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Agrociencia

versión On-line ISSN 2521-9766versión impresa ISSN 1405-3195

Agrociencia vol.52 no.4 México may./jun. 2018

 

Natural Renewable Resources

Physical and chemical characterization of organic materials for agricultural substrates

Salomé Gayosso-Rodríguez1 

Lizette Borges-Gómez1 

Eduardo Villanueva-Couoh1  * 

Maximiano A. Estrada-Botello2 

René Garruña1 

1 Instituto Tecnológico de Conkal, Avenida Tecnológico s/n. 9345. Conkal, Yucatán, México. (e_couoh@hotmail.com)

2 División Académica de Ciencias Agropecuarias de la Universidad Juárez Autónoma de Tabasco, Km 25 carretera Villahermosa-Teapa.

Abstract

Some substrates used to grow plants in containers are expensive and are extracted from natural ecosystems. Alternative materials should be inexpensive and innocuous. Some organic materials found in Yucatan, Mexico, could be potentially used as substrate for containers. The objective of this research was to evaluate the physical and chemical properties of pine sawdust (Pinus sp.) (≤2), pine shavings (≤5 and ≤10 mm), cocopeat (Cocos nucifera L.) (≤5 and ≤10 mm), gulfweed (Sargassum sp.) (≤5 and ≤10 mm), henequen pulp (Agave fourcyoydes Lem.) (≤10 mm), and dzidzilche leaf (Gimmopodium floribundum Rolfe) (≤10 mm). All these materials may be available in the region. The experimental design was completely random, with nine treatments and three replications. The following variables were evaluated: average particle diameter, apparent density, absolute porosity, ventilation porosity, water retention porosity, wettability, pH, specific conductance, organic material, caption exchange capacity, N concentration, and C, K+, Ca2+, Mg2+ and Na+ content. Additionally, water retention curves were developed and biological activity was measured. Pine sawdust, cocopeat, and gulfweed (with ≤5 mm particle size) had about 30 % of ventilation porosity and over 50 % of water retention porosity. Pine sawdust and shavings retained 24-40 % of the total available water. The pH of the materials almost reached 7 and their specific conductance was ≤1.5 dS m-1. The N, P, K+, and Na2+ content of henequen pulp and dzidzilche leaf matched the suggested interval for an ideal substrate. Therefore, sawdust, henequen pulp, dzilzidche leaf, cocopeat, and pine shavings with ≤5 mm particles have the appropriate characteristics to be used as substrate components.

Key words: Agave fourcyoydes lem.; retention curves; Gimmopodium floribundum Rolfe.; Sargassum sp.; alternative substrates; particle size

Introduction

In Mexico, forest topsoil and peat moss (Sphagnum genera) are the main substrates for ornamental plant production. The overexploitation of these resources causes erosion problems and the deterioration of ecosystems. Therefore, alternative substrates -that fulfill supporting and nutritional functions- are required. These substrates must be available, inexpensive, and eco-friendly (Urrestarazu, 2013). Some countries have promoted the reuse, recycling, and valorization of locally-available organic materials as substrates in the cultivation of container plants (Valenzuela et al., 2014; Gayosso et al., 2016a).

In Yucatan, several farming and agro-industrial wastes are available throughout the year and they could be used as substrates for the production of plants in containers. Some of these materials include: henequen pulp (Agave fourcyoydes Lem.), dzidzilche leaf (Gimmopodium floribundum Rolfe), cocopeat (Coco nucifera L.), gulfweed (Sargassum sp.), and pine timber waste (Pinus sp.) (Borges, 1998). The henequen pulp is a waste product from the defibration of the leaves of the henequen agave. Dzidzilche is a Mesoamerican native species (Polygonaceae family), naturally distributed in Yucatan. Both species are horticultural substrate components in vegetable gardens and seedbeds (Borges, 1998; Villanueva et al., 2010). In the 1980s, cocopeat was used as substrate and is currently one of the two most used substrates in Europe (Blok and Urrestarazu, 2010). Gulfweed arrives annually to the coastal zone of Yucatan and is used as a compost component to grow vegetables and ornamental plants in other countries (Phool, 1999). Pine sawdust is timber waste resulting from woodcutting and furniture manufacturing; combined with other materials, sawdust has been evaluated as a substrate for the cultivation of forest seedlings with favorable results (Mateo et al., 2011). The physical, chemical, and biological characterization of the materials for these purposes must be known, before they are used for cultivation (Gayosso, et al., 2016a).

