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Root/straw ratios for crops are urgently required by scientists wishing to estimate crop residue inputs to soil and for modeling C and N dynamics in the soil-plant-atmosphere system. In this paper we discuss the influence of moisture and rates of N application on such ratios for wheat (Triticum aestivum L.) grown in the semi-arid region of the Canadian prairies. Under natural rainfall root/straw (nongrain aboveground material), ratios decreased with increasing rates of N, but under irrigation these ratios were generally constant. We estimated root/straw ratios for roots measured at anthesis (their maximum mass) and straw measured at maturity, under natural rainfall conditions, to be 0.36-0.58 if roots are assessed for the 0 - to 120-cm depth, 0.21-0.34 for the 0 - to 30-cm depth, and 0.15-0.26 for the 0- to 15-cm depth. Under natural rainfall, if roots are measured at maturity (as is commonly done), the corresponding ratios were 0.29-0.37, 0.15-0.21, and 0.10-0.15, for the respective depths. Under irrigation, the ratios when roots were measured at anthesis were 0.36, 0.24 and 0.19 for 0- to 120-cm, 0- to 30-cm and 0- to 15-cm rooting depths, respectively; but when roots were measured at maturity these ratios were 0.30, 0.17 and 0.13, respectively. We suggest that the values based on roots measured at anthesis provide a more accurate estimate of root C available for decomposition. We propose that the ratio used should be dependent on the depth to which changes in soil C or N are being measured.