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. Key words: Soil organic C, roots, straw, fertilizer N, moisture