The physical characteristics of the substrate are more relevant than its chemical characteristics: once the crop is established, physical characteristics cannot be modified (Abad et al., 2005). Additionally, these characteristics determine the water, air, and nutrients content that the roots have available. Therefore, the physical characteristics have a direct relationship with the quantity of water and nutrients (fertigation) applied after the plant is established (Quintero et al., 2011). The relevant physical characteristic of the container substrate is its water retention and ventilation capacity. Both are directly related with porosity and depend on the distribution, composition, internal structure, shape, and size of the particle, which in turn determine the water-air ratio in the substrate (Anicua et al., 2009). On this matter, Cabrera (1999) pointed out that at least 20 % of substrate particles must be smaller than 0.5 mm. Vargas et al., (2008a) and Anicua et al. (2009) indicated that 0.25-1.00 mm particles are essential for the water-air balance. Morales and Casanova (2015) suggested that an appropriate water-air ratio is 10-30 % of air in the substrate and Abad et al. (2004) indicated that 20-30 % of water content is better, although it can change according to the species and the cultivar.

The assimilable nutrients content of organic substrates changes regarding its inert substrates, depending on their origin and degree of decomposition. Therefore, knowing the following information is important: the available elements content -such as NO3-, NH4+, P, K+, Ca2+, Mg2+, Fe3+, Mn2+, Mo, Zn2+, Cu2+ and B (Abad et al., 2005) -and the factors that affect their adsorption- such as their caption exchange capacity (CIC), pH, specific conductance (CE), C/N ratio, and phytotoxic element content (Burés, 1997). The 5.5-6.8 pH interval, ≤2 dS m-1 CE, and 20 meq 100 g-1 CIC are appropriate for vegetable cultivation; although this also can change according to the species (Abad et al., 2004; Quintero et al., 2011).

Usually, the study of substrates use is based on crops yielding or development results. Only a few include quality of substrate parameters. The following characteristics and materials were studied: the size of particles of tezontle (Vargas et al., 2008a); the physical, chemical, and biological characteristics of cocopeat particles (Vargas et al., 2008b); the physical and micromorphological characteristics of perlite and zeolite (Anicua et al., 2009); and the physical characteristics of pine bark and rice husks (Valenzuela et al., 2014). All these studies highlighted the importance of the physical and chemical characteristics of substrate in the establishment of crops. In Yucatan, several organic materials could fulfill the physical and chemical parameters needed for the cultivation of plants in containers, but they are not yet characterized. Therefore, the objective of this study was to characterize the physical and chemical properties of six organic materials available in the region to use them as substrate components for the cultivation of plants in containers.

Materials and Methods

The materials were characterized in the water-soil-plant lab of Instituto Tecnológico de Conkal, in Yucatan, Mexico. The materials were directly collected in the areas where they are generated. Their decomposition process had not started yet (non-composted). These materials were: 1) pine shavings (VP), wood brushing waste; 2) pine sawdust (AP), waste of wood cut with a compass saw (both 1 and 2 were collected from furniture manufacturing); 3) henequen pulp (BH), waste from the defibration of agave leaves; 4) cocopeat (FC) or dry coconut mesocarp; 5) dzidzilche leaf (HD), a product of the tree’s natural defoliation process; 6) gulfweed (SA) or seaweed deposited by the tide in the coast of Progreso, Yucatan. VP, FC, and SA were ground in a hammer mill (TRG 300G model) and sifted using a 10 mm and 5 mm diameter sieve; BH and HD were sifted using a 10 mm sieve; AP was not ground (the original size of the wood cut was ≤2 mm approximately). The nine materials evaluated were: VP10 (≤10 mm), VP5 (≤5 mm), AP (≤2 mm), BH (≤10 mm), FC10 (≤10 mm), FC5 (≤5 mm), HD (≤10 mm), SA10 (≤10 mm), and SA5 (≤5 mm).

FC’s and SA’s high specific conductance -resulting from its contact with sea water- (Burés, 1997) diminished after those materials were immersed in common water for 15 minutes, in a 1:2 v/v substrate-water and drain ratio. After FC and SA were washed five and four times, respectively (Gayosso et al., 2016b), they were dried outdoors in the shade.

