Introduction

Soil quality is defined as the ability of this resource to function within the limits of a natural or managed ecosystem, to maintain the productivity of plants and animals, to conserve or increase water and air quality, and to promote plant and animal health. Such quality is perceived through physical, chemical and biological properties in an environment determined by the climate and other components of the ecosystem (^{Doran, 2002}). New definitions have emerged more recently that also comprise soil properties, the ability to be sustainable, produce healthy food and mitigate environmental pollution (^{Benintende et al., 2012}). According to ^{Hengl et al. (2017)} ignorance about soil management leads to inadequate practices that result in the loss of physical properties, nutrients, and in general of soil fertility.

In particular, hillside farming requires attention, especially because improper handling accelerates degradation by water erosion (^{Camas et al., 2012)}. The tropical environment is added as another factor to consider in soil management, sinc under this condition, soil fertility is reduced in the short term, because the organic matter stores (MO) are small and are recycled quickly (^{Yoneyama et al., 2015}). Therefore, the combination of this environment and hillside relief in agriculture, require good conservation practices. One of theses is the contribution of organic waste, as this conservation practice allows the return of MO that is lost by erosion, as well as the improvement of structure and internal soil drainage (^{Six et al., 2004}).

Increased soil organic matter (MOS) also reduces runoff and improves infiltration, which is a condition reflected in a decrease of bulk density (ρ _b ), it also increments the total porosity (PT) and stability of soil aggregates (EAS) (^{Jordan et al., 2010}). Improving the physical properties mentioned above is important, because they are a functional entity of the soil structure which allows among other benefits, the emergence of plants and the increase of crop yield (^{Josa et al., 2010}).

The use of organic amendments to the soil would affect the activity of soil microorganisms, which are responsible of important processes such as mineralization and microbial immobilization, through which N, P and other nutrients are released (^{Orozco et al., 2016}). According to ^{Bourg and Sposito (2011)}, the source of the cation exchange capacity (CIC) of the soil is the MOS and the clays, so its conservation or increase through contributions of organic amendments, can positively affect this property physicochemical property. The pH is another property that is modified by the contributions of MO, because the decomposition of these amendments produce organic and inorganic acids that would affect that chemical property (^{Pérez-Esteban, 2014}).

Although the beneficial effects of organic amendments use have been extensively documented, the effect of organic composting can not be generalized, since the response and durability of changes that occur depend on soil characteristics (^{Yanardag et al., 2017}), especially in the tropical environment, where the soil diversity is very wide and the soil processes are very variable. For this reason, the objective of the research was to evaluate the impact of organic fertilization on the improvement of chemical and physical fertility in a Luvisol slope soil for agricultural use in the mountainous area of the municipality of Macuspana, Tabasco.

Materials and methods

Characterization of the experimental site. The experiment was established in the ejido Melchor Ocampo second section of the municipality of Macuspana, Tabasco, located at the coordinates 17° 34’ 41.372” north latitude; 92° 27’ 16.553” west longitude (Figure 1).

Figure 1 Location of the research area in the community of Melchor Ocampo, second section of the municipality of Macuspana Tabasco, Mexico.  

The experimental plot is located on a hillside whose slope is 5-10%, at an altitude of 280 m. The predominant climate corresponds to an Am (f) w, defined as warm humid tropical, with abundant rainfall in summer and with a dry and short season in spring and summer (^{García, 1973}). Precipitation and annual average temperature is 3 186 mm and 23.6 °C respectively. The vegetation on the site are native grasslands (Cynodon plectostachyus, Paspalum virgatum) maize (Zea mays) and some trees and shrubs from short acahuales.

Based on the criteria of the WRB version 2014 (^{WRB, 2015}) the soil was classified as Chromic Luvisols (cr LV) due to the presence of an argic diagnostic horizon at a depth of -100 cm. The first horizon was at a depth of 22 cm, which recorded a CIC of 39.79 Cmol⁽⁺⁾ kg^-1 considered as a high level. P and K levels of 5.57 mg kg^-1 and 0.38 Cmol ⁽⁺⁾ kg^-1 are located at a low level. Ca and Mg cations were found in high concentrations, with values of 10.3 and 10.5 Cmol⁽⁺⁾ kg^-1 respectively.

