Production of flowers in containers

^{Ansorena (1994)} indicated that the term container applies to any type of vessel that can contain a substrate, and in which a plant may develop. In ornamental flowers farming, containers can be used to produce: 1) flowers for cutting, such as agapanthus, gladiolus, lilium (azucenas), nardo (Polianthes tuberosa), bird of paradise, roses, sunflowers, Lissianthus, calla lily, estatice (Limonium sinuatum); 2) foliages, such as Ficus, ferns, araucarias, teléfonos (Epipremnum aureum), hydras (spp), coleos (Coleus blumei), cedars, Cissus, Phylodendrum; 3) plants with flowers in pots, such as impatiens, petunias, kalanchoes, poinsettias, anturios (Anthurium scherzerianum), chrysanthemo (Chrysanthemum morifolium Ramat), zempasúchil (Tagetes erecta), viola tricolor, begonias, vincas (Catharanthus roseus), calla lily, Spathiphyllum, Lilium, bromelias, orchids, hortensias, Gerberas, Cyclamen. The surface planted in containers is variable and in 2004 for the production of flowers for cutting in containers and pots: Europe 54 104 ha, Africa 5697 ha; Asia 224 418 ha, and America 45 980 ha; Mexico allocated 21 129 ha for the production of flowers for cutting and pots (^{SAGARPA, 2006}). However, in 2009, México was fourth in terms of surface allocated for the production of ornamental plants (23 417 ha) worldwide, out of which 75 % were cultivated in open areas, and included gladiolus, carnations, and gysophila; the remaining 25 % was grown in greenhouses, where roses, gerberas and potted plants were grown (^{SAGARPA, 2011}). Estado de México contributes 60 % of the value of the total production, and the rest is provided by Puebla, Morelos, Mexico City, Baja California, Chiapas, Jalisco, Colima, Veracruz, Yucatán, Michoacán, and Guerrero (^{SAGARPA, 2013}).

Mexico has an annual internal consumption value of over one billion dollars in cut flowers and foliage; occasionally, ornamental plants are exported with a value of 30 million dollars. Out of the total production, 95 % is sold domestically, and 3 to 7 % of the production is exported. Mexico also exports tulip and gladiolus bulbs, orchids, carnations, chrysanthemums, Gypsophilia, statice (Limonium sinuatum), gerbera (Gerbera jamesonii), daisies, anturio (Anthurium andreanum), bird of paradise, foliages, and leaves. Of the total 90 % is exported to the United States, and 10 %, to Canada, the Netherlands, Belgiun, Germany, Ukraine, South Africa (^{SAGARPA, 2009}). Also, Mexico imported ornamental plants, mainly roses from Ecuador, and live plants, rootless cuttings and grafts worth one million dollars in 2009, from Costa Rica, Germany, and Guatemala (^{AGEXPORT, 2010}).

The success of the crop depends on using the adequate container and substrates (^{Mascarini et al., 2012}; ^{Urrestarazu, 2015}). Plants grown in containers present high transpiration rates, they require plenty of water, and the probability of salinization due to the high rates of water loss. For these reasons, the physical, chemical, and biological characteristics of diverse substrates are necessary, along with the determination of whether they are adequate, on their own, or mixed (Valenzuela et al., 2004)⁴.

The physical characteristics must be determined before establishing the crop, since they will be difficult to correct afterwards (^{Ansorena, 1994}; ^{Baixauli and Aguilar, 2002}; ^{Castellanos, 2003}). Chemical characteristics can be modified after the crop has been established by applying nutritional elements (^{Abad et al., 2005}). The biological characteristics may be determined only in organic or active substrates, since they are thermodinamically unstable because organic matter is degraded by the action of microorganisms and chemical hydrolysis reactions (^{Bures, 1997}; ^{Cruz et al., 2013}).

