Studies of the high-ionization metal-line absorbers provide insights into hot diffuse gas that has been processed through stars in galaxies. In the ultraviolet and optical bands, these absorbers have been studied primarily using five-times ionized oxygen (OVI), six-times ionized nitrogen (NV), and seven-times ionized neon (NeVIII). Both OVI and NeVIII arise within the spectral range of the Ly α forest and are thus mostly visible at low redshifts where the Ly α forest line density is much smaller. NV is adjacent to the Ly α line and in principle can be surveyed over the full range of redshift; however, this ion is found in only a narrow range of astrophysical conditions. The population statistics measured include the redshift path density, the equivalent width and column density distributions, the cosmic mass densities, and the kinematics (broadening parameters, velocity splitting distributions, and absorber velocity widths). In this chapter, we discuss multiple observational programs and their reported findings for several of these ions.

Studies of the low-ionization metal-line absorbers provide insights into cool/warm higher-density gas that has been processed through stars in galaxies. These absorbers have been studied primarily using the abundant neutral atoms sodium, oxygen, and carbon (NaI, OI, and CI), as well as the singly ionized ions of carbon, silicon, calcium, and magnesium (CII, SiII, CaII, and MgII). For optical quasar spectroscopy, these ions have limited visibilities over different redshift ranges. The advent of sensitive UV and IR spectrographs expanded the redshift coverage of MgII absorbers from z = 0 to z = 7. However, the redshift visibility of OI, CI, CII, and SiII remain limited because of their far-ultraviolet transitions. The population statistics measured include the redshift path density, the equivalent width and column density distributions, the cosmic mass densities, and the kinematics (broadening parameters, velocity splitting distributions, and absorber velocity widths). In this chapter, we discuss multiple observational programs and their reported findings for several of the ions.

Studies of the intermediate-ionization metal-line absorbers provide insights into warm/hot lower-density gas that has been processed through stars in galaxies. These absorbers have been studied primarily using doubly and triply ionized carbon and silicon ions (CIII, CIV, SiIII, and SiIV). CIII arises deep within the spectral range of the Ly α forest and is thus mostly visible at low redshifts where the Ly α forest line density is much smaller. SiIII is adjacent to the Ly α line and is also best surveyed at low redshift. The CIV and SiIV lines are well redward of the Ly α line and thus have visibility over a wide range of redshift. UV and IR spectrographs expanded the redshift coverage from z = 0 to z = 7. The population statistics measured include the redshift path density, the equivalent width and column density distributions, the cosmic mass densities, and the kinematics (broadening parameters, velocity splitting distributions, and absorber velocity widths). In this chapter, we discuss multiple observational programs and their reported findings for several of these ions.

Crops are grown in certain types of soils, that is, not all soils are suitable for growing all crops and some crops can be grown in only certain types of soils. Irrigation of crops significantly depends on the type of soil. Soil is fundamental to our biosphere and requires proper management. This chapter discusses the basic soil properties that are relevant to crops and farm irrigation.