INTRODUCTION

The actual practice of agriculture with the use of chemical fertilizers is one of the main causes of today’s environmental diseases. Although technological packages have produced high increases in crop yields, they have also had collateral effects such as losses of thousands of hectares of fertile land, soil degradation and increases of more virulent pathogens (^{Cruse, 2012}). This technology is also responsible for eutrophication of bodies of water, increases in nitrate levels in ground and surface water, and increments of pesticide residues in water, soils and food (Tarigo et al., 2004)¹.

Moreover, huge amounts of organic residues produced by agricultural activities have become environmental pollutants since they increase salinity levels, nitrate lixiviation rate into underground and drain water as well as phosphate concentration in surface water (^{Flotats and Sole, 2008}). In addition, organic residues are associated with accumulation of lignin, aromatic oils and resins (^{Obied et al., 2005}), and several pests, weeds and diseases have been disseminated (^{Baffi et al., 2007}) and cases of toxicity in cultivated plants have increased (^{Zucconi et al., 1981}). As a result, the use of organic residues has become increasingly important as an efficient way to recycle nutrients and stimulate plant growth as well as to incorporate nutrients to degraded soils (^{Cerrato et al., 2007}). Hence, controlled degradation processes have been considered necessary to minimize environmental risks caused by organic residues as in the case of manure and urine (Lüebka and Lüebka, 2007)².

Composting and biodegradation is an effective way to manage organic residues to obtain a healthier, safer, more economically profitable product while improving soil fertility (Bernal et al., 2008; ^{Szabová et al., 2010}). The factors involved in the composting process, such as aeration, initial C/N relationship and moisture content, influence nutrient conservation, which are important in improving the potential of compost as organic fertilizer and its agronomical value (^{Guo et al., 2012}). Application of organic fertilizers to agricultural soils is an important practice for increasing crop yield (^{Saldaña et al., 2014}), and composts can improve physical, chemical and biological soil attributes (^{Soto and Melendez, 2003}). The objective of this study was to evaluate the contribution of soil macronutrients by composts made from organic residues (plant and livestock wastes) as compared with inorganic fertilizers.

Materials and Methods

Experimental design

The experimental design was completely randomized with ten fertilization treatments and five replications. The experimental unit was defined as a 6 L capacity pot. Treatments were set up with eight fertilizers based on four composts in two doses, a synthetic fertilizer (urea) and a control (no fertilizer). Composts were prepared from animal waste and plant residues and allowed to transform naturally for 25 weeks. For this purpose, we used hen manure (H) and cow manure (C) as animal source; sawdust (S) and maize stover (M) were used as plant residues. The characteristics of the applied composts are shown in Table 1. Compost doses were 35 and 75 Mg ha^-1, as recommended by ^{Castellanos et al. (2000)}, to achieve an experimental soil with medium and high organic matter (OM) content, 2 and 3% respectively, in this case based on an original content of 1.14% of OM and a sandy loam texture. The dose of urea (U) was 200 kg ha^‑1. Treatments and doses are shown in Table 2. Composts and fertilizer were applied at the time of planting blue corn. Harvest took place 130 days after planting, when the soil sampling was performed.

Table 1 Characteristics of four composts used as a source of organic fertilization. 

HS = hen manure and sawdust; CS = cow manure and sawdust; HM = hen manure and maize stover; CM = cow manure and maize stover.

Table 2 Treatments with organic and inorganic fertilization. 

HS = hen manure and sawdust; CS = cow manure and sawdust; HM = hen manure and maize stover; CM = cow manure and maize stover.

Results and Discussion

The analysis of soil macronutrients revealed significant changes in their concentrations in the different treatments (Table 3). Results of nine independent comparisons selected a priori reveal that only orthogonal contrast of all (all types of fertilizers) vs control (no fertilizer) significantly affected the concentration of six nutrients, while the HS-35 Mg ha^-1 vs HS-75 Mg ha^-1 contrast had no effect on the levels of the macronutrients.

Table 3 Mean squares and statistical significance of six macronutrients associated with application of different fertilizers (organic and inorganic). 

M = manure; V = plant residues; (*) significant at 5%.

These findings agree with those observed by other researchers. Bernal et al. (2008), for example, mentioned that bulking agents such as cereal straw and sawdust have high organic-C contents and high C/N ratios (above 50). Moreover, mature composts produced from cattle and poultry manure mixed with agricultural and forestry by-products provide beneficial effects on soil physical, chemical and biological properties as well as nutrient content. It has also been mentioned that bovine manure, being a rich source of carbon and nitrogen, increased microbial activity, and consequently led to increased availability of nutrients for the plant (^{Lincoff, 1981}).

