Soil compaction in areas of maize used for silage with the application of wastewater 1

The mechanical resistance of the soil to penetration (RP) was evaluated over two consecutive years in areas where off-season maize is produced for silage. Removal of the aerial part of the maize during the ensiling process leaves the soil exposed and subject to compaction, which may compromise a no-till system (NTS). The aim of this study therefore, was to monitor the RP of a soil under plant cover associated with the application of wastewater from pig farming (WPF) in areas of maize ensiling under NTS. The study was conducted in Matelândia, in the State of Paraná, employing seven management systems: soybean/silage/oats and turnip (SSOT); soybean/oats and turnip (SOT); soybean/silage/scarification + oats and turnip (SSSOT); soybean/silage and brachiaria (SSB); soybean/silage/oats (SSO); soybean and silage (SS) and soybean and maize (SM), repeated over two consecutive years. In the treatments that included WPF, 100 m3 ha-1 yr-1 were applied, split into two equal doses: one on the maize and the other on the cover crops. The experimental design was of randomised complete blocks (RCB), with four replications per treatment, and four samples per replication. The system with no cover crops had a higher RP; the 0.10 to 0.15 m layer of soil had the highest RP; whereas the application of WPF at a dose of 100 m3 ha-1 yr-1 decreased the RP.


INTRODUCTION
The use of silage is an efficient solution for periods of low fodder production, providing good-quality bulk, which is widely used in ruminant diets.Maize is one of the best plants for silage, due to its good dry-matter production per hectare and its high nutritional value (POSSENTI et al., 2005).Systems such as this, which includes intensive farming, require the use of forages with high productivity and good-quality biomass, as they employ intense machine traffic, which can cause soil compaction (CARRARA et al., 2007).In addition, inappropriate agricultural practices can adversely affect the quality of the soil, contributing to erosion and degradation (LÓPEZ-GARRIDO et al., 2014).
Compaction negatively alters several of the soil properties, such as root penetration capacity and water and nutrient availability to the plants, restricting the rate of photosynthesis and shoot growth, and consequently, crop yield (DRESCHER et al., 2012), especially in areas used for silage production.According to the same authors, by using a penetrometer it is possible to identify layers of higher RP and thereby infer the presence or lack of soil compaction, the degree of compaction and the depth of the compacted layer in the soil profile.With this information, the most suitable technology for decompaction can be chosen.
RP values critical to crop development vary according to the plant species and soil characteristics, such as for example, granulometric composition and structure.An RP value of 2.0 MPa has been widely quoted by several authors as critical for the growth of such plants as soybean and maize under different management systems (BOTTA et al., 2006;FUENTES-LLANILLO;GUIMARÃES;TAVARES FILHO, 2013;MAZURANA et al., 2013).Tormena et al. (2007) used 3.5 MPa as the maximum RP value in soil under a no-till system, based on the argument of the presence of continuous and effective biopores that result from this management system; whereas Chen et al. (2005) used 1.8 MPa as the agronomic limit for plant root development.
To set up the NTS, it is important to establish crops that produce plant mass in adequate quantities to provide ground cover.This becomes a problem in warmer regions with water restrictions due to the difficulty of establishing such crops and their accelerated process of decomposition (CRUZ et al., 2009).
The agricultural use of WPF has already been studied and documented by several authors (CERETTA et al., 2010;MENEGHETTI et al., 2012;PRIOR et al., 2009).For the most part, results indicate that this waste allows the cycling of nutrients and organic matter (OM).
Based on the above, the aim of this study was to evaluate the mechanical resistance of the soil to penetration, coupled with cultivating cover crops using wastewater from pig farming, in areas under a no-till system, used for whole maize plant silage.