Treatments. As a MO source a compost was applied, which showed a pH of 6.82, CE of 3.9 dS m^-1, 41.5% of MO equivalent to organic carbon content (CO) of 24.1%, nutritional contents of 0.73, 0.09, 0.64% of N, P and K respectively, and 2.14, 2.57, 1.07 Cmol⁺ kg^-1, of Ca Mg and Na respectively, the C/N ratio was 33. This compost was produced from gardening waste, which were subjected to a composting process in cells of 113 days, at which time the material reached a stable temperature of 25-45 °C (between ambient temperature and thermophilic range).

The moisture supply and aerobic turns were performed as required in the process to maintain humidity at 60% and 70 °C respectively. According to ^{Gallardo (2016)} a compost that comes from this type of materials, show low N contents, because it has a higher richness of lignocellulosic compounds, wichi is a favorable situation since it allows to maintain a C/N ratio above 16, which contributes to the better conservation of the soil, when reducing levels of fast mineralization. Compost was incorporated through a hand tillage made with a hoe and a shovel at 30 cm deep, taking care that the soil is softened to facilitate the incorporation and mixing of the soil with the organic material.

Four doses of compost (0, 20, 40, 60 Mg ha^-1) were evaluated through an experiment designed in a randomized complete block with four replications, where each experimental unit consisted of a plot of 5 * 5 m. 11 months after the addition of the compost, composite samples (from 10 subsamples) were collected at a depth of 0-30 cm.

Fertility indicators/analytical methods. The contents of MO were determined by wet oxidation (^{Walkley and Black, 1932}), pH by potentiometer method (Jackson, 1964), CE by conductmetry (^{Rhoades, 1993}), CIC and PSB by ammonium acetate extraction (^{Reeve y Sumner, 1971}), N-total by the Kjeldahl method (^{Bremmer, 1965}), phosphorus (P) by the Olsen method (^{Olsen and Sommers, 1982}), micronutrients (Cu²⁺, Fe²⁺, Mn²⁺, Zn²⁺) by atomic absorption spectrophotometry (^{Baker and Shur, 1982}). Also, physical properties such as ρ _b were determined by the double cylinder method (^{Blake and Hartage, 1986}), the ρ _r by the pycnometer method (^{Kunze and Dixon, 1986}) texture by Bouyoucos hydrometer (^{Bouyoucos, 1962} ), the hydraulic conductivity (Ks) by the constant head permeameter (^{Loveday, 1974}), mechanical resistance penetration (RMP) by cone penetrometer test (^{Dexter et al., 2007}), and size distribution of aggregates (DTA) by the dry sieving method (^{Chepil, 1953}). The total porosity was calculated by equation 1 proposed by ^{Skopp (2000)}.

Where: ρ_b= bulk density; ρ_r= actual density.

The size distribution of aggregates was determined by dry sieving method (^{Chepil, 1953}), from this the mean weight diameter (DMP) was calculated using equation 2 as ^{Eynard et al. (2004)}.

Where: DMP= weighted average diameter of dry sieved aggregates (mm); Xi= mean diameter of the fraction of each sieve (mm); Wi= total weight of the sample corresponding to each fraction size; STW= total weight of the sample.

The stability of aggregates was obtained by the Yoder method, modified by ^{Kemper and Rosenau (1986)}; through equation 3.

Where: EAH= water stable aggregates; Ma+s= mass of resistant aggregates in water plus sand (g); Ms= mass of the single sand fraction (g); mt= total mass of sieved soil (g).

Statistical processing. The information obtained was processed through analysis of variance (Andeva), correlation analysis, and Tukey’s mean comparison tests, using the statistical package SAS for Windows version 6.12

Results and discussion

Changes in physical fertility. The results indicate that the addition of the compost soil, in doses greater than 40 Mg ha^-1, caused changes in all physical properties evaluated (Table 1).

