Where Nf is plant N derived from the applied fertilizer, Ns is plant
N not derived from the applied fertilizer, N-uptake is total plant N,
N-grain is total N in grain, NUE is N use efficiency of fertilizer N,
N-fertilizer is the total amount of fertilizer N added to the soil, NHI
is N harvest index, HI is harvest index, DW-grain is dry yield of
grain, and TDW is total dry weight of aboveground biomass.