The physical characterization included:

Granulometry: A 1000-cm3 sample was sifted for 3 minutes, using an electric sifter, with 3.36, 2.0, 1.0, 0.5, and 0.25 mm sieves; based on the retained particles proportion, the average diameter of the predominant particle was determined (Dm), using the following formula:

Dm=Ʃi=1nvifiƩi=1nfi

where: Dm: average diameter (mm), vi : average value of particle range (mm), and fi : frequency (%).

Apparent density (Da). The material was dried 24 h using an air convection oven, at 100 °C. The following formula was used to calculate it: Da=dry substrate weight (g) / total volume (cm3).

Absolute porosity (Pt), ventilation porosity (Pai), and water retention porosity (Pra) were determined using the procedure described by Landis et al. (1990).

Water retention curves. These curves were obtained using a suction equipment with filtering funnels -with 0, 10, 50, and 100 cm, water column suction-, according the method of De Boodt et al. (1974). The equipment included Büchner porcelain funnels connected to a hosepipe with water; the hosepipe was moved to different heights. This method was used to generate the required pressures at different heights. The material was saturated with running water for 24 h; then, it was drained, placed in the funnels, and left to drain until the water level in the hosepipe was the same as the funnel base level (0 cm pressure). Then, it was lowered 10 cm and the water level was monitored until it stabilized (36-48 hours approximately). A sample was extracted, weighted, and dried in a stove at 70 °C for 24 h. After that, it was weighted again. This process was repeated with a 50 and 100 cm pressures. Using these values, the following data was determined: solid matter (MS), difference between total volume and absolute porosity; ventilation capacity (CA), difference between absolute porosity and water content at 10-cm pressure; easily available water (AFD), difference between retained water at 10- and 50-cm pressure; water reserve (AR), 50-100 cm pressure; scarce water (ADD), water retained at 100+ cm pressure; and total available water (ATD), equal to the addition of AFD and AR.

Wettability. This is the time (min) that a dry substrate sample requires to absorb water (Abad, et al., 2004). Five mL of distilled water were added to 10 g of organic material (dried at 40 °C) and the adsorption time was measured.

The chemical characterization included:

pH and specific conductance (CE). Both were measured using a potentiometer/conductivity meter (CONSORT C931) in the substrate’s aqueous medium (1:2 v/v for pH and 1:5 v/v for CE).

Humidity and organic material (MO). Two g of sample were dried at 100 °C, until a constant weight was achieved (in the first case) and then they were reduced to ashes, at 600 °C using a muffle (in the second case).

Total nitrogen (N). Total nitrogen was determined using the Kjiedalh method (Cottonie, 1994).

Total phosphorus (P). Total phosphorus was determined using the sodium molybdate method, with p-methylaminophenol sulfate, in an ultraviolet-visible spectrophotometer (UV2800 PC).

Total K+, Ca2+, Mg2+ and Na+ content. These elements were quantified through calcination, acid digestion (Cottenie, 1994), and atomic absorption spectrophotometry (GBC 932 plus).

Cation exchange capacity (CIC). For this variable, 1 N, pH 7 ammonium acetate was used (Cottenie, 1994).

Biological activity was measured based on the flux of CO2 in the substrates, using an automated soil gas flux system (IRGA, LI-8100, LICOR, Nebraska, United States). The substrate was measured according to field capacity in 20.32-cm wide pots. The readings in the gas flux system were carried out every 2 minutes per pot (time estimated during previous essays for the size of the pot).

The experiment had a fully-randomized design, with nine treatments (five materials and particles of various sizes) and three replications for all variables. The experimental unit was made of three pots. The values were evaluated to determine their normality. The values that were included in percentage were normalized using the square root of the arcsine. Afterwards, they were subject to a variance analysis and any statistical differences were subject to Fisher’s multiple range test (DMS; p≤0.05). The data were analyzed using Infostat/F.

Results and Discussion

Physical properties

HD and AP had approximately 20 % of ≤0.5 mm particles. Cabrera (1999) suggest this percentage to achieve a balanced air-water ratio. The size of the remaining particles ranged from 0.5 to 3.36 mm; their granulometry favored the retention of easily available water (Anicua et al., 2009) (Table 1).

Table 1 Granulometric distribution (percentage based on weight) of the size of particles from five organic materials found in Yucatan, Mexico. 

