Even in dry areas there is some rain. As this rain water infiltrates into the ground, it dissolves ions. However, rainfall events are of short duration. The ion-laden water returns to the surface due to capillary action; and evaporation resumes, leaving an accumulation of chemicals, especially the more readily mobilized salts of calcium (calcite or gypsum) or halides and related salts. As a group, these soils are called pedocals because of the dominance of calcium accumulation. The layer of minerals that accumulates as a result of evaporation is called a hard pan. When calcium is the dominant element, such a hard pan is called caliche. 

In semi-arid areas this process of salt accumulation is often aggravated by irrigation. Ground water, used as an additional water supply, always contains some dissolved ions. As greater amounts of water circulate through the soil and evaporate, adding their own supply of ions to those already present in the soil, rates of chemical accumulation increase. More than one desert or semi-desert which first bloomed under irrigation has seen its yields decrease over time as the result of salt accumulation. 

On the other end of the scale lie the soils which are watered with an ample supply of rain. In those soils the water continually leaches through the ground, removing soluble materials and leaving the more insoluble clays and compounds of iron and aluminum. This general category of soils, called pedalfers, is dominated by leaching. 

As temperature and rainfall increase, the leaching becomes more severe to the point where, eventually, the clays themselves break down into oxides and hydroxides. As can be expected, such soils called laterites, common in tropical areas, are not very fertile. Although the rain forests of the world appear at first glance to be luxurious, only limited amounts of nutrients are available and the rates of production of new biomass are low. Under even more extreme conditions, the iron compounds are leached out, leaving only a residue of aluminum oxides and hydroxides. This weathering residue is called a bauxite, and is one of our main sources of aluminum ore. 

Ideally of course, a soil should have all of its basic constituents. It should have sufficient leaching to prevent accumulation, but not enough so as to cause removal of the nutrients. Such a soil will naturally continue to promote growth in plants without requiring tending, as weathering processes renew the nutrients taken up by plants. Such soils are commonly found in areas with temperate, moderately watered climates, and are the bread baskets of the world. 

However, modern agricultural practices, especially those associated with intensive crop production, create greater nutrient demands upon soils than weathering processes can replenish. Today, we supplement these deficiencies with fertilizer. Because uptake of nutrients by plants is relatively slow, most of the fertilizer is leached out of the soil by rain water and finds its way into ground water supplies and surface waters. There, it causes chemical pollution and eutrophication. 

Likewise, poor farming practices have led to the loss of much of our topsoil, necessitating increased applications of fertilizer to maintain constant levels of production. The high cost of mismanagement of soils and intensive cultivation practices can best be illustrated if we consider that it takes nine or more calories of invested energy to produce one food calorie.