Use of Basic Sal Ts of Iron and Zinc as Plant Amend- Ments in Soils

In a greenhouse experiment, the effects of adding zinc (Zn) and iron (Fe) as either their basic sahs or as commercial chelates were compared for the production of leuuce. The basic sahs were found 10 be as effective as the chelates in providing Zn and Fe as well as being more environmentally safe. , INTRODUCTION Calcareous soils frequently supply insufficient quantities of Zn and Fe for normal plant growth. Therefore, a common practice is to apply chelates of Zn and Fe to such soils to satisfy the crap requirement (Wallace and Lunt, 1956; Wallace, 1983; Wallace and Wallace, 1983a). However, the synthetic chelating agents are frequently too expensive to be used in general field agrlcuhure and are difficuh to supply at the correct concentration. This has led to the search for alternative sources of these micronutrients (Mortvedt, 1986; Wempati and Loeppert, 1986; Parpian and Anderson, 1988). Chelating agents can be toxic to plants when used in excess and have been found to inhibit the uptake of other cations by plant roots (Wallace, 1983; Wallace and Wallace, 1983b). In adition, these heavy metals must not be applied in excess since they can have deleterious environmental effects, suth as the depression of the nitro gen-fixing microbial populations in soils (McGrath et al., 1988). Basic sahs of many of the heavy metal s are intermediately soluble between those of the simple salts and those of the hydroxides, so they could be considered as 'slow release' when added to soils as an alternative source of plant micronutrients. Unlike the chelates, basic salts are not stable in solution, so they cannot promote the movement of cations and other heavy metals down the soil profile. Basic salts are crystalline, easy to handle, and do not introduce any foreign component into the environment. Thus, they could be considered as being environmentally safe. In a previous paper (Oalvez et al., 1987), it was shown tha! addition of the basic salts of Zn, copper (Cu), and cobalt (Co) 10 a sandy soil resulted in values for their 'capacity factor' (I/Q) (Mattingly, 1965) lower than those for their simple salts but higher than for their corresponding hydroxides. In this paper, the basic salts of Fe [Fe4(OH)llN03·2H20] and Zn [ZnS(OH)8CI2·H20] were used in a greenhouse experiment to evaluate the effect of their addition to a sand and to a caleareous sandy soil on the weight and nutritional status of lettuce (Lactuca sativa L.) plants. The results were compared with those olJtained by using the commercially available chelates ofFe and Zd as soil amendments. The basic salt ofFe, Fe4(OH)l1N03·2H20, referred to as bFe, has recently being synthetized (Lopez-Delgado et al., 1988) and has been used as a plant nutrient for the first time in this experimento The use of the basic salt of Zn, Zns(OH)sC12·H20, referred 10 as bZn, to corree! deficiencies of this element in plants, animals, and human beings is protected by a patent (P9300705). MATERIALS AND METHODS The experiment was carried out in a greenhouse using ca. O.15/m3-capacity containers fulled with either sand or a caleareous sandy soil (from now on soil). Sorne characteristics of the sand and soil are shown in Table l. Four lettuce plants \Vere planted in each container, three to be sampled after 40,70, or 100 days, respectively, leaving the fourth to be sampled at the flower stage. A factorial experiment with five different treatments was set up to compare the effects of!he addition to the soil of the basic salts, bFe and bZn, with those of two commercial chelates (Sandofer R, 6% Fe as EDDHA; Bundolin Corrector Zn-14 R, 14% Zn as EDTA, referred to as chFe and chZn, respectively). In two of these, mixtures of the basic salts and the commercial chelates were applied: treatment bZnchFe (the basic salt of Zn plus chelate of Fe) and treatment chZnbFe TABLE 1. Sorne Characteristics of the Sand and the Calcareous Sandy Soil Used.