topsoil from a farm field is stolen by cousins

1. File FIR at local Police Station.  Police will determine the type of offence under IPC and other Laws, during investigation & filing prosecution.

Keep Smiling .... Hemant Agarwal 
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File compliant with local police as suggested by experts

Hello, Greetings of the day!
For the query you posted, I would suggest that:
The legal remedy that is available to you is that you can register your compliant under Section 441 of the Indian Penal Code. You can also claim for damages that is being occurred due to the criminal trespass. 
“According to Section 441 of The Indian Penal Code, whoever enters into property in the possession of another with the intent to commit an offence or to intimidate, insult or annoy any person in possession of such property, or having lawfully entered into such property, but remains there with intent thereby to intimidate, insult or any such person, or with an intent to commit an offence, is said to commit ‘criminal trespass’.”
There are two important ingredients to constitute an offence under Section 441 of the IPC, firstly entering into the property of another with criminal intent and secondly, entering lawfully but remaining in the property with a criminal intent to cause harm or annoyance or to commit an offence. Thus, these essential ingredients are necessary for committing Criminal trespass.

Section 447 of the Indian Penal Code deals with the Punishment for criminal trespass:
“According to Section 441 of The Indian Penal Code, whoever commits criminal trespass shall be punished with imprisonment of either descripttion for a term which may extend to three months, with fine or which may extend to five hundred rupees, or with both”.
As per Section 441 of the IPC, it prescribes the punishment either for imprisonment which may extend to three months, or fine which may extend to Rs. 500 or both.

Hope it helps
Regards,
Sneha Jaiswal