The infiltration of water into soil has profound importance as a central component of the hydrologic cycle and as the means of replenishing soil water that sustains terrestrial life. Systematic quantitative study of infiltration began in the 19th century and has continued through to the present as a central topic of soils, soil physics, and hydrology. Two forces drive infiltration: gravity, and capillarity, which results from the interaction of air-water surface tension with the solid components of soil. There are also two primary ways water moves into and within the soil. One is diffuse flow, through the pores between individual soil grains, moving from one to the next and so on. The other is preferential flow, through elongated channels such as those left by worms and roots. Diffuse flow is slow and continues as long as there is a net driving force. Preferential flow is fast and occurs only when water is supplied at high intensity, as during irrigation, major rainstorms, or floods. Both types are important in infiltration. Especially considering that preferential flow does not yet have a fully accepted theory, this means that infiltration entails multiple processes, some of them poorly understood. The soil at a given location has a limit to how much water it can absorb—the infiltration capacity. The interplay between the mode and rate of water supply, infiltration capacity, and characteristics of the soil and surrounding terrain determines infiltration into the soil. Much effort has gone into developing means of measuring and predicting both infiltration capacity and the actual infiltration rate. Various methods are available, and research is needed to improve their accuracy and ease of use.
Article
Infiltration of Water Into Soil
John Nimmo and Rose Shillito
Article
Measuring Soil Loss and Subsequent Nutrient and Organic Matter Loss on Farmland
Vincenzo Bagarello and Vito Ferro
Field plots are often used to obtain experimental data (soil loss values corresponding to different climate, soil, topographic, crop, and management conditions) for predicting and evaluating soil erosion and sediment yield. Plots are used to study physical phenomena affecting soil detachment and transport, and their sizes are determined according to the experimental objectives and the type of data to be obtained. Studies on interrill erosion due to rainfall impact and overland flow need small plot width (2–3 m) and length (< 10 m), while studies on rill erosion require plot lengths greater than 6–13 m. Sites must be selected to represent the range of uniform slopes prevailing in the farming area under consideration. Plots equipped to study interrill and rill erosion, like those used for developing the Universal Soil Loss Equation (USLE), measure erosion from the top of a slope where runoff begins; they must be wide enough to minimize the edge or border effects and long enough to develop downslope rills. Experimental stations generally include bounded runoff plots of known rea, slope steepness, slope length, and soil type, from which both runoff and soil loss can be monitored. Once the boundaries defining the plot area are fixed, a collecting equipment must be used to catch the plot runoff. A conveyance system (H-flume or pipe) carries total runoff to a unit sampling the sediment and a storage system, such as a sequence of tanks, in which sediments are accumulated. Simple methods have been developed for estimating the mean sediment concentration of all runoff stored in a tank by using the vertical concentration profile measured on a side of the tank. When a large number of plots are equipped, the sampling of suspension and consequent oven-drying in the laboratory are highly time-consuming. For this purpose, a sampler that can extract a column of suspension, extending from the free surface to the bottom of the tank, can be used. For large plots, or where runoff volumes are high, a divisor that splits the flow into equal parts and passes one part in a storage tank as a sample can be used. Examples of these devices include the Geib multislot divisor and the Coshocton wheel. Specific equipment and procedures must be employed to detect the soil removed by rill and gully erosion. Because most of the soil organic matter is found close to the soil surface, erosion significantly decreases soil organic matter content. Several studies have demonstrated that the soil removed by erosion is 1.3–5 times richer in organic matter than the remaining soil. Soil organic matter facilitates the formation of soil aggregates, increases soil porosity, and improves soil structure, facilitating water infiltration. The removal of organic matter content can influence soil infiltration, soil structure, and soil erodibility.
Article
Nomadism
Philip Carl Salzman
Nomadism is a technique of population movement used to accomplish a variety of goals. It is used for primary production when the resources to be tapped are distributed thinly over a wide space, or are located in different places in a large region. Commonly nomadism is a technique used in a spatially extensive adaptation. Pastoralists raising domestic animals on natural pasture move from grazed areas to areas with fresh pasture, and from dry areas to those with water.
