Let's delve into the details of each option:

Option 1: Phosphates have no biological significance in humans.

This statement is false. Phosphates play a crucial role in various biological processes in humans, including:

1. Bone Formation and Health: Phosphates are essential components of apatite, the mineral that gives bones their strength and rigidity. Deficiency in phosphates can lead to rickets, a condition characterized by weak and deformed bones.

2. Energy Production: Phosphates are integral components of ATP (adenosine triphosphate), the energy currency of cells. ATP provides the energy required for various cellular processes, including muscle contraction, nerve transmission, and protein synthesis.

3. Cell Membrane Structure: Phosphates are found in phospholipids, the major component of cell membranes. Phospholipids contribute to the flexibility and permeability of cell membranes, allowing essential molecules to pass in and out.

4. Nucleic Acid Synthesis: Phosphates are essential components of DNA and RNA, the molecules that carry genetic information. Phosphates form the backbone of these nucleic acids, linking the individual nucleotides.

5. Acid-Base Balance: Phosphates play a role in maintaining the body's acid-base balance. They act as buffers, absorbing excess protons and preventing excessive acidity or alkalinity in the body fluids.

6. Signal Transduction: Phosphates are involved in signal transduction pathways, relaying signals from the outside of the cell to the inside, triggering various cellular responses.

In conclusion, phosphates are far from insignificant in humans; they are vital for a multitude of biological processes that underpin our health and well-being.

Option 2: Between nitrates and phosphates, phosphates are less abundant in the earth's crust.

This statement is also false. Phosphates are more abundant in the earth's crust than nitrates. Phosphates make up about 0.09% of the earth's crust, while nitrates make up only about 0.0001%.

The higher abundance of phosphates is attributed to their chemical stability and ability to form various minerals. Phosphates readily form insoluble compounds with calcium, aluminum, and iron, leading to their concentration in sedimentary rocks.

Nitrates, on the other hand, are more soluble and easily lost through weathering and erosion. They are also readily consumed by microorganisms, further reducing their abundance in the earth's crust.

Option 4: Oxidation of nitrates is possible in soil.

This statement is incorrect. Nitrates are not oxidized in soil; they are actually reduced to nitrogen gas in soil through a process called denitrification.

Denitrification is an anaerobic process, meaning it occurs in the absence of oxygen. It is carried out by specialized bacteria that use nitrates as an alternative electron acceptor for respiration. This process converts nitrates into nitrogen gas, which is released into the atmosphere.

Oxidation of nitrates, on the other hand, is less common and typically occurs in specific conditions, such as in acidic soils or when nitrates are exposed to high levels of ultraviolet radiation.

Therefore, the only correct statement is option 3: Between nitrates and phosphates, nitrates are less abundant in the earth's crust. This is because nitrates are more soluble and easily lost through weathering and erosion, and they are also readily consumed by microorganisms. Phosphates, on the other hand, are more stable and abundant in the earth's crust due to their ability to form various minerals.