11. Sustaining Soil Resources While Managing Nutrients

Abstract

The focus of most nutrient management studies has logically been on economic viability and water quality. In this chapter, we examine the wider issue of sustaining soil resources when developing practices designed to improve water quality. Sustainability when applied to crop production is often an emotionally charged word that has been used in many contexts. It has been used interchangeably with terms such as low-input sustainable agriculture, alternative agriculture, organic farming, regenerative farming, best management practices, and maximum economic yield (Keeney, 1990). Here we wish to use it in a more formal sense as defined by the 1987 Iowa Groundwater Protection Act. The Act defined sustainable agriculture as “the appropriate use of crop and livestock systems and agricultural inputs supporting those activities, which maintain economic and social viability whereas preserving the high productivity and quality of Iowa’s land.” Similarly, the National Food, Agriculture, Conservation, and Trade Act of 1990, Section 1603, defines sustainable agriculture as “an integrated system of plant and animal production practices that will, over the long term, enhance environmental quality and the natural resource base upon which the agricultural economy depends.” Thus, to be sustainable in the Corn and Soybean Belt, a farming system not only needs to be economically viable and protect water quality but also must preserve or enhance the soil resource that makes the highly productive agriculture possible. How do various management practices affect yield, water quality, and the soil resource? When discussing soil productivity, we are primarily concerned with maintaining or building soil organic matter (SOM) within the topsoil, because SOM provides much of the nutrient reservoir (fertility); determines physical characteristics that control infiltration, aeration, and aggregation associated with good soil tilth; and provides the energy or substrate for biological processes. SOM can be lost from a soil through two primary mechanisms: soil erosion and in situ decomposition. Soil erosion is a natural process, but enhanced erosion has been a consequence of agriculture from its inception. Sediment derived from soil erosion is the primary pollutant of surface waters today and a major cost to society. As the topsoil erodes, the sediment is enriched in SOM and the nutrients required for crop production. Current soil conservation programs are targeted towards reducing soil erosion to the tolerable, or T, level as determined by the Revised Universal Soil Loss Equation, and all management practices targeted for nutrient loss reduction must also keep soil loss below T. As topsoil protection from erosion is covered elsewhere in this book (see chapter 9), we will not examine it further here except to point out that achieving T alone is not sufficient for sustaining soil resources. Instead, we will concentrate on the second loss mechanism for SOM: decomposition.