RESEARCH WORK

Influence of fertilization on the physical and chemical properties of a soil dedicated to the production of Megathyrsus maximus seed

J. F. Ramírez¹, Yousi Fernandez¹, P. J. González², Xiomara Salazar¹, J. M. Iglesias³ and Yuseika Olivera³

¹Estación Experimental de Pastos y Forrajes Cascajal. Carretera Central, Crucero Digna, CP 54 490, Santo Domingo, Villa Clara, Cuba
²Instituto Nacional de Ciencias Agrícolas, La Habana, Cuba
³Estación Experimental de Pastos y Forrajes Indio Hatuey, Universidad de Matanzas, Ministerio de Educación Superior, Matanzas, Cuba
E-mail: bibmansd@capiro.vcl.sld.cu

ABSTRACT

The influence exerted by different fertilizer doses on the physical and chemical properties of a ferruginous Gley Nodular soil, dedicated to the production of Megathyrsus maximus (Guinea grass) cv. Likoni seed, was studied. The design was randomized blocks, with four replications, and the following treatments were evaluated: control (without fertilizer); 3, 6 and 9 t of earthworm humus per hectare; 20, 40 and 60 t of cattle manure per hectare; and 180-50-75 kg of NPK per hectare. The values of the real and apparent density fluctuated among the treatments; but, in general, they were at the end higher (p > 0,05) in the control and in NPK, for the depths 0-10 and 10-20 cm, respectively. The natural humidity was higher (p d» 0,05) when applying 9 t of humus and 60 t of manure; while porosity did not differ among the treatments. The organic matter increased, and the best result (p d» 0,05) was obtained in the treatment of 9 t of humus. The seed yield with 9 t of humus was similar to that of NPK (between 41,0 and 91,3 kg of PGS/ha); however, the profits for the sale of seeds and the profit per hectare were higher in the treatment with 9 t of humus, which propitiated a lower production cost (10,30 vs. 16,50 pesos/kg of seed for humus and NPK, respectively). It is concluded that the use of organic fertilizers in the systems of Guinea grass seed production is feasible, from the productive and economic point of view.

Key words: Cattle manure, earthworm humus, seed production.

The tropical regions of the planet are considered large reserves of agricultural raw materials to satisfy worldwide consumption, food production, and even the demand of food in a population that reached 6 000 million inhabitants at the end of the millennium (FAO, 2009). Nevertheless, these reserves have progressively decreased, due to the indiscriminate exploitation of tropical agroecosystems in the most developed countries. The human activity in the livestock production sector has led to the assimilation of virgin and barren lands, with the intensive application of technical resources in agriculture, such as mechanization, irrigation and chemical fertilization. On the other hand, in the least developed countries, with high poverty and agricultural commercial deficit values (FAO, 2002), the lack of knowledge and the social need lead to the inadequate utilization of lands. This has propitiated the increase of soil degradation in diverse regions of the world (FAO, 2008); more than 20 % of the agricultural lands, 30 % of the forests and 10 % of pasturelands have been affected, which hinders the adaptation to and mitigation of the climate change, as due to the loss of biomass and organic matter (OM) of the soil, carbon is released to the atmosphere and this affects the quality of the soil and its capacity to hold water and nutrients (Steduto et al., 2012). Thus, it is necessary to monitor the evolution of the edaphological properties of agricultural systems (López, 2002), in order to predict the variations that occur due to certain exploitation practices, which would allow to apply amelioration measures in the face of the global changes induced by man.

In this sense, fertilization constitutes one of the most important agrotechnical factors to sustain and increase the seed production of forage plants (Gómez, 2002; Amberguer, 2006; Formoso, 2012). However, in Cuba the lack and high prices of chemical fertilizers, along with the need to preserve the environment, motivate the implementation of low-input systems for seed production, taking into consideration the limitations of soils. Of them, 65 % are included in categories III and IV according to the diagnosis of the Instituto de Suelos (Institute of Soils) (2006), which implies that they are affected by limiting factors and that causes that the potential yield is below 50 %. The factors with the highest influence on the low productivity of Cuban soils are: low content of nutrients and OM and trend to acidity (García et al., 2012), the latter comprises 27 % of the soils dedicated to livestock production.

In our country there are wide possibilities of obtaining manure from different species, which serves as raw material for the production of earthworm humus, and can be directly applied on the pasture production areas. In this sense, organic fertilizers have been used since ancient times in all the civilizations of the world, with good results, which has allowed food production in sufficient quantities (Peña et al., 1993). They improve the physical, chemical and biological properties of soils, by remediating the fertility problem. In addition, they are rich in microflora, which contributes a large quantity of microorganisms that activate the biological processes, for a better utilization of nutrients by the plants (Pacheco, 2000).