MATERIAL AND METHODS
The study was conducted on 50 ha of agricultural land in Matelândia, in western Paraná, at latitude 25º20' S and longitude 53º59' W, an average altitude of 360 m and a slope of 6.8%.The soil is a Eutrophic Red Latosol (EMBRAPA, 2013), with 585 g kg -1 clay, 175 g kg -1 silt, and 240 g kg -1 sand.Mean precipitation and temperature for the two years of the experiment were 1922 mm and 22.8 °C respectively (SIMEPAR, 2015).
Prior to introducing the NTS, the area had consisted of degraded pasture.When setting up the NTS, the necessary soil correction was carried out based on chemical analysis.For the last twelve years, the area has been under NTS, with soybean planted in the summer, and maize alternating with black oats in the winter.
The management systems used in the experiment consisted of soybean during the summer and maize used for whole plant silage during the winter, and, after harvesting the silage and maize for grain (Table 1), an intercrop of cover plants, with and without the application of WPF and scarification.
The experimental design was of randomised complete blocks (RCB), with four replications per treatment, collected at four points per replication, giving 56 experimental plots of 14 m 2 each.
In the treatments that included WPF, 100 m 3 ha -1 yr -1 were applied, divided into two equal doses, one for the maize and the other for the cover crops.The WPF used was available on the farm.The distance from the collection point to the site of application was 800 m.The WPF was placed in an anaerobic lagoon, covered by a waterproof sheet and, after spending 120 days in the lagoon, used on the crops.An applicator drawn by agricultural tractor was used to apply the WPF directly to the experimental plots.
The RP was determined to a depth of 0.4 m, using a Falker Solo Track automated soil compaction meter equipped with a number 2 shaft.The equipment recorded the RP values every 0.01 m; these were stored in the internal memory and then transferred to a computer for processing.The measurements were taken at four random points in each experimental plot, with the mean Soil compaction in areas of maize used for silage with the application of wastewater The soybean was planted after the cover-crops, which were desiccated twenty days before sowing the soybean using a non-selective, systemic, post-emergent glyphosate herbicide (N-(phosphonomethyl)glycine), at a dose of 4.0 L ha -1 .The Nidera 5909 RG cultivar was used in both years, at a population density of 320,000 plants ha -1 , or 16 plants m -1 , and distributed using an eight-row NTS seeder at a spacing of 0.50 m.A base dressing of 350 kg ha -1 NPK formulation 0-20-18 was incorporated by furrower to a depth of 0.12 m.Herbicides and insecticides were applied as recommended for the crop.The data were submitted to analysis of variance, and the mean values were compared by Scott-Knott test at a level of 5%.