Nomadism follows regular patterns where the resources tapped are reliable and thus predictable. This is common in macro-environmental adaptations to factors such as seasons and altitude. Some pastoralists have mountain adaptations, migrating to high altitudes in summer and low altitudes in winter, an adaptation called transhumance in Europe. Nomadic patterns are more irregular when rainfall patterns, and thus pasturage, are erratic and unpredictable, as is common in desert areas with low rainfall.
Among some pastoral peoples, all of the households in the community move together. Among other pastoral peoples, a sector of the populations is nomadic; young and/or mature men migrate with the livestock, while women, children, and elders remain in a stationary home settlement. This is also the pattern in European transhumance.
Many pastoral peoples produce primarily for their own subsistence; it is common that they have multi-resource or mixed economies, engaging also in hunting and gathering, horticulture, agriculture, and arboriculture. Economic activities are not limited to primary production; patterns of predation, including raiding and extortion, against other pastoralists, farmers, and traders are widespread. Other pastoral peoples are heavily market-oriented, producing for sale, or have symbiotic relations with hunters or cultivators; it is normal that they are more specialized in their production. But pastoralists can be found at all points on a continuum between subsistence- and market-oriented.
Article
Oats and Other Forage Crops
Daren Redfearn
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
Oats and the other small grains have been “rediscovered” with the drive towards intensifying agricultural production, integrating crops and livestock into diversified systems, and increasing environmental stewardship. Globally, oats and other winter annual small grains such as wheat, cereal rye, triticale, and barley, have been used primarily for grain production. The secondary market following grain production has been restricted to straw, used mainly as livestock bedding. In regions where livestock are economically important, oats and the other annual small grain crops can be used as a grazed forage or fodder crop, hay, or silage. There are several characteristics that make oats and other small grains suitable for multiple agricultural uses. All the small grains are fairly easy to establish, have rapid growth, can be productive, and have a high nutritional value for livestock. Recent improvements in cultivar development have allowed oats and wheat to be grown across a broader range of stressful environmental conditions. Similarly, cultivar development in oats and wheat has improved grazing tolerance, which is important in dual-purpose systems that emphasize both grazing and grain production. On a worldwide scale, oats and other annual small grains are economically and environmentally important forage crops, especially when used as focused components within intensified agricultural systems. Challenges include development of improved cultivars of oats and other small grains for use in intensified agricultural systems, including both grazing and no grazing, that serve as short rotation crops, dual-purpose crops, or are designed to mitigate a specific environmental issue.
Article
Organic Farming
Theodore J. K. Radovich
Organic farming occupies a unique position among the world’s agricultural systems. While not the only available model for sustainable food production, organic farmers and their supporters have been the most vocal advocates for a fully integrated agriculture that recognizes a link between the health of the land, the food it produces, and those that consume it. Advocacy for the biological basis of agriculture and the deliberate restriction or prohibition of many agricultural inputs arose in response to potential and observed negative environmental impacts of new agricultural technologies introduced in the 20th century. A primary focus of organic farming is to enhance soil ecological function by building soil organic matter that in turn enhances the biota that soil health and the health of the agroecosystem depends on.
The rapid growth in demand for organic products in the late 20th and early 21st centuries is based on consumer perception that organically grown food is better for the environment and human health. Although there have been some documented trends in chemical quality differences between organic and non-organic products, the meaningful impact of the magnitude of these differences is unclear. There is stronger evidence to suggest that organic systems pose less risk to the environment, particularly with regard to water quality; however, as intensity of management in organic farming increases, the potential risk to the environment is expected to also increase. In the early 21st century there has been much discussion centered on the apparent bifurcation of organic farming into two approaches: “input substitution” and “system redesign.” The former approach is a more recent phenomenon associated with pragmatic considerations of scaling up the size of operations and long distance shipping to take advantage of distant markets. Critics argue that this approach represents a “conventionalization” of organic agriculture that will erode potential benefits of organic farming to the environment, human health, and social welfare. A current challenge of organic farming systems is to reconcile the different views among organic producers regarding issues arising from the rapid growth of organic farming.
