Effects of Humic Substances from Different Sources on Growth and Nutrient Content of Cucumber Plants

Manuel Abad 1, Femando Fomes 2, Diego Garcfa 3, Juan Cegarra 3 and Asunci6n Roig 3

1 Universidad polit6cnica, Departamento de Producci6n Vegetal, 46020-Valencia, Spain 2 Infertosa, Unidad de Investigaci6n y Desarrollo, 46006-Valencia, Spain 3 Consejo Superior de Investigaciones Cientificas, CEBAS, Apartado 195, Murcia,

Spain

Abstract

Humic substances prepared from different sources of organic materials were tested for their effects on nutrient uptake and growth of cucumber plants. Plants were grown in a modified Hoagland solution (iron as soluble FeC13), with the addition of 50 mg/l of carbon in the form of humic substances derived from lignite, sphagnum moss or sedge peat. Humic substances produced highly significant increases in the growth of plant tops and roots, in the stem height, in the number of flowers per plant and in the leaf size. The addition of humic substances also resulted in an increase in the contents of N, P, K, Ca, Mg and Fe in the roots and also in the N, P and Fe contents in the shoots. Variation of effects of humic substances derived from different organic materials was not significant.

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Soil organic matter can, under certain conditions, have a beneficial effect on plant growth. The favourable effects of soil humus on plant growth result either (a) from influence on the physical, chemical and microbiological properties of the soil, or (b) from possible direct effects on the physiological processes of plants [1,2].

The effects of organic substances on soil properties have been studied extensively [3, 4], but the physiological effects have not received much attention. Furthermore, most of these investigations have been limited to seed germination and shoot growth of very young seedlings [2,5], and little is known about the specific effects of humic substances on growth and on nutrient uptake of plants over long periods of time. The response to humic substances depends on the source of the organic materials, and high concentrations of such substances are usually inhibitory for plant growth [5].

The main aim of this investigation was to study nutrient uptake and growth of cucumber plants grown in nutrient solutions containing various humic substances prepa- red from different sources of organic materials.

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Materials and Methods

Humic substances were isolated, as described below, from lignite, sphagnum moss and sedge peat. Both lignite and sphagnum moss were pretreated with 0.1 M HNO 3 and then centrifuged. The residue was subsequently extracted by shaking with 0.25 M KOH. Insoluble constituents were separated by centrifugation (7 900 g). Humic substances from sedge peat were directly extracted with 0.1 M KOH by violent shaking. After filtering through a mesh (<100 lxm2), the pH of the filtrate was adjusted to 1.5 with concentrated H2SO 4 and then left undisturbed for 12 h. The supernatant liquid fraction was removed by centrifugation. The lignite and the sphagnum and sedge peats, respectively, have been described as LIG, P-II and P-Ill in another work [6]. Details on the extraction of their humic substances and some of the characteristics of these substances have also been described. All three had very high contents of humic acids in comparison to their contents of fulvic acids.

Growth trials were carried out in a heated greenhouse during the winter (November 28-February 22). Aliquots of the corresponding humic substances, to give a final organic carbon concentration of 50 mg/1, were added to a half-strength, modified Hoagland.s nutrient solution (iron as soluble FeC13), and mixed thoroughly. The pH was adjusted to 6.0, and the solutions were divided into jars (900 ml/jar). The experiment was set up in 15 replicate jars with four different treatments: control (nutrient solution only) and nutrient solution with humic substances derived either from lignite, from undecomposed sphagnum peat or from strongly-decomposed sedge peat.