RESULTS AND DISCUSSION
The accumulated rainfall and the mean monthly maximum and minimum temperatures from February 2013 to February 2015 are shown in Figure 1.Rainfall was recorded each month, with low values occurring only during February and July of 2013 and July of 2014; however, plant development was not impaired and no irrigation was necessary.
The summary of the analysis of variance for the mechanical resistance to penetration (MPa) in the four evaluations (Table 2) showed a significant difference in the three factors under study (eight depths, with and without WPF and seven management systems).A double and triple interaction between the factors can also be seen.Only the interaction between depth and WPF showed no significant difference at a level of 5%, except in the evaluation made during September 2014.
In the first evaluation, carried out in September 2013 (Figure 2), a lower RP was found under the system including soybean, silage and scarification with oats and turnip (SSSOT) for the 0 to 0.2 m layer, than in the other treatments, both with and without the application of WPF.This result had been expected due to soil turning, which was carried out to a depth of 0.25 m.Gao et al. (2016) also found that for a system including soil turning, the RP was lower in the surface layer.Afzalinia and Zabihi (2014) found that bulk density and RP increased under NTS in relation to conventional tillage due to the lack of soil turning.However, studies show that turning decreases the RP during a certain period only.Botta et al. (2006), when evaluating scarification in the Pampas Region of Argentina, found soil re-compaction after two years.
The greatest RP levels for the remaining treatments under NTS were found in the layer down to 0.2 m.The same was found by Cunha, Cascão and Reis (2009), where the most influential layer was from 0 to 0.2 m, showing the effects of disaggregation caused by soil preparation.The systems with no mechanical interference showed similar behaviour in the soil profile.Down to the 0.05 m layer, the RP was less than 2.0 MPa.This value has been widely adopted by authors as critical for the growth of plants under different management systems (BOTTA et al., 2006;CUNHA;CASCÃO;REIS, 2009;FUENTES-LLANILLO;GUIMARÃES;TAVARES FILHO, 2013;MAZURANA et al., 2013).
However, Tormena et al. (2007) used 3.5 MPa as the maximum RP value in a soil under no-tillage, based on the argument of the presence of continuous and effective biopores that result from this management system.There is still uncertainty concerning these RP values, especially as different plants respond differently to these critical values, and although this evaluation is more sensitive in identifying compacted layers, it does not consider the bioporosity of the soil, which is so important for root growth in compacted soils (REICHERT; SUZUKI; REINERT, 2007).
Without WPF (Figure 2a), higher values for soil RP were seen in the 0.05 to 0.2 m layer, with an average of 2.5 MPa between the management systems, except for SSSOT.From 0.2 to 0.4 m the systems displayed similar behaviour, with the RP stabilising at under 2.0 MPa; except for SS (soybean and silage), in which the RP was higher, with only spontaneous plants and little increase in the roots, showing the importance of cover crops in protecting and forming biopores in the soil profile at the deeper layers.
With the application of WPF (Figure 2b), in the 0.05 to 0.2 m layer, higher RP values (over 2.5 MPa) were seen between management systems, except in the SSSOT and the soybean, silage and oats (SSO).In the 0.2 to 0.4 m layer, the systems showed similar behaviour, with values under 2.0 MPa, demonstrating that the machine traffic did not change the RP at this depth.These results reinforce those obtained by Cunha, Cascão and Reis (2009), highlighting that the effect of tractor traffic was mainly concentrated in the 0 to 0.2 m surface layer.
According to the values for water content, all the evaluations were carried out near field capacity (Figures 2, 3, 4 and 5), as recommended by Reichert, Suzuki and Reinert (2007).RP is dependent on the water content when determined in the field, and increases exponentially with its reduction (CONTE et al., 2011).Values over 2.0 MPa, even under conditions of high humidity, may be unfavourable to plants (STRECK et al., 2004).
In the evaluation of February 2014 (Figure 3), the same effect was seen as in the previous evaluation (Figure 2), i.e. the SSSOT system had an RP of less than 2.0 Mpa.In the 0.05 to 0.2 m profile, without the application of WPF (Figure 3a), the management systems under NTS had a RP value between 2.0 and 2.5 MPa; results that confirm those of Kumar et al. (2012), who found that the RP can reach up to 2.2 MPa in the 0 to 0.2 m layer under NTS.
Mean RP values before the soybean (Figure 2a) were higher than after the crop was harvested (Figure 3a).The same happened in the second year (Figure 4a and  5a).This was due to the intense machine traffic needed  to harvest the maize for whole plant silage, and to the aggressive root system of the soybean crop.
With the application of WPF (Figure 3b), higher RP values can be seen in the 0.05 to 0.2 m layer, of over 2.0 MPa between management systems, except for the SSSOT and SSO.Similar results were found by Streck et al. (2004), who saw maximum RP values at depths of 0.06 to 0.14 m for the treatments.In the 0.2 to 0.4 m layer, the systems were similar, with the RP stabilising at under 2.0 MPa.
Figure 4 shows RP values in September 2014 after the management of cover crops, without the application of WPF (a) and with application of WPF (b).The same effect can be seen as in the previous evaluations, with the SSSOT system displaying lower RP values of from 0 to 0.05 m, with the same characteristics as in the first two evaluations.However, in Figure 4a, from 0.05 to 0.1 m the SS system had a higher RP.This value shows that the lack of ground cover, as well as the smaller biopore volume under NTS, resulted in a higher RP value in the surface layer, which also occurred in the last evaluation in February 2015 (Figures 5a and 5b), and was statistically superior to the other treatments at 1% probability.
The system of soybean silage (SS) was the only system to leave the ground without cover crops after the completion of whole plant ensiling, with only spontaneous plants remaining.In areas under NTS, keeping the plant cover and the use of cover crops is fundamental to maintaining the physical quality of the soil, especially in the surface layer, which displays low density and high total porosity due to the higher concentration of roots and organic matter (REICHERT; SUZUKI; REINERT, 2007) from the remains of the main crop and/or from the cover crops planted for this purpose.
Because in areas intended for silage production the whole plant is ensiled, leaving only the roots and a small part of the stem, the soil is more susceptible to compaction.However, under the systems that included cover crops, after the maize was harvested for silage, the RP remained at similar levels to the soybean, oat and turnip (SOT) and soybean and maize (SM) systems, which were not subjected to the ensiling process.
