VI. Soil Organic Matter Maintenance

224



B. A. STEWART AND C. A. ROBINSON



clear but several of the reported studies did indicate that plowing increased the decline rate. Pieri (1995) proposed that there is a critical level for soil organic matter that is dependent on the soil organic matter%age and the sum of clay plus silt.

He states that if the soil organic matter percentage falls below the critical level, the

maintenance of soil structure is difficult to achieve. However, he disagrees with

agronomists that argue that if soil organic matter is important in soil quality, then

the higher soil organic matter content is, the better the soil is. Pieri states that in

semiarid Africa, where there are so many technical and economic constraints to

crop performance, it is fruitless to aim for a soil organic matter percentage above

the critical value.

Johnson er al. (1974) reported on a 29-year study at Bushland, Texas, where various cropping systems were compared for their effects on wheat production and

soil organic matter maintenance (Table 111). They clearly showed that organic matter decline was increased when the length of the fallow period was increased or

when tillage was intensified, and the greatest loss occurred when both circumstances were present. There was also a large accumulation of nitrate nitrogen in

the soil profile for all treatments but it was particularly large for the intensively

tilled fallow areas. The delayed subtilled plots, although not socially acceptable

because of the uncontrolled weed production, had the smallest decline in soil organic matter and yielded about the same amount of wheat as the systems that controlled weed growth.

Organic matter maintenance in semiarid regions is clearly one of the greatest

constraints in the development of sustainable agroecosystems. This challenge is

particularly great in many developing countries where the crop residues are so important as a source of animal feed and fuel for cooking. Whenever feasible, it is

best to let animals graze the crop residues so the manure will be distributed over

the area. When it is necessary to utilize the crop residues as animal feed away from

the land, every attempt should be made to return manure to the land whenever feasible. Otherwise, the soil organic matter level will continue to decline to the point

that long-term sustainability of the soil resource base will be threatened. Robinson er al. (1996) reported that the maintenance or enhancement of soil organic matter is proportional to the amounts of residues returned. Maintenance of soil organic

matter is important to maintain yield potential (Bauer and Black, 1994).



MI. SUMMARY

Achieving sustainable agroecosystems is the challenge of the coming century.

With increasing population and improved living standards, the demand for food

and fiber will force the development of agroecosystems into less favorable regions.

There is often an imbalance between natural resources, population, and basic human needs in many regions and this is often particularly true for semiarid regions.



AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 225

Agroecosystems in these areas can be developed and sustained, but careful management is required.

The prevention of soil degradation is the first and most important issue that must

be addressed in such areas. Soil degradation is a complex phenomenon. It is driven by strong interaction among socioeconomic and biophysical factors. It is fueled by increasing population, fragile economies, and poorly designed farm policies, and propelled by the fragility of the soil and harshness of the climate. Soil

degradation can be subtle and slow until a certain threshold is reached, and then

deterioration can occur quickly and, sometimes, irreversibly.

Soil organic matter is significantly correlated with soil productivity. Maintaining soil organic matter, therefore, is of critical importance. This is a tremendous

challenge in semiarid regions because insufficient precipitation seriously limits

carbon inputs and the often warm conditions accelerate the decomposition of native soil organic matter during periods of favorable soil water conditions. Extensive tillage generally increases the rate of decomposition.

There exists a considerable body of research knowledge and producer experiences. This information is sufficient in most cases to develop sustainable agroecosystems.The biggest challenge, however, is the implementation and execution

of sound management plans. Sustainable systems must focus on long-term goals,

but the reality is that short-term benefits and solutions almost always take precedence over long-term issues. Historically, agroecosystems have been developed

for short-term benefits without a thorough analysis of what long-term consequences would result. Scientists, producers, policymakers, and governments must

work together very closely in the future to meet the challenge of sustaining the natural resource base while producing adequate amounts of food and fiber.



