Sewage sludge compost as a substrate for croton seedlings production

The floricultural industry is always searching for new and low-cost materials to use as substrate for crop growth and development. Sewage sludge is an organic solid waste from sewage treatment and it has a good potential to be used as an organic fertilizer and/or substrate conditioner. The research aimed to evaluate the rooting and growth of Codiaeum variegatum ‘Andreanum’ (red variety) and Codiaeum variegatum ‘Punctatum’ (yellow variety) cuttings cultivated in the potting media: PM1 100% commercial potting media used as control (CS), PM2 100% organic compost of sewage sludge + sugarcane bagasse (SSSB), PM3 50% CS + 50% SSSB, PM4 100% organic compost of sewage sludge + eucalyptus bark (SSEB) and PM5 50% CS + 50% SSEB. After 90 days of experimentation the chemical analysis of the substrate solution (pH, electric conductivity, P, K, Ca, Mg, S, Na, B, Cu, Mn and Zn) and number of shoots, shoot fresh and dry mass, plant cutting fresh and dry masses, root fresh and dry masses, total fresh and dry masses and, length of the largest root were analyzed. Our results showed that sewage sludge compost can be used as a substrate for rooting and growth of croton cuttings when mixed with commercial substrate. The use of the pure sewage sludge compound with sugarcane bagasse or eucalyptus bark prejudiced croton cuttings to root.


INTRODUCTION
Sewage sludge is an organic solid waste from the treatment of sewage from sewage treatment plants (STP).The material is rich in organic matter and its use in agriculture as organic composite fertilizer is regulated by the Decree 4. 954/2004and limited by Resolutions 375/2006, 380/2006and 481/2017 (CONAMA) and by Normative Instructions 27/2006 and25/2009 (MAPA).
Currently the agricultural recycling is the most important destination for sewage sludge, once the material is rich in nutrients, consequently an excellent organic fertilizer (CONTIN et al., 2012), and has shown to increase production on several crops (SILVA et al., 2010).Sewage sludge can improve soil fertility by increasing CTC when incorporated into the soil, reducing concentrations of exchangeable aluminum and as a source of macro and micronutrients for plants (PRADO and CUNHA, 2011).Sewage sludge is also a soil conditioner due to its organic constitution improving soil particles aggregation and, consequently, increasing aeration and water retention capacity of the soils.In addition, sewage sludge may V. 24, N o .4, 2018 p. 380-386 increase soil microbial activity, which improves the physical and biological attributes of soils (WILSON et al., 1996;GALDOS et al., 2004, PRADO andCUNHA, 2011).
Croton (Codiaeum variegatum) is one of the most popular indoor plants, due to its adaptation to shade conditions and the variety of leaf shapes and color range (YOUNIS et al., 2010).Many species of croton are known for the brightly colored foliage ranging from red, orange and yellow, to green, with all combinations of variegated mixtures (MAHESH, et al., 2010).Croton plants are hardy, usually unaffected by insects and diseases, and highly adaptable to a wide range of environmental conditions and climate trends; which makes it recommended to be used as indoor plants for homes and buildings, potted plants, and by florists as fillers or greens in floral arrangements and bouquets (MAGDALITA et al., 2014).
This study aimed to evaluate the efficiency of sewage sludge on production of croton plantlets.