Tratamientos Tamaño de partícula (mm) Dm
<0.25 0.25-0.5 0.5-1.0 1.0-2.0 2.0-3.36 >3.36
Viruta de pino ≤10 mm 0.7 0.1 4.5 29.8 37.2 27.4 2.67
Viruta de pino ≤5 mm 6.6 1.8 34.6 55.7 0.7 0.2 1.14
Aserrín de pino 24.1 3.8 41.7 26.9 2.7 0.15 0.84
Bagazo de henequén 33.7 2.4 31.6 24.8 5.0 1.30 0.87
Fibra de coco ≤10 mm 4.8 2.2 10.4 9.0 2.0 70.6 6.17
Fibra de coco ≤5 mm 22.0 11.8 50.1 12.0 0.7 3.7 0.91
Hoja de dzidzilche 18.7 2.5 36.5 31.7 7.6 2.45 1.10
Sargazo ≤10 mm 4.1 1.0 15.9 50.3 22.9 5.7 1.76
Sargazo ≤5 mm 7.8 2.0 49.8 39 0.7 0.6 1.03

Dm: average diameter of the particle (mm).

AP, BH, and FC (≤5 mm) had over 25 % of <5 mm particles and a cumulative percentage of 27.9, 36.1, and 33.8 %, respectively. This can affect ventilation, since the ventilation capacity diminishes depending on the size of the particle (Prasad and Ni Chualáin, 2004). The decrease of up to 50 % of the ventilation capacity is the responsibility of 0.25-0.50 mm particles (Vargas et al., 2008a). On the contrary, FC10 had 70.6 % of >3.36 mm particles. On this matter, Anicua et al. (2009) and Vargas et al. (2008a) pointed out that >3.36 mm particles diminish humidity retention capacity.

The size of particles and the water retention porosity in pine waste had an inverse ratio, while the size of the particles and ventilation porosity had a positive ratio. These results match the findings for other organic materials (Prasad and Ni Chualáin, 2004), tezontle (Vargas et al., 2008a), perlite and zeolite (Anicua et al., 2009), and various mixtures of cocopeat and volcanic matter (Jiménez et al., 2014) (Table 2). This ratio varied in other materials studied for this research, because water retention depends on the size of the particles, tortuosity, and pore continuity (Burés, 1997; Gutiérrez et al., 2011).

Table 2 Percentages of: total porosity, ventilation porosity, water retention porosity, and apparent density (g cm-3) of organic materials with particles of different sizes.  

Tratamiento Porosidad total Porosidad de
aireación
Porosidad de
retención de agua
Densidad aparente
Viruta de pino ≤10 mm 91.21 a 74.08 a 17.13 e 0.08 d
Viruta de pino ≤5 mm 87.35 b 50.73 b 36.63 c 0.13 b
Aserrín de pino 86.83 b 29.74 d 57.09 a 0.14 b
Bagazo de henequén 44.86 g 10.37 f 34.49 cd 0.10 c
Fibra de coco ≤10 mm 52.45 f 46.06 c 6.39 f 0.02 e
Fibra de coco ≤5 mm 84.97 c 26.07 de 58.89 a 0.09 cd
Hoja de dzidzilche 72.11 e 22.27 e 49.84 b 0.15 a
Sargazo ≤10 mm 82.70 d 50.15 b 32.55 d 0.09 cd
Sargazo ≤5 mm 81.29 d 23.92 e 57.38 a 0.13 b
DMS 1.60 3.95 3.88 0.01

Different letters in a column indicate significant differences (p≤0.05). DMS: minimum significant difference.

Absolute porosity (Pt) of organic material must make up over 85 % of the substrate’s drained volume (Quintero et al., 2011), after it has been irrigated up to saturation point. This enables the ventilation of at least 10 % of the substrate’s volume, although it can reach 20 to 50 %, depending on the species that will be cultivated (Sánchez et al., 2008; Valenzuela et al., 2014). The treatments that approach these intervals were AP, FC (≤5 mm), and SA (≤5 mm), perhaps because 40 to 50 % of the particles involved in these treatments ranged from 0.5 to 1 mm. On this matter, Vargas et al. (2008a) pointed out that 0.25-1.00 mm particles determine the balance in the humidity-air ratio of the substrate.