Taking these antecedents into consideration, the objective of the research was to evaluate the effect exerted by different sources and doses of organic fertilizers on the hydrophysical and chemical properties of a soil dedicated to the production of Guinea grass seeds, as well as the economic results of these systems compared with the conventional system of chemical fertilization.

MATERIALS AND METHODS

Climate and soil. The research was conducted at the Pastures and Forages Research Station of Cascajal, located at 30º 21' East latitude and 20º 21' West longitude Santo Domingo municipality, Villa Clara province, in an alitic petroferric soil of low clayey activity, yellowish, ferruginous Gley Nodular, subtype Petroferric (Hernández et al., 1994), and at a height of 60 m.a.s.l.. The chemical characteristics of the soil, as well as the climate data of the locality, are shown in tables 1 and 2, respectively.

Treatments and design

The design was randomized blocks, with eight treatments and four replications, in 4 x 6-m plots of Guinea grass Likoni, with an experimental surface of 24 m² and a useful area of 15 m². The treatments were:

1- 0 fertilization (control)

2- Earthworm humus (3 t/ha)

3- Earthworm humus (6 t/ha)

4- Earthworm humus (9 t/ha)

5- Cattle manure (20 t/ha)

6- Cattle manure (40 t/ha)

7- Cattle manure (60 t/ha)

All of them in only one application per year.

8- Whole fertilization with 180-50-75 kg of NPK. The dose of N was divided into three cuttings per year, at a rate of 60 kg per cutting.

The chemical composition of the organic fertilizers is shown in table 3.

Experimental procedure

Sowing, fertilization and cultural attentions.After performing the conventional soil preparation (plowing, harrowing, crossing, harrowing and furrowing) and after creating an adequate bed for planting, it was carried out in September, 2000, with a distance of 1 m between furrows and 0,50 m between clumps. After planting and replanting the Guinea grass, clearing works were carried out with hoe, and one pass of cultivator with oxen, in order to keep the plots free from undesirable plants and guarantee the establishment phase. The organic fertilizers were applied only once at the moment of sowing, manually and on the bottom of the furrow; the humus was applied on dry basis and the manure, after lying in the manure heap for 90 days. The chemical fertilizer (PK) was applied during the spring, every year, in single doses of 50 and 75 kg/ha of P₂O₅ and K₂O, respectively; while N was fractioned and applied in doses of 60 kg/ha, after each seed harvest and the cutting of the remnant green mass, at a height of 15 cm of the soil.

The spaces between the plots and the sanitary cordons were kept clean, using plow, harrow and animal draught with oxen.

Seed harvest. The seed harvest was manually carried out in the periods March-April, May-June and September-October although there were other seed emissions that were not considered. The panicles were cut with a sickle or a knife, and they were put in woven nylon sacs to cause them to sweat. Afterwards, they were shaken, threshed, cleaned and dried until reaching 12 % of moisture. Afterwards, they were weighed and stored in sacs, in sheds with ambient temperature.

The yield (kg/ha/year) of each treatment was calculated from the sum of the weight of the seeds of each harvest. From all the harvested, dried and processed material the yield of pure germinable seed (PGS) was determined, according to ISTA rules (1995).

Soil sampling. The soil was sampled at the beginning of the study, before applying the treatments and after carrying out the seed harvests. For such purpose, five subsamples were taken in each plot and a homogenous sample was formed, which was analyzed in the laboratory to know the variation of NPK, pH, cations and OM in time. The samples were taken at the depths 0-10 and 10-20 cm, and the analyses were made at the laboratory of the National Institute of Agricultural Sciences (INCA) Mayabeque, Cuba.The methods used are shown in table 4.

Economic evaluation. To evaluate the economic impact of the application of systems with fertilization, the cost of the fertilization with earthworm humus was compared with that of NPK. The prices of these fertilizers were taken into consideration, as well as the cost of humus handling and carrying, besides the incomes for the sale of seeds and green forage.

Statistical analysis.An ANOVA was applied, and the means were compared through Duncan's (1955) multiple range test. The program Statistica for Windows, Release 4.3 (1993) was used.

RESULTS AND DISCUSSION

The real density is the relation between the weight unit and the volume unit of the solid phase of the soil, and it turns out to be more or less constant, because it is determined by the chemical and mineralogical composition of the solid phase. In this indicator, in the first year as well as at the end of the research, the highest values were obtained in the control and in the treatment with NPK, which differed from the other treatments (p > 0,05) at the depths 0-10 and 10-20 cm (table 5). In general, the values found during the first year (between 2,39 and 2,63 g/cm³) are in correspondence with the ones reported in literature about clayey, quartzitic or feldespar soils (Heredia, 2015). However, after three years of exploitation the soil of the experimental area decreased its density (below 2,3 g/cm³) and it indicates the increase of the OM content (Thompson and Troeh, 1988; García et al., 2009), mainly in the treatments in which the amendments with humus and manure were applied.

