Chapter 4 - Food and Agriculture (Part II) PDF

#Es Part II  Soil degradation & conservation A rich soil is much more than dirt  Soil: solid material of geological and biological origin  Chemical, biological, and physical processes change it and give it the ability to support plant growth  Soil has multiple functions: ...

#Es Part II  Soil degradation & conservation A rich soil is much more than dirt  Soil: solid material of geological and biological origin  Chemical, biological, and physical processes change it and give it the ability to support plant growth  Soil has multiple functions:  Supporting crop growth, filtering water, helping Earth’s nutrient cycles and exchange of gases  Soil is a fundamental part of terrestrial ecosystems  It is home to 25% of all species, including detritus feeders and decomposers  It involves dynamic interactions among organisms, detritus, and mineral particles Soil characteristics: soil texture  Most soil is hundreds or thousands of years old  Soil texture: relative proportions of each soil type  Parent material: mineral material of the soil  Weathering: physical and chemical breakdown of parent material into smaller fragments  Sand: particles from 2.0 to 0.063 mm  Silt: particles range from 0.063 to 0.004 mm  Clay: anything finer than 0.004 mm  Gravel, cobbles, boulders are larger than sand  Loam: ideal for farming—holds nutrients and water  40% sand, 40% silt, and 20% clay The relative sizes of soil particles Soil particles range in size from very small (clay) to larger (sand) particles © 2017 Pearson Education, Inc. Topsoil formation Soil production involves dynamic interactions among minerals, detritus, and detritivores © 2017 Pearson Education, Inc. Soil components affect its properties  Soil properties are influenced by its texture  Larger particles have larger spaces between them  Small ones have more surface area relative to their volume  Nutrient ions and water molecules cling to surfaces  These properties affect water infiltration, nutrient- and water-holding capacity, and aeration  Workability: how easily soil can be cultivated  Clay soils: hard to work with—too sticky or too hard  Sandy soils are easy to work with Soil profiles  Soil horizons: horizontal layers of soil  They can be quite distinct  Soil profile: a vertical slice through the soil horizons  Reveals the interacting factors in soil formation  O horizon: topmost layer of soil  Dead organic matter (detritus) deposited by plants  High in organic content  Primary source of energy for the soil community  Humus: decomposed dark material at the bottom of the O horizon Soil and plant growth  For best growth, plants need a root environment that supplies mineral nutrients, water, oxygen  Plus, the proper pH and salinity (salt content)  Soil fertility: soil’s ability to support plant growth  The presence of nutrients and all other needs  Tilth of soil: its ability to support plant growth  Nutrients (phosphate, potassium, calcium, etc.) become available through:  Rock weathering: much too slow for plant growth  Breakdown and release (recycling) of detritus Loss and gain of nutrients and water  Leaching: nutrients are washed from soil by water  Decreases soil fertility and pollutes waterways  Nutrient-holding capacity: the soil’s ability to bind nutrient ions until they are absorbed by roots  Transpiration: movement of water through plants  Water is absorbed by roots  Water vapor and oxygen exit, carbon dioxide enters, through pores (stomata; s = stoma) in the leaves  Loss of water through stomata can be dramatic  Plants with inadequate water wilt What affects a soil’s ability to get water?  Water is resupplied to the soil by rainfall or irrigation  Infiltration: water soaks into the soil  Runoff is useless to plants and may cause erosion  Water-holding capacity: soil’s ability to hold water after it infiltrates  Poor holding capacity: water flows below root level  Sandy soils  Evaporative water loss depletes soil of water  The O horizon reduces water loss by covering the soil Plant-soil-water relationships © 2017 Pearson Education, Inc.  