Chapter 5 is a unique book chapter. The purpose is to introduce the reader to the array of chemical, physical, and mineralogical data commonly provided by the Natural Resource Conservation Service (NRCS) as well as most international soil survey programs. This database is enormous, containing a wealth of still untapped potential. The chapter provides the reader with a concise overview of what is on the soil characterization data sheets and how the data are obtained. These data then serve as the basis for any number of research questions and explorations.

Chapter 7 presents the soil carbon cycle. The chapter largely by-passes the still uncertain processes that occur at the molecular scale. The focus is on macroscopic properties and how they vary with space and time. Soil C storage is first examined from a box model perspective, which introduces mass balance equations and how they are useful, when coupled with data, in beginning to understanding soil C dynamics. The chapter includes an introductory perspective on the vertical trends in soil C and the transport-reaction models that are needed to fully explain these patterns. Soil organic C is largely removed from soil as CO2, and production-diffusion models are introduced to explain observable CO2 depth profiles and to calculate the fluxes to the atmosphere. Diffusion impacts the C isotope composition of soil CO2 and any CaCO3 minerals that subsequently form. These are examined through the lens of diffusion modeling, which is now common, and critical, in any examination of soil properties with depth.

The best thing that happened during this long gestation period was the outstanding work of my graduate students, which enriches this book and has transformed my understanding of soils. I mention those whose work is directly linked to this book. Gene Kelly helped expose me to the soils of the Great Plains and introduced me to biomineralization. Stephanie Ewing, Justine Owen, Kari Finstad, and Marco Pfeiffer have collectively provided a quantum leap forward in our understanding of the driest desert on Earth, in northern Chile. As a group, they have contributed to understanding the climate threshold that exists between the biotic and abiotic parts of our planet – and its profound impact on soil and landscape biogeochemistry. Erik Oerter advanced the ability of soils to provide paleoclimate information from carbonate using micro-sampling and dating techniques.

Chapter 2 is a deep dive into the threads that link the chemical and mineralogical makeup of soils to that of the surrounding cosmos. The first section examines elements across the periodic table, and how they systematically change in abundance due to a variety of cosmic processes. The second part of the chapter examines how the elements are combined into minerals, and especially the silicate minerals. Discussion of the factors that dictate mineral stability in soils is introduced, and these will be examined in even greater depth in Chapter 8. Secondary minerals are also introduced, again with a strong focus on the silicate group. Cation exchange is examined. The chapter ends with the effect of plants, and biology, on soil chemistry, which is expanded upon in Chapter 3. The activities at the conclusion of the chapter provide students with an opportunity to use spreadsheets and data analyses in order to gain experience and confidence in data analysis.

Chapter 1 is an abridged and concise overview of Jenny’s Factors of Soil Formation theory. The soil system is defined and explored from a state factor perspective. The ways in which this theory can be used to pose research questions and to design observation studies to address them are discussed. The relevance to the current Critical Zone science program is considered. The use of the state factor approach to global soil C science and to a climosequence of soils along the Mississippi River corridor is illustrated. The activities at the end of the chapter include a detailed tutorial on the solution of the state factor model for soil N in the Great Plains, first examined by Hans Jenny in a 1930 research paper.

Chapter 8 focuses on major advances in data acquisition and modeling of soil chemical weathering - at both the humid and the very dry end of the Earth’s climatic spectrum. The behavior of elements in aqueous solutions is examined in more detail. The now widely used mass balance method of calculating elemental gains and losses by comparison with an immobile index element is introduced and discussed. The concept of weathering fronts, and ways to understand what creates them and the rates at which they move through soils, is introduced - a relatively new concept from geoscientists who focus on soil geochemical processes and their rates.

Chapter 4 is an introduction to the architecture and makeup of soils as observed during study and sampling. This chapter is abridged in such a way as to provide an understandable foundation for the following chapters. In the classroom, this chapter can be presented and illustrated with any number of soil profile photos, and associated data, all of which are readily accessible on the internet. Additionally, even in urban settings, a class can be taught outdoors near the classroom, and a soil core can be extracted, laid sequentially on a tarp, and examined using the concepts and terms introduced in the chapter.

Chapter 9 is devoted to one of the most important new developments in soil research in the last 50 years: the linkage of geomorphological models with soils on hillslopes. Soils on hillslopes are dynamic entities that are constantly in motion and that exhibit aspects of complex systems, with feedbacks that stabilize them and help ensure their presence on all but the more extreme conditions on Earth. The processes (and models) to describe hillslope mass balance (erosion and production) are introduced. The importance of hillslope erosion and soil production to biogeochemistry is considered. Finally, soils on depositional landforms, which receive the material from the hillslopes, are also considered.

Chapter 3 is devoted to the biology of soil biogeochemistry. This is a rapidly evolving field. The chapter begins with our present understanding of the tree of life, and how little of it we have been able to detect. The second section examines the role of biology, its enzymatic impacts on the nature of chemistry over geological time, and the impacts it has had via oxidation-reduction pathways. The section considers how minerals and compounds that are now common on Earth are present largely (or only) because of biological processes. The next section examines what is presently known about the geography of soil microbiology, and what that means for the spatial diversity of metabolic capabilities. The chapter considers the challenges and emerging opportunities of explicitly embedding microbial parameters into biogeochemical models. Finally, the role and impact of vascular plants on nutrient cycling, distributions, and weathering are introduced.

Chapter 6 is an introduction to the importance and role of time in soil biogeochemistry. Time is one of the five major factors of soil formation, but many students are unfamiliar with the concept and terms of geological time, and with the concept and understanding of soil age. New developments in geomorphology and geochemistry in the last two decades have further added important insights into the concept and determination of age on hillslopes, which is also introduced here.

Chapter 10 focuses on what might be the most important reason for studying soil biogeochemistry: its importance for and interactions with us. First, the sheer magnitude of human disturbance of the soil mantle on Earth is introduced, especially the effect of farming. The impact of farming on soil C is examined in detail, and the effects of irrigated agriculture on soil geochemistry are used as an example of further comparisons that can be made by the reader. The uncertain and potential impact of our warming climate system on soil C, and potential feedbacks that might be ensuing, are considered. The recently renewed interest in accelerated soil erosion on managed landscapes is examined in light of new methods to measure the rates of erosion and also our emerging concepts and data on the true rate of soil production and regeneration.