Physical and chemical characteristics of the substrate

Studies with unconventional substrates for ornamental plants and flowers in Latin America are based on the characteristics of plants, and less on the quality of the substrates (Valenzuela and Gallardo, 2006)⁵. It is convenient to characterize the new materials before proposing them as an option (Masaguer, 2013)⁶. The physical characteristics that must be considered to select a substrate are apparent density, granulometry, porosity, moisture retention (^{Ansorena, 1994}; ^{Urrestarazu, 2015}), type of packaging, and permeability (^{Burés, 1997}). The apparent density is defined as the dry mass contained in a 1 cm³ of the culture media, and volume of the container is dependent upon this density (^{Cruz et al., 2013}). For plants in containers in greenhouses, ^{Abad and Noguera (2000)} recommend 0.15 g cm^-3, whereas ^{Baudoin et al. (2002)} mention 0.22 g cm^-3, and ^{Quintero et al. (2011)} 0.50 to 0.75 g cm^-3. Porosity is dependent upon the density of the substrate, and this characteristic directly affects the speed of filtration of water and retention of moisture; and it would be the most significant physical characteristic for ornamental horticulture in containers (^{Cabrera, 1999}; ^{Mascarini et al., 2012}). Porosity is determined by the percentage of volume that is not occupied by the solid phase (^{Ansorena, 1994}; ^{Burés, 1997}); the total porosity is recommended to be over 85 % of the volume (^{Morales and Casanova, 2015}).

The distribution of the particle size in a material defines the granulometry which, in turn, determines the size of the pores; particles from 0.25 to 1 mm are essential in the relation water-air, the reduction of the particle size reduces the total porosity and consequently, the capacity for retaining water (^{Vargas et al., 2008}; ^{Anicua et al., 2009}). It is important for not all pores to be covered in water, in order to allow the oxygenating of roots and the gas exchange between the atmosphere and the substrate; therefore, between 10 and 30 % of the volume of substrate with air is recommended (^{Morales and Casanova, 2015}). This characteristic is known as the capacity of ventilation and is the proportion of the volume of substrate that contains air after being saturated with water and allowed to drain (^{Abad et al., 2004}). The distribution of substrate particles between 0.25 and 2.5 mm is adequate for vegetable crops (^{Quintero et al., 2011}). The mixture of organic and inorganic materials with particles greater than 1 mm favors the formation of pores with complex packing, which, in turn, favor the retention of humidty (^{Gutiérrez et al., 2011}; ^{Morales and Casanova, 2015}).

The physical barriers and the limited space imposed by the container reduce the volume of the roots and the pH buffering capacity, increase the requirement of water and nutrients, and the sensitivity to factors that affect the root system, such as air, water, nutrients, pH and electrical conductivity (EC) (^{Terés, 2000}). The water retained by the substrate is not uniform throughout the container (^{Ansorena, 1994}; ^{Mascarini, 2012}); therefore, the substrate must have an adequate moisture retaining capacity, which is directly related to porosity, and both depend on the distribution, composition, internal structure, shape and size of the particles, which also influence the relation water-air of the substrate (^{Anicua et al., 2009}). Usable moisture is the difference between the amount of water retained by the substrate after wetting and drained, and the water retained by the substrate which the plant cannot extract; the optimum value is between 20 and 30 % of the volume of the substrate, although there are variations, depending on the needs and tolerance of each species (^{Abad et al., 2004}; ^{Cruz, 2013}).

A few of the chemical characteristics of the substrates that must be considered are the content of nutrients, cationic exchange capacity (CEC), pH, EC, C/N ratio and content of phytotoxic elements (^{Ansorena, 1994}; ^{Burés, 1997}; ^{Puerta et al, 2012}). CEC is the capability of a substrate to adsorb and exchange ions between the negatively and positively charged colloids in the middle, and is related to pH and nutrient availability (^{Burés, 1997}). The optimum value will depend on the frequency of fertilization/irrigation; if it is permanent, the value of CEC will have no effect, but if it is intermittent, the value should be medium or high (>20 meq 100 g^-1) (^{Abad et al., 2004}). CEC is determined by the amount of ions in the solution. A high concentration will cause a low water potential, which may cause the plant to lose water, which is why the substrate must have a low salt content (≤2 dS m^-1) (Boudin et al., 2002).

