ROOTING CHARACTERISTICS OF LETTUCE GROWN IN IRRIGATED SAND BEDS

To avoid the current water pollution from intensive glasshouse horticu lture, closed systems have to be developed with recirculating drainage water. For crops with a high planting density, such as lettuce, shall ow beds of coarse sand may be used if water and nutrient supply can be regulated adequately. The aim of the present study was to determine t he rooting characteristics and root distribution of lettuce in sand be ds, as affected by substrate depth, the distance to a drain, drip line s and drip points, and the excess of nutrient solution applied. The hy pothesis was tested that a small excess and a large distance between d rip points leads to local salt accumulations in the root environment a nd thus to a less homogeneous root distribution. The data confirmed bo th parts of the hypothesis: spatial patterns in salt distribution were found. Detailed measurements in a sand bed with only one drip line pe r two crop rows and an amount of fertigation solution added of 2 times the estimated evapotranspiration, showed that root length density was negatively correlated with salt content when comparisons were made wi thin the same layer. Crop yield per row was influenced in the extreme treatment, i.e, one drip line per two crop rows and an amount of ferti gation solution added of 1.3 times the estimated evapotranspiration, b ut yield per bed was still unaffected. The increased heterogeneity of the crop will cause problems at harvest and indicates that the most ex treme treatment included in the comparison is just beyond the limit of acceptable heterogeneity in the root medium. Lettuce can be grown on sand beds with a recirculating nutrient solution provided that drip li nes are well distributed in the bed and the daily nutrient solution ex cess is more than 30% of demand.