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Chapter 3 gives examples of how grapevines, being woody perennials, have the potential to develop extensive, deep root systems when soil conditions are favorable. One of the most important factors governing root growth is a soil’s structure, the essential attributes of which are • Spaces (collectively called the pore space or porosity) through which roots grow, gases diffuse, and water flows • Storage of water and natural drainage following rain or irrigation • Stable aggregation • Strength that not only enables moist soil to bear the weight of machinery and resist compaction but also influences the ease with which roots can push through the soil The key attributes of porosity, aeration and drainage, water storage, aggregation, and soil strength are discussed in turn. Various forces exerted by growing roots, burrowing animals and insects, the movement of water and its change of state (e.g., from liquid to ice) together organize the primary soil particles—clay, silt, and sand—into larger units called aggregates. Between and within these aggregates exists a network of spaces called pores. Total soil porosity is defined by the ratio . . . Porosity = Volume of pores/Volume of soil . . . A soil’s A horizon, containing organic matter, typically has a porosity between 0.5 and 0.6 cubic meter per cubic meter (m3/m3)—also expressed as 50% to 60%. In subsoils, where there is little organic matter and usually more clay, the porosity is typically 40% to 50%. Box 4.1 describes a simple way of estimating a soil’s porosity. Total porosity is important because it determines how much of the soil volume water, air, and roots can occupy. Equally important are the shape and size of the pores. The pores created by burrowing earthworms, plant roots, and fungal hyphae are roughly cylindrical, whereas those created by alternate wetting and drying appear as cracks. Overall, however, we express pore size in terms of diameter (equivalent to a width for cracks). Table 4.1 gives a classification of pore size based on pore function.

Oxygen has returned to some dead zones, but many problems remain. As this chapter explains, nutrient input from agriculture in some regions has decreased because farmers use buffer zones, cover crops, and precision agriculture. But voluntary efforts to minimize nutrient pollution aren’t enough. In Iowa, the Des Moines Water Works, led by a charismatic CEO, Bill Stowe, unsuccessfully sued to reduce nitrate leaching from local farms. The value of government action has been demonstrated in Denmark, whereas its absence has led to many environmental problems in China. The chapter argues that one solution is tied to human health and climate change: our diet. Eating less, especially eating less red meat, would be better for our health, and it would reduce nutrient pollution and abate climate change. Agriculture accounts for nearly a third of all greenhouse gas emissions. The chapter ends by suggesting that the successful bans against DDT and phosphorus detergents are among the reasons to be optimistic about solving the dead-zone problem.

This chapter gives a basic introduction to soil formation and fundamental soil processes in agroecosystems. The types of soils found in agroecosystems and their importance for agriculture is explored, with a principal focus on soil biodiversity, i.e. soil-dwelling organisms, their variety and function, and the interaction between soil biology, agriculture, and food production. The chapter describes some of the issues associated with soils in agroecosystems. These include interactions between agricultural practices and soil erosion and soil quality issues such as salinization and desertification. The major challenges to maintaining ‘healthy’ soils on productive land are outlined, and approaches and techniques for managing soils described.