Yield and quality of leek in response to compost applied as a mulch or incorporated into the soil

Compost is widely used to increase soil fertility, usually practiced by incorporating the compost into the upper soil layer. This study questions the rationale behind this practice. Compost was applied as a mulch and compared with compost worked into the soil in a growth experiment with leek (Allium porrum L. Var. 'Siegfried Frost'). The compost used was made of source separated organic waste from either gardens and parks, or households. Garden-park compost was applied in 2.5 times greater volumes than household compost to compensate for its lower content of nutrients. The soil was either sandy loam or clay loam. Each of the eight combinations of variables (application method, compost type, and soil type) was repeated three times with 20 leeks in each replicate. Significantly higher yields were obtained with compost applied as a mulch. Here, the yield averaged 78 g fresh weight per leek, compared to 59 g per leek from plots with compost incorporated. Compost mulching also resulted in a significantly higher quality leeks, including more first class leeks, longer and thicker shafts, and a generally better appearance. The advantage of placing the compost on the soil surface rather than thoroughly mixing it with the soil can be attributed to a higher availability of plant nutrients. No significant effect of compost type on leek yield was observed, indicating that the 2.5 times higher volumetric dose of garden-park compost provided the same amount of available nutrients as a single dose of household compost. The soil type did not significantly influence the yields either, which is attributable to both soils being well structured prior to compost amendment.