Aeration  Roots must breathe to obtain oxygen for energy  Land plants depend on loose, porous soil  Soil aeration: allows diffusion of oxygen into, and carbon dioxide out of, the soil  Overwatering fills air spaces and drowns plants  Compaction: packing of the soil due to excessive foot or vehicular traffic  Reduces soil air and infiltration and increases runoff  It is strongly influenced by soil texture Relative acidity (pH) and salt in the soil  pH refers to the acidity or alkalinity of any solution  The pH scale runs from 1 to 14 (7 is neutral)  Different plants are adapted to different pH ranges  Most do best with a pH near neutral  Some plants do better with acidic or alkaline soils  Too much salt prevents roots from taking up water  High salt levels dehydrate and kill plants  Only specially adapted plants grow in saline soils  None of them are crops  Irrigation can cause salinization (salt buildup in soil) Soil and carbon storage  Soils hold carbon from dead organisms  three times as much as held in the atmosphere and plants  What drives the rate of organic decay in soils?  Temperature, moisture, separation of particles, placement of roots, microbial communities  Scientists want to know how soil will change due to:  Changes in leaf litter, climate change, and the effects of soil biota The soil community  To support plants, soils must:  Have nutrients and good nutrient-holding capacity  Allow infiltration, have good water-holding capacity, and resist evaporative water loss  Have a porous structure that allows aeration  Have a near-neutral pH  Have low salt content  According to the principle of limiting factors, the poorest attribute is the limiting factor  Silts and loams are best for crops because limiting factors are modified by organic matter Soil organisms and organic matter  Dead leaves, roots, other detritus on and in soil support a complex food web  Bacteria, fungi, protozoans, mites, millipedes, insects, spiders, earthworms, snails, slugs, moles  Millions of bacteria are in a gram of soil  Humus: residue of partly decomposed organic matter at the bottom of the O layer  Extraordinary capacity for holding water and nutrients  Composting: fosters decay of organic wastes  Compost is essentially humus Soil enrichment  Most detritus comes from green plants  Green plants support soil organisms  Soil organisms create the chemical and physical soil environment beneficial to plants  Green plants further protect the soil by:  Reducing erosion and evaporative water loss  So keep an organic mulch around garden vegetables  The mutually supportive relationship between plants and soil is easily broken  Keeping topsoil depends on addition of detritus What happens if detritus is lost?  Soil organisms starve  Soil will no longer be kept loose and nutrient-rich  As humus decomposes, the clumpy aggregate structure of glued soil particles breaks down  Water- and nutrient-holding capacities, infiltration, and aeration decline  Mineralization: humus is lost and topsoil collapses  All that remains are the minerals (sand, silt, clay)  Topsoil results from balancing detritus and humus additions and breakdown  The importance of humus to topsoil Detritus additions and humus-forming processes are balanced against their breakdown and loss © 2017 Pearson Education, Inc. Fertilizer  Fertilizer: material that contains one or more necessary nutrients (nitrogen, phosphorous)  Replenishes nutrients in topsoil  Organic fertilizer: plant and animal wastes  Manure and compost, leguminous fallow crops (alfalfa, clover), food crops (lentils, peas)  Inorganic fertilizer: nutrients without organic compounds  Does not support soil organisms or build soil structure Soil degradation  In natural ecosystems, new detritus is always added to topsoil  Cutting forests, grazing livestock, growing crops: soil is at the mercy of management or mismanagement  Soil degradation: a reduction in the capacity of soil to support plants and perform ecosystem functions  A worldwide problem  Erosion is one of the worst forms of soil degradation Soil degradation affects about 200 million hectares—about 38% of the world’s cropland© 2017 Pearson Education, Inc. Erosion: a critical land degradation  Erosion: soil and humus particles are picked up and carried away by water and wind  Occurs any time soil is exposed to the elements  Soil removal may be slow and gradual (by wind) or dramatic (e.g., gullies formed by a single storm)  Vegetative cover prevents erosion by:  Reducing the energy of raindrops  Allowing slow infiltration into the soil  Minimizing and slowing runoff  Slowing wind velocity and holding soil particles Erosion in Mexico Severe erosion due to poor farming practices © 2017 Pearson Education, Inc. The other end of the erosion problem  Water that can’t infiltrate enters streams and rivers, overfilling them and causing flooding  Sediment: eroding soil that enters surface waters  Clogging channels and intensifying floods  Filling reservoirs, killing fish, damaging ecosystems  The greatest source of surface-water pollution?  