Valores con la misma letra son estadísticamente iguales con un valor de probabilidad ≤ 0.05 determinado por pruebas de comparación de medias de Tukey

Table 1 Changes in the soil physical properties due to the addition of compost. 

It is observed that ρ _b decreased when increasing the dose of compost, however, only the dose of 60 Mg ha^-1 showed statistically significant differences. The ρ _b went from 1 Mg m^-3 in the control treatment, to 0.9 Mg m^-3 when adding 60 Mg ha^-1 compost. According to ^{Paredes et al. (2010)}, the ρ _b is a physical property that requires adding more than 40 Mg ha^-1 compost in order to be able to be modified. However this decrease brings other benefits such as reduced compaction and increases the macroporosity, the aggregate size, PT, K_s and air permeability. ^{Verhulst et al. (2010)} indicate that organic fertilization to the soil increases the size of the aggregates, although the size and stability of the aggregates depends on the amount and stability of the organic sources used.

For example, the higher the content of labile soluble organic carbon (SOC), the aggregate’s size is greater (^{Lutzow et al., 2007}), while smaller aggregates may be associated with the most humified fraction with periods of residence in the soil greater than seven years. Therefore microaggregates are considered as carbon storages (C) more stable on the soil (^{Six et al., 2004}). The results of this research (Table 5) indicate that compound dose increases produced smaller and less stable aggregates. The analysis of aggregate size distribution indicates that the diameter of the aggregates in the treatment of 60 Mg ha^-1 was 5.76 mm, being smaller than those recorded in the control soil (0 Mg ha^-1) which was 6.97 mm.

This behavior in the aggregate size may be a reflection of the fact that the compost is a more labile source of MO, with respect to the more stable MO that is found naturally in the control soil as described by ^{Gallardo (2016)}. The conceptual model proposed by ^{Six et al. (2004)} indicates that the fresh MO supplied to the soil through an organic amendment, joins the macroaggregates and constitutes the thick intra-aggregate particulate organic matter (MOP_i) which degrade into into thin MOP_i fragments within the aggregates, this MOP_i is the nuclei of new microaggregates physically protected from decomposition, so that the C content and microbial activity is reduced and the production of the binding agents decreases (^{Six et al., 2004}).

The reduction in microbial activity leads to destabilization and potential disaggregation of macroaggregates; after disaggregation, microaggregates, mineral fraction and MOP are released. Subsequently, these fractions can be reincorporated into new macroaggregates when fresh organic wastes are added. Some macroaggregates may follow the same sequence under conventional tillage, however most of them are disturbed and its life cycle is shortened by a faster MO cycling, causing a smaller proportion of macroaggregates enriched with MOP_i compared to those that can be formed under conservation tillage (^{Six et al., 2004}).

It was observed that the change in the stability degree and aggregate size, had no effect on the hydraulic conductivity (K_s). As shown in Table 1, by reducing the DMP of the aggregates the K_s increases, which is demonstrated by the correlation level recorded between the two variables (R= -0.58; p≤ 0.02). According to ^{Ben-Hur et al. (2009)}, the increase in the number and size of soil aggregates influences the water flows within the profile, since if the number of smaller aggregates (<1 mm) increases, K_s is considerably reduced. The results shown in Table 1 reinforce the above, since higher K_s were observed in treatments with higher doses of compost, where the aggregates were smaller. Similar to the results of this research were reported by ^{Slawinski et al. (2011)}, who indicated that the K_s was greater in soils where <0.25 mm aggregates dominated, compared to soils with larger aggregates because the aggregates fraction <0.25 mm and 0.25-0.5 mm allow the transport of water to occur among aggregates, whereas in fractions greater than those sizes, transport is carried out within the aggregates.

