1. Soil: Foundation for Land Ecosystems Chapter 8

2. Soil Characteristics The processes of soil formation create a vertical gradient of layers that are often quite distinct. These horizontal layers are known as horizons. A vertical slice through the different horizons is called the soil profile.

3. Soil Particles As rock weathers, it breaks down into smaller and smaller fragments. Below the size of small stones, these fragments are classified as sand, silt and clay. Sand – 2.0 -.02 mm Silt -.02 -.002 mm Clay – anything finer than.002 mm

4. Soil Textures Soil Texture refers to the relative proportions of each type of particle in a given soil.

5. Soil Properties 1.Larger particles have larger spaces separating them than smaller particles have. 2.Smaller particles have more surface area relative to their volume than larger particles have. 3.Nutrient ions and water molecules tend to cling to surfaces.

6. Soil Properties These properties of matter profoundly affect such soil properties as infiltration, nutrient and water holding capacity, and aeration.

7. Workability of a Soil Workability is the ease with which a soil can be cultivated. Has a tremendous impact on agriculture. Ex: clay soils are very difficult to work with because they can go from being too sticky and muddy to being too hard and even bricklike with only the slightest change in moisture.

8. Soil Classes Soils come in an almost infinite variety of vertical structures and textures. To give some order to this diversity, soil scientists have created a taxonomy of soils. If you are classifying a soil you first find your order and work your way downwards through the taxonomic categories – suborders, groups, subgroups, families, to soil class.

9. Soil and Plant Growth For their best growth, plants need a root environment that supplies optimal amounts of mineral nutrients, water, and air (oxygen). The pH and salinity of the soil are also critically important. Soil’s fertility, the soil’s ability to support plant growth, often refers specifically to the proper amounts of nutrients.

10. Leaching Nutrients may be literally washed away from the soil as the water moves through it. Leaching not only lessens soil fertility, but also contributes to pollution when materials removed from the soil enter waterways. So, the soil’s capacity to bind and hold nutrient ions until absorbed by roots is just as important as the initial supply of those ions. This is referred to as the soil’s nutrient holding capacity or its ion exchange capacity.

11. Water and Water-Holding Capacity Water is constantly being absorbed by the roots of plants, passing up through the plant and exiting as water vapor through microscopic pores in the leaves – a process called transpiration.

12. Water and Water-Holding Capacity Water is re-supplied to the soil naturally by rainfall or artificially by irrigation. Three attributes of the soil are significant in this respect: 1. The soil’s ability to allow water to infiltrate, or soak in. 2. The soil’s ability to hold the water as it infiltrates (water holding capacity). 3. The soil’s evaporative water loss from the soil surface.

14. Aeration Roots need to breathe. Land plants depend on the soil being loose and porous enough to allow the diffusion of oxygen into, and carbon dioxide out of the soil – a property called soil aeration. Over-watering fills the air spaces in the soil, preventing adequate aeration. So does compaction, or the packing of the soil. It also reduces infiltration and increases runoff.

15. Salt and Water Uptake A build up of salts in the soil makes it impossible for the roots of a plant to take in water. If salt levels get high enough, water can be drawn out of the plant, resulting in dehydration and death. Only plants with special adaptations can survive in saline soils, none of those are crop plants.

16. Soil Structure As animals feed on the detritus on or in the soil, they often ingest mineral soil particles as well. It is estimated that as much as 15 tons per acre of soil pass through earthworms each year in the course of their feedings. As the mineral particles go through the worm’s gut, they actually become glued together by indigestible humus compounds. The earthworm’s excrements are called castings, which are relatively stable clumps of inorganic particles plus humus. The burrowing activity of organisms keeps the clumps loose.

17. Soil Structure Loose, lumpy characteristic of the soil is referred to as soil structure or arrangement of the particles. A loose soil structure is ideal for infiltration, aeration, and workability. Humus, has an extraordinary capacity for holding both water and nutrients. The clumpy, loose, humus-rich soil is the best for supporting plant life.

18. Interactions A highly significant interaction is between the roots of some plants and a certain fungi called mycorrhizae. Drawing some nourishment from the roots, mychorrhizae penetrates the detritus, absorbs nutrients and transfers them directly to the plant. There is no loss of nutrients to leaching.

19. Interactions Not all interactions are good – there are nematodes, small worms, that feed on living roots and are highly destructive to agricultural crops. However, the population of nematodes are kept in check by a fungus that feeds on the worms.

20. Soil Enrichment or Mineralization Now you can see how the aboveground portion and the soil portion of an ecosystem support each other. The detritus, which supports the soil organisms, is from the green plant producers, so green plants support the soil organisms. By feeding on the detritus, the soil organisms create the chemical and physical soil environment that is most beneficial to the growth of the producers.

22. Green Plants and Soil Green plants protect the soil and themselves in two other ways: 1. Protects the soil from erosion. 2. Reduces evaporative water loss.

