[Salado (OP)]

I heard that somewhere and i wonder if it's true and why.

[CliffordK]

Of course, there is a lot of nitrogen in the air of the form N₂.  However, few plants can use this nitrogen directly.  The plants require reduced nitrogen in the form of NH₃, NH₄⁺, (NH₂)₂C=O (Urea), and etc.

Most of the amino acids (protein building blocks) contain nitrogen.  In order for the plants to grow, and produce the proteins, they must absorb nitrogen.

And, thus, in the USA, we use over 10 billion tons of nitrogen on our crops every year.

[Salado (OP)]

Does that mean that plants can grow where ever there's corpses of animals?

[CliffordK]

All living things have proteins.  Plants, Animals, Bacteria, etc.  And, thus also nitrogen, and other nutrients.  Animals, and muscle is high in protein, but certainly not the only protein source.  However, generally the plants can make their own amino acids, and thus would not be dependent on essential amino acids like humans.

Likely the plants would be better off as a carcass decomposes.  However, there are stories that some American Indians had used fish (whole fish?  or perhaps entrails) as fertilizer for their crops.

Some plants including legumes have nitrogen fixing bacteria on their roots, and can absorb most or all of their nitrogen from the atmosphere.

[Bored chemist]

Yes, they can (and they pretty much do)
http://www.yates.com.au/commercial/products/plant-food/organic-based/blood-bone-based-fertiliser/

[Lab Rat]

Plants rely quite a bit on bacteria and other organisms that will break things down in the soil around their roots into a form of nitrogen they can absorb and use.

[alexgreen12]

Nitrogen is actually considered the most important component for supporting plant growth. Nitrogen is found in soils and in the leaves, grain, plant tissue, and roots of plants. Nitrogen deficiency can be corrected by applying either organic or inorganic fertilizers, but nitrogen-fixing bacteria or ammonium-based fertilizers work the most quickly.

[evan_au]

nitrogen-fixing bacteria or ammonium-based fertilizers work the most quickly

Some plants like legumes have structures that host nitrogen-fixing bacteria, allowing them to grow in soils with low levels of "fixed" nitrogen. These plants enrich the soil so that other types of crops will grow better in the same soil.
- A species of corn was discovered recently which also has this property, allowing it to be farmed with less fertilizer (less CO2 emissions)
- However, the plant must supply food/energy to the nitrogen-fixing bacteria, so the plant has less resources to spend producing edible seeds or leaves.

So there are considerable benefits to nitrogen-fixing plants - but also some limitations...
See: https://en.wikipedia.org/wiki/Nitrogen_fixation#Root_nodule_symbioses