INTRODUCTION

The ground component, is one of the most important and determinant resources in the agricultural activities. It is the base for the development in the communities; therefore, it provides and disposes food and resources to their use. In the department of Nariño, during the last years, its areas dedicate to these kind of activities, has found degradation and decreases in their productive capacity (approx. 2000 hc/year). It has seen in the lower productive capacity to the harvest, Ambiental conflicts and erosive processes due to the inadequate management to the resource and the low technological in the territory (^{Burbano, 2016}).

Nariño's ground have been affected by the implementation and management of monocultures by asking more quantity of agrochemist , taking into account this problematic the attention has been focused in the searching of alternatives in the sustainable production, to reduce some impacts such as the evidence in the application of agroforestal science that promote to the improve and maintenance to the grounds'characteristics, however the knowledge to these dynamics and interactions to the high andean region, it is low and had not allowed to generate some strategies according to the needs and local terms, so a low level to acceptability and acceptance. (^{Alvarado et al., 2011}; ^{Urbano et al., 2019}).

In this way, the combination of trees with crops between their stripes or alleys is constituted as a practice of great potential where the perennial woody multipurpose being nitrogen fixators, allow to incorporate this element naturally into the soil and crops (^{Morales, 2018}; ^{Cherubin et al., 2019}). The wax laurel (Morella pusbecens (Humb. & Bonpl.) ex Willd.) Wilbur), a tree species, with a high adaptability in the area, tolerance to external factors, which presents at its roots the actinomycete of the genus Frankia, responsible for the capture of nitrogen (^{Molina et al., 2000}). In associate with the yacon species (Smallanthus sonchifolius (Poepp.) H.Rob), presents great regrowth capacity, low demand in agronomic management and fertilizers, resistant to competition with invasive weeds (^{Nieto et al., 2004}).

^{Rojas and Parra (2018)} mentioned the importance of the implementation of native trees, through their goods and services contribute to the farmer and his family, serve as windbreaker barriers to crops such as S. sonchifolius, which has a fragile stem and is susceptible to overturning. The search for alternatives of primary and secondary transformation of this valuable species, of plant genetic resources, and that in the future can be part of a process of agro-industrial productive conversion for agroecosystems of high Andean mountain.

^{Sáenz (2019)} argues that in the Latin American context, there is a promising future for Andean highland plants such as S. sonchifolius, with immediate and medium-term possibilities for commercialization in several markets. Being a viable economic alternative for producers, incorporating it into its production systems, however, the country does not have a large-scale production of this tuber, and its low knowledge in the market has been a consequence of the poor research and development of it (^{Calderón et al., 2017}).

According to the above, it is important to study the physical properties of the soil, because they largely determine the capacity of many of the uses that man makes; to understand the extent to which and how they influence plant growth (^{Morales, 2018}).

Therefore, the present research evaluated some physical soil properties such as apparent density, actual density, total porosity, aggregate distribution and structural stability in an agroforestry system of M. pubescens in associated with S. sonchifolius, in the Experimental Botana Farm, before the establishment and at the end of a production cycle of S. sonchifolius, according to the established planting distances.

METHODS AND MATERIALS

Localization. This research was done in the experimental of the University of Nariño, located in the path Botana, municipality of Pasto, the study site locates at coordinates 77º 18’ 58’’O y 1º 10’ 11.4’’N, at 2820 msnm of altitude, average temperature of 12 ºC, annual rainfall of 800 to 1000 mm, relative humidity 70 to 80 % with 900 hours sun approximately (^{IDEAM, 2010}). According to ^{IGAC (2004)}. It belongs to the life zone, low wet forest montano (hb- BM); according to the taxonomic group, this soil is classified as Acrudoxic Fulvudands with a rolling topography.

A minimum farm work was done to a natural cropped, to the component agricultural sowing and to the forest tree component harvest it was used to dig to holes in the experimental plot two samples, the first one in phase 1 (F1: before the crops implementation) and the second one in phase 2 (F2: harvest to S. sanchifolius) the results were implemented in the physics lab of soils of the University of Nariño. Where an evaluation about certain physical properties such as bulk density (Ba), real density (Dr) total porosity (p), structural stability (EE), and aggregates distribution (GAD).