The C/N ratio is used as an indicator of the origin, the degree of maturity and the stability of the organic matter (^{Burés, 1997}; ^{Domeño et al., 2011}). The composted substrates with a value below 40 are considered mature and stable (^{Abad et al., 2004}). In raw materials for substrates (pine sawdust and coconut fiber), the highest ratio C/N will mean greater stability, and the recommended C/N ratio is between 30 and 300. The materials with a high C/N ratio are more stable and they avoid N losses due to fixation, phytotoxicity due to the presence of organic compounds produced in the degradation process, changes in the CEC or increases in salinity (^{Domeño et al., 2011}).

Substrates used worldwide

The use of substrates is due to the need to transport plants from one place to another, agricultural soil depletion, salinity, and the risk of diseases (^{Pastor, 2000}; ^{Cruz et al., 2013}). Substrate production began in the 1960's in the Netherlands, with substrates such as peat moss, sand, clay, pearlite and vermiculite; in the 1980's substrates became diversified and residues and subproducts appeared, such as coconut fiber (^{Petit and Villegas, 2004}; ^{Urrestarazu, 2013}).

In Northern Europe, the peat moss is the main substrate, followed by coconut fiber (which surpassed mineral wool in the past two decades), bark and pearlite. In the south, they use mixtures of peat moss with barks, sand, wood products, volcanic products, compost with plant residues, and manures (^{López et al., 2007}; ^{Blok and Urrestarazu, 2010}; ^{Alonso et al., 2012}). In Spain, mineral wool, pearlite, and coconut fiber are used; and the application and commercialization of volcanic rocks, due to their availability in deposits of the Iberian Peninsula is a source of interest (^{Urrestarazu, 2013}; ^{Pozo et al., 2014}). In Argentina pine bark, soil, sand, peat moss, pine needles, cow manure, horse manure, soft wood shavings, and rice husk are used (Oryza sativa); few producers use pearlite and vermiculite (^{Acosta et al., 2008}; ^{Barbaro et al., 2011}; ^{Barbaro et al., 2014}). In the Dominican Republic, they use rice charcoal and coconut fiber (Pérez et al., 2010)⁷, similar to Venezuela, where they also use sugarcane pulp (^{Cásares and Maciel, 2009}). In Brazil soil, sand, bovine manure, poultry litter, rice husk, conifer bark, organic composts, vermiculite, pearlite, mineral wool and coconut powder are utilized (^{Acosta et al., 2008}). In Colombia the rice cascarilla, partially burned or toasted, is the main substrate, and it is used with mineral wool, pearlite, coconut fiber, and coal waste (^{Quintero et al., 2011}).

In Mexico, the most commonly used substrate is hillside soil, peat moss, wood products (bark, sawdust, shavings), compost made from organic matter or gardening waste, coconut powder, sewage mud, sludge, manure, hay, rice and peanut husk, and inert materials such as tepojal, tezontle, basalt, pearlite, sand, vermiculite, charred clay, and pumice stone (Iskander, 2002⁸; Zamudio, 2008; ^{Ojodeagua et al., 2008}). However, the search for local materials is permanent for recycling them, with an emphasis on those with the lowest cost, and without an environmental impact (^{Bracho et al., 2009}; Avilés et al, 2010⁹; Urresterazu, 2013; ^{Valenzuela et al, 2014}; ^{Cruz et al. 2016}).

The states that produce ornamental plants (Estado de México, Puebla, Distrito Federal, Morelos, and Michoacán, followed by Baja California, Guerrero, Jalisco, Querétaro, and Oaxaca) use inert substrates mixed with hillside soil and peat moss (^{García et al., 2001}). In Veracruz, ^{Benítez et al. (2002)} reported that in government-owned conifer greenhouses, hillside soil is used in combination with sand, leaf soil or tepezil, in a proportion of 1:1. The mixture of rice husk with leaf soil (1:1) is used for tropical species. The producers of poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch) from Morelos use leaf soil from oak (Quercus resinosa, Q. insicnis, Q. crassipes, and Q. mexicana), "ocochal" obtained from ocote (Pinus montezumae), mixtures of leaf soil-compost-sawdust (50:30:20 v/v) and leaf soil-ocochal (pine ocoxal leaves) (50:50 v/v) (^{Galindo et al., 2012}). The commercial greenhouses for ornamental plants in containers in Tabasco use the cocoa husk mixed with soil taken from the area. In Yucatán, agave pulp (Agave fourcyoydes Lem.), chicken droppings, pig manure, Tsi'tsilche (Gimnopodium floribundum) and soil are used to grow vegetables in containers (^{Borges, 1998}). Organic materials are generally used in mixtures, and inorganic materials are more used individually in hydroponic crops8.