The total porosity (PT) was another physical variable that changed due to the increase in the dose of compost, but only in the treatment of 60 Mg ha^-1 statistical difference was found (Table 1). According to ^{Curaqueo et al. (2010)}, the soil structure involves the shape, degree and size of the aggregates, consequently this property regulates the porosity, and therefore, the retention and availability of water, in addition to its capacity to contain air, as well as the growth of crop roots. The water retention capacity in the soil depends on the number of pores, the pore size distribution and specific surface area of each floor (^{Malamoud et al., 2009}). Therefore, the MOS generally has a positive effect on the ability to retain water (^{Thierfelder and Wall, 2009}), although the synergistic effect of MOS on these and other properties is not entirely clear (^{Malamoud et al., 2009}).

It was observed that the dose of 60 Mg ha^-1 of compost significantly reduced penetration resistance (RP), it was particularly more observable at -10 cm depth, while the depth of 20-30 cm, only with the dose of 40 Mg ha^-1 of compost a compaction reduction was observed (Figure 2) was observed. This effect could be explained since the MOS increases the interaggregated macroporosity and consequently allows the rearrangement of the aggregates when the soil is penetrated by the measuring instrument.

Figure 2 Levels of mechanical resistance to penetration at three depths in a chromic Luvisol treated with different doses of compost.  

Soil compaction occurs due to a reduction of pore space caused by a load applied to the soil surface, this condition affects soil properties that are directly associated with the development of plants and agricultural work, particularly the ρ _b (^{Dexter et al., 2007}). The RP depends on the resistance to soil deformation, compressibility and soil-metal friction, C/N ratio, soil group, as well as environmental conditions of humidity and temperature, and can be inferred indirectly through easy to measure properties as ρ _b , water content, MOS and quantity of cementitious agents (Dexter et al., 2007). According to ^{Chen et al. (2012)} the penetration resistance increases with decreasing soil water, and decreases with an increase in ρ _b . Some practices that may reduce RP are depth tillage as well as incorporation of MO. Therefore the contribution of organic fertilizers has favorable effects on this property (^{Reichert et al., 2009)}.

Changes in chemical fertility

MO increased when levels of compost increased, however, only with doses higher than 40 Mg ha^-1 statistically significant differences (Table 2) were observed.

Valores con la misma letra son estadísticamente iguales con un valor de probabilidad ≤ 0.05 determinado por pruebas de comparación de media de Tukey.

Table 2 Changes in the chemical properties of the soil due to the addition of compost. 

The MO in the control treatment is 6.92% of MO considered as high. From that level, all the treatments that received the organic amendments, increased their MO contents. A calculation based on the ρ _b and MO content of each treatment at a depth of 30 cm, resulted in an amount of 206, 225, 303 and 295 Mg of MO ha^-1 for inputs of 0, 20, 40 and 60 Mg ha^-1 of compost respectively. That is, for the case of treatment of 20 Mg ha^-1 of compost, MO gain in the soil was 19 Mg of MO ha^-1 compared to the initial content, the content of MO is almost equal to the amount provided, so that this dose can be considered as maintenance to recover the contents that are transferred within the MO cycle.

However, the additions of 40 and 60 Mg ha^-1 of compost recorded gains of 97 and 89 Mg of MO ha^-1 equivalent to 47 and 43% compared to the initial content of MO. According to ^{Liu et al. (2010)}, the increase of MO in soils amended with organic fertilizers, is due to an increase in the microbial biomass, in addition to an increase in the biomass productivity and therefore there is a greater amount of residues particularly of roots that are reused in the soil, as well as exudates that add to the reserves of edaphic MO, while considering that a fraction of the MO from the organic fertilizer supplied does not mineralize and therefore it gets accumulated in the soil.

A close correlation was observed between MO with physical properties, as well as with nutrient contents, CIC and pH. Regarding to the latter, ^{Pérez-Esteban et al. (2014)}, indicated that the formation of low molecular weight organic acids, during the humification process of MO, causes a decrease in pH; however, if during humification there is an abundance of functional groups OH^- phenolics and OH^- alcoholics which are a source ofnegatively charged sites at pH> 7, the pH of the soil tends to remain or increase after the addition of an organic fertilizer (^{Pérez-Esteban et al., 2014}). In this research, the addition of the compost had no effect on the pH variable. It is probable that this lack of change has been due to the buffer effect of MO on drastic soil changes.

