Take one step further.  Cereals are mostly derived from hardy grasses while other field crops have similar abilities to thrive while being offered relatively low levels of nutrients.  With good management, fertile soils are able to present these lower nutritional levels to growing plants without amendment or fortification with potent, concentrated nutrient sources.  But most vegetables demand far higher levels of support.  Few soils, even fertile soils that have never been farmed, will grow vegetables without improvement.  Farmers and gardeners must increase fertility significantly if they want to grow great vegetables.  The choices they make while doing this can have a strong effect, not only on their immediate success or failure, but on the actual nutritional quality of the food that they produce.

How Humus Benefits Soil

The roots of plants, soil animals, and most soil microorganisms need to breathe oxygen.  Like other oxygen burners, they expel carbon dioxide.  For all of them to grow well and be healthy, the earth must remain open, allowing air to enter and leave freely.  Otherwise, carbon dioxide builds up to toxic levels.  Imagine yourself being suffocated by a plastic bag tied around your neck.  It would be about the same thing to a root trying to live in compacted soil.

A soil consisting only of rock particles tends to be airless.  A scientist would say it had a high bulk density or lacked pore space.  Only coarse sandy soil remains light and open without organic matter.  Few soils are formed only of coarse sand, most are mixtures of sand, silt and clay.  Sands are sharp-sided, relatively large rock particles similar to table salt or refined white sugar.  Irregular edges keep sand particles separated, and allow the free movement of air and moisture.

Silt is formed from sand that has weathered to much smaller sizes, similar to powdered sugar or talcum powder.  Through a magnifying lens, the edges of silt particles appear rounded because weak soil acids have actually dissolved them away.  A significant amount of the nutrient content of these decomposed rock particles has become plant food or clay.  Silt particles can compact tightly, leaving little space for air.

As soil acids break down silts, the less-soluble portions recombine into clay crystals.  Clay particles are much smaller than silt grains.  It takes an electron microscope to see the flat, layered structures of clay molecules.  Shales and slates are rocks formed by heating and compressing clay.  Their layered fracture planes mimic the molecules from which they were made.  Pure clay is heavy, airless and a very poor medium for plant growth.

Humusless soils that are mixtures of sand, silt, and clay can become extremely compacted and airless because the smaller silt and clay particles sift between the larger sand bits and densely fill all the pore spaces.  These soils can also form very hard crusts that resist the infiltration of air, rain, or irrigation water and prevent the emergence of seedlings.  Surface crusts form exactly the same way that concrete is finished.