Estimation of agricultural productivity loss due to soil loss in Buayan-Malungon River Basin, Philippines: A foundation for watershed protection and agricultural development

The Buayan-Malungon River Basin (BMRB) is currently experiencing intolerable soil loss due to improper agricultural practices, and unsustainable land uses. This problem disrupts important ecosystem services such as provisioning, regulating, cultural, and supporting. This article provides an estimate of the agricultural crop productivity losses due to soil loss. The study estimates productivity losses through regression analysis by using nitrogen (N) fertilizer as a proxy for soil quality. The result revealed that N-fertilizer had a significant effect on corn yield. The predicted soil loss in BMRB in 2020 reached 41,308 tons/ha. Hence, the total soil loss cost in the river basin reached PhP 218,023,624 in 2020. This cost can be minimized if farmers practice a contour farming system as it helps to improve soil quality and control soil loss, which leads to sustainable farming and environmental sustainability.


Introduction
Soil is an essential element in the production of crops.Soil and soil fertility play a significant role in the natural ecosystem and agricultural production.The productivity of any farm crop is determined by soil fertility as well as other factors such as seed quality, weather, and so on.However, with the perpetual cultivation of soil targeted in boosting farm production and improving agriculture to maintain food security, soil fertility may degrade.In developing countries like the Philippines, developments and improvements in the agricultural sector are crucial as most people depend on agriculture as their primary source of income and livelihood.However, most farmers consider only the direct costs of farm products like fertilizer, pesticides, cost of labour, and capital (i.e., visible cost) in the production, while the implicit expenses like loss of soil nutrients and fertility (i.e., invisible cost) are oftentimes not fully accounted for and most taken for granted if not neglected.Continuous land cultivation (i.e., intensive farming) without employing soil conservation practices leads to land degradation (i.e., loss of soil fertility), reduces productivity, and disrupts environmental stability.The lack of required nutrients for plants because of nutrient-depleted soils posed a threat to agricultural sustainability [1].
The economic consequences of the degradation of soil and associated soil resources affect stakeholders, particularly small farmers, landholders, local government organizations, and the business sector.In general, the degradation of land and its topsoil lessens the desired economic rent, threatening IOP Publishing doi:10.1088/1755-1315/1302/1/012130 2 farmers' welfare and society.Eroded soils require more inputs such as labour and fertilizer on the part of the farmers to maintain the same level of productivity.Onsite effects of soil loss include loss of natural soil fertility, loss of organic matter, and market prices of agricultural inputs and outputs while the off-site effects of soil erosion caused by sedimentation include reduced fish catch for downstream users, increased sediment of dead storage in reservoirs, and water pollution for downstream users [2].The estimated soil loss from paddies of around 4.23 tons per ha/year using the Universal Soil Loss Equation (USLE) in China [3], while the estimated soil loss in Taiwan reached 8.3 tons per ha/year for the paddies with the slope of approximately 25 degrees [4].Further, if nitrogen (N) could be replenished annually through fertilizer application in developing countries, crops could be grown for at least four years, and the amount of inorganic fertilizer required would cost US$116 over that time [5].In the Philippines, soil erosion is considered the most significant culprit of soil degradation.The rates are higher than the set tolerable soil loss based on crop production maintenance, ranging from 1 to 11 t/ha/yr [6].At an average organic matter, (OM) content of 2.5% and a bulk density of 1.3 g/cc, N lost through soil erosion is approximately 568 kg/ha/yr.A study was conducted to measure the onsite economic cost of soil erosion in the Magat watershed and found that the estimated cost of soil erosion was around PhP15/ton/ha nominal price in 1988 [7].
Buayan-Malungon River Basin has experienced rapid development from the past to the present, particularly in General Santos City (South Cotabato) and Sarangani Province, Philippines.BRMB has four administrative boundaries (i.e., South Cotabato, Sarangani, Davao Occidental, and Davao del Sur).Thus, it faces interconnected biophysical, political, institutional, and socio-economic problems.These problems disrupt the ecosystem services such as supporting, provisioning, regulating, and cultural benefits of the river basin.The River Basin Control Office of the Department of Environment and Natural Resources (RBCO-DENR) identified current concerns of the river basin such as inadequate and inappropriate infrastructure to manage water and land resources, water quality deterioration, wetland resource unsustainability, unappreciated land rights, and watershed degradation.The current state of the river basin is caused by several fundamental issues such as land tenure issues, poor health services, increasing population, weak regulatory mechanisms, limited inter-sectoral collaboration, conflicts due to political boundary disputes, continuous agro-industrial expansion, and highly politicized land use development.As a result, there is a loss of livelihood and income opportunities, property damage, conflicting resource use, over-extraction of resources, biodiversity loss, unsustainable land-use conversion, and erosion.It is estimated that around 79,935 ha of areas within the river basin have severe soil erosion, and about 36,797.85ha and 27,767.79ha have moderate and slight erosion, respectively [8].In Europe, a tolerable soil loss reached 0.3 to 1.4 tons/ha/year using European soil formation data [9].
The most widely used, however, comes from research conducted by the United States Department of Agriculture (USDA), which established tolerable soil loss values ranging from 4.5 to 11.2 ton/ha/year as the maximum level of soil loss that allows for economically and sustainably high crop productivity [10].Following the concerns faced by the BMRB on soil loss, this article provides an estimate of the agricultural productivity loss due to soil loss in the river basin.It focuses on the effect of soil loss on corn productivity in the area.
Estimating the value of soil loss would help farmers, the business sector, the community, and government institutions in planning out the right strategies to lessen and avoid the cost of soil loss.It may also help farmers plan suitable crops to be planted in the area with severe, moderate, and slight erosion, and consider investing in soil conservation practices.It encourages them to invest in a diverse farming system rather than a monocropping system.Moreover, it helps farmers with the best production and management practices and strategies to reduce sediments and nutrient loss from production.This study also helps policymakers, particularly in barangays, municipalities, provinces, and regions to analyze the trade-off due to soil loss in the river basin.It further helps them plan for future agricultural development and sustainable land management of the river basin.It also serves as a guide in planning using an integrative approach or ridge-to-reef approach, which implies multisectoral coordination among the different agencies such as DA, DENR, and various Local 3 Government Units (LGU) from the barangay level to the regional level in protecting and conserving the river basin.