The contrast all vs control, significantly affected all nutrients. Some contrasts (CS-35 vs CS-75; HM-35 vs HM-75), however, had significant effect on at least one of the nutrients. The concentrations of NO₃ ^- and Ca⁺⁺ were affected significantly by six of the nine contrasts, while the level of K⁺ responded only to the contrast all vs control, probably because most of the soils in temperate zones in Mexico are rich in this macronutrient, the reason that potassium fertilizers are usually not applied or the required doses are low. The interaction M*V had a significant effect on macro-nutrients NO₃ ^-, Ca⁺⁺ and Mg⁺⁺.

The coefficients of variation (CV) observed were 1.6 and 42.4% for Na⁺ and P⁼, respectively. This suggests that the concentration of the first was relatively constant. The second, on the contrary, was more affected by the application of organic and inorganic fertilizers, particularly by the composts prepared in this study.

The highest coefficient of determination (R2) was estimated for Ca⁺⁺ (0.968). At the other extreme, the macronutrient Na⁺ exhibited the lowest (0.42). For Ca⁺⁺, the percentage of the total variation (96.8%) explained by the experimental design was close to 100%, while the value of 42% for Na⁺, indicated that the error component (random causes) was proportionally higher than that, due to the effect of the treatments. The coefficients of determination of the rest of macronutrients ranged between 0.662 and 0.906. All of these values were higher than 0.5; that is, more than 50% of the variation was explained by the statistical model used in the study.

By comparing the concentrations of nutrients NO₃, P⁼ and K⁺ (Table 4), it was found that the control had the lowest means 75.4, 5 and 777.5 mg kg^-1, respectively, while the highest means, 268.2, 33.8 and 1267.5 mg kg^‑1, were recorded for treatments CS-75, HS-75 and CM-75, respectively, in all three cases with the highest dose of compost (75 Mg ha^-1).

Table 4 Means comparison and significant groups of six soil macronutrients (mg kg^-1) from organic and inorganic sources. 

^† In each column the same letters indicate the same group of significance. Tukey Test (α = 0.05).

For nutrients Ca⁺⁺ and Mg⁺⁺, the means of the control (4215 and 252 mg kg^-1) were among the top three, although they were statistically equal (Table 4), while treatments CM-75 and U-200 were, respectively, first and second with the means 4420 and 4117 mg kg‑1for the element Ca⁺⁺; 272.5 and 245.5 mg kg^-1 for the nutrient Mg⁺⁺. These results clearly demonstrate a substantial improvement in the concentration of the above mentioned nutrients, in response to the application of compost obtained by mixing manure and plant residues, as has been pointed out by several researchers (^{Fox et al., 1989}; ^{Magdoff, 1991}; ^{Blackmer, 1992}).

The comparison of means test of the control (Table 4) revealed the presence of at least two significance groups for K⁺ and Ca⁺⁺ and up to five groups for P⁼ and Mg⁺⁺ averages. As mentioned above, the control group had the lowest average for nutrients NO₃ ^-, P⁼, K⁺ and Na⁺; however, it can be seen that in five of the six nutrients, the control was statistically equal to at least one other treatment, as with the element K⁺, 777.5 and 840 mg kg^-1, for the control and inorganic fertilization, respectively. When analyzing the Na⁺ element, the presence of three groups of significance is remarkable: group a, formed by treatments T₁ to T₇ (means varied from 2.87 to 3.12 mg kg^-1), group b, formed by treatments T₈ and T₉ (1.85 and 1.77 mg kg‑1); group c, the control (T₁₀), with the lowest mean (1.26 mg kg^-1).

The responses observed in the present study agreed with other studies carried out by several researchers, who reported significant increases in the concentration of nutrients. For instance, ^{López et al. (2001)} reported values of N of 2.18 and 1.80%, when organic fertilizer made from hen manure and cow manure were applied. In a critical review, Bernal et al. (2008) discussed the important role of bulking agents such as cereal straw and wood by-products (sawdust), which have high organic-C contents and high C/N ratios, on maturity index of compost produced when they are mixed with cattle manure and poultry manure. This also supports the idea of the effect of the interaction manure × plant residues on soil mineral content. These results have also been observed in other studies, where responses to compost application were evaluated in the stages of growth and fruit development (^{Ortega et al., 2010}). Similar results have been reported by Apolinar (2006)³, who applied a mixture of sawdust-compost to a soil cultivated with tomato, demonstrating the benefits of sawdust as a bulking agent used to produce composts. In addition, this researcher reported improvement in physical-chemical properties of the soil fertilized with compost mixed with sawdust.

