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INTRODUCTION
For the sustainable production of cane sugar must be considered its impact from the environmental, economic and social point of view, as fundamental pillars of sustainability. The compaction of soils dedicated for sugarcane plantations is one of the causes of the fall in agricultural yields, having a negative effect on the growth and development of the new sprout, being one of the main problems to solve in actual sugar cane agriculture (R. Prado et al., 2018; Colombi y Keller, 2019).
Soils with high moisture content to respect of plastic limit are more susceptible to compaction than dry soils in presence of external loads, normally generated by the pressures on the soil by means of tillage, the wheels of harvest machinery and transport. In addition, their properties such as clay content, porosity and bulk density also increase the compaction processes. As the good integration of the technologies involved in mechanized harvesting is achieved, the harmful effects on the soil can be minimized (González et al., 2013; Matos et al., 2014; Aguilera Esteban et al., 2019).
Technological systems have been introduced in Cuba that include innovative harvesting machines, seeders and transport with auto-tipper. With the aim of reducing the damage of sugarcane fields during mechanized harvesting, the transportation system called split travel or sugar cane transhipment was introduced by means of self-tipping trailers or semi-trailers pulled by medium and high power tractor within the field and trucks with trailers to pull towards the industry tipper. This technology is intended to reduce the costs of the harvest and improve the quality of the row material for the industry. In the same way, subsoiling and deep cultivation of the soils are used as part of the agrotechnical attentions (Morejón et al., 2016; Martínez et al., 2020).
Beside, farmers have developed methods to prevent damaging soil disturbance due to excessive cultivation and to rebuild soils that have already been severely damaged (Colombi y Keller, 2019; Emmet-Booth et al., 2020). The decrease in the agricultural yield of sugarcane fields has been linked to the reduction in soil fertility and modification of its physical and mechanical properties. These modifications have special emphasis on compacted soils, which includes a drastic reduction in water infiltration capacity, drainage and porosity as well as an increase in apparent density. On the other hand, soil compaction is mainly caused by mechanized operations such as harvesting, transport and agrotechnical treatments where the weight of the machinery is transformed into greater densification of the soils, damage of stumps, sprouts and geometry of the furrows (Ahmed Chacón-Iznaga, 2019; Emmet-Booth et al., 2020). The objective of the present work is to determine the main effects on the soil and stumps of sugar cane caused by mechanized activities in the sugarcane plantation.
MATERIALS AND METHODS
The study took place in field number 2 of 17,5 ha of the UBPC ¨Máximo Gómez¨, located at coordinates 22,4827° North and 79,8070° West of the municipality of Camajuaní, Villa Clara (Figure 1). Which has an area of 203 ha dedicated of sugarcane plantation of Cuba 86-12 (C86-12) variety, wide on the island and favourable adaptation to areas with water stress, production of sprout as well as adaptable to poorly drained soils (González R.M. et al., 2011). The predominant soil in the field is brown with carbonates; with low stony index and undulating geography. Measurements were made 26 days after harvest, for which a KTP-2M harvester was used. As agrotechnical attention, the cultivation was carried out 16 days after harvest using the M-160 implement forming aggregates with MTZ-80 tractor. As part of the study, the determination of the microrelief basic on furrow geometry, cone index, soil moisture content and damage on the stumps was carried out, for which a total of 20 experimental points distributed diagonally in the field at 30 m of distance and faraway of the guardrail.
For the measurements of the field microrelief, the profilometer of metal rods of 2 m long equipped with depth markers, 26 cm of separation between rods and height of 50 cm was used in order to determine the depth in furrow and street (Cruz Díaz et al., 2015).
The moisture content measurements were taken in the midpoint of the furrow, and near to the root zone in the ridge. At each point, four measurement were taken at depths of 0-10, 10-20, 20-30 and 30-40 cm (Figure 2).
Soil moisture was determined based on dry soil (% hbss) using the gravimetric method, which is based on the lost weight of a wet sample subjected at 105° C to the oven for 8 hours (Figure 3), The final weight of the sample was obtained using analytical balance under precision of 0,0001 g. The data obtained by the gravimetric method were processed using the software: StatGraphics ver. 5,1 for a confidence level of 95%. The following expression was used for calculation:
The penetration soil resistance was determined by the cone index, known as IC index, measurements were made in the midpoint of the furrow, and next to the root zone in the ridge. For each point, four measurements were made at depths of 0, 10, 20, and 30 cm, three replicates of each measurement were made. The direct reading penetrometer with conical tip of 1,3 cm2 area were used, the penetration force was determined using the following expression:
The penetration stress (F) was determined by the following expression:
For mechanical effects on the sugarcane stumps, a total of 10 samples were selected in the field in correspondence with the selected experimental points. Pits arranged in a longitudinal orientation to the furrows with depth between of 50 cm were excavated, allowing the visualization of the root structure. The quantity of sprout was determined in the selected stumps and macroscopic examination was carried out, using digital images in order to determine the structural damage and vitality of the root system. The damaged stumps were sectioned in order to evaluate their internal development.