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Index

A

Activity reports, 186

Administrators, evaluating, 188

Agricultural research, see Ethics

Agricultural themes, ethical dimensions, 155

Agriculture, sustainable, see Sustainability

Agroecosystems, 191-225

climatic effect, 201-202

increasing plant-available water, 205-223

crop calendars, 2 19-223

lengthening fallow period, 206-213

mulches, 2 13-2 I7

tillage, 217-219

productivity, 193

socioeconomic effect. 203-205

soil degradative processes, 200-201

soil effect, 202-204

soil organic matter maintenance, 223-224

stability, 194

sustainability. 194

Alfalfa. salt tolerance, 95

Animakrop mix, changing, 62

Animal feeds, promoting more efficient use of

nutrients, 62

Animal products, reducing consumption,

62-63

Animals

ethical treatment in research, 177-179

integrating into cropping system, 64

waste management, 53

Aridity index, 195

Authorship, ethics, I73

Avocado, salt tolerance, 100- I0 I



B

Bacteria, nutrient uptake stimulation, 19

Bermuda grasses, salt tolerance, 95

Berries, genetic variability and salt tolerance,

98-101

Boundaries, nutrient flows and cycles, 8-9



C

Canola, salt tolerance, 92

Carbon, decomposition dynamics, 123, 127

Chinampas, 61

Chloride, toxicity in woody species, 98-101

Citrus, salt tolerance, 100

Clover

introduction in Europe, 49

salt tolerance, 95

Commerce, ethics codes, 161

Communication, honest, with constituents,

189-190

Competition, ethics and, 179-180

Composting, low-input, on-farm, 141-144

Conflicts of interest, research ethics, 174-175

Consulting, ethics, 184

Copyright, infringement, 176

Corn

relation of yield and growing season evapotranspiration, 222

salt tolerance, 90

Costs, indirect, recovery, ethics, 171-173

Cotton, salt tolerance, 91

Cover crops, 35-36

minimizing leaching losses, 59

Credibility, establishing and maintaining, researchers, 168-169

Cropping

double, 221-222

pattern, matching with climate, 22G221

Crop residue

burning, 141

chemical composition, 122

decomposition, modeling, 125-129

expert systems and erosion models. 126

RESMAN, theory in, 126-129

as nutrient cycling, 32-33

soil protection by, 122

surface managed, decomposition, 122-125

Crops

calendars, plant-available water and, 219-223



229



230



INDEX



Crops (conrinued)

