6HELAL H M,ARISHA H,RIETZ E. The uptake of trace elements by spinach and bean varieties of different root parameters [ J ]. Plant and Soil, 1990,123(2) :229 -232.
2Femandez-Escobar R, Benlloch M, Barranco D, et al. Response of olive trees to foliar application of humic substances extracted from leonardite[J]. ScientiaHorticulturae, 1996, 66 : 191-200.
3Piccolo A, Pietrumellara G, Mbagwu J S C. Use of humic substances as soil conditioners to increase aggregate stability[J]. Geoderma, 1997, 75 : 267-277.