^{Millaleo et al. (2006)} reported increases in the concentration and availability of P⁼ when high doses of organic fertilizers made from livestock manure and crop residues were applied. The same authors observed a significant increase in crop foliage biomass and availability of several nutrients. Other authors (^{Laboski and Lamb, 2003}) observed an increase in the availability of P⁼ when livestock manure was applied because its concentration in the organic amendment can be equal to or greater than. Another factor that may explain the results reported in this research is the production of organic acids during microbial degradation of organic fertilizer applied to soil, as they can increase nutrient availability to plants (^{Gale et al., 2000}). Beneficial effects on soil-plant system properties, such as improved physical characteristics, increased nutrient contents, soil fertility, higher biological and enzymatic activity, have been observed after applying composts (Roca et al., 2008).

Average P⁼ content in hen manure-sawdust mixture composts was HS-35 (26.8 mg kg^-1) and HS‑75 (33.8 mg kg^-1); in cow manure-sawdust composts it was CS-35 (13.2 mg kg^-1) and CS-75 (17.3 mg kg^‑1). It was observed that in the first group the means were almost double the value of the second group (Table 4). Similar studies (^{Jiménez et al., 2004}), where cattle manure was used without mixing with sawdust, reported concentrations of P⁼ between 16.54 mg kg^-1 and 38.10 mg kg^-1, far below the values observed here. This suggests a positive significant effect of the interaction manure x sawdust. ^{Pérez et al. (2008)} recorded a concentration of 247.45 Mg ha^-1 P⁼ in response to compost made from mixtures of hen droppings and cow manure. These values exceed the results presented in our paper by nearly 60%.

For K⁺, averages from 777.5 mg kg^-1 (control) to 1267.5 mg k^-1 (BE-75) were reported in our work. However, eight of the 10 treatments compared were part of the group of significance, which includes the highest average (Table 4). Arrieche (2008)⁴ reported a K⁺ concentration average of 278.8 mg kg^-1, contribution achieved with composts were made with chicken manure and cachaza. The composts were applied to improve soil properties; however, these values were lower than those reported in our study.

Two groups of significance for the nutrient Ca⁺⁺ were observed, fluctuating between the treatment means of 1105 and 4420 mg kg^-1 (Table 4). Other researchers (^{Olivares et al., 2012}) have reported values close to 4800 mg kg^-1 in soils fertilized with compost made from cattle manure; this is greater than the average of the treatments compared in our study. Similarly, the results of this study agree with those reported by ^{Carmo et al. (2016)}, who observed that only chicken and quail manures, substrate and compost, had a significant effect on Ca⁺⁺ concentration. Also, Olivares et al. (2012), using cattle manure mixed with sawdust, reported an average contribution to soil of Ca⁺⁺ of 4500 mg kg^-1, similar to those of some of the treatments analyzed in our work.

The average concentrations of Mg⁺⁺ in treatments varied between 165 and 252 mg kg^-1. These results differ from ^{Olivares et al., 2012}, who reported an average of 5200 mg kg^-1 when manure-based compost and sawdust were applied. This value was higher than the best treatment of our study. ^{Cortés et al. (2008)} reported values of 604 to 797 mg kg^-1 with fertilizers based on cattle manure; these values were also higher than the results presented in Table 4. ^{Arrieche and Ramírez (1997)}, ^{Lora (1998)} and ^{Brito et al. (2004)} noted that values between 100 and 150 mg kg^-1 Mg⁺⁺ are suitable for a sandy loam soil, such as that used in this study; thus, the values obtained in this study can be considered acceptable.

The average concentration of Na⁺ varied from 1.26 to 3.12 mg kg^-1 (Table 4), values that differ from those observed in other studies (^{Olivares et al., 2012}), where a combination of manure and sawdust was applied, yielding an average of 900 mg kg^-1, much higher than the value found in our study. ^{Canet et al. (1998)} reported lower concentrations than those observed here (0.22 mg kg^-1) when they applied ovine manure. It is important to avoid high values of Na⁺ because the osmotic pressure generated in the groundwater interferes with the growth of most crops.

Conclusions

Five treatments of compost applied to soils significantly increased concentrations of NO₃ ^- compared with inorganic fertilization and the control. Compost produced from hen manure provided a more consistent effect on the concentration of NO₃ ^- and P⁼ in soil, relative to cow manure. Sawdust-based composts had a greater effect on the content of NO₃ ^-, compared with maize stover-based compost. The average concentration of Ca⁺⁺ and Mg⁺⁺ in soils with organic fertilizers was lower than those caused by inorganic fertilization. The average concentration of Na in soils with organic fertilizers was higher than that observed with inorganic fertilization.