management, practices and soil ecology,

20-2 1

mix, changing, 62

rotation, 34-35

substitution as method of dealing with salinity, 76



D

Data

analysis, ethics, 167-168

collecting and reporting, ethics, 167

Decomposition

crop residue, modeling, 125-129

surface-managed crop residues, 122-125

Desertification, 204

Dust mulch. 212



E

Ecosystem relations, 9-12

Energy, use and nutrient flows, 50-52

Environmental factor, residue decomposition,

127

Environmental stresses, interactions with salinity, 84

Ethical behavior, practical principles, 152-1 54

Ethical codes, as rules, 160

Ethical disputes, resolving, 156-157

Ethics, 149-1 90

choosing research subject matter, 154157

dimensions of agricultural themes, 155

resolving ethical disputes, 156-157

science paradigm criticism, 156

sustainable agriculture, 154-155

in conduct of research, 165- I84

authorship and shared recognition, 173

collecting and reporting data, 167

competition, 179-180

conflicts of interest, 174-175

consulting, 184

data analysis, 167-168

designing experiments, 166-167

drawing and reporting inferences, 168

establishing and maintaining credibility,

168- 169

ethical treatment of animals, 177-179

indirect cost recovery, 171-173

intellectual property rights, 175-177



peer review, 173

performing to specifications, 180

proposal budgets, 17I

proposal preparation, 169-170

technology transfer, 180-183

topic selection, 166

whistle-blowing, 173-174

definitions, 151

difficultieswith utilitarian approach, 158-162

abiding by rules, 162

difficulty in evaluating outcomes, 158

ethics codes as rules, 160-162

evaluation of principles, 159-160

sea of uncertainty, 158-159

personal and group, 151-152

research administration, 184-190

activity reports, 186

equity and merit, 189

evaluating administratorsand managers,

188

hiring and termination, 184-185

honest communication with constituents,

189-1 90

job applications, 188

letters of recommendation,support, and

evaluation, 186- I87

nurturing scientists, 185-186

promotion documents and decisions, 187

scientific misconduct, 152

world food situation and, 162-165

driving forces, 162-164

message for agronomists, 165

moot questions, 164-165

sources of research support, 165

Experiments

designing, ethics, 166167

drawing and reporting inferences, 168

Expert system, residue decomposition models,

126



F

Fairness, 153, 189

Fallow period

efficiency and tillage, 218

lengthening, 206-2 13

mulch and plant-availablewater, 214-217

Fertilizers

overuse, 2-3

utilizing more efficiently, 60



23 1



INDEX

Field crops, genetic variability and salt tolerance, 9 1-92

Field screening techniques, salt tolerance, 103

Flow nutrients, nearby, agricultural use, 61

Food, consuming local produce, 64

Forages, genetic variability and salt tolerance,

94-95

Fruits, genetic variability and salt tolerance,

98-101

Fungi, nutrient uptake stimulation, 19



G

Genes, salt tolerance, 101-102

Geologic deposits, nutrient dynamics, 50

Grains, genetic variability and salt tolerance,

88-91

Grasses, genetic variability and salt tolerance,

94-95



H

Harvest, nutrient loss, 24-26

Heritability, salt tolerance, 103

Hiring, ethical, 184-185

Honesty, 152-153

Human waste, land application, 53



I

Immobilization, inorganic nutrients, 18-19

Inorganic nutrients, immobilization, 18-19

Integrity, 153

Intellectual property rights, ethics and,

175-177

Ion

accumulation, salt tolerance and, 86

selectivity, salt tolerance and, 85-86

Irrigation, increasing salinity of lands and, 76



J

Job applications. I88



K

Kentucky bluegrasses, salt tolerance, 95



L

Land, increased yield, 192

Letters of recommendation, support, and evaluation, 186-187

Lettuce, salt tolerance, 98

Linseed, salt tolerance, 93



M

Managers, evaluating, 188

Manure

nutrient flow, 40

utilizing more efficiently, 60

Melon, salt tolerance, 97-98

Merit, rewarding, equity and, 189

Military, ethics codes, 161-162

Mineralization, 14-15

soil organic matter, 18

Modeling, salt tolerance, 107

Molecular biology, salt tolerance, 106-107

Mulches, increasing plant-available water,

21 3-2 I7



N

Nitrate leaching, 26.42

Nitrogen

decomposition dynamics, 123, 127

fixation, 19

by symbiotic and nonsymbiotic organisms,

6M1

recommendations, 30-3 1

Nondisclosure agreements, 177

No-till cropping systems, 121-144

deleterious rhizobacteria for weed control,

137-141

domination by fungi and earthworms, 136

low-input, on-farm composting, 141-144

root-microbial relationships, 129-1 37

Nutrient cycle, 8

crop residues, 32-33

ecology, 17-2 1

efficiency, 9-10

plant strategies, 10

simplified managed system, 10-1 1

simplified natural system, 9-10

Nutrient dynamics, 1-66: see also Soil-plant

system

definitions, 7-9



232



INDEX



Nutrient dynamics (continued)

