SAT Reading: Practice Test 3

Questions 1–10 are based on the following passage.

This passage is adapted from R. Smith’s “The Organic Way of Life.”

Compared to the atmosphere, soil is a place where temperature fluctuations are small and slow. Consequently, soil animals are generally intolerant to sudden temperature changes and may not function well over a very wide range. That’s why leaving bare earth exposed to the hot summer sun often slows plant growth and why many thoughtful composters either put down a thin mulch in summer or try to rapidly establish a cooling leaf canopy to shade raised beds. Except for a few microorganisms, soil animals breathe oxygen just like other living things and so are dependent on an adequate air supply. Where soil is airless due to compaction, poor drainage, or large proportions of very fine clay, soil animals are few in number.

The soil environment is generally quite moist, and even when the soil seems dry the relative humidity of soil air usually approaches 100 percent. Soil animals consequently have not developed the ability to conserve their body moisture and are speedily killed by dry conditions. When faced with desiccation they retreat deeper into the soil if there is oxygen and pore spaces large enough to move about. So we see another reason why a thin mulch that preserves surface moisture can greatly increase the beneficial population of soil animals. Some single-cell animals and roundworms are capable of surviving stress by encysting themselves, forming a little “seed” that preserves their genetic material and enough food to reactivate it, coming back to life when conditions improve. These cysts may endure long periods of severe freezing and sometimes temperatures of over 150 degree F.

Inhabitants of leaf litter reside close to the surface and so must be able to experience exposure to dryer air and light for short times without damage. These are called primary decomposers. They spend most of their time chewing on the thick reserve of moist leaves contacting the forest floor. Primary decomposers are unable to digest the entire leaf. They extract only the easily-assimilated substances from their food: proteins, sugars and other simple carbohydrates and fats. Cellulose and lignin are the two substances that make up the hard, permanent, and woody parts of plants; these materials cannot be digested by most soil animals. Interestingly, there are a few larvae whose digestive tract contains cellulose-decomposing bacteria but these larvae have little overall effect.

By the time the primary decomposers are finished, the leaves have been mechanically disintegrated and thoroughly moistened, worked over, chewed to tiny pieces and converted into minuscule bits of moist excrement still containing active digestive enzymes. Many of the bacteria and fungi that were present on the leaf surfaces have passed through this initial digestion process alive or as spores waiting and ready to activate. Digestive wastes of primary decomposers are thoroughly inoculated with microorganisms that can consume cellulose and lignin. Even though it looks broken down, it has not yet fully decomposed. It does have a water-retentive, granular structure that facilitates the presence of air and moisture throughout the mass creating perfect conditions for microbial digestion to proceed. Both secondary and primary decomposers are necessary to complete the composting process.