The values seen in the management systems from the evaluation carried out in February 2015 (Figure 5), showed a significant difference at a level of 1%.The SSSOT system had a lower RP value in the 0.05 to 0.20 m layer (Figures 5a and 5b), which was the depth Soil compaction in areas of maize used for silage with the application of wastewater that had the greatest effect on the evaluations.This demonstrates the efficiency of scarification in reducing the RP.These results agree with those of Cunha, Cascão and Reis (2009), who found the same effect of a reduction in RP with the use of a scarifier in preparing the soil.In the 0.2 to 0.4 m layer, no system showed any significant difference in either evaluation, with an RP below 2.0 MPa both with and without WPF.
With no WPF (Figure 5a), a significant difference can be seen between the management systems and the depth of 0.10 to 0.15 m, which is statistically superior to the other layers under evaluation.According to Reichert, Suzuki and Reinert (2007), compaction in agricultural areas and pastures usually occurs to a maximum depth of 0.2 m.The highest RP (over 2.5 MPa) was found in the SS system, which was significant at a level of 1% compared to the other systems under evaluation.This result was due to the absence of ground cover under NTS, highlighting the importance of cover crops, which provide necessary protection, avoid the surface compaction caused by machine traffic and reduce bulk density.
Similar results were found in the management systems with WPF (Figure 5b).The SS system showed an RP over 2.5 MPa, but only from 0.05 to 0.1 m.Under this management system, surface soil compaction was caused by the lack of ground cover.Such RP values under the SS system may compromise crop productivity.According to Beutler et al. (2006), soybean productivity decreases at RP values of >2.24 MPa, and maize productivity at RP values of >1.65 MPa, restricting shoot growth and crop productivity (FREDDI et al., 2007).Similar results were found by Rossetti and Centurion (2013), who stated that plant height, stalk diameter, height of the first ear insertion and maize productivity showed a decreasing linear relationship for increases in the RP.There is still no consensus among researchers.For Betioli Júnior et al. (2012), under NTS, the bioporosity resulting from less mechanical movement of the soil may offer alternatives for root growth, compensating for the higher resistance of the soil matrix; because of this, the critical resistance limit of 2.0 MPa can be increased.
The systems that presented values below 2.0 MPa at a depth of 0.10 to 0.15 m, were SSO and SSSOT (Figure 5b).Among the management systems under NTS only, with no mechanical interference of the soil, it can be seen that SSO appears in all the evaluations which include the application of WPF (Figures 2b,3b,4b and 5c), with RP values below 2.0 MPa in the 0.10 to 0.15 m layer.This layer, under the other management systems, had the highest RP values.The application of WPF under the SSO system, which used black oats as ground cover, reduced the RP in the 0.10 to 0.15 m layer, and was statistically lower than the other management systems under NTS.This is probably due to the response of the black oats to the application of WPF, with greater shoot and root-system development, the creation of biopores and an increase in soil aeration, thereby reducing the RP, especially at this depth.Valicheski et al. (2012) studied the effect of compaction on the physical properties of the soil and the development of cover crops (forage turnip and black oats).They found that the high resistance of the soil to penetration, mainly at the more intense levels of traffic, restricted root development in the forage turnip that, because it is a plant with a tuberous root system, exploited a smaller volume of soil, thereby absorbing a smaller amount of nutrients and showing less shoot development.According to the same authors, black oats, due to its fasciculated root system (which allowed the roots to develop in small cracks), exploited a larger volume of soil, resulting in greater shoot development.
The mean RP values under the management systems with and without WPF are shown in Figure 6, together with the values collected before the start of the experiment.In each of the collections, the mean values were greater in the 0.10 to 0.15 m layer, which may impair crop development.These values agree with those of Mazurana et al. (2013), who found that the traffic conditions imposed on the soil by the tractor in an area of oats + vetch, increased the RP to a depth of 0.15 m.
The behaviour of the RP in the first evaluation (Figure 6a) was similar to that at the start of the experiment to a depth of 0.2 m.This did not occur from 0.2 to 0.3 m, where the mean values both with and without WPF were higher than the data obtained prior to the experiment, with a mean RP of 2.0 MPa.There was a significant difference between the mean values of the treatments with and without WPF in the first evaluation (Figure 6a).The same was seen in the other evaluations (Figures 6b, 6c and 6d).The application of WPF resulted in a significant difference in RP in the 0 to 0.4 m profile, which disagreed with Oliveira, Lima and Verburg (2015) and Costa et al. (2011), who found no alteration in the physical attributes of the soil due to the application of WPF.
Soil compaction in areas of maize used for silage with the application of wastewater According to Arruda et al. (2010), applying WPF at doses of 50 and 100 m 3 ha -1 in relation to the control with no fertiliser, did not alter the physical attributes of the soil after four years, showing that the agricultural use of WPF preserves the physical quality of the soil.
In each of the evaluations (Figures 6a, 6b, 6c and 6d), there was a significant difference seen at the depth of 0.10 to 0.15 m.
There was no variation between mean values with and without WPF for any of the evaluations (Figures 6a, 6b, 6c and 6d), compared to initial values before the start of the experiment.This shows that under NTS there is no effect on RP in production areas of whole plant silage if the management is suitable and includes cover crops.
These results demonstrate that in areas already established under NTS, the effect of compaction caused by intense machine traffic can be controlled and still preserve the physical quality of the soil.Ralisch et al. (2008) found that for 14 consecutive years, in systems under NTS in areas of pasture with no soil turning, the RP approached the values for native forest, showing that in areas of established NTS the effect is minimised.
The continuous use of cover crops under SPD, and monitoring the physical conditions of the soil over time, are fundamental for the evaluation and development of management systems.Biological action in reducing the effects of compaction by the use of cover crops and crop rotation may have a beneficial effect in the medium-and long-term, where this effect is not detectable by the routine evaluation of such physical properties of the soil as the RP (REICHERT; SUZUKI; REINERT, 2007).
Before carrying out scarification of the area, there must be a real need for the operation.In most cases, simply planting cover crops, or changes in the type of management, are sufficient to recover the physical quality of the soil.A well-managed NTS is the best way of reducing soil loss through surface runoff, protecting the soil from the direct action of raindrops, and preventing the water from evaporating.
When scarification is carried out, in addition to increasing costs, especially of fuel, a part of the benefits from NTS is lost.In agricultural areas, NTS is not established during the early years, but only over time (4 to 5 years) and, depending on the type of management, the physical properties of the soil can be maintained in conditions that do not impede root development in such crops as soybean or maize.