Benefits of substrates on the production of ornamental plants

The contributions of organic residues as substrates to produce ornamental plants are diverse, such as the presence of nutrients easily absorbed by the plant, plant growth regulators, microorganisms that help in nutrient absorption, and are an aide for the growth of organisms which help control plant pathogens (^{Puertas and Hidalgo, 2009}; ^{Puerta et al., 2012}). ^{Ayala and Valdez (2008)} used coconut fiber-based substrate to evaluate the first stage of growth of ornamental flower species, such as Dianthus chinensis (China pink), Gazania rigens (gazania), Tagetes erecta (marigold), Viola wittrockiana (viola tricolor), Antirrhinum majus (snapdragon), and Petunia x hybrida (petunia). Results showed that plants grown in coconut powder delayed their growth, and the authors pointed this out as an advantage, since tall plants cause the containers to tip over (^{García et al., 2010}) and flower quality was not affected.

^{Flores et al. (2008)} used coconut powder to produce Cyclamen persicum Mill. and no differences were observed in the foliar area or dry matter with regard to plants grown in a commercial peat moss based substrate. ^{Jaramillo et al. (2004)} used coconut fiber mixed with leaf soil and pearlite to grow malvón (Pelargonium sp.); the mixture enhanced plant growth (radicle, biomass, and number of leaves) and moisture retention improved in comparison to when using the same individual substrates. Coconut fiber was also evaluated in a mixture with rice husk to produce Anthurium x Cultorum cv. Arizona (^{Cásares and Maciel, 2009}); results showed a greater physical stability of the substrate (no contraction), which favored growth and the number of inflorescences in comparison to the combination of rice husk and sugarcane pulp. Vermicompost enhanced growth, flowering, and root development in Ageratum hustonianum and Petunia hybrida. The cachaza in the form of mud, derived from the process of clarification of sugarcane juice during the manufacturing of sugar, mixed with peat-mossâ and tezontle was used to produce apical stakes of Kalanchoe blossfeldiana (Poelln.) Sensation; results showed that the mixture increased the number of roots, root exploration, diameter of stem, and K absorption (^{Villanueva et al., 1998}). Mixtures of agave pulp, pig manure, and dzilziché (Gimnopodium floribundum), combined with different proportions of soil were used to produce chrysantemums. Out of these, the combination soil:agave pulp (70:30) produced plants with a higher inflorescence quality (^{Villanueva et al., 2010}). ^{Alonso et al. (2012)} evaluated mixtures of chicken droppings with hay or sawdust, plus black and blonde peat moss, in yellow chrysanthemums cv. Albanor in containers. The application of chicken droppings to black peat moss increased the amount of flowers significantly, but some plants died with the increase of chicken droppings.

Organic residues are unstable, studies related to the necessary proportions should be increased. The use of organic residues for vegetable production led to several investigations; besides, inorganic materials such as tezontle, are of interest as an agricultural substrate (^{Ojodeagua et al., 2008}; ^{Vargas et al., 2008}; ^{Anicua et al., 2009}; ^{Gutiérrez et al., 2011}; ^{Rodríguez et al., 2013}; ^{Trejo et al., 2013}).

Studies on the use of substrates have also shown unfavorable results. Such is the case of the suppression of seed production in petunias (Petunia x hybrida Hort), growth reduction in viola tricolor (Viola x wittrockiana subsp. Delta) and primrose (Primula acaulis subsp. Oriental) (^{García et al., 2010}; ^{Lazcano y Domínguez, 2010}; ^{Acosta et al., 2014}).

