Introduction

In Mexico, up to 10,000 t of residual mushroom (A. bisporum) compost waste are produced annually. Each kilogram of product can generate up to five kilograms of compost, where high nutrient concentrations are conserved (^{Polat et al., 2009}; ^{Servicio de Información Agroalimentaria y Pesquera [SIAP], 2013}). This residual compost waste contains biomolecules which are slowly released during the mineralization process of the organic matter, producing adequate levels of nitrates, phosphates, and sulphates in the soil during extended periods (^{Pardo-Giménez, Cunha-Zied, & Pardo- González, 2010}). Under this perspective, the residues can be considered as byproducts ready to be utilized, which can represent a potential source of nutrients available to plants and the soil, when they are recycled during the composing process (^{Mondini, Dell’ Abate, Leita, & Benedetti, 2003}).

To produce organic tomatoes under the sustainable agriculture scheme, the use of compost, vermicompost tea and compost extracts has been considered, given the fact that their biological processes transform organic waste from different material sources into a more relatively stable end product. (^{Claassen & Carey, 2004}). The benefits of these products are as follows: they help improve soil characteristics such as fertility, water storage capacity, and mineralization of elements such as nitrogen, phosphorus, and potassium; they assist in maintaining optimum pH values for plant growth and they promote microbial activity (^{Nieto-Garibay, Murillo-Amador, Troyo-Diéguez, Larrinaga- Mayoral, & García-Hernández, 2002}). Vermicompost tea is an aqueous extract of high biological quality obtained from aerobic fermentation (^{Domínguez, Lazcano, & Gómez-Brandón, 2010}); moreover, it contains active substances that act as growth regulators, increase cation exchange capacity, have high levels of humic acids and increase soil moisture retention capacity and porosity, which facilitates proper soil aeration and drainage (^{Hashemimajd, Kalbasi, Golchin, & Shariatmadari, 2004}; ^{Rodríguez-Dimas et al., 2008}).

On the other hand, the compost extract is also aqueous but rich in beneficial microorganisms, soluble nutrients and fine particles of organic matter. This extract can be obtained by means of aerobic fermentation (^{Rodríguez-Torres, Venegas-González, Angoa, & Montañez-Soto, 2010}). Its chemical composition, in terms of the nutrients it provides to crops and their effects on the soil, varies according to available raw material, compost age, and moisture management and content (^{Ferreira-Araújo, Marçal-Silva, Carvalho-Leite, Fernando-de Araújo, & da Silva-Dias, 2013}). The advantage of these aqueous extracts is that they can be applied by means of a pressurized irrigation system, so their use can be adapted to greenhouse production practices. The aim of this study was to determine whether the compost, the vermicompost tea and the compost extract are capable of improving soil characteristics and maintaining tomato yields.

Materials and methods

The experiment was conducted under shade house conditions on soil at the Experimental and Technology Transfer Center (ETTC) at the Sonora Institute of Technology, located in Cd. Obregon in Sonora, Mexico, situated at 27° 21’ 56.3’’ NL and 109° 54’ 53.4’’ WL, between August, 2013 and May, 2014. A completely randomized experimental design with five treatments and three replications was used. Each experimental unit consisted of one plant (3.3 plants per square meter). The planting beds were 30 m long and spaced 1.8 m apart, and the plants were conducted to one main stem. An analysis of variance with ^{STAT-GRAPHICS, version 16.1.11 (StatPoint Technologies, 2010)} and Tukey´s range test (P ≤ 0.05) were performed.

The compost used was obtained from substrate employed for mushroom production (Table 1) by the company Fertilizantes Nitrogenados y Fosfatados, S. de R.L. de C.V., located in La Barca, Jalisco. The compost application was carried out during pre-sowing, on the same day as transplanting (September 27, 2013).

The vermicompost tea was made from humus generated by earthworms of the species Eisinia foetida; their basic diet was exclusively residual mushroom waste. Five kg of humus were taken and placed in a mesh bag, which was then put in a tank with 200 L of lixiviated material and shaken for 24 hours at room temperature.

The compost extract was obtained from the compost ⁄ water mixture with a 3:10 ratio, and was subjected to an intermittent shaking process for five days at room temperature. The vermicompost tea and compost extract were applied weekly using a drip irrigation system. The physico-chemical and nutritional characteristics of the compost, vermicompost tea and compost extract are described in Table 1.

The tomato cv “Grandella” was used to evaluate the treatments (Table 2). Sowing took place on August 10, 2013 and subsequently the plants were transplanted in a zig-zag pattern onto soil covered by plastic mulch.

Physicochemical analysis of the soil was carried out according to ^{NOM-021-RECNAT-2000 specifications (Diario Oficial de la Federación, 2002)}. One week before and after the production cycle, soil samples were collected at a depth of 20 cm. Organic matter, moisture content, pH, electrical conductivity (EC), cation exchange capacity (CEC) and bulk density were the parameters assessed.

