The use of the earthworm (Eisenia foetida Savigny, 1826) is an environmentally friendly alternative to transform organic resources in decomposition and improve the quality of agricultural soils (^{Atiyeh etal, 2000}). Plants develop in a complex system where a suitable substrate has a structure with high porosity, aeration and water retention (^{Olivares-Campos et al., 2012}).

Vermicompost (VC) has a direct effect on plant growth, and it is a product with great commercial potential (^{Atiyeh et al., 2002}). It consists of a chemical mixture of minerals composed of low amounts of salts having a large ion exchange capacity (^{Albanell et al, 1988}), and provide substances involved in plant growth regulation (^{Tomati et al, 1990}). These active substances, such as nitrates, phosphorus and soluble aggregates (potassium, calcium and magnesium), are easily assimilated by plants (^{Orozco et al, 1996}).

^{Ghosh et al. (1999)} indicate that the mixture of vermicompost with inorganic fertilizers increase crop yields, compared to those who only use the latter; (^{Chakraborty et al., 2008}) its application also improves the quality of vegetables and nutrient levels are higher than in traditional crops (^{Kale, 1998}). In the vermicomposting process many nutrients are changed to their simplest form, facilitating its absorption by the plant (^{Chanda et al., 2011}). Therefore, the objective of this study was to evaluate different vermicompost doses for tomato cultivation in soil with low organic matter concentration impacted by long periods of inorganic fertilization in northern Sinaloa.

In this experiment an organic fertilizer (vermicompost) based on cow manure and organic kitchen waste in a 9:1 ratio, and inorganic fertilizer as a nitrogen source (46% urea) was used. The organic fertilizer was vermicomposted for six months. Seedlings of the Missouri (Solanum lycopersicum L.) variety were used, which were produced in the greenhouse of the Interdisciplinary Research Centre for Integral Development of the Region (CIIDIR-IPN), Sinaloa Unit located in Guasave, Sinaloa, Mexico was used. The experiment was conducted during the fall-winter crop cycle in the experimental areas of CIIDIR -IPN. Plant materials were transplanted 40 days after germination.

In the field experiment six treatments were evaluated. The treatments were: 0, 500, 1 000, 1 600, 2 000 and 4 000 kg VC ha^-1. Each of the treatments were added 600 kg ha^-1 of urea. The design was a randomized complete block design with three replications. The vermicompost was applied in two portions, the first portion three days before planting, applied on the transplantation site and the second portion after 45 days, the fertilizer was added into a hole made on one side of the plants, and covered with the same cultivation soil. Drip irrigation was performed every third day for 3 consecutive hours. Experimental plots consisted of five rows 10 m long spaced at 1.50 m. In each of the grooves three tomato seedlings were planted per meter.

Sampling was done randomly taking five tomato plants per replication. Three samplings were performed at 40, 65 and 100 days after transplantation, the number of fruits were counted, fruit size was measured in inches with a CAL50-80 fruit caliper and fruit weight in grams using an analytical balance (OHAUS plus AP210). The experiment lasted 100 days. Data were subjected to analysis of variance (p≤ 0.05 ANOVA) and when required, the means were compared by the Tukey test (p≤ 0.05), these analyzes were carried out using the Statistica package.

During sampling it was observed that the first fruits appeared at the dose of 4 000 kg VC ha^-1 compared to the other treatments. ^{Togun and Akanbi (2003)} indicate that the first fruits appear in a relatively short time after the first flowering when nutrient content in the soil is high and diverse. The number of fruits/plant at the end of the experiment at the dose of4 000 kg VC ha^-1 was 170, showing a significant difference with respect to the treatment of 0 kg VC ha^-1 which was 132 fruits / plant (Table 1). In other doses, the number of fruits was less than 120 tomatoes per plant.

Table 1 Average number of fruits produced per plant in each treatment throughout the crop cycle. 

Promedios con letra diferente son significativamente diferentes (p≤ 0.05).

