Improvement of Soil Quality through Minimum Tillage for Sen Cropping Pattern in Indonesia

Small farmers in East Nusa Tenggara, Indonesia still preserve their local knowledge or local wisdom in maintaining the food security of semi-arid dry land. Levis et al., (2017) revealed that 71.24% of farmers still practiced local wisdom known as the “Sen” or “Salome” cropping pattern. The Sen cropping pattern is a planting method in which several main annual food crops are planted together in one planting hole. The plants planted are corn (Zea mays L.), cowpea (Vigna unguiculate spp) and pumpkin (Cucurbita moschata Durch). In composition, this cropping pattern is mixed cropping and it has a mutually beneficial symbiotic relationship (Basuki and deRosari, 2017).
       
In the Sen cropping pattern, corn acts as the main food crop and becomes a stake for the cowpea crop, while the cowpea acts as a nitrogen-fixing plant. The pumpkin plant acts as a cover crop to inhibit the growth of weeds, dispels several types of pest insects, provides mulch and becomes a source of organic material for the topsoil. The pumpkin plant also maintains humidity and reduces water loss and erosion.
       
Other advantages of the Sen cropping pattern are food diversification, reducing the risk of crop failure and saving labor, time and production costs. The absence of technological input such as tillage and the absence of amendment applications result in low crop productivity in the Sen cropping pattern. Neo and Ceunfin (2018) reported that corn productivity in the Sen cropping pattern was 0.56 tons/ha. The lack of further tests on soil quality and crop productivity in the Sen cropping pattern leads to a study on the Sen cropping pattern model integrated with minimum tillage.
       
Minimum tillage by making a relatively large planting holes in the Sen cropping pattern is expected to create a better growing medium. Relatively large planting holes will increase soil porosity to support aeration, drainage and better root growth. The relatively large planting holes also facilitate the application of amendments that are more efficiently concentrated in the rooting zone. The amendment application helps create nutrient availability and triggers beneficial soil biota life in improving soil quality.
       
The sustainability of plant productivity and a healthy environment produced by the interaction between soil properties, both physical and chemical properties of the soil, reflects the quality of the soil. Semi-arid dry land in East Nusa Tenggara, which has an air temperature of 23.6-37°C and a dry period of 7-8 months with erratic rainfall of 500-1200 mm/year (Faqih et al., 2015), has low soil quality in general. The developing soil is dominated by soils with limestone, shallow, rocky and rough rocky material with a weak structure and become very vulnerable to erosion, compacting, evaporation, quick leaching processes and strong P adsorption.

The study was conducted on dry land owned by farmers of Kuanheum Village, Amabi Oefeto, Kupang Regency, East Nusa Tenggara from August to November 2018, coinciding with the end of the dry season. The study site is located at 123°54’19 “E and -10°7’22"S with an elevation of 150 meters above sea level. The study site is upland that has not been used for three years and is left to be open land for cattle grazing.
       
The research was arranged in a randomized block design with four treatments in three experimental blocks. There were three minimum tillage treatments, namely planting hole of 20x20x20 cm, 30x30x20 cm and 40x40x20 cm and one untreated planting hole as control. In the treatment control, the planting hole was made as deep as 5 cm with a diameter of 2.5 cm without amendment application (Fig 1). In the minimum tillage treatment, the soil was only dug in the area of tillage according to the size of the planting hole tested (Fig 2). The dug soil was then mixed with cow manure (C / N = 11.9) 2.5 tons/ha according to the needs of the corn plant and given to the planting hole. The plants tested were corn, cowpea and pumpkin with the same harvest age of 3 months. Two seeds/types of plants/holes were planted according to the Sen cropping pattern (Fig 3). Mineral fertilizers such as Urea (300 kg/ha), SP-36 (100 kg/ha) and KCl (75 kg/ha), according to the needs of corn plants, were applied to the minimum tillage treatment. The mineral fertilizers were given gradually, at 7 and 35 days after planting by burying them in a half-circle around the plant. Watering, as much as 1 liter, was done once a day during the initial vegetative period and the watering volume was increased to 3 liters as the temperature rose.
 

 

 

       
The improvement of soil physical and chemical properties was measured by comparing the initial soil properties with the soil properties observed at 14 days after planting. The physical properties observed were bulk density, specific gravity, porosity and aggregate stability, while the chemical properties included pH, organic C, total N, available P, K, Ca, Mg, Na and CEC. A total of 24 soil samples were tested in the laboratory. The effectiveness of soil quality improvement was measured by plant productivity. The data were analyzed statistically with ANOVA, mean and standard deviation tests using SPSS 23.

Initial physical and chemical properties of the soil
 
Based on Table 1, the soil of the study site has a sandy loam texture with a percentage of sand, silt and clay fraction of 59.56%, 27.11% and 13.33%, respectively. USDA-NRSC (2014) suggested that the ideal bulk density of sandy loam textured soil for growing plant roots is <1.40 g/cm³. However, the soil bulk density of > 1.63 g/cm³ can be an indicator of soil compaction. The soil bulk density in the study site was quite high (1.68 g/cm³), indicating that the soil had been compacted (Table 1). Kinetic energy from high rainfall intensity and livestock movement activities are considered to be the main agents in the process of soil compacting. The results of the analysis of specific gravity, porosity and aggregate stability were 2.60 g/cm3, 35.26% and 299.85 KJ, respectively.
 

