Study the Effects of Humic Substances on Growth, Chemical Constituents, Yield and Quality of Two Lettuce Cultivars (cv.s. Dark Green and Big Bell)

An experiment was conducted at the Experimental Station of Faculty of Agriculture, Cairo University, Giza, Egypt, during two successive seasons (2012 and 2013) to study the effects of humic substances on yield and quality of two lettuce cultivars (Dark Green and Big-Bell). The experiment included 7 treatments; (T1) 50% mineral recommended NPK with humic substances extracted from biogas manure as foliar treatment (HBF); (T2) 50% NPK with humic substances extracted from the compost as foliar treatment (HCF); (T3) 50% NPK with humic substances of biogas manure as drench soil treatment (HBS); (T4) 50% NPK with humic substances of compost as drench soil treatment (HCS); (T5) 50% NPK+HBF+HCS; (T6) 50% NPK+HBS+HCF, and (T7) 100 % mineral recommended NPK (control). Dark Green was taller than Big-Bell with two combination treatments, while Big-Bell had higher fresh weight. Meanwhile, Dark Green was double in dry weight. Treatment 50% NPK+HCF+HBS was superior in chlorophyll content in both cultivars. Big-Bell achieved the highest total crop yield particularly with foliar application of humic substances extracted from compost or biogas manure with 50%NPK. Humate of compost either by foliar or soil application with Dark Green cultivar and humate biogas by soil application with Big-Bell cultivar gave the lowest nitrate content in the leaves. Combination treatment, 50% NPK+HCF+HBS (T6) was the highest in total count of bacteria, fungi and actinomycetes, while combination treatment, 50% NPK +HCS+HBF (T5) was the best treatment in values of dehydrogenase and nitrogenase enzymes in the rhizosphere region in both cultivars. Also, the combination treatment (T6) recorded the highest level of total soluble solids in both Dark Green and Big-Bell cultivars, respectively. NPK contents in lettuce leaves were the highest with T1, T6, T4 and T5 for both cultivars. It is concluded that the best treatments were T1 (50% NPK+HBF), T2 (50% NPK+HCF) and T5 (50% NPK+HBF+HCS) which gave highest fresh yield and low levels of nitrate.