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).

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}).

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}).

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).

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.

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.