^{Weaver et al. (2004)} estimated the buffer grade of a soil against pH changes, it was determined that in soils with low CO level, the variations of this CO would generate large changes in buffer capacity. However, in soils with a high content of CO, its variations only generates marginal changes of the buffer capacity. This confirms that due to the richness of MO that the soil already had, an increase due to the contribution of the compost, could only cause a rapid and temporary change over the pH variable. As can be seen, the contribution of the compost increased CE in ranges between 1.24-1.43 dS m^-1. This effect is due to the fact that during the mineralization of the MO there are active groups that can modify the medium.

CE level of 3.9 dS m^-1 found in the compost represents the main source of salinity, as this level corresponds to the maximum permissible limit in an organic fertilizer. Therefore, with this level of CE, it is important to monitor the effect of the compost on the soil. The CIC is a variable that is sensitive to the contributions of MO because the main source of this property is the specific surface of the MOS and the clays. However, in this research only the dose of 60 Mg ha^-1 showed statistically significant differences (Table 2). This suggests that in soils with high MO and clay contents as the one involved in this study, changes in CIC caused by composting are not expressed drastically or prolonged.

It was observed that with high doses of compost is posible to improve the soil’s PSB, bringing it from a ~ 55% level of low doses (0, 20 Mg ha^-1), to ~73% of the high dose (40 60 Mg ha^-1), which can be considered as beneficial, since the percentage of base saturation (PSB) provides useful information about acidity, nutrient availability and soil fertility in general, plus it allows to determine the capacity of the soil to act as a buffer against acid accumulation and mineral leaching potential. The PSB represents the percentage of the exchange sites in the soil, which are occupied by the basic ions Ca, Mg, Na and K.

The difference between that number and 100 is the percentage of exchange sites occupied by H and Al acid cations. In most situations, relatively high base saturation (> 60%) is desirable. As the nutrient content of the soil after the organic fertilizing, no statistical differences in the case of P were observed, although a tendency to increase as the dose of compost was increased was observed, particularly from 40mg ha^-1. It was possible to indicate that the contributions of the compost generate a tendency to improve the contents of K; allowing carry a low content of 0.38 kg Cmol⁺ kg^-1 up to 0.41 kg Cmol⁺ kg^-1 at the end of the experiment. A similar situation was observed in the case of Ca and Mg (Table 3).

Valores con la misma letra son estadísticamente iguales con un valor de probabilidad ≤ 0.05 determinado por pruebas de comparación de media de Tukey.

Table 3.  Changes in nutrient contents of the soil due to the addition of compost 

Higher levels of these elements were observed, at high doses (40 and 60 Mg ha^-1) compared to low doses (0 and 20 Mg ha^-1). The lowest value was recorded in treatment of 20 Mg ha^-1 In the case of Fe and Zn microelements, it was observed that increasing the dose of compost increased the concentration of these elements. As for Cu and Mn, no statistically significant differences were observed.

Conclusions

Contributions over 40 Mg ha^-1 of compost to a slope chromic Luvisol soil caused a decrease in the size (DMP) and wet aggregate stability (EAH), which is interpreted as the formation and destruction of macroaggregates corresponding to the intermediate stage of a rapid process of aggregation-disaggregation-aggregation. Those macroaggregates tended to stabilize as the newly introduced fresh MO is depleted. Changes in soil structure caused an increase in hydraulic conductivity (K_s) and a reduction in the penetration resistance (compaction), particularly in the surface layer (-10 cm). MO, CIC and PSB increases were observed in soils treated with more than 40 Mg ha^-1. CE of 3.9 dS m^-1 of compost caused a slight increase in soil salinity. It was observed that the contribution of compost improved the contents of K, Ca, Mg, Fe and Zn, while in P, Cu and Mn they were very light. No statistically significant differences were observed in pH.