Site Description
The Buayan-Malungon River Basin is located on the island of Mindanao, particularly in the central and southern parts of the island.It passes through the different municipalities/cities of Sarangani, South Cotabato, Davao del Sur, and Davao Occidental.It is one of the 18 major river basins in the Philippines.The Buayan-Malungon River basin covers an area of approximately 150,509 hectares.Although there are several rivers in General Santos City, South Cotabato, and Sarangani that serve as tributaries to the bay, the Buayan-Malungon river basin is the main tributary to the bay.The large rivers are the primary source of sediments in the bay.It is due to severe erosion in the upland portion of a watershed caused by improper agricultural land use and deforestation.Mt.Matutum, located in South Cotabato, contributes to water consumption in Sarangani province [11].
The study area has an upland-lowland landscape.The upland region is made up of forest and agroforestry.Most developments (i.e., infrastructure, buildings, and roads) are found in lowland areas.The lowlands are a more densely populated part of the landscape.Agricultural production activities can also be found in upland and lowland areas, while aquaculture and fishing are found on the coast.Figure 1 shows the location of the Buayan-Malungon River Basin.

Type and Sources of Data
In estimating the agricultural productivity loss due to soil loss in the BMRB, the researcher collected data through personal interviews with the farmers about their production, inputs used and corresponding prices, farm care and maintenance, yield, price of the crop, and income from production.Some production and price data were obtained from the Philippine Statistics Authority (PSA).

Sampling and Analytical Procedure
The corn productivity loss due to soil loss was calculated using information gathered from personal interviews with farmers.To estimate the loss in corn agricultural productivity, researchers selected some barangays within the river basin using the soil erosion map provided by the RBCO in the area.Personal judgment was used to determine which area should be selected based on the data, as long as the area is experiencing slight, moderate to severe soil erosion, and is accessible and secure.In identifying the respondents of the study, barangays with the highest elevation, and experiencing slight, moderate to severe soil loss were chosen.The chosen barangays and randomly selected respondents are given in Table 1 with their corresponding population as of the 2020 Census.Given the total population of 29,563, the researcher set the margin of error to 6%, this was the maximum deviation desired between the sample estimate and the true value.The confidence level was set to 90% or a z value of 1.645.The sample size was computed using the formula below: n=( 2 ×  (1 − ))/ 2  (1) where: z is the z score e is the margin of error n is the population size p is the population proportion Therefore, n=(1.645 2  0.5 (1 − 0.5))/0.06 2 = 188 -total respondents (2) Before conducting the interview, the researcher obtains permission from the respective municipal mayors and barangay captains.A total of 192 farmers were interviewed about their crops and production practices which exceeds the computed 188 total respondents to be interviewed.Out of 192 respondents interviewed, there were 141 corn farmers, 82 banana farmers, and 74 coconut farmers interviewed.The analysis on the estimation of soil loss in agricultural productivity focused on corn productivity because the data on the respondents' production practices were complete, and the majority of the respondents interviewed were involved in corn production.
The data collected from the respondents were encoded, cleaned, and entered STATA software to analyze the effect of soil loss on corn productivity/yield.Using the production function shown below, the researcher estimated the effect of soil loss on corn productivity.The assumptions of the regression error term were checked and tested in the presented model, and no violations of the regression assumptions were found because the error term was normally distributed, with no problems in heteroscedasticity, multicollinearity, or autocorrelation.
where: Crop productivity was influenced by soil quality and other factors.The production function approach was used to analyze corn productivity, with the environment (i.e., soil quality) serving as an input.The use of fertilizer (NPK) and its effect on productivity were used as a proxy for environmental quality (i.e., soil quality) because the effect on productivity is minimal or within limits with good soil quality or when soil loss is within tolerable limits.The effect of using fertilizer to replace nutrient loss due to soil loss was measured using regression analysis.