Article
Origin and Development of Agriculture in New Guinea, Island Melanesia, and Polynesia
Tim Denham
Early agricultural and arboricultural practices in the Pacific are based on vegetative principles, namely, the asexual propagation and transplantation of plants. A vegetative orientation is reflected in the exploitation of underground storage organs (USOs) within Near Oceania, as well as Island Southeast Asia, during the Pleistocene. During the early Holocene, people in the New Guinea region (including Near Oceania) began to intensify the management of plant resources in different landscapes. The increased degree of plant management, as well as associated environmental transformation, is most clearly manifest in the agricultural chronology at Kuk Swamp in the highlands of Papua New Guinea. At Kuk, shifting cultivation was potentially practiced during the early Holocene, with mounded cultivation by c. 7000–6400 cal BP and ditched drainage of wetlands for cultivation by c. 4400–4000 cal BP. Comparable agricultural records are lacking for other regions of Near Oceania; lowland sites indicate a range of arboricultural practices focused on fruit- and nut-bearing trees during the Terminal Pleistocene and throughout the Holocene, as well as potentially sago during the late Holocene. By c. 4000–3000 cal BP, indigenous agricultural and arboricultural elements were integrated with new cultural traits from Southeast Asia, including domestic animals, pottery and potentially new varieties of traditional crops. From c. 3250 to 2800 cal BP, different elements of agricultural and arboricultural practices from lowland New Guinea and Island Melanesia were taken by Lapita pottery–bearing colonists into the western Pacific. A later period of agricultural expansion occurred around c. 1000–750 cal BP with the colonization of eastern Polynesia. Agricultural practices and crops were variably taken and adapted to different islands and island groups across the Pacific. Additional transformations to agriculture occurred with the Polynesian adoption of the sweet potato (Ipomoea batatas), a South American domesticate, as well as following protohistoric and historic encounters.
Article
Prehistoric Agriculture in China: Food Globalization in Prehistory
Giedrė Motuzaitė Matuzevičiūtė and Xinyi Liu
It is commonly recognised that farming activities initiated independently in different parts of the world between approximately 12,000 and 8,000 years ago. Two of such agricultural centres is situated in modern-day China, where systems based on the cultivation of plants and animal husbandry has developed. Recent investigations have shown that between 5000 and 1500 cal. bce, the Eurasian and African landmass underpinned a continental-scale process of food “globalisation of staple crops. In the narrative of food domestication and global food dispersal processes, China has played a particularly important role, contributing key staple food domesticates such as rice, broomcorn, and foxtail millet. The millets dispersed from China across Eurasia during the Bronze Age, becoming an essential food for many ancient communities. In counterpoise, southwest Asian crops, such as wheat or barley, found new habitats among the ancient populations of China, dramatically changing the course of its development. The processes of plant domestication and prehistoric agriculture in China have been a topic of extensive research, review, and discussion by many scholars around the world, and there is a great deal of literature on these topics. One of the consequences of these discoveries concerning the origins of agriculture in China has been to undermine the notion of a single centre of origin for civilisation, agriculture, and urbanism, which was a popular and widespread narrative in the past. It has become clear that agricultural centres of development in China were concurrent with, rather than after, the Fertile Crescent.
Article
Prehistoric and Traditional Agriculture in Lowland Mesoamerica
Clarissa Cagnato
Mesoamerica is one of the world’s primary centers of domestication where agriculture arose independently. Paleoethnobotany (or archaeobotany), along with archaeology, epigraphy, and ethnohistorical and ethnobotanical data, provide increasingly important insights into the ancient agriculture of Lowland Mesoamerica (below 1000 m above sea level). Moreover, new advances in the analysis of microbotanical remains in the form of pollen, phytoliths, and starch-grain analysis and chemical analysis of organic residues have further contributed to our understanding of ancient plant use in this region. Prehistoric and traditional agriculture in the lowlands of Mesoamerica—notably the Maya lowlands, the Gulf Coast, and the Pacific Coast of southern Chiapas (Mexico) and Guatemala—from the Archaic (ca. 8000/7000–2000 bc) through the Preclassic/Formative (2000 bc–ad 250) and into the Classic (ad 250–900) period, are covered. During the late Archaic, these lowland regions were inhabited by people who took full advantage of the rich natural biodiversity but also grew domesticates before becoming fully sedentary. Through time, they developed diverse management strategies to produce food, from the forest management system (which includes swidden agriculture), to larger scale land modifications such as terraces, and continued to rely on semidomesticated and wild plant resources. Although lowland populations came to eventually rely on maize as a staple, other resources such as root crops and fruit trees were also cultivated, encouraged, and consumed. The need for additional research that includes systematic collection of paleoethnobotanical data, along with other lines of evidence, will be key to continue refining the understanding of ancient subsistence systems and how these changed through time and across lowland Mesoamerica.