Seeds of cucumber (cv. Sensation F1) were soaked in distilled water for 2 h, sown in wet vermiculite and kept in the greenhouse for 12 days. Healthy and homogeneous plants, supported by split foam plugs, were inserted into lid holes of 1 1 plastic jars. Seedlings were first grown in distilled water for two days to acclimatize the system and then transferred to the treatment solutions. Solutions were replaced every two days in order to maintain oxygenation of the roots and to avoid precipitation of humic substances. Plants were grown for 8 weeks, and then 10 plants per treatment were harvested and divided into roots and shoots. Fresh and dry weights of roots and tops, stem height, number of flowers and leaves per plant and leaf size of the largest leaf were measured and recorded. Plant material was dried at 60°C for analysis of mineral nutrients. Nitrogen was determined by the micro-Kjeldahl method; Ca, Mg, K, Mn, Fe and Zn by atomic absorption spectro- metry after dry combustion of the samples, and P by the molybdovanadate method. Five plants per treatment were allowed to grow for an additional 4 weeks, and then total yield and marketable yield (over 50 g/fruit) per plant were recorded.

Results and Discussion

Compared to the control, the addition of 50 mg/1 of carbon, in the form of humic substances, to nutrient solutions brought about significant increases in the dry weights of the roots, tops and whole plants (Table 1), and in the height of the stems, the fresh weight of the roots, the number of flowers per plant and the leaf size (Table 2). It seemed that, under the experimental conditions used, the humic substances directly stimulated plant growth and development (short-term fertilizer effect). These results are in agreement with

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Table I Effect of humic substances from different organic materials on the dry matter yield.

Plant tops Plant roots Whole plant Treatment* g/plant % yield g/plant % yield g/plant % yield

Control 5.44 100 0.93 100 6.37 100 L I 9.26 170 1.93 208 11.19 176 S M 9.11 168 1.73 186 10.83 170 S P 8.96 165 2.28 245 11.24 176 LSD 5% 2.53 --- 0.56 --- 2.99 ---

* LI nutrient solution with humic substances from lignite; SM, from sphagnum moss; SP, from sedge peat. LSD 5% indicates the least significant difference at the 5% probability level.

Table 2 Vegetative growth and reproductive development of cucumber plants as affected by humic substances from different organic materials,

Stem Fresh weight Number/plant Leaf length height or roots

Treatment* (cm) (g/plant) Leaves Flowers (cm)

Control 67.1 19.05 10.6 9.4 15.3 L I 81.5 43.85 11.1 11.5 18.7 S M 78.3 41.20 12.6 11.3 17.8 S P 84.8 43.65 12.5 12.4 17.9 LSD 5% 8.4 8.91 NS 1.8 1.9

* Key symbols like in Table 1 ; NS = non-significant

those of Rauthan and Schnitzer [7] for cucumber in nutrient culture and with those for young seedlings on a variety of plant species (all Angiosperms) in water and nutrient-so- lution culture conditions [2].

The effect of humic substances on plant growth depends on the parameter measured. Humic substances enhanced the increases in fresh and dry weights of roots by 125% and 113%, respectively, whereas the dry weight of plant tops was increased by 68%, the number of flowers per plant by 25%, the height of the stems by 22%, and the leaf size by a mere 18% (Tables 1 and 2). On the other hand, the number of leaves per plant was unaffected by humic substances. Thus, it can be concluded that different parts of the plants react to humic substances to different extents. Other studies have also shown that different organs of intact plants respond differently to humic substances [7,8].

An increase in crop value may come through an increase in total yield and/or a higher proportion of marketable fruits. As these parameters were significantly enhanced by the application of humic substances prepared from sedge peat (Table 3), it may be possible

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g/plant Treatment* Total yield Marketable

yield

Control 212.6 208.5 L I 272.4 265,1 S M 282.5 271.9 S P 332.5 323.3 LSD 5% 77.1 74.1

* Key symbols like in Table 1.

Table 3 The influence of humic substances from different organic materials on total yield and mark- etable yield (over 50g/fruit) of cucumbers.

to increase plant yield to an extent above that based on the application of fertilizer equivalent nutrients.

Growth and development of cucumber did not show significant differences upon the addition of diverse humic substances derived from the three, above-mentioned sources of organic materials (Tables 1-3). Sedge peat did, however, cause a slight improvement in growth and development in comparison to lignite and sphagnum moss, although this effect was mostly inconsistent and not statistically significant.