Loss of agricultural productivity
The application of fertilizer served as a proxy for measuring soil quality in the area.Farmers do not need to apply macronutrient fertilizers such as nitrogen and phosphorus (P) to maintain productivity if soil loss is avoided.According to the interview, farmers used this fertilizer to increase yield because productivity without fertilizer was very low which cannot cover production costs.As a result, farmers used fertilizer to compensate for the nutrient loss due to soil loss.
Total K, gender, educational attainment, and farmer conservation techniques were also variables in the original model.When the model was analyzed using STATA, these variables were not significant thereby left out of the model.The final model and result of regression analysis is given in Table 2.The regression model explains 67% of the total variation in corn yield (Y).At the 1% level of significance, the overall model is significant.The findings revealed that farm size has a substantial significant impact on corn yield at a 1% level of significance.Thus, for every hectare increase in cultivated corn area, yield increases by 1,189.48kg.This implies that the higher the land holdings dedicated to corn production, the greater the yield.Moreover, nitrogen (N) is also significant at the 1% level of significance.This implies that the nitrogen used has a significant impact on corn yield.Thus, for every kilogram of nitrogen applied to a corn field, the yield/ha will increase by 7.6 kg and vice versa, holding other variables constant.Plants require this macronutrient for growth and development.Thus, to maintain production, farmers replace nutrient loss due to soil loss with fertilizer that the plant requires.
At a 10% level of significance, total Phosphorus (P) is also significant.However, the effect of P on corn yield is negative implying that phosphorus-containing fertilizer in production will negatively affect yield.As a result, every kg of P used in the corn field reduces yield by 14 kg/ha, holding other variables constant.This indicates that the soil quality in the area does not require P or that P is already present in the soil.As a result, despite the presence of soil loss, P is not a problem in the area.The findings indicated that P-containing fertilizer should be reduced because it is not needed in the area.The P uptake is effective in the enhanced soil moisture and temperature.Thus, dry soil conditions like in some parts of the area can negatively impact uptake by the root system.
Corn yield is also influenced by household size.It is significant at the 5% level of significance.Thus, for every increase in the number of households working on the farm, yield increases by 193.66 kg/ha/cropping, holding other variables constant.The higher the number of family members working on the farm, the higher the yield.This is true because all of the household members were actively engaged in farm activities.The wife and children were assisting the family's head (the father) in farming activities.Because the area is elevated/sloping, family members assisted each other in production activities from land preparation, planting, crop care, maintenance, and harvesting.The larger the number of household workers on the farm, the larger the area that the household can plant, and thus production will increase.Furthermore, the household member serves as a proxy for labor.Farmers did not hire laborers to work on their farms because they could not afford to pay for labor; therefore, instead of hiring laborers, all members of the household assisted each other in working on their farms.
Furthermore, farmers who participated in training gained skills that significantly increased crop yield.Crop yield will increase by 572.75kg for every training attended by farmers, holding other variables constant.It is valid because the results show that farmer training has a positive effect on corn while controlling for other variables, and it is significant at the 5% level of significance.Farmers attended training that was usually sponsored by the Department of Agriculture (DA), the Agricultural Training Institute (ATI), and the Department of Environment and Natural Resources (DENR), in collaboration with the municipality and respective barangays.DA usually extended and facilitated farming training for various crops, such as corn, rice, and other high-value commercial crops.The ATI provided training on upland farming systems and fertilizer application, while the DENR provided training on forest resource management and agroforestry.Training provided by various government agencies assists farmers in developing and improving their farming skills, particularly in mountainous areas.The training also teaches farmers how to use techniques to reduce soil loss and maintain soil quality, resulting in increased productivity.
The author calculated the effect of soil loss on the productivity of corn.The result of the study revealed that the average N used by farmers for a farm size of 2 hectares was 108.38 kg/cropping.As a result, 108.38kg of N divided by 2 equals 47.74kg of N used per hectare of cropping.The estimated price per kg of N amounted to PhP110.56.Hence, the estimated cost per hectare of nutrient loss due to soil loss reached PhP 5,278.13.