This result agrees with that reported by ^{Togun and Akanbi (2003)}, who mentions that the number of tomato fruits per plant at concentrations of 4 000 VC kg ha^-1 was ~ 20% higher than the 0 kg VC ha^-1 treatment, thus these vermicompost concentrations meet the nutritional requirements of the plant.

With regard to the weight and size of tomato fruits, plants with 4 000 kg VC ha^-1 increased from ~ 3 000 to ~ 20 000 g/ plant over a period of six weeks (Table 2). During the first sampling weights were similar above the ~ 2 000 g/plant between treatments except for the 500 kg VC ha^-1 which was ~ 1 600 g/plant.

Table 2 Average fruit weight per plant in grams. 

Promedios con letra diferente son significativamente diferentes (p≤ 0.05).

During the second sampling, the plants with the 1 600 kg VC ha^-1 dose recorded the highest weight with the least number of fruits. For the third sampling, the 4 000 kg VC ha^-1 dose averaged a production of ~ 20 000 g/plant, showing a significant difference, greater than ~ 5 000 g/plant over the control and ~ 7 000 g/plant relative to the other treatments. This difference is due to the number of fruits and average fruit weight per plant. The fruit with the highest weight was found in the 500 kg VC ha^-1 dose weighing ~ 245 g and the lower weight was at 0 kg VC ha^-1 with ~ 64.62 g of the last count. This is in agreement with ^{Akanbi et al. (2000)} and ^{Shankar et al. (2012)}.

The amounts of nutrients present in vermicompost increase the tomatoes weight and volume, and unlike reports from ^{Maynard (1995)} and ^{Togun and Akanbi (2003)}, total fruit production in the higher doses compared to control increase production up to ~ 5 000 g, confirming that vermicompost complements the nutrients quality required for plant development. According to reports from Togun and Akanbi (2003), tomato fruits fertilized with vermicompost reach an average individual weight of ~ 120 g, because the nutrients available to the plant from vermicompost favor the production of larger and heavier fruits (^{Shankar et al., 2012}). In addition, high doses of vermicompost maintain an adequate pH level and salt content, which improves soil physical conditions promoting plant development (^{Lazcano et al., 2009}). These results differ from those reported by ^{Domínguez et al. (2010)} who state that at higher concentrations of humic substances there will be greater plant growth due to an increased hormonal response from the plant; since a lower fruit production was observed in the intermediate vermicompost doses.

Concerning fruit size, the 1 000, 1 600 and 2 000 kg VC ha^-1 doses showed fruits of4 cm in diameter and all doses except the 1 600 kg VC ha^-1 produced fruits of 7 cm (Table 3). In the 0 kg VC ha^-1 dose, 75% of the fruits is between 6 and 7 cm being the dose with most fruits in large sizes. In those plants with high vermicompost concentrations a greater number of tomatoes of ~ 6 cm were harvested, except for the 2 000 kg VC ha^-1 dose in which most fruits were below 5.6 cm.

Table 3 Fruit size of tomatoes produced per vermicompost dose. 

Vermicompost had an effect on tomato weight and size, since the essential nutrients in the applied vermicompost are similar to those reported by ^{Atiyeh et al. (2002)} and ^{Shankar et al. (2012)}, who mention that vermicompost provide the nutrients needed for a good plant development. The structural improvement ofthe cultivation soil favors the activity of microorganisms, which influence the exchange of nutrients and the plant potential to grow and increase the production volume of tomato (^{Alarcón and Ferrera-Cerrato, 2000}; ^{Arancon et al. 2004}, ²⁰⁰⁶). The nutrients in a medium such as vermicompost are assimilated by the plant in higher concentration since they have greater bioavailability as observed in the present study, thereby supporting plant growth and development in a relatively short time, leading to an earlier onset of fruit formation and production compared to the control plants.

The vermicompost effect on tomato development had an important influence on plant yield, increasing fruit number and weight. Plants with the 4 000 kg VC ha^-1 dose outperformed the plants in the control treatment. Vermicompost can be an excellent complement for chemical fertilizers promoting higher yield and fruit quality for both the national and international market, but an economic analysis is required to consider vermicompost as a viable product for tomato cultivation.