Potassium sulphate (K2SO4) is a valuable fertilizer that provides two essential plant nutrients: potassium and sulphur. It typically contains about 50% potassium (as K2O) and approximately 17% sulphur. The presence of sulphur is particularly beneficial for crops that have high sulphur requirements, such as oilseeds and legumes, and it is often preferred in saline soils because it does not contribute chloride ions, which can be detrimental to certain sensitive crops.

When DDT is ingested by mammals, including rats and humans, it undergoes metabolic transformation. The primary metabolite formed is DDA (2,2-bis(p-chlorophenyl)acetic acid), which is a water-soluble compound that can be excreted from the body. While DDE is a common environmental degradation product of DDT found in soil and fatty tissues, DDA represents the specific metabolic pathway product in mammalian systems, facilitating the detoxification and elimination of the chemical.

New Zealand's agricultural system, particularly its grassland farming, relies significantly on biological nitrogen fixation by legumes like clover. In this system, fertilizer application is often limited to non-nitrogenous sources and lime. When properly nodulated and provided with adequate mineral nutrition, clover-rhizobia symbiosis effectively supplies the necessary nitrogen for pasture growth, reducing the need for synthetic nitrogen fertilizers.

Stabilizing shifting sand dunes requires a multi-faceted approach. Mechanical stabilization, such as creating barriers in a grid or chess-board pattern using local brushwood, is essential to prevent sand movement and protect young seedlings. Additionally, excluding biotic interference (like grazing) is critical to allow the vegetation to establish successfully and provide long-term biological stabilization of the dunes.

Boron is an essential micronutrient but becomes toxic to many crops at relatively low concentrations. Irrigation water quality standards generally classify concentrations below 0.7 ppm as safe for sensitive crops, while higher levels may cause toxicity symptoms such as leaf tip burn and reduced yields.

Azolla, a genus of aquatic ferns in the family Salviniaceae, is highly beneficial as a biofertilizer in rice cultivation due to its nitrogen-fixing symbiosis with Anabaena azollae. However, its practical application is constrained by several factors, including the requirement for consistent water availability, poor survival rates during high-temperature summer months, and logistical challenges associated with the transportation of fresh biomass to field sites.

Sodium ions (Na+) are known for their high hydration energy and ability to influence soil physical properties. In soil chemistry, high concentrations of sodium can lead to dispersion, which affects water movement and absorption. While sodium is essential for various physiological processes in plants, its presence in soil can alter osmotic pressure and water retention characteristics, often leading to challenges in saline-sodic soil management.

Humic substances are complex organic compounds formed during the decomposition of plant and animal residues. They are typically fractionated based on their solubility in acids and bases. Hymatomelanic acid is defined as the ethanol-soluble fraction of the humic acid complex. This fractionation is important for soil chemists to understand the structural heterogeneity and functional properties of soil organic matter, which plays a crucial role in soil structure, nutrient retention, and overall soil health.

The phosphorus cycle is unique because it does not have a significant atmospheric component. The largest reservoir of phosphorus is found in sedimentary rocks, specifically phosphate rocks. Through the processes of weathering and erosion, phosphorus is released from these rocks into the soil and water, where it becomes available for uptake by plants and subsequent integration into the food chain. This makes rocks the primary long-term storage site for phosphorus.

Legumes form symbiotic relationships with nitrogen-fixing bacteria in their root nodules. These bacteria are collectively known as rhizobia. Among these, the genus Bradyrhizobium is characterized by its slow-growing nature compared to the fast-growing Rhizobium species, making it a distinct group within the root-nodule bacteria family.