Is the arable land a mean of production? Wikipedia claims that "capitalism is an economic system and an ideology based on private ownership of the means of production and their operation for profit".

And then, the land used for agriculture is a mean of production? Are the farmers capitalists?
Yes.

Anyone who utilizes their privately owned property or assets as a means of production for the sake of profit is a capitalist.

In this case the farmer's capital  is the land which they use for agricultural production.