Article
Pros and Cons of GMO Crop Farming
Rene Van Acker, M. Motior Rahman, and S. Zahra H. Cici
The global area sown to genetically modified (GM) varieties of leading commercial crops (soybean, maize, canola, and cotton) has expanded over 100-fold over two decades. Thirty countries are producing GM crops and just five countries (United States, Brazil, Argentina, Canada, and India) account for almost 90% of the GM production. Only four crops account for 99% of worldwide GM crop area. Almost 100% of GM crops on the market are genetically engineered with herbicide tolerance (HT), and insect resistance (IR) traits. Approximately 70% of cultivated GM crops are HT, and GM HT crops have been credited with facilitating no-tillage and conservation tillage practices that conserve soil moisture and control soil erosion, and that also support carbon sequestration and reduced greenhouse gas emissions. Crop production and productivity increased significantly during the era of the adoption of GM crops; some of this increase can be attributed to GM technology and the yield protection traits that it has made possible even if the GM traits implemented to-date are not yield traits per se. GM crops have also been credited with helping to improve farm incomes and reduce pesticide use. Practical concerns around GM crops include the rise of insect pests and weeds that are resistant to pesticides. Other concerns around GM crops include broad seed variety access for farmers and rising seed costs as well as increased dependency on multinational seed companies. Citizens in many countries and especially in European countries are opposed to GM crops and have voiced concerns about possible impacts on human and environmental health. Nonetheless, proponents of GM crops argue that they are needed to enhance worldwide food production. The novelty of the technology and its potential to bring almost any trait into crops mean that there needs to remain dedicated diligence on the part of regulators to ensure that no GM crops are deregulated that may in fact pose risks to human health or the environment. The same will be true for the next wave of new breeding technologies, which include gene editing technologies.
Article
The Role of Cover Crops in Agriculture and Their Environmental Significance
Helena Aronsson
Growing a cover crop between main crops imitates natural ecosystems where the soil is continuously covered with vegetation. This is an important management practice in preserving soil nutrient resources and reducing nitrogen (N) losses to waters. Cover crops also provide other functions that are important for the resilience and long-term stability of cropping systems, such as reduced erosion, increased soil fertility, carbon sequestration, increased soil phosphorus (P) availability, and suppression of weeds and pathogens.
Much is known about how to use cover crops to reduce N leaching, for climates where there is a water surplus outside the growing season. Non-legume cover crops reduce N leaching by 20%–80% and legumes reduce it by, on average, 23%. There are both synergies and possible conflicts between different environmental and production aspects that should be considered when developing efficient and multifunctional cover crop systems, but contradictions about different functions provided by cover crops can sometimes be overcome with site-specific adaptation of measures. One example is cover crop effects on P losses. Cover crops reduce losses of total P, but extract soil P to available forms and may increase losses of dissolved P. How to use this effect to increase soil P availability on subtropical soils needs further studies. Knowledge and examples of how to maximize the positive effects of cover crops on cropping systems are improving, thereby increasing the sustainability of agriculture. One example is combined weed suppression in order to reduce dependence on herbicides or intensive mechanical treatment.
Article
Seed Banking as Future Insurance Against Crop Collapses
Fiona Hay
Food security is dependent on the work of plant scientists and breeders who develop new varieties of crops that are high yielding, nutritious, and tolerate a range of biotic and abiotic stresses. These scientists and breeders need access to novel genetic material to evaluate and to use in their breeding programs; seed- (gene-)banks are the main source of novel genetic material. There are more than 1,750 genebanks around the world that are storing the orthodox (desiccation tolerant) seeds of crops and their wild relatives. These seeds are stored at low moisture content and low temperature to extend their longevity and ensure that seeds with high viability can be distributed to end-users. Thus, seed genebanks serve two purposes: the long-term conservation of plant genetic resources, and the distribution of seed samples.