The addition of humic substances to nutrient solutions yielded highly significant increases, in comparison to the controls, in the contents of N, P, K, Ca, Mg and Fe in roots and also in the N, P and Fe contents of shoots (Tables 4 and 5). Under these conditions, the content of the mentioned elements in the roots, with the exception of N and Mg, was increased by more than 55%. Also, contents of N, P and Fe in plant tops were greatly increased. Zn content in roots was significantly decreased by the administration of humic substances (Table 5). Conflicting results were obtained for the effect of humic substances on Mn content in roots, since it was significantly increased in the case of sedge peat and decreased with lignite and sphagnum moss.

Table 4 Effects of humic substances from different organic materials on the elemental contents of plant tops. Results expressed in terms of dry weight.

Total content mg/plant i~g/plant

Treatment* N P K Ca Mg Fe Mn Zn

Control 145.2 31.2 183.7 88.3 49.1 482.4 1,302.1 365.7 LI 211.9 51.8 267.3 87.5 47.6 1,044.2 1,010.1 364.2 SM 133.4 46.1 280.4 81.4 51.1 734.8 1,041.1 329.0 SP 282.4 30.1 253.4 96.6 58.6 715.3 1,180.6 389.3 LSD 5% 35.9 8.9 NS NS NS 170.1 NS NS

* Key symbols like in.Table 1. NS = non-significant.

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The main effect of humic substances on the elemental nutrient content of plants is related to Fe content. Futhermore, plants grown in the absence of humic substances (controls) showed the typical interveinal chlorotic leaves which result from Fe deficien- cy. In plant tops, the increase in Fe content brought about by humic substances ranged between 117% (lignite) and 48% (sedge peat) (Table 4), which suggests that Fe uptake by plants depended on the nature of the carbonaceous materials from which the humic substances were isolated. Also, the higher uptake of Fe by plants grown in nutrient solution containing the humic substances from lignite could be related to the special characteristics of these substances. Lignite humic substances showed the highest contents of functional groups in comparison to the other two types of substances tested in the experiment [6].

In plant roots, Fe uptake was more than doubled by humic substances from lignite and sedge peat, but it was unaffected by sphagnum moss (Table 5). On the other hand, Fe content in roots of control plants was unusually high and probably resulted from the surface precipitation of FeC13 colloids [9]. In the same way, a portion of the Fe content in roots of plants that had received the application of the humic substances, could be superficially absorbed, owing to the precipitation of low soluble Fe-humate.

Table 5 Effects of humic substances from different organic materials on the elemental contents of plant roots. Results expressed in terms of dry weight.

Total content mg/plant i.tg/plant

Treatment* N P K Ca Mg Fe Mn Zn

Control 66.0 6.2 59.6 3.7 3.7 1,408.7 822.6 143.2 LI 79.8 10.7 118.9 8.4 3.4 3,050.2 213.9 74.8 SM 69.3 7.0 81.2 2.7 2.9 1,173.3 397.7 27.7 SP 90.2 11.3 137.8 10.3 6.4 4,673.3 1,392.9 107.0 LSD 5% 9.2 1.8 22.8 3.7 2.4 653.7 169.7 19.4

* Key symbols like in Table 1.

The above results show that applying of humic substances to the nutrient solution in which cucumber plants are grown, significantly stimulates (a) the vegetative growth and reproductive development of plants and (b) the uptake of nutrient elements from the nutrient solution. Hence, humic substances can exert their effects indirectly and/or directly. In the former case, they can, for instance, form complexes with metal ions and thus increase their solubility and thereby their availability to plant roots. In the case of a direct effect, humic substances must be taken up by the plant, and then they can affect a number of biochemical mechanisms, such as active ion uptake or protein synthesis. Further research is needed to find out which of these explanations is most likely.

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