Experimental Design. To evaluate the effects in distances of harvest of S. sanchifolius, a silviculture system was established, using a complete block using a design with three treatments and three repetitions. (3x3). The treatments corresponded to the density to the crops, the first treatment (T1) S. sonchifolius the first treatment, was cropped to 1 meter between plants, treatment two (T2) S. sonchifolius 0.80m between plants, and the treatment 3 (T3) S. sonchifolius cropped to 0.50m between plants and the absolute control (a lot without any ban) the forest tree component was established two months before the crops of S. sanchifolius 4 x 4 between sledding and 9 meters between lines.

Materials. The plants of S. sanchifolius was acquired for articulated productors to the “Red de Guardianes de las Semillas” in the department of Nariño. The seedlings of M. pubesncses were acquired in some nurse rings to the zone, with an initial high average of 15cm.

Soil sampling. According to the ^{Chaves and Delgado methodology (2015)}, for physical soil analysis, random zigzag sites were located in the sampling area, where the organic surface layer of the soil was removed. With the help of a shovel, the six subsamples were extracted for each treatment at 20cm each, homogenized in a plastic container free of impurities and then a representative sample of 1kg was taken which was placed in a plastic bag with airtight closure and subsequently the samples were labeled for each treatment (Table 1).

Statistics Analysis. A t-test was performed to make comparisons between f1 (before crop planting) and f2 (S. sonchifolius harvest). To assess the statistical differences between treatments in f2, a variance analysis and Tukey test were performed to separate the averages from the treatments. Statistical analyses were carried out with the Infostat Statistical Package program (^{Di Rienzo et al., 2010}) and the Excel program.

DISCUSSION AND RESULTS

Bulk density, actual density and total porosity before and after the establishment of the farming arrangement in alleys of M. pubescens and S. sonchifolius. The results of the t test (Table 2) indicated that there were no significant statistical differences for the variables Apparent Density (Da), Actual Density (Dr) and Total Porosity (%P) between the witness (F1: prior to the establishment of the agroforestry system) and treatments (f2: Sowing distances of S. sonchifolius).

These results are consistent with those found by ^{Gaviria and Herrera (2013)}, in the study of the physical properties of the soil in an agroforestry arrangement laurel wax (M. pubescens), established at 4 x 4m, with chocho (Lupinus mutabilis) in the municipality of Pasto, in three different agroforestry arrangements; (fix 1: M. pubescens, fix 2: M. pubescens x L. mutabilis 0.15 x 0.20m and arrangement 3: M. pubescens x L. mutabilis 0.30 x 0.60m); in which they did not show significant differences for the same variables evaluated.

^{Ramírez and Vélez (2002)}, ^{Zuñiga et al. (2011)} mention that the physical properties of the soil are not affected, because the changes depend to a large extent on the origin of the soil, its evolution and the contents of organic matter, waiting for changes in these properties in a longer evaluation period than that contemplated in this research, since the actual and apparent density and total porosity, are very stable soil properties, which in a short period of time of use are not modified.

Similarly, ^{Novillo et al. (2017)} evaluated five land use systems, analyzed the variables Dr and %P, which did not present significant statistical differences between coverages and soil depth, also, apparent density tends to increase depending on the type of use, so it can be inferred that the properties change according to soil use (^{Morales, 2018}).

However, total porosity showed some tendency to decline (60.41 to 59.09%), although the data were not significant, related to the slight increase in Da between f1 and f2 (0.92 to 0.99Mg m^-3). Nariño soils are characterized by densities less than 1Mg m^-3 (^{IGAC, 1986}). The Da over a long period of time (45 years) increases with the type of land use and depth, reorganizing the removed particles and occupying the space corresponding to water and air; which differs from the soils of the Botana farm, with a tendency to compact, which affects the moisture retention capacity and limits the root development of the plants (^{Volverás et al., 2016}).

Moreover, ^{Leyva et al. (2018)}, report lower values of apparent soil density in agroforestry systems with respect to grasslands, stating that the tree component improves this property, confirming that land use, vegetation and soil type are factors that are interrelated and determine the physical and chemical properties of soils (^{Gaspar and Navas, 2013}).

Structural stability before and after to the establishment to the alley cropping. The obtained results showed that the soil suffered a change in the structural stability, the higher in F II with 2.52 W.M.D. in relation with F1 with 0.97 W.P.D. (Figure 1).

Figure 1. Structural stability before and after to the establishment in the alley cropping. 

According to the classification ^{IGAC (2006)} (Table 3), the structural stability of the soil in this study ranges from slightly stable to moderately stable. This is due to the resistance to deterioration and erosion offered by the intensity, type of use and handling of soil machining (^{Buenaver and Rodríguez, 2016}).

