▁tools ▁exc av ated ▁from ▁arch ae ological ▁rice ▁fields ▁at ▁the ▁Maj e on - ni ▁Site . ▁However , ▁iron ▁tools ▁for ▁p addy - field ▁far ming ▁were ▁not ▁introduced ▁until ▁som etime ▁after ▁ 2 0 0 ▁BC . ▁The ▁spatial ▁scale ▁of ▁p addy - fields ▁increased , ▁with ▁the ▁regular ▁use ▁of ▁iron ▁tools , ▁in ▁the ▁Three ▁Kingdom s ▁of ▁Korea ▁Period ▁( c . ▁AD ▁ 3 0 0 / 4 0 0 - 6 6 8 ). ▁ ▁Japan ▁The ▁first ▁p addy ▁fields ▁in ▁Japan ▁date ▁to ▁the ▁Early ▁Y ay oi ▁period ▁( 3 0 0 ▁BC ▁– ▁ 2 5 0 ▁AD ). ▁The ▁Early ▁Y ay oi ▁has ▁been ▁re - dated , ▁and ▁it ▁appears ▁that ▁wet - field ▁agricult ure ▁developed ▁at ▁about ▁the ▁same ▁time ▁as ▁in ▁the ▁Korean ▁pen ins ula . ▁ ▁Culture ▁ ▁China ▁▁ ▁Although ▁China ' s ▁agricult ural ▁output ▁is ▁the ▁largest ▁in ▁the ▁world , ▁only ▁about ▁ 1 5 % ▁of ▁its ▁total ▁land ▁area ▁can ▁be ▁cultiv ated . ▁About ▁ 7 5 % ▁of ▁the ▁cultiv ated ▁area ▁is ▁used ▁for ▁food ▁cro ps . ▁R ice ▁is ▁China ' s ▁most ▁important ▁c rop , ▁raised ▁on ▁about ▁ 2 5 % ▁of ▁the ▁cultiv ated ▁area . ▁Most ▁rice ▁is ▁grown ▁south ▁of ▁the ▁Hu ai ▁River , ▁in ▁the ▁Yang t ze ▁valley , ▁the ▁Z hu ▁Ji ang ▁delta , ▁and ▁in ▁Y