Excess sediments and nutrients from erosion  Rainfall runoff also depletes groundwater  The tight connection between healthy soil and availability and quality of water is obvious Causes of erosion: overcultivation  Plowing exposes soil to wind and water erosion  Soil remains bare before planting and after harvest  Plowing causes splash erosion  Destroying soil’s aggregate structure  Decreasing aeration and infiltration  Tractors compact soil, reducing aeration and infiltration  Evaporative water loss is increased  Crop rotation is sustainable—planting a cash crop (e.g., corn) one year, but other years planting hay and clover (fixes nitrogen, adds organic matter) Causes of erosion: overgrazing  Livestock graze on land that can’t grow crops  Overgrazed plants can’t keep up with consumption  Reduced grasses lead to erosion and barren land  In the 1800s, American bison were slaughtered  Rangelands stocked with cattle were overgrazed  Leading to erosion and growth of unpalatable plants  U.S. rangelands produce less than 50% of the forage they produced before commercial grazing Causes of erosion: deforestation  Porous, humus-rich forest soil efficiently holds and recycles nutrients and absorbs and holds water  Removing forests exposes the soils  Increasing runoff and nutrient leaching  Saturated topsoil slides off the slope  Subsoil continues to erode  Cutting tropical rain forests causes acute problems  Tropical soils (oxisols) lack nutrients due to leaching  Rains wash the thin layer of humus away  Leaving only the nutrient-poor subsoil Conserving and restoring soil  Healthy soils are essential for human societies  Sustainability means doing all we can to reduce erosion and prevent degradation  Soil conservation must be practiced at many levels to bring about sustainable stewardship of soil resources  Individual landholders can preserve soil by using traditional knowledge and conservation techniques  Public policies can lead to conservation or disaster Techniques to reduce soil erosion  Contour plowing: plowing and cultivating at right angles to the contour of slopes  Slows downhill flow of water  Shelterbelts: protective rows of trees and shrubs planted beside plowed fields  Protects fields from wind and blowing snow  Plants grown along waterways and fields filter runoff  No-tillfarming: allows continuous cropping while minimizing erosion  Routinely practiced in the United States  Contour farming and shelterbelts Contour farming and strip Shelterbelts slow the wind and cropping (planting different protect soil from erosion kinds of plants in one field) © 2017 Pearson Education, Inc. No-till farming minimizes erosion  After spraying a field with herbicide to kill weeds  A planting apparatus cuts a furrow through the mulch  It drops in seeds and fertilizer and closes the furrow  The waste from the previous crop becomes detritus  So the soil is never exposed  Erosion and evaporation are reduced and detritus maintains the topsoil  But, herbicides are used, which create their own problems  Apparatus for no-till planting This no-till planter is planting corn over stubble, with minimal disturbance to the protective vegetation covering the soil © 2017 Pearson Education, Inc. In some cases, soil can be restored  Addition of organic material, use of rooted plants, and introduction of soil biota  Compost, manure, other plant products, and earthworms are added to enhance soils  Removal of soil contaminants  Bioremediation: using organisms (microbes, plants) to remove pollutants Restoring Soil Fertility  By using organic fertilizers derived from plant and animal materials  Animal manure  Green manure  Compost  Biochar  By using synthetic inorganic fertilizers made of inorganic compounds  By rotating crops We must protect soil health  Development goals are related to soil health  Sustainable Development Goal (SDG) 2: food security and sustainable agriculture relates to soils  SDG 15: combats desertification and ends land degradation  Most protection efforts focus on agriculture  But must include urban soils, mining and waste degradation, the effects of climate change on soil