Ownership of something doesn't really mean it's a good, it's the right to exclude others from using what you own. So unless other people can do things to you without your consent, you do own your body.
I don't understand what the qualitative difference between a field and an apple is. Aren't they both things you find useful and are produced by mixing labour with natural resources? When you put a fence around a field you're excluding others from using something you made from the earth and when you put that apple in a locked container, you do the same, don't you?
Also I'm not sure what you mean when you say that the fruits of the land are reproducible. You can't grow more than X apples per orchard and you can't mine more than X tons of iron from a mine. Land is limited and its resources are limited.
Ownership implies that something can be bought and sold and I don't think it is right that people can be bought and sold.
I appreciate that you 'don't understand' the difference between the land and an apple produced from that land but that's not really my fault, I've used plain language. Land is not reproducable in that if you have an acre you can't grow two acres from it, if you have a tree you have the potential to grow a multiplicity of apples and further trees. This is why monopolisation of land alienates in a substantive way but claiming the fruits of your labour, the apple, doesn't. You could be pedantic and claim that there is, ultimately, only a finite number of apples that can be grown, but I'm not really interested in pedantry, it's only evidence that you've lost your argument.
Capitalism? Capitalism is a social and economic system in which the earth, its resources and the productive forces dependent upon them, are coercively monopolised by the capitalist class for their maximised benefit, facilitating the alienation and exploitation of everyone else who must work for the owning class or suffer the consequences.