Globally, there are more than 7,400,000 accessions held in genebanks; an accession is a supposedly distinct, uniquely identifiable germplasm sample which represents a particular landrace, variety, breeding line, or population. Genebank staff manage their collections to ensure that suitable material is available and that the viability of the seeds remains high. Accessions are regenerated if viability declines or if stocks run low due to distribution. Many crops come under the auspices of the International Treaty on Plant Genetic Resources for Food and Agriculture and germplasm is shared using the Standard Material Transfer Agreement. The Treaty collates information on the sharing of germplasm with a view to ensuring that farmers ultimately benefit from making their agrobiodiversity available.
Ongoing research related to genebanks covers a range of disciplines, including botany, seed and plant physiology, genetics, geographic information science, and law.
Article
Social and Environmental Implications of Plantation Agriculture in Malaysia and Indonesia
Jean-François Bissonnette and Rodolphe De Koninck
Plantation farming emerged as a large-scale system of specialized agriculture in the tropics under European colonialism, in opposition to smallholding subsistence agriculture. Despite large-scale plantations in the tropics, smallholdings have consistently formed the backbone of rural economies, to the extent that they have become the main producers of some of the former plantation crops. In the early 21st century, oil palm has become the third most important cash crop in the world in terms of area cultivated, largely due to the expansion of this crop in Malaysia and Indonesia. Although in these countries, oil palm is primarily cultivated in large plantations, smallholders cultivate a large share of the territory devoted to this crop. This is related to the programs set up by governments of Malaysia and Indonesia during the second half of the 20th century, to provide smallholders with land plots in capital intensive large-scale oil palm schemes. Despite the relative success encountered by these programs in both countries, policymakers have continued to insist on the development of private centrally managed large-scale plantations. Yet, smallholding family farming has remained the most resilient economic activity in rural areas of the tropics. This system has proven adaptive to environmental change and, given proper access to markets and capital, particularly responsive to market signals. Today, many small-holdings are still characterized by the diversity of crops cultivated, low use of chemical inputs, reliance on family labor, and high levels of ecological knowledge. These are some of the main factors explaining why small family farms have proven more efficient than large plantations and, in the long term, more economically and ecologically resilient. Yet, large-scale land acquisitions for monocrop production remain a current issue, highlighting the paradox of the latest stage of agrarian capitalism and of its persistent built-in disregard for environmental deterioration.
Article
Soil Quality as Affected by Intensive Versus Conservative Agricultural Managements
Luigi Badalucco
Soils, the earth’s skin, are at the intersection of the lithosphere, hydrosphere, atmosphere, and biosphere. The persistence of life on our planet depends on the maintenance of soils as they constitute the biological engines of earth. Human population has increased exponentially in recent decades, along with the demand for food, materials, and energy, which have caused a shift from low-yield and subsistence agriculture to a more productive, high-cost, and intensive agriculture. However, soils are very fragile ecosystems and require centuries for their development, thus within the human timescale they are not renewable resources. Modern and intensive agriculture implies serious concern about the conservation of soil as living organism, i.e., of its capacity to perform the vast number of biochemical processes needed to complete the biogeochemical cycles of plant nutrients, such as nitrogen and phosphorus, crucial for crop primary production. Most practices related to intensive agriculture determine a deterioration even in the short-middle term of their physical, chemical, and biological properties, which all together contribute to soil quality, along with an overexploitation of soils as living organisms. Recent trends are turning toward styles of agriculture management that are more sustainable or conservative for soil quality.
Usually, use of soils for agricultural purposes deflect them at various degrees from the “natural” soil development processes (pedogenesis), and this shift may be assumed as a divergence from soil sustainability principles. For decades, the misuse of land due to intensive crop management has deteriorated soil health and quality. A huge plethora of microorganisms inhabits soils, thus acting as “the biological engine of the earth”; indeed, this microbiota serves the soil ecosystem, performing several fundamental functions. Therefore, management practices might be planned looking at the safeguard of soil microbial diversity and resilience. In addition, each unexpected alteration in numberless soil biochemical processes, being regulated by microbial communities, may represent an early and sensible signal of soil homeostasis weakening and, consequently, warn about soil conservation. Within the vast number of soil biochemical processes and connected features (bioindicators) virtually effective to measure the sustainable soil exploitation, those related to the mineralization or immobilization of the main nutrients (C and N), including enzyme activity (functioning) and composition (diversity) of microbial communities, exert a fundamental role because of their involvement in soil metabolism. Comparing the influence of many cropping factors (tillage, mulching and cover crops, rotations, mineral and organic fertilization) under both intensive and sustainable managements on soil microbial diversity and functioning, through both chemical and biological soil quality indicators, makes it possible to identify the most hazardous diversions from soil sustainability principles.