energy use and nutrient flows, 50-52

farm-level changes, 60-62

farm-scale cycling and Rows, 3 8 4 7

to and from farms, 4 0 4 3

between farms, 43-44

nutrient exports > imports, 44-46

nutrient exports < imports, 46

nutrient exports = imports, 46-47

within-farm, 3 9 4 0

at field level, 23-38

changes in nutrient, 36-37

changing to biologically based nutrient

sources, 37-38

cover crops, 35-36

crop residues, 32-33

crop rotation, 34-35

inadvertent nutrient losses, 26-27

nutrient additions, 27-3 1

nutrient losses, 24-27

pastures, 35

tillage systems, 33-34

field-level changes, 58-60

finite geologic deposits, 50

harvest removal, 24-26

historical overview, 5-7

increasing soil nutrient availability, 5 9 4 0

influences on flow patterns, 54-56

intercontinental flows, 49-50

landscape position, 12-13

possible changes in large-scale flows, 52-54

seasonal patterns, 12-1 3

societal-level changes, 6 2 4 5

spatial cycle and ecosystem relations, 9-12

spatial scale of changes and time needed to

complete, 57-58

utilizing fertilizers and manures more efficiently, 60

utilizing more efficiently taken up nutrient

sources, 60-6 I

watersheds, 4 7 4 8

Nutrient flow, 8

ecology, 17-2 1

field, changes in, 3 6 3 7

patterns, potential implications, 44-45

Nutrient leaks, 62-63

Nutrients

added, quantity, 29-3 I

application timing and methods, 28-29

balances, mixed crop and livestock farm, MI



degree of synchronization between availability and uptake needs, 58-59

enhancing uptake efficiency, 58-59

management issues, 2-4

sources

biologically based, 37-38

soluble, sparing use, 61

transformations, ecology, 17-2 I

transporting back to farmland, 64-65

Nuts, genetic variability and salt tolerance, 98-101



0

Oil seed crops, genetic variability and salt tolerance, 92-93

Organic matter

maintenance, semiarid regions, 223-224

semiarid regions, 21 1

Ornamentals, genetic variability and salt tolerance, 101

Osmotic adjustment, salt tolerance and, 87



P

Pastures, 35

Patent, infringement, 176

Peer review, ethics, 173

Performing to specifications, I80

Phosphorus, solubility, 19

Plant-animal-human trophic pyramid, segment

separation, 6 3 4 5

Plant nutrition, 13-17

Potato, salt tolerance, 92

Practices

advocating, 182-183

testing and comparing, 181-182

Products

advocating, 182-1 83

testing and comparing, 18 1-1 82

Promotion, documents and decisions, 187

Proposals

budgets, ethics and, 17 1

preparing, ethics and, 169-1 70



R

Research, see Ethics

RESMAN, theory used in, 126-129

Rhizobacteria, deleterious, 129-1 32

weed control, 137-141



INDEX

Rice, salt tolerance, 90-91

Root environment, optimizing, 59

Root-microbial relationships, 128-1 37

deleterious rhizobacteria, 129-132

root zone temperature and, 133

Ruminant livestock, biological nitrogen fixation,

4041



S

Safflower, salt tolerance, 92-93

Salinity problems, semiarid regions, 212

Salt, accumulation, 86

Salt stress, short- and long-term effects, 85

Salt tolerance, 75-108

breeding methods, 101-105

field screening techniques, 103

genes for tolerance, 101- 102

heritability, 103

selection methods. 104-105

crop species, 77

genetic variability, 88-101

field crops, 91-92

fruit, nuts, and berries, 98-101

grains, 88-9 I

grasses and forages, 94-95

oil seed crops. 92-93

ornamentals, 101

vegetable crops, 95-98

in low-yielding varieties, 81

measurement, 79-80

mechanisms, 84-88

ion accumulation, 86

ion selectivity,85-86

organic solutes, 87

osmotic adjustment, 87

water use efficiency. 87-88

modeling, 107

molecular biology, 106-107

rationale for breeding for, 77-78

selection for. 78-84

environmentalinteractions, 84

growth stage, 82-83

specific ion tolerance, 83-84

yield and productivity, 80-82

tissue cultures, 105-106

Science, pure, ethics codes, 161

Science paradigm. criticism, 156

Scientific misconduct, 152

Scientists, nurturing. 185-1 86



233



Seasonal patterns, nutrient dynamics, 12-13

Selection methods, salt tolerance, 104-105

Semiarid regions, 194-198

aridity index, 195

characterization, 192

example locations, 196-198

length of growing period, 195-196

soil organic matter maintenance, 223-224

Soil

chemical properties, 21

degradation, interdependenceon biological

and socioeconomicfactors, 203, 205

effect on sustainability,202-204

erosion, 26

fertility, maintaining long-term, 15-17

management, practices and soil ecology,

20-2 1

nutrients, increasing availability,5 9 4 0

nutrient stocks, 13-17

organic matter

depletion, 3

dynamics, 17

maintaining high levels, 15-16

mineralization, 18

physical properties, 2 1-22

Soil-plant system, 13-23

biological, chemical, and physical interactions, 22-23

ecology of nutrient flows, transformations,

and cycles, 17-21

maintaining long-term soil fertility, 15-1 7

satisfying short-term fertility needs, 14-15

simplified nutrient cycle, flows, and transformations, 14

Soil resources, wasteful use, 62

Solutes, organic, salt tolerance and, 87

Sorghum

relation of yield and seasonal evapotranspiration, 208

salt tolerance, 90

Soybean, salt tolerance, 92

Spacial scale, 9-12

Stocks, 7-8

Straw mulch, 215-216

Stubble mulching, 213-215

Sugar beet, salt tolerance, 91-92

Summer fallow, 206.21 1-213

Sunflower, salt tolerance, 93

Sustainability,4, 56-57, 194, 198-205

climatic effect, 201-202



2 34



INDEX



Sustainability (conrinued)

definition, 198

ethics and, 154-155

reasons for importance in policy agenda,

199

socioeconomic effect, 203-205

soil effect, 202-203



T

Technology transfer, ethical issues, 180-1 83

Technology transfer agents, responsibility,

181



,



Temperature function, residue decomposition,

128

Termination, of employees, 185

Tillage, plant-available water and, 217-219

Tillage systems, 33-34

Tissue cultures, salt tolerance, 105-106

Tomato, salt tolerance, 97

Trade secrets, ethics and, 176177

Transformations, 8

Trophic pyramid, 5



V

Vegetable crops, genetic variability and salt tolerance, 95-98



W

Water, plant-available, technologies for increasing, 205-225

crop calendars, 2 19-223

lengthening fallow period, 206-213

mulches, 2 13-2 17

tillage, 217-219

Water function, 127

Watersheds, nutrient dynamics, 47-48

Water use, efficiency and salt tolerance, 87-88

Weed control, deleterious rhizobacteria,

137-14 1

Wheat

relation of yield and growing season evapotranspiration, 222

relation of yield to seasonal evapotranspiration, 208-210

salt tolerance, 89-90, 102

Wheatgrass, salt tolerance, 94

Whistle-blowing, ethics, 173-174

Winter wheat

deleterious rhizobacteria effect, 129-132

high crown set, 141-142

rhizoplane populations of inhibitory

pseudomonads, 134-1 35

yield from fields inoculated with rhizobacteria, 138

World food, ethics and, 162-165