For nutrient analysis, the third leaf from the apical bud was taken, obtaining two to three leaves per replicate of each treatment, at 64 days after transplanting (DAT), when the plants had developed 75 % of their floral buds. The wet digestion technique proposed by ^{Alcántar and Sandoval (1999)} was used for this analysis, taking 0.25 g of leaf tissue. Each sample was analyzed with a plant tissue analysis kit (^{DR ⁄2500; Hach company, Loveland, Colorado, USA}) on the basis of the manufacturer’s technical specifications (^{HACH, 2003}). The elements determined were nitrogen (N), phosphorus (P), potassium (Y), calcium (Ca), magnesium (Mg), iron (Fe), copper (Cu), and zinc (Zn).

At 154 DAT, a SPAD 502 Plus^® meter (model 2900P, Spectrum Technologies Inc., Plainfield, Illinois, USA) was used to take chlorophyll readings on the third leaf from the apical bud. The average value of three points on each leaf was taken into account per experimental unit, expressed in SPAD units, taking into consideration the adjustments made by ^{Bruinsma (1963)} and ^{Krugh, Bichham, and Miles (1994)}.

For fruit number and weight, the total number of fruits obtained in 14 cuts were counted and weighed using a Torrey® scale (model L-EQ series, Monterrey, Mexico), whereas the yields were obtained by calculating the total weight (kg) of the fruits harvested from each treatment per square meter.

Results and discussion

The analysis of variance performed on soil physicochemical parameters showed a significant (P ≤ 0.05) effect, with the exception of organic matter content and bulk density (Table 3). Soil moisture increased by 9.6 and 8.6 % for T3 and T5, respectively, in relation to the control (T1). ^{Aguilar-Benitez et al. (2012)} stated that the use of 3 % vermicompost has an effect on moisture deficit, increasing bean yields by up to 50 %. Furthermore, ^{Gajalakshmi, Ramasamy, and Abbasi (2001)} indicated that vermicompost use incorporated into the soil improves its moisture retention capacity. The combination of vermicompost tea, compost and compost extract increased soil organic matter, which in turn contributes to moisture retention due to aggregate formation (^{Murray-Nunez et al., 2011}).

In all cases, the soil presented moderately alkaline pH; however, T2, T4, and T5 were highly significant (P ≤ 0.001), with increases of up to 4 % with respect to T1. The pH increase observed in the soil after applying organic treatments can be attributed to the basic cation content (Ca, Mg and K) found in the composted materials (^{Orozco-Rodríguez & Muñoz- Hernández, 2012}). ^{Daza (2014)} showed that, among other properties, compost application improves soil pH. ^{Durán-Umaña and Henríquez-Henríquez (2010)} also reported a similar case.

The soil EC presented significant (P ≤ 0.05) differences, increasing after treatment application. The compost, vermicompost tea and compost extract all demonstrated high EC (Table 1). Despite this, ^{Hashemimajd, Mohamadi, & Jamaati-e-Somarin (2012)} mentioned that the compost EC levels can range between 0.1 and 10 dS·m-1. The T5 EC increased 80.9 % and T3 EC rose 71 %, while T1 and T2 EC (33.3 %) did not present significant differences. Finally, T4 EC improved by 28.5 % compared to the control. Despite the EC increments, the soil presented low salinity, according to NOM-021- RECNAT-2000 classification.

The CEC showed statistical differences (P ≤ 0.05) among treatments, being higher due to organic matter application. The T3 and T4 treatments resulted in an increase of 2.5 and 3.4 % relative to the control (T1), respectively. ^{Arrigo, Jiménez, Palma, Benito, and Tortarolo (2005)} also reported similar results after incorporating organic matter into the soil. ^{Félix-Herrán, Sañudo-Torres, Olalde-Portugal, Rojo-Martínez, and Martínez-Ruiz (2008)} mentioned that as a result of the mineralization of composted waste being incorporated into the soil, CEC is increased prior to applying the treatments. In this regard, ^{Escudey, Förster, and Galindo (2004)} confirm that an increase in soil organic matter is a consequence of increased CEC.

The nutrient content analysis at 102 DAT showed that T2 and T5 increased leaf N content by 5.4 % and 7.0 %, respectively, with respect to T1; however, there were no statistical differences (P ≤ 0.05). It is important to mention that these values fall within the sufficiency levels (2.5 to 5 %) for tomato production during the flowering stage. ^{Ochoa-Martínez et al. (2009)} showed similar leaf N concentration results to those obtained in this study with the use of compost tea. For its part, P had similar values in T1, T2, and T4, being statistically different from T3 and T5, slightly exceeding the recommended limits (0.35 to 0.9 %). Nonetheless, the other treatments presented a sufficient leaf P concentration for the development of principal physiological functions.

