This field study investigated the impact of various fertilization strategies with red clover ( Trifolium pratense L.) green manure on the levels of S-alk(en)yl- l-cysteine sulfoxides (ACSO) and l-ascorbic acid in leek. Two of the 12 treatments were controls, one without fertilizers and the other with a commercial mineral fertilizer. The remaining 10 treatments were different forms and quantities of green manure prepared from red clover. One treatment consisted of direct incorporation into soil of the preceding red clover crop. The other 9 treatments comprised three types of red clover green manure [anaerobically digested red clover biomass (biodigestate), composted red clover, fresh red clover as mulch] applied at three different doses. Yield was increased only at the highest dose of compost and the highest dose of mulch. High doses of green manure decreased dry matter content in leek. The fertilizer treatments increased the nitrogen uptake and the nitrogen content of leek. Sulfur uptake and sulfur levels were increased only by the mineral fertilizer and by the compost. Nonfertilized leek contained 20.4 +/- 5.8 g/kg of dry weight (dw) ACSOs as determined by LC-MS/MS and 1.57 +/- 0.01 g/kg of dw ascorbic acid as determined by HPLC. The ACSOs were to 92-96% isoalliin, the rest being methiin. Alliin was identified in only 1 of 72 samples. The ACSO level was increased by 37% by the mineral fertilizer. Whereas direct incorporation of red clover, mulch, and red clover biodigestate had no influence on the ACSO level, the highest dose of compost increased the ACSO level by 55%. Ascorbic acid levels were not influenced by the mineral treatment. Green manures increased ascorbic acid levels only on a dry weight basis. A high correlation between the content of sulfur and ACSO indicated that delivering capacity of sulfur from the manure to the plant strongly affected the ASCO content of the leek. In conclusion, the composted green manure was the most useful organic fertilizer in this study and reached at least the efficiency of the mineral fertilizer.