Step-by-step analysis of each option:

A: Phosphates have no biological significance in humans.

1. This statement is factually incorrect. Phosphates are essential for life.
2. They are a fundamental component of adenosine triphosphate (ATP), which is the primary energy carrier in all cells.
3. They form the sugar-phosphate backbone of DNA and RNA, the molecules that carry genetic information.
4. Calcium phosphate, in the form of hydroxyapatite ($\text{Ca}_5(\text{PO}_4)_3(\text{OH})$), is the main structural component of bones and teeth.
5. Therefore, this statement is false.

B: Between nitrates and phosphates, phosphates are less abundant in earth's crust.

1. The preamble states that nitrates are more soluble in water. Most common nitrate salts (like NaNO₃, KNO₃, Ca(NO₃)₂) are highly soluble.
2. In contrast, many common phosphate minerals, such as apatite ($\text{Ca}_5(\text{PO}_4)_3(\text{F, Cl, OH})$), are largely insoluble in water.
3. Because of their high solubility, nitrate deposits are easily washed away by rain and water, and are therefore much rarer than phosphate deposits. Significant nitrate deposits are found only in very arid regions, like the Atacama Desert in Chile.
4. Phosphate rock, on the other hand, is a common mineral and is found in extensive deposits worldwide.
5. Therefore, phosphates are more abundant than nitrates in the earth's crust. This makes the statement that phosphates are less abundant incorrect.

C: Between nitrates and phosphates, nitrates are less abundant in earth's crust.

1. As explained in the analysis of option B, nitrates are highly soluble in water. This property causes them to be leached from soils and rocks and transported to oceans or consumed by organisms.
2. Consequently, large, stable mineral deposits of nitrates are rare compared to the less soluble phosphate mineral deposits.
3. Therefore, nitrates are indeed less abundant than phosphates in the Earth's crust. This statement is correct.

D: Oxidation of nitrates is possible in soil.

1. To determine if oxidation is possible, we need to find the oxidation state of nitrogen in the nitrate ion ($\text{NO}_3^-$).
2. Let the oxidation state of nitrogen be $x$. Oxygen usually has an oxidation state of -2. The overall charge of the ion is -1.
3. The equation is: $x + 3(-2) = -1$.
4. Solving for $x$: $x - 6 = -1 \implies x = +5$.
5. Nitrogen is in Group 15 of the periodic table, and its highest possible oxidation state is +5.
6. Since nitrogen in the nitrate ion is already in its maximum oxidation state, it cannot be further oxidized.
7. Therefore, this statement is false.

Conclusion:

Based on the analysis, only statement C is correct.