The income is exempt, whoever earns it
Agricultural income from land situated in India is exempt from income tax under Section 10(1), and what the law means by agricultural income is set out in Section 2(1A): rent or revenue from agricultural land in India, income from actually carrying out agricultural operations on it, and income from a qualifying farm building. Crucially, the exemption turns on the land being in India and used for agriculture, not on who earns from it. So your residency makes no difference here. An NRI's agricultural income from Indian land is exempt exactly as a resident's would be.
Where NRIs go wrong is at the edges of the definition. Selling the agricultural land is a capital gains event, not agricultural income. Selling standing trees or spontaneously grown produce, where there was no real cultivation, is not agricultural income either. And processing beyond what is ordinarily needed to make the produce marketable can turn part of the receipt into taxable business income. So the exemption is generous but narrow: it covers the farming, not everything that happens on or to the land.
The catch: it can raise the tax on your other income
Even though agricultural income is exempt, it does not always leave your tax bill untouched. Through a mechanism called partial integration, agricultural income is added to your other income for one limited purpose: to decide the rate. It applies only when both tests are met, your net agricultural income is more than ₹5,000, and your other, non-agricultural Indian income is above the basic exemption limit. When both are true, the agricultural income is notionally stacked on top of your taxable income to fix the slab, so your taxable income is charged at a higher average rate, while the agricultural income itself stays exempt.
One subtlety worth stating plainly: this rate rule is not in Section 10(1). The exemption is statutory, but the aggregation is provided each year by the Finance Act, so it is a live rule for the current year too, not something you can assume away. For an NRI, the practical effect is that agricultural income back home can raise the tax on your Indian rent, interest or capital gains, which is exactly the interaction people miss when they treat the two as separate. On the ownership side, FEMA lets an NRI or OCI inherit agricultural land, a plantation or a farmhouse, but not buy one, so if you are earning agricultural income it should be from land you inherited or held from before you became an NRI.