Absolute porosity and ventilation of BH were 44.86 and 10.37 %, respectively, because 36 % of their particles were smaller than 0.5 mm; consequently, their ventilation porosity diminished. Jiménez et al. (2014) reported that -in a mixture of cocopeat and volcanic rock- ≤0.6 mm particles diminished the total porous space. The treatments with greater ventilation porosity and less water retention porosity were VP, FC, and SA, which had ≤10 mm particles and greater Dm (2.67, 6.17, and 1.76 mm). This determined the formation of macropores that facilitated draining water through percolation.

Da increased in inverse proportion to the size of the particles. These results were similar to the findings of Anicua et al. (2009) for perlite and Jiménez et al. (2014) for mixtures of cocopeat and volcanic rock. Non-solid materials have internal pores (Ansorena, 1994) and their particles remain of the same size. However, when the size of particles diminishes after the materials have been milled, their pores are broken, the organization of the particles is modified, the porous space is reduced, and solid matter and Da increase. Additionally, particles are not spherical; some are organized in slices with heterogeneous forms and sizes; smaller particles can occupy less space and have a greater Da. The Da of VP (≤5 mm), AP, and SA (≤5mm) were close to 0.15 g cm-3. This is the recommended Da for substrates in containers (Abad et al., 2004); the Da for FC (≤5 mm) was similar to the one reported by Vargas et al. (2008b) for commercially-available cocopeat (0.7 to 0.11). The Da of VP, FC, and ≤10 mm SA was low; this is related with the distribution of over 70 % of >1.0 mm particles. Adding these materials does not provide the mechanical support that roots need to strengthen their hold on the ground and using them as substrates may overturn plants.

Owing to their biological activity, organic substrates require a greater content of oxygen and ventilation capacity. Abad et al. (2004) reported an optimum 20-30 % level of air in the substrate volume. In water retention curves, VP (≤5 mm), AP, FC (≤5 mm), HD, ≤5 and ≤10 mm SA had 28 to 44 % CA. This percentage is slightly higher than the one suggested by Abad et al. (2004). This could be the result of material granulometry, since the size of over 60 % of their particles ranged from 0.50 to 2.00 mm. This information matches the distribution recommended by Cabrera (1999) for the preparation of substrates for containers.

AP, ≤5 mm and ≤10 mm VP, BH, ≤5mm FC, and HD had a 10-16 % retention of easily available water (AFD) (Table 3). The size of the particles influenced AFD values: 20 to 36 % of its particles were ≤0.5 mm. Vargas et al. (2008b) pointed out that cocopeat of this size has the greatest influence in AFD. Additionally, Anicua et al. (2009) reported the highest AFD percentages for 0.25-5.0 mm perlite and zeolite particles. However, Gutiérrez et al. (2011) and Jiménez et al. (2014) agreed that the highest AFD retention occurs in 1 and 2 mm granulometries, because water retention is determined by the size of the particles, the different kinds of pores that appear between particles, which influence water movement (Ansorena, 1994; Gutiérrez et al., 2011).

Table 3 Percentage of the substrate volume (cm3) occupied by the ventilation capacity, easily available water, reservoir water, and scarce water with organic materials with particle of various sizes.  

Tratamientos Capacidad
de aireación
Agua fácilmente
disponible
Agua de reserva Agua difícilmente
disponible
Viruta de pino ≤10 mm 49 a 15 ab 20 b 8 g
Viruta de pino ≤5 mm 44 b 14 b 18 c 11 f
Aserrín de pino 40 c 12 c 24 a 11 f
Bagazo de henequén 21 f 10 d 7 d 7 g
Fibra de coco ≤10 mm 19 g 1 e 1 gh 33 c
Fibra de coco ≤5 mm 44 b 15 ab 3 ef 28 d
Hoja de dzidzilche 38 d 16 a 2 fg 16 e
Sargazo ≤10 mm 27 e 1 e 4 e 61 a
Sargazo ≤5 mm 28 e 2 e 0 h 51 b
DMS 1.71 1.71 1.61 1.71

Different letters in a column indicate significant differences (p≤0.05). DMS: minimum significant difference.