In the first year, with the application of the three doses of cattle manure (p > 0,05), the best values were obtained at the depth 0-10 cm; while in the horizon 10-20 cm there were no differences between 20 t of this product and the different doses of earthworm humus, which was more evident at the end of the experiment as there were no significant differences among the organic fertilizers and the control, although they differed with regards to NPK. In the horizon 0-10 cm there were no differences among the organic fertilizers at the end of the research (year 3); nevertheless, they differed from the control and the chemical formulation (p > 0,05), with values that oscillated between 2,06 and 2,13 g/cm³.

The apparent density of a soil can be used as a measure of its structure. A low density, generally, is equivalent to the existence of more porosity and aggregates, and implies higher stability, lower compaction and, probably, higher humidity content with regards to a soil with high density (Doran, 2000). This indicator showed negative results in all the treatments, because it increased after three years, with the highest values (p > 0,05) in the control and in NPK; this indicates that this soil shows high levels of compaction (Anon, 1982; Jaramillo, 2002), mainly where no organic fertilizers were applied, which can be a limitation for seed production. The obtained values were even higher than the ones reported by Cairo and Fundora (2002) for sandy soils populated with pine trees in the Pinar delRíoprovince, and the ones described byPadrón et al. (2012) in alfisols with pastures, in Yaracuy, Venezuela.

In general, the application of manure and humus allowed to obtain the best values of apparent and real density of the soil, which coincides with the report by Baldock and Nelson (2000), and is mainly related to the OM content that both contribute to the soil (Medina et al., 2001), which favored the highest development of the roots and their respiration, as well as the absorption of the nutrients made by Guinea grass (Matías, 1999).

On the other hand, the treatments did not affect porosity at the beginning or the end of the research, although it tended to decrease during the years of evaluation, which was related to the increase of apparent density (Rubio, 2010) which is caused by the progressive compaction, caused in turn by the trampling and unduly use of the tillage implements (Arias, 2001). The porosity values obtained during the first year are within the category from normal to moderate (Klimes, 1970; Moreno, 2015) although they were not in correspondence with the obtained apparent density values, which cause a pore volume higher than 50 % (Thompson and Troeh, 1988; Rucks et al., 2004). In the successive years the porosity was very low, which could have caused the decrease of macroporosity, the water infiltration rate and the aeration. This could cause waterlogging, anoxia and difficulty of the roots to elongate and penetrate until reaching water and the necessary nutrients (Mora and Toro, 2000). Under these conditions, the development and growth of the plants is inhibited or delayed (Donoso et al., 2000), which influences the progressive decrease of the grass yield (Pérez et al., 2000; Pérez et al., 2006).

With regards to the natural humidity of the soil, during the first year 9 t of humus and 60 t of manure differed (p > 0,05) from the control and from NPK in both horizons; just like 40 t of manure in 10-20 cm. After three years of research, something similar occurred in the horizon 0-10 cm with 9 t of humus, 60 t of manure and 6 t of humus; this last one differed, in turn from the others. In 10-20 cm, 60 t of manure differed from the chemical fertilizer and from 6 t of humus, but it was equal to the other treatments. This indicator did not show a marked trend to the increase or decrease of humidity during the experimental period, although it was favored by the higher doses of organic fertilizers; which, according to Ibrahim and Mora (2006), allows the improvement of the edaphic fauna, as well as the increase of the population of macro- and microorganisms that inhabit and benefit the physical status of the soil. These populations exert a vital function with regards to the structure and cycle of nutrients in the tropical ecosystems.

Table 6 shows the results of the evaluated chemical variables. The dose of 60 t of manure and 9 t of humus influenced directly the values of K, M and OM in the first year (p > 0,05) with regards to the control, and differed in the OM content with regards to the treatment NPK, just like the dose of 40 t of manure. The lowest applications of humus and manure did not generate changes in the OM content, with regards to the soil without fertilization and the one fertilized with NPK. Influence of the different treatments on the Ca and Mg contents and the pH of the soil, was not shown either.

In this sense, it is necessary to state that the study was conducted on a desaturated soil which showed, before applying the treatments, low contents of OM and assimilable P and K, as well as high acidity (table 1), with pH lower than 5. Such characteristics are closely related to the genesis of this soil type, formed from metamorphic rocks (quartzitic schists) and in which ferratization and lixiviation processes have occurred causing a strong loss of bases through the profile and the appearance of H⁺ in the complex of change (Jouravleva, 1996; Zapata, 2004); situation which, if maintained, constitutes a constant source of acidification (hydrolytic acidity).

