Reported values of percent fertilizer-N absorption (PNA) by crops grown in the fields are somewhere in between 30 and 70%, although there are many extremes. In some crops, such as rice, winter wheat, pearl millet and sugarbeet, the PNA values based on the difference method are generally larger than those on the ¹⁵N method, while, in other crops such as spring wheat, maize and sorghum, the trend is opposite. In the other group of crops, such as potato and sudangrass, the PNA shows little difference between those two methods. Among various causes, root-development characteristics, being specific to each crop species, seems to play an important role on the PNA difference by the two methods, since the PNAs are often larger in the pot tests than in the field experiments. There are, however, great variations among the crops in the relative contributions of root development and growth characteristics to the PNA as well as in their N responses. It is concluded that in pursuing the PNA difference caused by those two methods, the following matters will have to be further studied: (a) identification of all aspects of the fertilizer-N distribution in fields, including plant absorption, demineralization and losses through gases and leaching; and (b) concurrent influences of environmental conditions.