Furthermore, the leaf K content varied between 3.43 and 4.91 %, without presenting significant differences compared to the control. However, T5 was statistically different from the rest of the treatments, staying within the 2.5 - 5 % tomato sufficiency ranges (^{Benton, 2012}). Likewise, Ca concentrations showed similar behavior and significant (P ≤ 0.05) differences with T5, slightly exceeding the maximum level of recommended leaf Ca content (1.5 to 3 %). Mg concentration showed significant (P ≤ 0.05) differences, but all treatments were between the ideal values (0.15 to 1 % Mg), maintaining normal plant development without the presence of physiological disorders.

The microelements (Cu, Fe and Zn) presented significant (P ≤ 0.05) differences. Leaf Fe and Zn content fluctuated within the established 60 to 300 mg∙kg^-1 Fe and 20 to 250 mg∙kg^-1 Zn tomato ranges. T2 exceeded by 25 % the maximum Fe concentration value reported by ^{Benton (2012)}; Zn was 20 % higher than the optimum tomato sufficiency values. For its part, Cu was up to 56 % higher in the other treatments compared to the control (^{Benton, 2012}). Thus, these concentration levels, despite being high, did not display toxicity symptoms or antagonism during the evaluated phenological stage (Table 4).

Incorporating the treatments produced the same effect as that obtained by conventional fertilization. This reaction can be attributed to an increase in soil nutrient availability, after applying compost and vermicompost, due to their effects on pH and soil structure, thus increasing fertility (^{Olivares-Campos, Hernández-Rodríguez, Vences- Contreras, Jáquez-Balderrama, & Ojeda-Barrios, 2012}). ^{Brown and Cotton (2011)} found that compost use has a positive effect on soil properties. ^{Sarwar et al. (2008)} reported similar values to those obtained in the present study and demonstrated that there were more nutrients available to plants treated with compost.

Similarly, ^{Ahmed and Khan (2013)}, after applying vermicompost, increased tomato yields, so they suggest the use of this option as a potential means of sustainable production. Likewise, ^{Tringovska and Dintcheva (2012)} have shown the benefits of vermicompost, attributing the plant reaction to a change in soil physical and biological properties (^{Robledo, Grosso, Zoppolo, Lercari, & Etchebehere, 2010}).

Leaf chlorophyll readings after applying the organic treatments (T2, T4 and T5) did not show statistical differences (P ≤ 0.05) compared to the control (T1). ^{Degli-Esposti et al. (2003)} noted that readings above 40 SPAD units were directly related to the good nutritional status of the plant. Various studies also indicate that there is a high correlation between extractable leaf N concentration and the chlorophyll units measured with the SPAD meter (^{Rodríguez-Mendoza, Alcántar- González Aguilar-Santelises, Etchevers-Barra, & Santizo- Rincón, 1998}; ^{Rezende-Fontes & de Araujo, 2006}). This finding supports the data obtained in the present study, where the higher the leaf N concentration the higher the chlorophyll units. ^{Preciado-Rangel et al. (2011)} also obtained very similar results to those of the present study, finding a good response with the organic treatments, thereby representing a viable option as a nutrient source for tomato plants.

In terms of fruit number and yields, no significant (P ≤ 0.05) differences were found. Fruit weight was statistically different (P ≤ 0.05) among treatments, with a 10 % difference between T4 and T5. ^{Vega-Ronquillo, Rodríguez-Guzmán, de Cárdenas-López, Almaguer, and Serrano-González (2006)} were able to increase cucumber yields after compost and earthworm humus application, and found that fruits of the treated plants increased in weight. ^{Roblero-Ramírez Nava-Pérez Valenzuela-Quiñónez, Camach-Báez, and Rodríguez- Quiroz (2014)} demonstrated similar effects in tomato fruits, where vermicompost use had an important influence on fruit number and weight. ^{Tejada, González, Hernández and Garcia (2008)} also coincided with the findings of the present study, confirming that the application of leachates from vermicomposting increases tomato yields.

Yields remained statistically unchanged (P ≤ 0.05) compared to the control. These results coincide with those of ^{Ortega-Martínez et al. (2010)} who maintained tomato production with compost application. This indicates that the use of compost and aqueous extracts has an effect on the physicochemical properties of the soil. These changes in the soil result in reduced nutrient leaching from the soil matrix due to greater water retention and increased CEC (^{de Grazia, Tittonell, & Chiesa, 2004}).

Conclusions

Residual mushroom waste, in its solid state, as aqueous extract, vermicompost tea, and compost extract, showed potential as a soil conditioner, particularly the compost and compost extract, which directly improved soil physicochemical characteristics such as organic matter, bulk density and CEC. In addition, they satisfied crop macro and micronutrient requirements, and thus maintain yields. Consequently, residual mushroom waste is a viable material source to be composted; therefore, they are an effective alternative for use as a nutrient source and soil conditioner for tomato production. Thus, these alternatives allow reducing agrochemical use as much as possible by making better use of renewable resources.