Materials that come from pine timber waste were the only ones that showed 20-40 % of totally available water in the substrate (Abad et al., 2004); the other materials were not able to hold water in a column with 50-cm pressure and did not keep enough AFD to provide adequate growth and development for the plant. Therefore, they are inadequate substrates. SA showed high ADD retention at a 100-cm pressure water column, 61 % of ≤10 mm and 51 % of ≤5 mm particles. BH had the highest content of solid materials (55 % of its volume), which reduced the space available for water (14 %) and air (13 %); if it were used as a substrate, it could diminish water supply and affect root growth, metabolic activity, and water and nutrient absorption (Vargas et al., 2008a).

The materials had a tendency to retain more water as the size of the particle diminished. This matched the conclusions of Vargas et al. (2008a) and Gutiérrez et al. (2011). However, water is not always available for the plant, such as in the case of pine waste (pine sawdust, pine shavings with ≤10 mm and ≤5 mm particles); as the size of particles diminished, ADD increased and AFD diminished. Additionally, it surpasses the 10 % AR suggested by Abad et al. (2005). On this matter, Gutiérrez et al. (2011) recommend performing a micromorphological analysis that determines the kind, size, and frequency of the pores, in order to achieve a better understanding of the complexity of water retention in pure and mixed materials.

The suggested wettability for a substrate is 5 min or less (Abad et al., 2004). The evaluated materials adsorbed water in less than five minutes, except for the henequen pulp and the dzidzilche leaf (20 and 22 min, respectively). Water moved on dzidzilche leaves at a slow, but uniform pace; however, water moved slowly on henequen pulp towards the walls of the container; its 33.7 % content of <0.25 mm particles likely caused the appearance of small pores than prevented water from entering and encouraged water to drip on henequen pulp.

Chemical properties of the substrates

All treatments had a pH that was slightly higher than the recommendations made by Abad et al. (2004) and there were no statistical differences between them. pH can influence the availability of nutrients in the substrate which the plant can use. Therefore, pH must be regulated when these materials are used (Table 4).

Table 4 Chemical characteristics of organic materials available in Yucatan, Mexico. 

Contenido Aserrín de
pino
Bagazo de
henequén
Fibra de coco Hoja de
dzidzilche
Sargazo DMS
pH 7.1 a 7.2 a 7.2 a 7.2 a 7.4 a 0.66
CE (dS m-1) 1.43 a 1.14 a 1.49 a 1.39 a 1.31 a 0.71
Humedad (%) 9.7 c 6.9 d 13.8 b 14.6 b 17.6 a 0.96
MO (%) 99.9 a 64.7 c 97.8 a 77.2 b 76.5 b 3.15
C (%) 57.9 a 37.6 c 56.7 a 44.8 b 44.3 b 1.83
C/N 2759 a 23 b 253 b 24 b 30 b 972.1
CIC (meq 100 g-1) 9 d 19 c 22 c 77.67 b 82.67 a 3.41

Different letters in a row indicate significant differences (p≤0.05). DMS=minimum significant difference.

For every material, CE was less than 1.5 dS m-1: an acceptable value for the cultivation of plants in containers (Abad et al., 2004). Burés (1997) mentions that FC in CE can vary from 0.1 to 6 dS m-1, as a result of its contact with sea water in its zone of origin. However, the number of times that cocopeat and gulfweed were rinsed diminished their CE from 2.71 to 1.49 dS m-1 (FC) and from 6.13 dS m-1 to 1.31 dS m-1 (SA). There were no statistical differences between treatments.

The C/N ratio indicates origin, degree of ripeness, and stability of the organic material and it decreases as it decomposes (Burés, 1997). Among the materials studied, pine sawdust and cocopeat had the highest C/N ratio. Their nature makes then rich in lignin and cellulose; they have a high carbon concentration and a low nitrogen concentration, resulting in their high C/N ratio (Borges et al., 2003; Burés, 1997; Quintero et al., 2011).

The C/N ratio in fresh materials is high and the former diminishes as the latter decomposes (Burés, 1997). Some authors mention a 300 C/N ratio in non-composted pine bark (Ansorena, 1994; Quintero et al., 2011) and 361 for pine sawdust that had previously been used as litter in poultry farms (Barbazan et al., 2011); the fact that those researches used sawdust made only from bark and sawdust with traces of poultry manure favored the decomposition of carbon structures and increased nitrogen content, resulting in a lower C/N ratio than the one found in our research. However, non-composted fresh pine sawdust with no added materials was used in our study. A greater C/N ratio can limit the amount of N available and reduce the cation exchange capacity (Landis et al., 1990); however, it provides greater stability, and it reduces the phytoxicity caused by the formation of new organic compounds, resulting from the degradation process, changes in CIC, or increases in salinity (Domeño et al., 2011).