Article
Soil Resources, the Delivery of Ecosystem Services and Value
David A. Robinson, Fiona Seaton, Katrina Sharps, Amy Thomas, Francis Parry Roberts, Martine van der Ploeg, Laurence Jones, Jannes Stolte, Maria Puig de la Bellacasa, Paula Harrison, and Bridget Emmett
Soils provide important functions, which according to the European Commission include: biomass production (e.g., agriculture and forestry); storing, filtering, and transforming nutrients, substances, and water; harboring biodiversity (habitats, species, and genes); forming the physical and cultural environment for humans and their activities; providing raw materials; acting as a carbon pool; and forming an archive of geological and archaeological heritage, all of which support human society and planetary life. The basis of these functions is the soil natural capital, the stocks of soil material. Soil functions feed into a range of ecosystem services which in turn contribute to the United Nations sustainable development goals (SDGs). This overarching framework hides a range of complex, often nonlinear, biophysical interactions with feedbacks and perhaps yet to be discovered tipping points. Moreover, interwoven with this biophysical complexity are the interactions with human society and the socioeconomic system which often drives our attitudes toward, and the management and exploitation of, our environment.
Challenges abound, both social and environmental, in terms of how to feed an increasingly populous and material world, while maintaining some semblance of thriving ecosystems to pass on to future generations. How do we best steward the resources we have, keep them from degradation, and restore them where necessary as soils underpin life? How do we measure and quantify the soil resources we have, how are they changing in time and space, what can we predict about their future use and function? What is the value of soil resources, and how should we express it? This article explores how soil properties and processes underpin ecosystem services, how to measure and model them, and how to identify the wider benefits they provide to society. Furthermore, it considers value frameworks, including caring for our resources.
Article
Soil Salinization
Pichu Rengasamy
Salt accumulation in soils, affecting agricultural productivity, environmental health, and the economy of the community, is a global phenomenon since the decline of ancient Mesopotamian civilization by salinity. The global distribution of salt-affected soils is estimated to be around 830 million hectares extending over all the continents, including Africa, Asia, Australasia, and the Americas. The concentration and composition of salts depend on several resources and processes of salt accumulation in soil layers. Major types of soil salinization include groundwater associated salinity, non–groundwater-associated salinity, and irrigation-induced salinity. There are several soil processes which lead to salt build-up in the root zone interfering with the growth and physiological functions of plants.
Salts, depending on the ionic composition and concentration, can also affect many soil processes, such as soil water dynamics, soil structural stability, solubility of essential nutrients, and pH and pE of soil water—all indirectly hindering plant growth. The direct effect of salinity includes the osmotic effect affecting water and nutrient uptake and the toxicity or deficiency due to high concentration of certain ions. The plan of action to resolve the problems associated with soil salinization should focus on prevention of salt accumulation, removal of accumulated salts, and adaptation to a saline environment. Successful utilization of salinized soils needs appropriate soil and irrigation management and improvement of plants by breeding and genetic engineering techniques to tolerate different levels of salinity and associated abiotic stress.
Article
Soil Sediment Loading and Related Environmental Impacts from Farms
Vito Ferro
Beyond damage to rainfed agricultural and forestry ecosystems, soil erosion due to water affects surrounding environments. Large amounts of eroded soil are deposited in streams, lakes, and other ecosystems. The most costly off-site damages occur when eroded particles, transported along the hillslopes of a basin, arrive at the river network or are deposited in lakes. The negative effects of soil erosion include water pollution and siltation, organic matter loss, nutrient loss, and reduction in water storage capacity. Sediment deposition raises the bottom of waterways, making them more prone to overflowing and flooding. Sediments contaminate water ecosystems with soil particles and the fertilizer and pesticide chemicals they contain. Siltation of reservoirs and dams reduces water storage, increases the maintenance cost of dams, and shortens the lifetime of reservoirs. Sediment yield is the quantity of transported sediments, in a given time interval, from eroding sources through the hillslopes and river network to a basin outlet. Chemicals can also be transported together with the eroded sediments. Sediment deposition inside a reservoir reduces the water storage of a dam.