CONCLUSIONS
1.The application of wastewater from pig farming decreased the mechanical resistance of the soil to penetration; the system of soybean, silage and scarification with oats and turnip had the lowest RP; and at the depth of 0.2 to 0.4 m, all the treatments under evaluation displayed similar behaviour, with no signs of compaction; 2. The greatest mechanical resistance of the soil to penetration was in the 0.10 to 0.15 m layer and under the system with no cover crops (SS); 3. The study led to the conclusion that the management system under NTS including black oats associated with the use of wastewater from pig farming is the most suitable for reducing the RP in production areas of whole plant silage.

Figure 1 -Figure 2 -Figure 3 -
Figure 1 -Accumulated rainfall (mm) and mean maximum and minimum temperatures from February 2013 to February 2015.Data obtained from the SIMEPAR weather station (2015)

Figure 4 -
Figure 4 -Mechanical resistance to penetration (MPa) obtained in the third evaluation of the experimental area in September 2014 after the management of cover crops, without the application of WPF (a) and with the application of WPF (b)

Figure 5 -
Figure 5 -Mechanical resistance to penetration (MPa) obtained in the fourth evaluation of the experimental area in February 2015 after the soybean harvest, without the application of WPF (a) and with the application of WPF (b)

Figure 6 -
Figure 6 -Mean values for mechanical resistance to penetration (Mpa), with WPF and without WPF, and the initial value characterising the experimental area, obtained in the first evaluation (a), second evaluation (b), third evaluation (c) and fourth evaluation (d)

Table 1 -
Management systems used in the experiment

Table 2 -
Summary of the analysis of variance of the mechanical resistance to penetration (MPa) in the four evaluations Considering the F-test, ns not significant at 5% probability; ** significant at 1%; * significant at 5% Notes: