Environmental Chemistry Chapter 18: Soil, Agriculture, and Food Production Copyright © 2011 by DBS

Environmental Chemistry

Chapter 18:Soil, Agriculture, and Food Production

Copyright © 2011 by DBS

Contents

• Soil and Agriculture• Soil: Essential for Life, Key to Sustainability• Nature and Composition of Soil• Acid-Base and Ion-Exchange Reactions in Soils• Macronutrients in Soil• Nitrogen, Phosphorus, and Potassium in Soil• Micronutrients in Soil• Soil Loss and Deterioration• Wastes and Pollutants in Soil• Saving the Land• Process Intensification in Agriculture• Sustainable Agricultural Management• Agroforestry

Soil and Agriculture


• Soil layer is extremely thin

• Essential to support most life on Earth, cf. ozone layer


Approximately 10,000 years ago, the harsh circumstances described above changed when humans in the Fertile Crescent, an area now called the Middle East, learned to cultivate certain grasses that produced grain that could provide a source of food. Furthermore, this grain could be stored for long periods of time in dry granaries, providing a stable source of food. About that time as well, humans domesticated some animals, including sheep that provided wool and meat, goats that provided milk and meat, and donkeys that could be used for transport and to provide power to cultivate land.

Humans had discovered agriculture, the production of food and fiber by raising plants and animals. The changes brought about by agriculture were many and profound. The practice of agriculture meant that human populations needed to stay in particular locations conducive to the growing of crops. It freed significant numbers of people from the task of getting food, so that human ingenuity could be devoted to other pursuits, such as the development of wheeled vehicles, the construction of sailing boats, and the discovery of writing.


Agriculture

• Agriculture is the production of food and fiber by raising plants and animals

– Crop farming to produce food and fiber from plants

– Livestock farming for meat, milk, hide, other animal products

• Production greatly increased by breeding improved varieties

– Hybrid plants

– Now recombinant DNA technology and genetic engineering

• Agriculture extremely productive

– Top five food plants: Wheat, corn, rice, potatoes, soybeans


Agriculture

• Enormous environmental effects from agriculture

(i) Conversion of most forest and grasslands to crop production

– Displacement of native plants

– Destruction of wildlife habitat

– Erosion

– Pesticide pollution

(ii) Domestic animals effects on environment

– Huge, concentrated operations with thousands of animals to produce meat, milk, and eggs

– Conversion of forest lands to grazing land

– Destruction of grasslands by the grazing practices of goats and sheep

– Methane to the atmosphere from ruminant animals and rice paddies

– Enormous consumption of water to produce animal protein

• Ruminant animals enable utilization of cellulose as a food source


• Sustainable Agriculture

– Irrigation with minimal quantities of water

– Fertilizer and pesticides to increase yields

– Use of herbicides to enable minimal tillage agriculture

– With population growth, amount of arable land per person decreases

– Collapse of world fisheries from poor management and over-exploitation


Paul MacCready’s graph and his ‘50 year time bubble’

Soil: Essential for Life, Key to Sustainability


• Heavy pressures on the soil resource

– Less soil per person

– Drought from global warming

– Loss of soil fertility

– Desertification of soil

• Soil conservation has been a generally successful environmental movement in the U.S. since about 1900

– Now concerned with urbanization


What is soil?

• Soil is a relatively incohesive material consisting of a mixture of minerals, organic matter, air spaces, and water capable of supporting plant life

– From rock weathered by physical, chemical, biological processes

– Organic matter is mostly partially decayed plant biomass

• Five ecological roles for soil

1. Medium for plant growth

2. Habitat for soil-dwelling organisms

3. Medium for decay of biomass leading to nutrient recycle

4. Key component of the hydrologic cycle in water transfer and purification

5. Key component of the anthrosphere in engineered soil


Figure 18.1. Ecological Roles for Soil


• In addition to being the site of most food production, soil is the receptor of large quantities of pollutants, such as particulate matter from power plant smokestacks, fertilizers, pesticides, and some other materials applied to soil often contribute to water and air pollution.

• Soil is a key component of environmental chemical cycles. It is a valuable part of Earth’s natural capital.

Nature and Composition of Soil


Soil: A 3-phase material

minerals, organic matter, water, air, mircobes, plant roots


Soil Formation

• ‘Pedogenesis’ – physical, chemical and biological processes

• Important factors in soil formation:

– Parent material

– Climate

– Landscape topography

– Biotic Factors – vegetation, animals

– Timescale

Results in a large variety of soils in different locations, each with a set of distinct layers (horizons) that make up a soil profile


Soil Horizons

• Rainwater carries dissolved and colloidal solids to lower horizons

• Biological processes (decay of biomass) produce CO2, organic acids, and metal-binding complexing agents which move to lower horizons altering the properties of the minerals.

• 5 master horizons:

– O horizon – plant debris in various stages of decay (misisng in grasslands)

– A horizon – called topsoil, rich in organic matter and humus, bacteria, fungi, plant roots and worms

– E horizon – site of maximum eluviation (washout), depleted of clay and Al Fe oxides, leaving resistant minerals such as quartz

– B horizon – zone of accumulation, repository of organic matter, salts, and clay particles leached from higher layers

– C horizon – comprised of fractured and weathered parent materials



Water and Air in Soil

• Large quantities of water required for plant production• Transport medium for carrying nutrients to plants• Transfer to plants is governed by capillary and gravitational forces• Strong interaction between clays and water in soil



Figure 18.4. Plants transport water from soil to the atmosphere by transpiration, carrying plant nutrients with it



• Waterlogging of soil can cause toxic metals accumulation

• Reducing conditions

MnO2(s) + 4H+ + 2e- Mn2+(aq) + 2H2O

Fe2O3(s) + 6H+ + 2e- 2Fe2+(aq) + 3H2O

– Later exposure to air precipitates MnO2 and Fe2O3

• Air in soil depleted of O2 and enriched in CO2

• Due to decay of biomass: {CH2O} + O2 CO2 + H2O

• Lowers pH and contributes to weathering or carbonates, also affects rate of metal uptake by roots


The Inorganic Components of Soil

• Typically 95% of solid soil

• Typically form colloidal secondary minerals

• Repositories of nutrients

• There are several important inorganic minerals in soil

– Finely divided quartz, SiO2

– Orthoclase, KAlSi3O8

– Albite, NaAlSi3O8

– Epidote, 4CaO•3(AlFe)2O3 •6SiO2•H2O

– Geothite, FeO(OH)

– Magnetite, Fe3O4

– Calcium and magnesium carbonates, CaCO3, CaCO3•MgCO3

– Oxides of manganese and titanium


Organic Matter in Soil

• Typically 5% of soil solids

• e.g. Soil humus, oxalate ion (C2O42-), organic acids, sugars, amino acids, organo

N, S and P compounds, terpenes, etc.

• Largely determines soil productivity

– Food for microorganisms

– Chemical reactions such as ion exchange

– Holds and releases plant nutrients

– Holds soil water

– Influences soil physical properties

– Aids weathering of mineral matter


Soil Humus

• Makes up bulk of soil organic matter

• Biodegradation product of plant matter, largely from plant lignin

• Solid humus is largely the insoluble humin fraction

• Strongly influences soil properties

• Strongly binds metals

– Hold micronutrient cations

– Serve as acid-base buffers

– Stabilize aggregates of soil particles

– Increase sorption and retention of soil water


The Soil Solution

• Soil solution is aqueous component of soil

– Contains dissolved matter and plant nutrients

– Transports substances between soil solids and plant roots

• Dissolved mineral matter in soil is largely present as ions

– Cations: H+, Ca2+, Mg2+, K+, Na+, some Fe2+, Mn2+, Al3+ (usually as hydroxy complexes, such as AlOH2+)

– Anions: HCO3-, CO3

2-, HSO4-, SO4

2-, Cl-, F-

– Complexed anions: AlF2+

– Ion pairs: CaSO4, FeSO4

Acid-Base and Ion-Exchange Reactions in Soils


• Soil exchanges cations between soil solids and soil solution

– Negative sites on soil exchange cations

– Cation exchange capacity, milliequivalents cation/100 g dry soil

– Dependent on pE and pH

– Both mineral (clay) and organic (humic) fractions involved

– Because of H+ exchange soil acts as a buffer

• Uptake of metal cations from soil by plant roots

Soil}Ca2+ + 2CO2 + 2H2O Soil}(H+)2 + Ca2+(root) + 2HCO3-

• Ion Exchange Reactions on Soil

– Ion exchange reaction such as: Soil}Na+ + K+ Soil}K+ + Na+

– Extent of reaction expressed by an ion exchange constant, K


Production of Mineral Acid in Soil

• Microbially mediated oxidation of pyrite in soil

FeS2 + 7/2O2 + H2O Fe2+ + 2H+(root) + 2SO42-

• Soils in sediments may be enriched in iron sulfides and produce acid when exposed to air

• Treatment with Lime (CaCO3) to Eliminate Excess Soil Acid

Soil}(H+)2 + CaCO3 Soil}Ca2+ + CO2 + H2O

• Treatment with Al(III) or Fe(III) Sulfate to Neutralize Alkali

2Fe3+ + 3SO42- + 6H2O 2Fe(OH)3(s) + 6H+ + 3SO4

2-

Macronutrients in Soil

Macronutrients in Soil

• Essential plant macronutrients

– C, H, and O from atmosphere or water

– N, P, K, Ca, Mg, and S from soil

– N, P, and K most likely to be deficient, added as fertilizer

– In some cases SO42- is deficient

Nitrogen, Phosphorus, and Potassium in Soil


• A

Figure 18.5. Nitrogen Sinks and Pathways in Soil


• Phosphorus

• Plants assimilate P as inorganic H2PO4- or HPO4

2-

• Potassium

• Potassium utilized as K+ ion

• K is abundant in Earth’s crust, but often deficient in soil

Figure 18.6. Bacteria Growing Synergistically with Legumes Such as Soybeans Fix Nitrogen in a Form Utilized by Plants

Micronutrients in Soil

Micronutrients in Soil

• Boron, chlorine, copper, iron, manganese, molybdenum, and zinc are essential plant micronutrients

• Micronutrients from soil

– Rarely deficient

– Iron solubility can be a problem

• Hyperaccumulators are plants that accumulate very high levels of some metals

– Usually copper or nickel

• Copper flower,” Aeolanthus biformifolius DeWild can contain up to 1.3% (dry mass) copper when grown on copper-rich soils

– Heavy-metal-tolerant hyperaccumulators have been used for phytoremediation of soils contaminated with heavy metals

Fertilizers

Fertilizers

• Nitrogen Fertilizer

• Most made by reacting atmospheric N2 with H2 over a catalyst

– High temperature around 500˚C

– Very high pressure around 1000 atm

– N2 + 3H2 2NH3

• Nitrogen can be applied as anhydrous NH3

– Toxicity hazard

– Very soluble in soil water

– Readily forms NH4+

• Safer to apply N as 30% NH3 in water

– Can be put into irrigation water

• Solid N fertilizer as ammonium nitrate, NH4NO3

– Explosive when mixed with hydrocarbon

• Urea, CO(NH2)2 is a good solid N fertilizer

• Other solid N fertilizers

– NaNO3 (from Chilean deposits)

– KNO3

– Ca(NO3)2

– (NH4)2SO4 from coke manufacture

– Ammonium phosphates

Fertilizers

Phosphate Fertilizer

• Phosphate occurs largely as fluorapatite mineral, Ca5(PO4)3F

• Converted to more soluble superphosphates by phosphoric or sulfuric acid

2Ca5(PO4)3F + 7H2SO4 + 3H2O 2HF(g) + 3Ca(H2PO4)2•H2O + 7CaSO4

HF byproduct can create air pollution problems

• Some micronutrient elements are lost in processing

• Potassium Fertilizer

• Primarily as KCl mined or in some brines

• Generally returned to soil, but may be lost with forage crops

Soil Loss and Deterioration


• More soil into production from fragile lands

• Forest land • Arid grasslands • Steeply sloping land

• Increase cropping intensity of existing land

• Rain forests maintain most nutrients in plant biomass or thin soil

– Nutrients readily lost in cultivation

• Cultivation of arid grasslands causes wind erosion and conversion to desert (desertification)

• Steeply sloping land is eroded by water

– Topsoil lost

• Intense cultivation of land can be very harmful

– If properly done, may even enhance soil quality

– May add desirable organic matter to soil


Shifting Cultivation: Slash and Burn

• Shifting cultivation entails clearing vegetation and cultivation until nutrients are depleted

– Land allowed to restore with native species and process repeated

• The most common shifting cultivation process is slash and burn

– Trees killed by slashing, burned to add nutrients to soil

– Used on about 30% of Earth’s arable land

– Fallow periods becoming shorter, land less fertile

• Slash and burn has numerous adverse effects

– About 70% of deforestation in Africa

– Carbon dioxide and methane releases contribute to global warming


Soil Degradation

Indicators of soil degradation

– Texture and bulk density indicate ability to anchor roots, hold water during dry periods, resist erosion

– Water-holding capacity important in resisting erosion and transporting water

– Soil organic matter important for fertility, structural stability, food for earthworms

– pH around neutral is desirable

– Electrical conductivity indicates nutrient salts

– Extractable nitrogen, phosphorus, and potassium reflect plant nutrient content

– Relatively high levels of microbial activity reflect healthy soil


Factors in Soil Sustainability

• Soil resistance refers to soil’s capacity to resist detrimental effects

– Example: High levels of potassium resist effects of forage removal

• Soil resilience is soil’s ability to recover from insults

– Example: Quick reversion to forest or grassland after cultivation

• Desertification

– Drought conditions and loss of ability to grow plants

– Destruction by bad grazing practices, such as by goats

– Erosion • Adverse climate • Inavailability of water

– Fertility loss • Soil humus loss • Adverse soil chemical properties

– Salinization

• Deforestation

– Loss of trees and ability to grow them

– Loss of many species

– Especially severe in many tropical areas


Soil Erosion

• Soil erosion is loss of productive topsoil by wind and water action

• Soil eroded from ancient times in Greece and Rome

• About 1/3 of U.S. topsoil lost from erosion

• Wind erosion in high plains areas

– Exacerbated by cultivation of grasslands to grow high value crops

Figure 18.7. Pattern of Soil Erosion in Continental U.S.


• Soil Sustainability and Water Resources

• Precipitation falls on soil in a watershed

• Watershed and soil in it determine

– Fate of water

– Water retention in usable condition

– Degree of erosion by water

– Infiltration to groundwater

– Tendency to flash flooding

• Measures taken to improve watershed

– Modification of contour

– Construction of waterways

– Planting grass on waterways

– Water-retaining ponds

– Reforestation

– Controlled grazing practices

Wastes and Pollutants in Soil


• Soil is a receptor of many wastes and pollutants

– Atmospheric SO2/H2SO4

– Atmospheric NOx

– CO metabolized by soil microorganisms

– Metals including lead

– Hazardous wastes from landfill leachate, lagoons

– Volatile organic compounds from leakage

– Persistent polychlorinated biphenyls (PCBs)

– Large quantities of pesticides, especially herbicides

• Deliberate land farming of some wastes degraded on soil


• Degradation of Pesticides on Soil

• Strongly influenced by pesticide properties

– Solubility

– Volatility

– Charge

– Polarity

– Molecular structure

• Influenced by soil

– Nature of soil surface

– Degree of pesticide binding to soil

• Pesticides degraded by

– Biodegradation

– Chemical processes

– Photochemical processes

• Biodegradation is by far most important

Saving the Land

Saving the Land

• Soil conservation is the most established environmental movement

– Emphasizes prevention of erosion

• Important aspects of soil conservation

• • Terracing • Contour plowing • Cover crops such as clover

• Conservation tillage is planting crops in residue from preceding year’s crops without cultivation

– Makes use of minimal herbicides to control weeds

– Surface residue of plant biomass and roots prevents soil erosion

• Cultivation of perennial plants may eventually enable growing grain without cultivation

– Grass used for forage is an example

– Trees are an important perennial plant

Saving the Land

• A

Figure 18.8. Terraces and Contour Planting are Important Soil Conservation Practices

Saving the Land

• Soil Restoration

• Many ways in which soil becomes impaired

– Loss of fertility

– Erosion

– Salinity buildup

– Phytotoxin contamination (metals)

• Soil restoration or restoration ecology

– Physical alteration to reduce erosion, aid drainage

– Adding organic matter, such as by growing clover (high biomass)

– Flush salts

– Neutralize acids or bases

Process Intensification in Agriculture


• Process intensification in agriculture involves greater production from less land

• From 1950 to present, more food has been produced by agriculture than in the preceding 10,000 years

• Green revolution in agriculture from about 1950

– Intensified management of crops, soil, and water

– Dramatically increased production, especially in Asia

– Grain production increased three-fold

– Grain prices actually fell

• Most important were high-yielding hybrid and dwarf varieties of wheat, rice, and corn

– Two or three crop cycles per year in climates free of freezing weather


• Benefits of Process Intensification in Agriculture

– More food, massive starvation averted

– More crop residues recycled to soil

– Higher efficiency of nutrient utilization

• Downside of Process Intensification in Agriculture

– Depletion of some nutrients, metals

– Eutrophication of water from fertilizer runoff

– Accumulation of salts from irrigation (salinization)

– Reduced crop gene pool

– Increased vulnerability to disease

• Buildup of pesticides, especially herbicides, in soil

• Reduced production of protein rich pulses (beans, peas, lentils), vegetables, and fruits relative to high-starch grains

Sustainable Agricultural Management

Sustainable Agricultural Management

• Involves both soil and crop management techniques

• Major aspects are the following:

1. Increase biological productivity and diversity

2. Prevent soil degradation including erosion, salinization and desertification

3. Reduce pollution of soil and other environmental spheres

4. Decrease quantities of nutrients and water used per unit of production by increasing efficiency of nutrient and water utilization

5. Increase amounts and quality of soil organic matter

6. Increase desirable biological activity in the soil subsurface by earthworms, plant roots, nitrogen-fixing bacteria, and other organisms

Agroforestry

Agroforestry

• Crops grown in strips between trees (Figure 18.9)

• Trees stabilize soil

• Trees with nitrogen-fixing capability add fertility

Material buffer can be returned to higher levels

Documents

Environmental Chemistry Chapter 18: Soil, Agriculture, and Food Production Copyright © 2011 by DBS