23. If the cycle breaks… Without the addition of detritus, soil organisms will starve and the benefit to the soil will be lost. As the soil’s humus is lost, water and nutrient holding capacity, infiltration, and aeration decline correspondingly. This loss of humus and the consequent collapse of topsoil is known as mineralization, because what is left is just the gritty, mineral content – sand, silt and clay (no humus).

25. 8.2 Soil Degradation How is topsoil lost? Erosion – the process of soil and humus particles being picked up and carried away by water or wind. Splash erosion – impact of falling raindrops breaks up the clumpy structure of the topsoil. The dislodged particles wash into spaces between other aggregates, clogging pores and thereby decreasing infiltration and aeration.

27. How is topsoil lost? Sheet erosion – the decreased infiltration results in more runoff and carrying away fine particles of the surface. Gully erosion – the water converges into streams, which have greater volume, velocity and energy. The result is the erosion into gullies.

29. Desert Pavement The lighter particles of humus and clay are the first to be carried away, while rocks, stones, and coarse sand remain behind. As erosion removes the finer particles, the remaining soil becomes progressively coarser. The sand is what remains after the finer, lighter clay and silt particles have blown away. In some deserts the wind has left a thin surface layer of stones and coarse sand is called a desert pavement.

31. Desertification As clay and humus are removed, nutrients are also removed. As these soil properties are diminished by the erosion of the topsoil, such areas become deserts. Desertification is used to refer to this process. Definition – the formation and expansion of degraded areas of soil and vegetation cover in arid, semiarid, and seasonally dry areas, caused by climatic variations and human activities.

32. Causing and Correcting Erosion The three major practices that expose soil to erosion and lead to desertification are overcultivation, overgrazing, and deforestation.

33. Overcultivation Plow to control weeds – soil is then exposed to wind and water erosion. It may remain bare for a long time before newly planted crop forms a complete layer. Plow to loosen the soil and improve aeration and infiltration – splash erosion destroys the soil’s structure and seals in the surface. Weight of the Tractors – increases the compaction of the soil.

34. Correction No – till agriculture – the field is sprayed with herbicide to kill weeds, and then a planting apparatus is pulled behind a tractor to accomplish several operations at once. A steel disk cuts a furrow through the mulch of deed weeds, drops seed and fertilizer into the furrow and then closes it. At harvest, the waste from the previous crop becomes the mulch to cover the next. The soil is never left uncovered and erosion and evaporative water loss is reduced.

35. Overgrazing Grasslands that receive too little rainfall to support cultivated crops have traditionally been used for grazing livestock. Likewise, forested slopes that are too steep for cropping are commonly cleared and put into grass for grazing. These lands are too often overgrazed. As the grass production fails to keep up with consumption, the land becomes barren. Desertification results.

36. Public Lands Overgrazing occurs because the rangelands are public lands not owned by the people who own the animals. Herders who choose to withdraw their livestock sacrifice income. Others continue to overgraze the range. This is known as the “Tragedy of the Commons”.

37. Taylor Grazing Act of 1934 The U.S. Bureau of Land Management and the Forest Service lease 2 million km 2 of federal lands for grazing rights at a nominal fee of $1.35 per animal unit per month. The fee structure was established under the Taylor Grazing Act of 1934, which effectively prohibited any reduction in grazing levels and kept fees below market value. The government loses about $124 million dollars a year and the amount of livestock is considered too high by nearly all range experts.

38. Deforestation Forest Ecosystems are extremely efficient systems both for holding and recycling nutrients and for absorbing and holding water, because they maintain and protect a very porous, humus-rich topsoil. Investigators at the Hubbard Brook Forest project in NH found that by converting a hillside from forest to grass doubled the amount of runoff and increased the leaching of nutrients.

39. Deforestation If a forest is simply cut and the soil is left exposed pounding raindrops quickly seal the soil. The topsoil becomes saturated with water and slides off the slope in a muddy mass into waterways, leaving barren subsoil that continues to erode.

40. Global Loss Forests continue to be cleared at an alarming rate - about 23.2 million acres per year in the 1990s, most of which is in the developing countries. About 80% of deforestation is for agricultural purposes directed toward the production of cash crops or the conversion of grass to grow beef. Less than 2% of tropical forests are under any kind of management plan for the protection or harvesting of forest products on a sustainable basis.

42. Irrigation Supplying water to croplands by artificial means. Center pivot irrigation – water is pumped from a gigantic sprinkler that slowly pivots itself around the well. Total irrigated land is about 667 million acres globally.

43. Salinization The accumulation of salts in or on the soil to the point where plant growth is suppressed. Salinization occurs because even the freshest of fresh water contains.02%-.05% of dissolved salts. As the water leaves by evaporation or transpiration, the salts remain behind and gradually accumulate as a precipitate. Because this happens mostly in drylands, salinization is considered a form of desertification, since it leaves the land less productive or useless.