MATERIAL AND METHODS
The research was conducted in a plastic greenhouse (22º51'S and 48º26'W) with an automated nebulizer irrigation system.
After 90 days of experimentation the variables number of shoots (NS), shoot fresh mass (SFM), shoot dry mass (SDM), plant cutting fresh mass (PCFM), plant cutting dry V. 24, N o .4, 2018 p. 380-386 SEwAgE SLUDgE COMPOST AS A SUBSTRATE fOR CROTON SEEDLINgS PRODUCTION mass (PCDM), root fresh mass (RFM), root dry mass (RDM), total fresh mass (TFM), total dry mass (TDM) and, length of the largest root (LLR) were analyzed.The chemical analysis of the substrate solution (pH, electric conductivity, P, K, Ca, Mg, S, Na, B, Cu, Mn and Zn) were analyzed according to the methodology of IN nº 17/2007(BRASIL, 2007).
The experimental design was a completely randomized block with 5 potting media and 2 croton varieties on a factorial scheme 5 x 2 (potting media x varieties), with 3 replicates, each plot having 10 cuttings set in individual tubes.Data were submitted to test of normality, heterogeneity and analysis of variance (ANOVA) and means were compared using the Tukey test at 5% probability with the Minitab 13.0 software.
Means followed by the same letters, lowercase in rows and uppercase in columns, do not differ amongst themselves (p ≤ 0.05) by Tukey's test.
The red variety, unlike the yellow variety sprouted in PM2 (SSSB); however, there was no root system formation to support the cuttings (Table 3).The yellow and red croton varieties showed to be sensible of to the use of organic compost of sewage sludge (PM2 and PM4).The use of compost with sewage sludge used in the commercial production of container grown nursery crops (flowering annuals, herbaceous perennials and woody ornamentals) was tested and the optimal plant growth generally occurred in media containing 50 and 100 percent compost (BUGBEE, 2013).
The highest root dry mass (RDM) was obtained on substrates PM1 (CS) and PM5 (50% CS + 50% SSEB), while the other substrates did not show significant difference for the variable.The total dry mass was similar among the substrates, even so the red croton variety showed higher values.The plants total mass (TFM and TDM) was composed by the mass of the cuttings (PCFM and PCDM); thus, showed the same characteristics.
The croton varieties differed statistically for some plants characteristics according to the potting media.The mass of roots and shoot had the same performance, with the highest values on PM1 (CS), PM3 (50% CS + 50% SSSB) and PM5 (50% CS + 50% SSEB) for the red variety.
However, the length of the largest root (LLR) had no significant difference among varieties for all potting media.Rooting of croton cuttings depends of substrate; substrate with low density, high CTC and air spaces as medium grade vermiculite medium was the most suitable for the rooting while soil and sand presented the lowest rooting rates (TILLMAN et al., 1994).
The potting media have chemical and physical characteristics, which interfere on plant development.The potting media must have the pH close to neutrality, nutrient concentration sufficient for the initial growth of the plants; have a good relation between water retention and drainage, and porosity that allows root system formation.
Chemical analyzes of the solution in equilibrium of the substrate (substrate solution) were carried out to characterize the nutrient availability at the beginning of the experiment (without plant intervention) and at the end of the experiment, when the plants completed 90 days in the nursery (to verify the interference of plants in nutrients levels for each substrate studied).The chemical analyzes were performed to characterize only the two instants; however, the concentration of nutrients in the substrate solution fluctuates over time, since they are constantly adsorbed and accessible.Thus, the values of analyzes demonstrate the behavior of each nutrient in the substrate solution on that moment, thus not what is adsorbed in the substrate.
During the experimentation, the composition of the substrate solution may have altered according to external influences (temperature, humidity) and internal influences (plant absorption, leaching, CTC).Some values of the final chemical analysis of substrates solutions showed an increase when compared with the initial chemical analysis.These results may have taken place by three hypotheses: some nutrients were available for the substrate solution over time increasing their concentration; decomposition of the organic material in the potting media initiating an increase of the amount of mineralized nutrients available to plants, or the decomposition of larger particles into smaller particles increasing the density of the material and consequently the amount of material to be analyzed once the analysis was performed based on sample's volume.Since there were no nutrients incoming into the system through fertilization, the exit of nutrients were only carried out by plant absorption or leaching.However, leaching was controlled using a nebulization system for irrigation; thus, most of the nutrient output of the system was performed via absorption of croton plants.
The values in Table 4 represent what availability of nutrients at that time in substrate solution and not what is adsorbed at CTC or still unavailable on organic form.Thus, the chemical constitution of the substrate solution was different over time, according to nutrient availability, nutrient absorption by plants or loss by leaching.
The potting media PM4 (SSEB) had higher CE value and K, Ca, Mg and S contents in the final substrate solution; however, the cuttings did not root well.Consequently, the high concentration of nutrients at the end occurred by the non-absorption by plants.The highest values of P were observed in PM1 (CS).
Soluble salt concentrations of substrates increase as a function of sludge amendment and were within the range that can inhibit the growth of many plant species at the high levels of sludge application (RODGERS and ANDERSON, 1995).However, this was not the case, once the commercial substrate (PM1) had highest EC value (Table 1).The highest concentrations of some nutrients in PM1 (CS) at the initial analysis of the substrate solution results of the addition of soluble minerals at the moment of fertilizer production, i.e., readily available nutrients.

Table 1 .
Chemical characteristics of samples from each potting media used as treatment.

Table 3 .
Characteristics of plant cutting from each variety of croton and potting media used as treatment.

Table 4 .
Chemical characteristics of samples at the end of the 90 days from each potting media used as treatment for each croton variety.