Abad et al. (2004) recommended an optimum CIC level of 20 meq 100 g-1 or higher. In our study, FC, HD, and SA showed favorable CIC since it contributes to the capacity to soften quick changes in the availability of nutrients and pH. The CIC of AP (9 meq 100 g-1) was low and average (19 meq 100 g-1) for BH, according to Quintero et al. (2011).

The concentration of N in BH, HD, and SA was greater than 1.4% (Table 5); in contrast, the concentration of N and P in AP was lower than in the evaluated materials. As a result of these low contents and their lack of decomposition, it would be necessary to apply N to these materials, before they were used as substrates, in order to prevent competition between the microorganisms and the plants (Burés, 1997). The content of N in BH (1.6 %) was lower than the 0.5 % content reported by Borges et al. (2003). Borges (1998) pointed out that the content of N and P diminishes as the degree of decomposition increases. When compared with the other materials, BH had a higher content of P and CA. Gulfweed had the highest contents of K, Mg, and Na; this could be related to its high ADD retention.

Table 5 Total mineral content in organic materials available in Yucatan, Mexico.  

Contenido Aserrín de pino Bagazo de
henequén
Fibra de coco Hoja de
dzidzilche
Sargazo DMS
N (%) 0.023 e 1.669 b 0.224 d 1.819 a 1.465 c 0.05
P (mg Kg-1) 0 d 4539.56 a 833.86 c 3789.28 b 3542.21 b 522.91
K (mg Kg-1) 1600.48 e 11999.21 c 5149.06 d 15478.76 b 20765.03 a 3062.57
Ca (mg Kg-1) 815.21 c 172150.6 a 4642.45 c 76698.56 b 71925.55 b 14497.03
Mg (mg Kg-1) 103.66 c 1738.76 b 573.61 c 1822.31b 10474.73 a 1043.75
Na (mg Kg-1) 70.38 d 178.55 c 363.65 b 209.43 c 2862.60 a 82.54

Different letters in a row indicate significant differences (p≤0.05). DMS=minimum significant difference.

FC and AP had low N, P, and K concentrations, perhaps as a result of the tissue origin (fruit mesocarp and lignified stalk, respectively). BH, HD, and SA are leaf tissues with photosynthetic activity and they could contain more simple carbohydrates. Crespo et al. (2013) pointed out that blue agave has total non-structural soluble and polysaccharides sugar reservoirs in its heart (piña) and stalk; this could explain the high mineral content in the pulp of henequen agave.

The differences in the flux of CO2 reached statistical levels only between AP (2.2±0.60 μmol m-2 s-1) and the other substrates (SA: 8.0±0.78; BH: 6.8±0.75; FC: 6.7±0.84; and HD: 4.9±0.78 μmol m-2 s-1). Consequently, the flux of CO2 is a result of biological activity and its greater intensity can cause volume loss, compacting, diminishing of the ventilation capacity, and alteration of the size of the particles (Abad et al., 2004; Burés, 1997). Therefore, AP’s low biological activity guarantees that some of its physical characteristics will remain stable through time. On this regard, Pineda et al. (2012) used AP mixtures to grow tomatoes, keeping total adequate porosity, water retention, apparent density, and ventilation capacity, for up to 24 months of cultivation. As a result of their high CO2 flux values, BH and SA could require prior composting, before they were used as substrates, in order to prevent a high biological activity and the alteration of certain physical characteristics.

Conclusions

The origin and processing of organic materials affects the shape and size of particles, as well as their porosity and their water retention capacity. According to their granulometry, AP and HD are the organic materials that contribute to the balance in the air-water ratio. When AP, FC, and SA particles are ≤5mm, their size favors ventilation and water retention. Gulfweed has high K+ and Mg2+ content, but low Na+ content, scarce water, and low biological activity which make it inadequate as a substrate. Pine timber waste had an acceptable percentage of total available water, while its low CO2 guarantees that its degradation will be slow and that it will keep a stable volume through time. BH and HD have higher N, P, and Ca2+ concentrations.

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Received: March 2017; Accepted: February 2018

* Author for correspondence: e_couoh@hotmail.com

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