The prediction of sediment yield can be carried out by coupling an erosion model with a mathematical operator which expresses the sediment transport efficiency of the hillslopes and the channel network. The sediment lag between sediment yield and erosion can be simply represented by the sediment delivery ratio, which can be calculated at the outlet of the considered basin, or by using a distributed approach. The former procedure couples the evaluation of basin soil loss with an estimate of the sediment delivery ratio SDRW for the whole watershed. The latter procedure requires that the watershed be discretized into morphological units, areas having a constant steepness and a clearly defined length, for which the corresponding sediment delivery ratio is calculated. When rainfall reaches the surface horizon of the soil, some pollutants are desorbed and go into solution while others remain adsorbed and move with soil particles. The spatial distribution of the loading of nitrogen, phosphorous, and total organic carbon can be deduced using the spatial distribution of sediment yield and the pollutant content measured on soil samples. The enrichment concept is applied to clay, organic matter, and all pollutants adsorbed by soil particles, such as nitrogen and phosphorous. Knowledge of both the rate and pattern of sediment deposition in a reservoir is required to establish the remedial strategies which may be practicable. Repeated reservoir capacity surveys are used to determine the total volume occupied by sediment, the sedimentation pattern, and the shift in the stage-area and stage-storage curves. By converting the sedimentation volume to sediment mass, on the basis of estimated or measured bulk density, and correcting for trap efficiency, the sediment yield from the basin can be computed.
Article
Soils, Science, Society, and the Environment
Colin R. Robins
Soils are the complex, dynamic, spatially diverse, living, and environmentally sensitive foundations of terrestrial ecosystems as well as human civilizations. The modern, environmental study of soil is a truly young scientific discipline that emerged only in the late 19th century from foundations in agricultural chemistry, land resource mapping, and geology. Today, little more than a century later, soil science is a rigorously interdisciplinary field with a wide range of exciting applications in agronomy, ecology, environmental policy, geology, public health, and many other environmentally relevant disciplines. Soils form slowly, in response to five inter-related factors: climate, organisms, topography, parent material, and time. Consequently, many soils are chemically, biologically, and/or geologically unique. The profound importance of soil, combined with the threats of erosion, urban development, pollution, climate change, and other factors, are now prompting soil scientists to consider the application of endangered species concepts to rare or threatened soil around the world.
Article
Soil Tilth and Management
Lars J. Munkholm, Mansonia Pulido-Moncada, and Peter Bilson Obour
Soil tilth is a dynamic and multifaceted concept that refers to the suitability of a soil for planting and growing crops. A soil with good tilth is “usually loose, friable and well granulated”; a condition that can also be described as the soil’s having a good “self-mulching” ability. On the other hand soils with poor tilth are usually dense (compacted), with hard, blocky, or massive structural characteristics. Poor soil tilth is generally associated with compaction, induced by wheel traffic, animal trampling, and/or to natural soil consolidation (i.e., so-called hard-setting behavior). The soil-tilth concept dates back to the early days of arable farming and has been addressed in soil-science literature since the 1920s. Soil tilth is generally associated with soil’s physical properties and processes rather than the more holistic concepts of soil quality and soil health. Improved soil tilth has been associated with deep and intensive tillage, as those practices were traditionally considered the primary method for creating a suitable soil condition for plant growth. Therefore, for millennia there has been a strong focus both in practice and in research on developing tillage tools that create suitable growing conditions for different crops, soil types, and climatic conditions. Deep and intensive tillage may be appropriate for producing a good, short-term tilth, but may also lead to severe long-term degradation of the soil structure. The failure of methods relying on physical manipulation as means of sustaining good tilth has increased the recognition given to the important role that soil biota have in soil-structure formation and stabilization. Soil biology has only received substantial attention in soil science during the last few decades. One result of this is that this knowledge is now being used to optimize soil management through strategies such as more diverse rotations, cover crops, and crop-residue management, with these being applied either as single management components or more preferably as part of an integrated system (i.e., either conservation agriculture or organic farming).Traditionally, farmers have evaluated soil tilth qualitatively in the field. However, a number of quantitative or semi-quantitative procedures for assessing soil tilth has been developed over the last 80 years. These procedures vary from simply determining soil cloddiness to more detailed evaluations whereby soil’s physical properties (e.g., porosity, strength, and aggregate characteristics) are combined with its consistency and organic-matter measurements in soil-tilth indices. Semi-quantitative visual soil-evaluation methods have also been developed for field evaluation of soil tilth, and are now used in many countries worldwide.