       
Table 1 shows that the study site has low soil fertility. Soil chemical properties such as the contents of organic C (1.30%), total N (0.11%), K (0.38 me/100g) and Na (0.10 me/100g) are low. In sandy loam textured soils, nutrients are easily lost through leaching and evaporation. The low content of clay and organic matter in the study site causes low CEC (12.90 me/100g).
       
Table 1 shows that Ca (32.39 me/100g) and Mg (7.93 me/100g) concentrations are high, pH (8.4) is slightly alkaline and available P (10.26 ppm) is low. The high concentration of Ca and Mg is closely related to the parent material of the study site, namely limestone sediments, which contain a lot of CaCO₃ and CaMg(CO₃)₂. CaCO₃ hydrolysis reaction causes soil pH to increase and leads to the formation of strong Ca-P bonds so that P is not available to plants (Turner, 1958; Hopkins and Ellsworth, 2005; Wibowo et al., 2019).
 
Effects of the treatment on the improvement of soil chemical properties
 
Minimum tillage by making a relatively larger planting hole aims to facilitate a more efficient amendment application in providing organic material and essential nutrients around the roots. Table 2 shows that the minimum tillage treatment tends to improve soil chemical quality more optimally than the control treatment.
 

       
The treatment of holes with a size of 20x20x20 cm could improve soil chemical quality more optimally than other planting hole sizes. This treatment tends to be able to provide an increase in organic C, total N, available P, K, Na and CEC reaching 335%, 172%, 265%, 123%, 270% and 83% of the initial conditions, sequentially. The same treatment also tends to provide a decrease in pH, Ca and Mg 20%, 15% and 46% of the initial soil conditions, consecutively. The tendency produced is related to the high concentration of organic matter and mineral fertilizers.
       
The difference in the size of the planting hole in the minimum tillage treatment causes a difference in the ratio of the soil and the amendments between each hole size in spite of the same dose of the amendment given. The smaller planting hole size (20x20x20 cm) will automatically have a higher concentration of organic matter as indicated by a higher organic C concentration. The carboxylic and OH-phenolate functional groups in organic acids, resulted from the mineralization of organic matter, act as agents in decreasing pH and as inhibitors in strong P adsorption, so that Ca and Mg concentrations decrease in alkaline soils (Kumar et al., 2015; Adeleke et al., 2016; Utami et al., 2017; Wadu et al., 2017). High concentrations of organic matter can support an increase in soil CEC, contribute to macro and micronutrients as well as increase the efficiency of mineral fertilizers application (Gosavi et al., 2009; Angelova et al., 2013; Kidinda et al., 2015).
 
Effects of the treatment on the improvement of soil physical properties
 
Based on Table 2, minimum tillage tends to create more optimal growing media compared to soil without tillage. The plowing process by digging relatively large planting holes around the root zone can reduce soil compacting and provide soil porosity that supports optimal aeration, drainage and root growth.
       
The treatment of planting hole with a size of 20x20x20 cm significantly improved soil physical quality compared to other planting hole sizes. This treatment tends to provide 30% and 4% reduction in soil bulk density and specific gravity, respectively and an increase in soil porosity of 55% of the initial soil condition. The tendency produced is related to the support of high concentrations of organic matter. Organic matter has a lower bulk density and specific gravity compared to mineral material, therefore an increase in organic matter in the soil can reduce the bulk density and specific gravity of the soil (Bauer, 1974; Kumar et al., 2009; Cercioglu et al., 2012). Increased organic matter in each tillage treatment was observed to be able to stimulate the life of beneficial soil biota such as termites. Termite activity plays a role in improving soil structure and porosity (Mando and Miedema, 1997).
       
Based on Table 2, the highest aggregate stability at 14 days after planting was observed in control treatment. The control treatment is considered not to be able to reduce soil compacting. The high density of soil particles due to the compacting process in the treatment control generates the need for greater water energy to break the density between particles. In the minimum tillage treatment, the process of destroying the density of soil particles occurs, resulting in lower aggregate stability. Table 2 shows that the high concentration of organic matter in the planting hole with a size of 20x20x20 cm is able to support aggregate stability, which is better than other minimum tillage treatments. Organic compounds from organic matter act as a cement in binding soil particles, forming stable aggregates (Kumar et al., 2013). It takes time for the organic matter to decompose further to contribute to a more stable cementing agent.
 
Effects of the treatments in the crop productivity
 
Based on Table 2, the improvement of soil chemical and physical properties by minimum tillage treatment succeeded in providing an optimal increase in production. Minimum tillage increased the productivity of corn, cowpea and pumpkin, respectively 3.08-4.31 tons/ha; 0.37-0.41 ton/ha; and 37.57-63.03 tons/ha of the crop productivity without tillage.
       
Table 2 shows that a planting hole with a size of 20x20x20 cm gave the highest soil quality improvement. Nevertheless, the highest crop productivity results were obtained in the planting hole of 40x40x20 cm. The treatment increased the production of corn, cowpea and pumpkin 430%, 279% and 402% of the production in control treatment, consecutively. The tendency proves that the planting hole of 20x20x20 cm is able to support optimum plant growth and development in the initial vegetative phase. The treatment of larger holes (40x40x20 cm) allows roots to grow more optimally and minimize competition between plants within a planting hole in the final vegetative phase to the generative phase. Vincent and Davies (2002) revealed that increasing the size of the planting hole might be beneficial through a decrease in the competition of roots.

The results showed that the best improvement of soil quality to support plant growth during the vegetative period was achieved in the treatment of a planting hole of 20x20x20 cm. Meanwhile, the planting hole of 40x40x20 cm, with the same dose of fertilizer and watering, could provide higher plant productivity.