals could breakdown physically and chemically. Soils can form from materials already in place or materials deposited by wind, water, glaciers. The soils of NE Kansas formed from loess, silt deposited by wind after glaciers retreated. The delta soils of rivers formed from soil particles carried by water. The parent material determines the mineral composition of the soil soils, what type of clays can form, and what the native fertility is.
· C – While all factors in the equation work together, parent material is the most important factor followed by Climate. Climate, especially temperature and precipitation, determine not only the rates of parent material weathering but also how the remaining factors work. Soil formation is termed a bio- and geo- chemical process meaning that water and heat are needed for formation to proceed. When starting from primary minerals, wet-dry and warm-cold cycles speed up weathering.
· B – The biota or living organisms play a critical role in the rate of soil formation and the type of soil you end up with. Precipitation or lack of it determine whether plant life moves to trees or grasses and this is important in determining soil pH (acidity), fertility, and soil structure. The type of soil microorganisms is a function of the plant life, climate, and soil chemistry which in turn affects a host of soil properties.
· LS – Landscape position or Topography is where the soil is forming in the landscape. North facing slope, south facing slope, flood plain, steep slope, and so on, all have an impact of the microclimate, the biota, and therefore the rate of soil formation
· T – Soil formation takes time. However, the “age” of the soil isn’t just a function of so many years but of how developed a soil is. A young, poorly developed soil in the desert Southwest may be 10,000 years old while a mature, well developed soil in the South may have matured in less than half of that time. So time here is relative and a function of what the soil is forming form and what the climatic conditions are.
Putting all this together in a nutshell means that even under the best of conditions it takes centuries to develop a mature, productive soil. For areas like the Great Plains where conditions aren’t as favorable, that time frame is extended to thousands of years. So every time an inch of topsoil is lost by wind or water, not only is productivity but the result of centuries of soil development.