Soil porosity represents the fraction of the total soil volume occupied by pores. It is calculated using the formula: Porosity = (1 - (Bulk Density / Particle Density)) * 100. Substituting the given values: (1 - (1.3 / 2.6)) * 100 = (1 - 0.5) * 100 = 50%. This indicates that half of the soil volume consists of pore spaces, which are critical for air and water movement within the soil matrix.

Saline soils are characterized by high concentrations of soluble salts that interfere with plant growth. The criteria provided (pH < 8.5, ESP < 15, and high electrical conductivity) are the standard diagnostic thresholds for saline soils. These soils do not have excessive exchangeable sodium, which distinguishes them from sodic or saline-sodic soils, and their management focuses primarily on leaching excess salts from the root zone.

The average particle density of most mineral soils is approximately 2.65 Mg/m³ (or 2.65 g/cm³). This value is largely determined by the density of common soil minerals like quartz, feldspar, and mica. It represents the mass per unit volume of the solid soil particles alone, excluding pore space.

Cation exchange capacity (CEC) is the ability of soil particles to attract and hold positively charged ions. This property in colloidal clay is primarily driven by isomorphous substitution (the replacement of one structural cation by another of similar size) and the presence of broken edges on clay lattices, which create net negative charges. These two mechanisms are the fundamental sources of the negative charge that facilitates cation exchange in soil minerals.

Gypsum (calcium sulfate) is applied to sodic soils to replace exchangeable sodium with calcium. The gypsum requirement (GR) is calculated based on the difference between the initial and desired final Exchangeable Sodium Percentage (ESP), multiplied by the Cation Exchange Capacity (CEC) of the soil. The formula provided in option C correctly represents the standard calculation method used in soil science to determine the amount of gypsum needed for reclamation.

Electrical conductivity (EC) is a measure of the salt concentration in soil solution. While various units exist, deciSiemens per meter (dS/m) is the standard SI-derived unit used in modern agricultural soil testing to assess salinity levels.

Reclamation disease is a physiological disorder in plants characterized by stunted growth and leaf tip necrosis, often linked to copper deficiency in newly reclaimed soils. While the source identifies Aridisols, it is worth noting that this condition is also historically associated with organic-rich Histosols (peat soils) where copper is strongly bound. The provided answer is Aridisols, which are common in arid regions where soil management and reclamation are frequent.

Aridisols are the dominant soil order in Pakistan because the country experiences predominantly arid and semi-arid climatic conditions. These soils are characterized by low organic matter and limited moisture, which are typical of the vast desert and dryland regions found throughout the country's geography.

Gravitational water is the excess water in the soil that moves downward through the soil profile by the force of gravity, beyond the capacity of the soil to hold it as hygroscopic or capillary water. This movement is essential for groundwater recharge and leaching processes, as it drains out of the root zone after saturation.

Apparent Nutrient Recovery is one of the more complex forms of NUE expressions and is defined as the difference in nutrient uptake in above-ground parts of the plants between the fertilized and unfertilized crop relative to the quantity of nutrient applied.This can be calculated if you have values of Nutrients removed from the fertilized plot, nutrients removed from the unfertilized plot, and nutrients added.