After three years of evaluation changes were observed in Ca, Mg and pH of the soil due to the treatments (p > 0,05), as well as in the OM content; while the trend in the K levels was maintained, with the best values for 9 t of earthworm humus. Although the P values increased with time, no significant differences were found among the treatments at the end of the research. In this sense, Daza et al. (2008) stated that the addition of organic and inorganic fertilizers is a temporary solution to P fixation, for which a periodical application is required to increase the quantity of P in solution and its availability for the plants, because a very fast mineralization of OM and the lixiviation of the applied fertilizers are originated, which limits the effect of the addition of these materials on the adsorption of P.

In general, the best treatments were 9 t of humus and 60 t of manure, because they always differed from the control, and with the former the highest OM value (3,34 %) was obtained. An important aspect was the decrease of acidity in the treatments with organic fertilizers, due to the increase of the Ca and Mg proportions (FAO, 1999) and of the OM content in the soil and its mineralogical transformation; while in the control acidity increased slightly and in the NPK treatment it maintained the same level as the first year.

When applying the treatments it was found that the organic fertilizers caused important changes in the content and stability of nutrients in the soil, which did not occurred in the non-fertilized soil. This is typical in this type of fertilization (Watabase, 1993), and it is ascribable to the improvement in the nutrient availability, as well as to a possible increase of the microorganism population, which can interact beneficially with the soil and the plants (Johnson et al., 2003; Ouédraogo et al., 2004). Cattle manure and earthworm humus contain the highest part of nutrients and mineral elements in assimilable form, which makes them very effective to ameliorate the soil. In relatively low doses they can correct the deficiencies and exert a positive effect on the edaphic properties, which allows to substitute chemical fertilizers totally or partially, as well as to attenuate the effects of environmental contamination, lower the costs and obtain acceptable yields with less quantity of fertilizers (Ramírez et al., 2002). In this an important role is played by the organic carbon of the soil, globally acknowledged as a fundamental factor for its health, because it is an essential part of the carbon cycle and is highly important in the mitigation of the effects of climate change (FAO, 2015).

Table 7 shows some elements related to the economic evaluation of the seed production by applying fertilizers. In this case the application of 9 t of earthworm humus was compared with the use of NPK.

The highest profits in the three years were obtained with the application of earthworm humus, which was not related to the highest production or incomes for seeds, but with the reduction of the expenses for the use of fertilizer only in the first year and partly, because of a higher income for the sale of green forage. The seed yield was similar in all the years for both treatments, with a trend to decrease in the third year (values below 45 kg of PGS/ha). The production values are considered relatively high for this variety in the first two years, because in the literature between 280 and 750 kg of total seed/ha or a production potential of 51 kg of PGS kg/ha is reported (Ramírez, 2002; Pérez et al., 2006). Although in the analysis the price of seed, which generally represents around 30 % of the system implantation costs (Arbito, 2011), was not taken into consideration, with the above-explained facts it is proven that seed production is a complicated activity, which shows high establishment costs. However, the productive cycle of the seed declines from the second year (Seré, 1985).

In studies conducted by Oquendo et al. (2008), the value of the production obtained for the sale of Guinea grass Likoni seed was 2 028, 1 692 and 1 512 pesos/ha, for areas planted with botanical seeds, transplants and clumps, respectively. These values are higher than the ones reached in this research, which could be related to the fact that in that study irrigation was used and the production was higher. Such authors reported and establishment cost of 634 pesos/ha, similar to that of the treatment with earthworm humus, if the costs for the seed purchase are calculated.

The reduction of the expenses in the treatment with earthworm humus resulted in a lower cost per peso (0,36 vs. 0,53) and cost per kilogram of seed (8,1 vs. 11,1) in that production system, if it is compared to the NPK. This allows to lower the prices of seed that is commercialized for the farmers, which exceeds 15,0 USD/kg with technologies that use chemical fertilizers (Pérez et al., 2006).

It is concluded that organic fertilizers, mainly the doses of 9 t of earthworm humus and 60 t of manure, exerted a positive effect on the physical properties of the soil, as well as an increase of the contents of P, K, Ca, Mg and of the OM, with a sensitive increase of pH, which brought about a seed production very similar to the one achieved with chemical fertilizers. This, in turn, propitiated better economic profits and reductions in the production cost and the cost of the kilogram of produced seed, besides the reduction of toxicity in the soil due to the non-utilization of agricultural chemicals.

Received: March 3, 2014
Accepted: July 24, 2015