Article
Subsurface (Tile) Agricultural Drainage
Gary R. Sands, Srinivasulu Ale, Laura E. Christianson, and Nathan Utt
Agricultural (tile) drainage enables agricultural production on millions of hectares of arable lands worldwide. Lands where drainage or irrigation (and sometimes both) are implemented, generate a disproportionately large share of global agricultural production compared to dry land or rain-fed agricultural lands and thus, these water management tools are vital for meeting the food demands of today and the future. Future food demands will likely require irrigation and drainage to be practiced on an even greater share of the world’s agricultural lands. The practice of agricultural drainage finds its roots in ancient societies and has evolved greatly to incorporate modern technologies and materials, including the modern drainage plow, plastic drainage pipe and tubing, laser and GPS-guided installation equipment, and computer-aided design tools. Although drainage brings important agricultural production and environmental benefits to poorly drained and salt-affected arable lands, it can also give rise to the transport of nutrients and other constituents to downstream waters. Other unwanted ecological and hydrologic environmental effects may also be associated with the practice. The goal of this article is to familiarize the reader with the practice of subsurface agricultural drainage, the history and extent of its application, and the benefits commonly associated with it. In addition, environmental effects associated with subsurface drainage including hydrologic and water quality effects are presented, and conservation practices for mitigating these unwanted effects are described. These conservation practices are categorized by whether they are implemented in-field (such as controlled drainage) versus edge-of-field (such as bioreactors). The literature cited and reviewed herein is not meant to be exhaustive, but seminal and key literary works are identified where possible.
Article
Surface Irrigation
Luis S. Pereira and José M. Gonçalves
Surface irrigation is the oldest and most widely used irrigation method, more than 83% of the world’s irrigated area. It comprises traditional systems, developed over millennia, and modern systems with mechanized and often automated water application and adopting precise land-leveling. It adapts well to non-sloping conditions, low to medium soil infiltration characteristics, most crops, and crop mechanization as well as environmental conditions. Modern methods provide for water and energy saving, control of environmental impacts, labor saving, and cropping economic success, thus for competing with pressurized irrigation methods. Surface irrigation refers to a variety of gravity application of the irrigation water, which infiltrates into the soil while flowing over the field surface. The ways and timings of how water flows over the field and infiltrates the soil determine the irrigation phases—advance, maintenance or ponding, depletion, and recession—which vary with the irrigation method, namely paddy basin, leveled basin, border and furrow irrigation, generally used for field crops, and wild flooding and water spreading from contour ditches, used for pasture lands. System performance is commonly assessed using the distribution uniformity indicator, while management performance is assessed with the application efficiency or the beneficial water use fraction. The factors influencing system performance are multiple and interacting—inflow rate, field length and shape, soil hydraulics roughness, field slope, soil infiltration rate, and cutoff time—while management performance, in addition to these factors, depends upon the soil water deficit at time of irrigation, thus on the way farmers are able to manage irrigation. The process of surface irrigation is complex to describe because it combines surface flow with infiltration into the soil profile. Numerous mathematical computer models have therefore been developed for its simulation, aimed at both design adopting a target performance and field evaluation of actual performance. The use of models in design allows taking into consideration the factors referred to before and, when adopting any type of decision support system or multicriteria analysis, also taking into consideration economic and environmental constraints and issues.
There are various aspects favoring and limiting the adoption of surface irrigation. Favorable aspects include the simplicity of its adoption at farm in flat lands with low infiltration rates, namely when water conveyance and distribution are performed with canal and/or low-pressure pipe systems, low capital investment, and low energy consumption. Most significant limitations include high soil infiltration and high variability of infiltration throughout the field, land leveling requirements, need for control of a constant inflow rate, difficulties in matching irrigation time duration with soil water deficit at time of irrigation, and difficult access to equipment for mechanized and automated water application and distribution. The modernization of surface irrigation systems and design models, as well as models and tools usable to support surface irrigation management, have significantly impacted water use and productivity, and thus competitiveness of surface irrigation.