For this research the use of plant material of S. sonchifolius was decisive in the arrangement of the solid phase of the soil, since the distribution of its aggregates and porous space makes possible the preservation of this property; likewise, organic matter and the coverage provided, decreased the agroclimatic impact on the system, avoiding the structural damage of the soil by direct radiation from the sun and the effect of rain on an uncovered soil (^{Lal and Shukla, 2004}; ^{Molina and Moreno, 2017}).

These results are consistent with ^{Gaviria and Herrera (2013)}, who found significant differences for wet aggregates, in an agroforestry system of M. pubescens associated with L. mutabilis in the municipality of Pasto, which shows that there is greater protection of the soil against raindrops. A minimum state of aggregation is presented in plots at full exposure, with minimal tillage and crop rotation, by possessing finer aggregates that absorb more energy from raindrops, while thicker aggregates offer greater resistance to separability. Decreasing the values of apparent density and resistance to penetration after soil machining (^{Méndez et al., 2016}).

Distribution of aggregates (granulometry) before and after the establishment of the farming arrangement in alleys of M. pubescens and S. sonchifolius. The t-test found significant statistical differences between f1, with 60% of aggregates (>0.5mm) and f2 with 70.3% aggregates (>0.5mm), according to the rating IGAC (2006), the data obtained correspond to an average aggregation state, as shown in Table 4.

^{Whalen and Chang (2002)}, claim the increase in soil organic matter is the cause of the higher proportion of the soil being concentrated in macroaggregates; promoting soil stability and aggregation; Likewise, ^{Martínez et al. (2008)} and ^{Gaviria and Herrera (2013)} refer to average size aggregates <0.5mm, which can block thicker pores, making it difficult to penetrate air into the ground; this is also due to the organic carbon of the soil, which has a positive correlation with the size of the aggregates.

The distribution of aggregates in f II was higher (Figure 2), which may have been improved by the penetration of the roots of S. sonchifolius; although the system has been in place for a very short time, it can be inferred that the successive decomposition of microbial biomass, possibly attracted by the exuded produced by the agricultural component, improves the distribution of soil aggregates, confirming what is reported by ^{Zuñiga et al. (2011)} and ^{Volverás et al. (2016)} who claim that increasing the proportion of macro over microplates increases soil quality.

Figure 2. Distribution to the aggregated DAG (gronunoletre) before and after to the establishment from alley crops. 

Physical comparation to the soil between the treatments to the phase II using soil in alley crops. According to the analysis to variance (Table 5), there is not any significant statistical differences between treatments.

This is consistent with the information reported by Ramírez and Vélez (2002), in the evaluation of an agroforestry arrangement of M. pubescens with Solanum tuberosum L. and Allium sativum L., a tendency was found to improve the physical properties of the soil over time; by the effect of the roots can improve the aggregation of the soil in the long term, by effects of root penetration, modification of the water regime, production of exudates, entanglement of aggregates caused by the roots and decomposition of dead roots (^{Six et al., 2004}; ^{Bronick and Lal, 2005}; ^{Molina and Moreno, 2017}).

Aggregate distribution and structural stability showed a tendency to improve between f2 treatments. ^{Benavides et al. (2015)}, found significant differences for three types of use T1 (Kikuyo prairie (Pennisetum clandestinum Hochst. ex Chiov) and grama dulce (Paspalum distichu L.)), T2 (trees dispersed within the pastures, quillotocto (Tecoma stans Griseb.), arrayan (Myrcianthes leucoxyla (Ortega) McVaugh) and pichuelo (Senna pistacifolia (Kunth) H.S. Irwin & Barneby)) and T3 (the encino forest (Weinmannia tomentosa L. f.), campanillo (Delostoma integrifolium D. Don)) and cucharo (Myrsine guianensis (Aubl.) Kuntze)), for the properties apparent density (Da), actual density (Dr), total porosity (Pt), aggregate distribution (DMP), being able to identify that these properties change according to the different uses of the soil over time.

CONCLUSIONS

The physical properties of the soil at the stage of establishment of the farming arrangement in alleys M. pubescens with S. sonchifolius, presented minimal differences for the variables: structural stability and distribution of aggregates, which will allow in the long term to improve the structure of the soil by incorporating the plant material of M. pubescens, minimal tillage and the addition of organic matter.

There were no differences in soil properties by the effect of planting distances in phase II (f2), however, the values indicate that these properties tend to improve over time.