Alternative substrates for the production of ornamental plants

Investigations to find options and replace the use of leaf soil as a substrate in ornamental plants have been diverse. Zeolite is an inert material, of sedimentary origin, of the group of the aluminosilicates, which is used with this purpose. In Mexico, there are deposits in at least 13 states (^{Urbina et al., 2011}). The physical characteristics of zeolite were evaluated by ^{Anicua et al. (2009)} and compared with those of tezontle. Results showed that the hydric and osmotic potentials change with the size of the zeolite particles. The authors concluded that zeolite could be used as an alternative, with results similar to those obtained with tezontle, since there are no effects or interaction with the granulometry.

Pine bark (abundant in the state of Oaxaca) was physically and chemically characterized and evaluated, composted and mixed with bovine manure and peat moss, clay, vermiculite, maguey pulp, raw sawdust, and commercial substrate. The mixture with peat moss, vermiculite and maguey pulp showed physical characteristics in the adequate intervals for plant production (Masaguer et al., 2013)¹⁰. Coffee (Coffea arabica L.) pulp and the residue from soyate (Brahea dulcis) (^{Sustaita, 2009}) were also characterized, and both have appropriate physical and chemical characteristics. Tequila agave (Agave tequilana Weber) pulp in vermicompost and compost mixed with sheep manure (4:1; V/V) is a potential substrate for agriculture (^{Rodríguez et al., 2010}).

^{Padrón et al. (2004)} incorporated cocoa husk to polyurethane foam and observed that the physical and mechanical characteristics of resistance to deformation by compression improved, and there was as increments of degradability as well as water absorption; therefore, it can be used as a substrate for semi-deserted areas. ^{García et al. (2010)} documented the mitigation of the negative effect of salinity (sodium and chlorine contents, mainly) of the composts in plant growth.

Challenges to the production of substrates

Despite the progress in the composition and characterization of some substrates, their uses still face challenges such as the environmental impact, costs, availability, and standardization (^{Burés, 1997}; ^{Abad et al., 2004}). Regarding the environmental impact, the exploitation of natural non-renewable resources as material for substrates, there are still some problems to be solved regarding the irrational use of hillside soil, since it would cause serious erosion and losses of soil productivity (^{Diario Oficial de la Federación, 1996}). Likewise, the peat moss reserves are limited (^{Fernández et al., 2006}), and the high cost and exploitation do not sustain the production of peat-mossâ and may impact the environment (^{Abad et al., 2004}; Block and Urrestarazu, 2010). The use of inert substrates also causes problems, such as the drainage of nutrient dissolutions that pollute the soil and water tables, Block and Urrestarazu (2010) indicated some of the issues in the production of substrates, such as the difficulty to produce the particle size (8 mm) in pearlite and the loss of volume due to the reorientation of the particles of wood fiber planting boards.

High prices, the environmental impact, and the questionable future availability of the materials currently used as substrates highlight the importance of the use of autochthonous organic and locally available materials. The use of materials of plant and animal origin in organic agriculture, such as green fertilizers, mulch, compost, manures, and others, is an alternative to fertilize, improve the soil and satisfy the need to grow plants with sustainable purposes (FIRA, 2003)¹¹. Also, locally available organic residues also have drawbacks, such as the unstable supply through time and the heterogeneity of the material (^{Abad et al., 2004}). ^{Pineda et al. (2012)} evaluated the variation of the physical characteristics of the substrate based on pine sawdust (Pinus sp.) mixed with tezontle, in five plantation cycles for tomato (Licopersicum esculentum L.) and they observed significant changes in the physical characteristics of porosity, retention of moisture, and capacity of ventilation. Therefore, the stabilization of the materials should be studied before they can be considered as substrates.

Block and Urrestarazu (2010) point out aspects that must be taken into account in the production of substrates, such as energy consumption, CO₂ release, water use and transportation for a sustainable, innocuous, and economically sustainable production. ^{Pastor (2000)} and ^{Abad et al. (2004)} state that, regarding the manufacture, challenges are the efficiency in the process of composting and the production of mixtures, mechanizing procedures and characterization of final mixtures. The materials must also be easy to handle, obtainable, inexpensive, stable, rehydratable, and easy to mix.