RESULTS AND DISCUSSION
Results of soil compaction
Resistance to soil penetration showed an increase associated with the depth taken in the root zone of the stumps and in the street (Figure 4). However, this increase does not manifest in a similar way in both cases and for the different depths. In the root zone of the sprouts, the IC penetration resistance was 1,2 kPa on the surface and had a linear increase up to 20 cm depth, the increase being less between 20 and 30 cm until reaching the 3,5 kPa. This resistance behavior is justified by the lateral pressures, generated by the passage of the tires to which the ridge is subjected, compacting the soil in the root zone. These pressures are lower as the depth increases, which leads to less resistance of the soil just below the root zone being part of the results obtained with the modelling the effects of tires described by González et al. (2013). On the other hand, the resistance in the street was lower, as a consequence of the cultivation operation, the surface layer of the soil is separated, being divided by the mechanical action of the implement, the subsequent effect of water and heat, among other factors. This fact justifies the value of 0,8 kPa on surface, shows a slight increase to the depth of 10 cm. However, between 20 and 30 cm in depth the resistance increases in a linear way, showing a tensions accumulation, until reaching 3,2 kPa.
Variation in soil moisture
Soil moisture, taken on the ridge and the street, shows great variability with respect to depth as well as a difference in behaviour for both cases (Figure 5).
The moisture of soil in the ridge was higher than the moisture in street in the surface of the furrow, this behaviour is sustained up to 28 cm deep, where the moisture in the street exceeds the moisture in the ridge. The moisture on the ridge close to stump is favoured by the accumulation of dry leaves and crop residues, which avoids the direct incidence of solar rays and constitutes one of the methods for the conservation and protection of soil according to Colombi y Keller (2019). A significant decrease in humidity is also shown after 25 cm of depth, reaching values of 31,2%. On the contrary, the exposure to the air and the sun from the street after mechanized cultivation, as well as the fragmentation of the aggregates, favours the reduction of surface humidity up to 15 cm deep, the water passes to lower strata increasing the humidity in a non-linear way up to 35 cm.
Soil profile evaluation
As a result of profile measurements, the characteristic curves obtained in field were digitalized. Figure 6 shows one of the measurements made where micro-relief of the furrow and the ridge is shown. The profile obtained shows an irregular distribution, reaching values of 17,1 cm in maximum height and minimum of 4,9 cm in the median with respect to the bottom of the street, which is represented as a zero line. The irregularities of the profile on the street are result of the cultivation activity in this section the lifting of clods predominates, causing appreciable differences in depth. The furrow profile analysis showed that there is considerable variability in the geometry and height of the ridges in the average profile of the field. Figure 7 shows the graphic processing of the profile results in one of the sampling points, showing that in all cases the height values of the ridge are less than 25 cm for an average of 18 cm. Such conditions are linked to deficiencies for root development of the stumps due to insufficient nutrients, as well as allowing erosion by rain and wind in the area of the sprouts (Y. Betancourt Rodríguez, 2019; Awe et al., 2020). Similarly, the average height of all the profile measurements made did not exceed 25 cm, maintaining the irregular behaviour, both in the furrow and in the ridge.
Effects on sugar cane stumps
In the studied samples, a maximum depth of the main roots of 45 cm was obtained, although only up to 30 cm was sufficient vitality and quantity (Figure 8a). The presence of stumps with low foliar development were also identified, representing 8,2% of the study area, which showed severe limitations of the root zone (Figure 8b). On the other hand, the secondary or capillary roots (Figure 9a) show a low development in the damaged stumps, which mean a deficient suction process of water and nutrients from the soil. These stumps are characterized by only having between one and three sprouts, while the stumps in better conditions have between six and twelve sprouts and a better development of the root zone is observe.
Finally, the stumps with severe limitations in their development showed insufficient root volume, caused mainly by the detachment from the soil, because of the direct contact with mechanized tools in cultivation activities or lateral pressures during harvest. Figure 9b shows a sample of the collected stumps, where the effects of internal dry up is appreciated, this process ending with the eventual death of the stump.
CONCLUSIONS
Penetration soil resistance, showed values of 0,8 and 1,2 kPa on the surface of the street and the area of the stumps respectively, in both cases the IC increases with the depth of the soil up to 30 cm, reaching values of 3,2 and 3,5 kPa. Resistance in the street was lower than the ridge as a result of mechanized cultivation.
Soil moisture in the ridge of the furrow was higher than humidity in the street up to 25 cm deep, with values of 28,6 and 30% respectively on the surface, favored by the accumulation of crop residues.
The analysis of soil profile of the furrows showed a considerable variability in geometry and height of the ridges, which do not exceed 25 cm and irregularities in the furrow street related to soil cultivation.
The radical system of the stumps showed a maximum depth of the main roots of 45 cm, with lack of vitality and their quantity in the damaged stumps, while the secondary or capillary roots show insufficient development.