The recommended corn fertilization rate (kg/ha) was N=58, P=16, and K=10 [5].The study's findings on total N used by farmers in the area were 47.74 kg/ha, which is less than the recommended rate by about 10.26 kg/ha.Given that the area is experiencing soil loss, the amount used was lower than the recommended rate.As a result, yield suffers, and without fertilization, production may fall.The 47.74 kg of N multiplied by PhP 110.56 per unit price of pure nitrogen is equal to PhP 5,278.13.This is the per hectare cost of the farmers due to nutrient loss brought about by soil loss.This is the cost of soil loss which is being replaced using N-fertilizer to maintain productivity.The total area planted with corn in the Buayan Malungon River Basin was 31,697.59.This was based on the 2015 and 2020 data from different municipalities across the river basin.Thus, 31,697.59ha multiplied by PhP 5,278 cost of pure nitrogen/ha/cropping resulted in a total cost of nitrogen loss amounted to PhP 167.304 million/cropping season.This is the total cost of pure nitrogen which can be minimized or avoided if soil loss is within a tolerable limit.
The yield, on the other hand, will increase by 7.70kg for every 1 kg of N used by the farmer, according to the results of the regression analysis.Farmers used 47.74 kg of N/ha/cropping, which resulted in a 367.589 kg increase in yield per cropping when multiplied by 7.70 kg (an increase for every kg used of N).Thus, 367.598kg multiplied by PhP 13.45 (corn price per kg) increased the farmer's income to Php 4,944.19/ha/cropping.The cost of pure N per hectare was PhP 5,278 which was higher than the increase in income.Therefore, even if farmers increase their use of pure N fertilizer, the returns remain negative.This could be due to the area's severe soil losses, in which the N used to compensate for the losses cannot compensate for the damage caused by the soil and nutrient losses.On the other hand, farmers may also use manure as a substitute for synthetic fertilizer to improve soil quality.It is also beneficial to our environment [12].
Furthermore, based on the author's calculation, the total area with intolerable loss (i.e., 12-50 tons/ha) reached 44,908 in 2015 while in 2020, the soil loss reached approximately 41,308 ha.This was lower compared to the 2015 data.From 2016 to 2020, the total area planted with corn in the various municipalities was 31,158 ha (CLUP, 2015).Corn production was typically planted in upland/slopy areas, particularly in the Municipalities of Alabel, Malungon, and Malita where soil loss was too high ranging from 20 to 50 tons per hectare per year.This might be due to farmers' production practices, which cause severe soil loss aside from the physical characteristics of the area.
Slashing and burning during land preparation, as well as excessive spraying with chemical pesticides to kill weeds, are common corn production practices in the area.After clearing the area, the farmers can immediately proceed to plant corn.When the weeds began to grow, the second round of chemical spraying was performed to eliminate weeds while protecting the corn plant.Herbicides were applied at least three to four times per cropping.Further, only a few farmers who live in low-elevation areas cultivated their land using carabao and a plow.
Given the farming practices mentioned above, this contributed significantly to soil loss in the river basin.This increases the cost of production by increasing fertilizer to replace the nutrients lost.Thus, if all of this land is planted with corn, the total loss reaches PhP 237,024,424 (Table 3).On the other hand, the unacceptable soil loss cost reached PhP 218,023,624 in 2020.This could be due to farmers' improved production practices as well as other factors affecting soil loss such as rainfall, LULC, and so on.Farmers bore the cost of nutrient loss because they did not receive fertilizer subsidies from the government or non-governmental organizations.To minimize these losses farmers might practice the contour farming system as it helps to improve soil quality, control soil erosion, and increase income per hectare by around 5-10% due to soil conservation benefits derived from the contour farming system [13].To reduce soil erosion, farmers could use several forage species have been used as contour hedgerow species.Among these are Gliricidia sepium, Leucaena leucocephala, Setaria spp., Napier grass (Pennisetum purpureum), and vetiver grass (Vetiveria zizanioides).These species were chosen as hedgerow species because of their value as livestock feedstuffs, as well as their roles in minimizing soil loss, attempting to control the growth of weeds, and enhancing and stabilizing fallow areas [14].These practices will lead to production and watershed sustainability.

Conclusions
Based on the estimated soil loss, approximately 44,909 ha and 41,308 ha of the BMRB area experienced intolerable soil loss ranging from 12 tons/ha/yr to 50 tons/ha/yr in 2015 and 2020, respectively.These areas are typically found in Malungon, Alabel Sarangani, and Malita in Davao Occidental.Furthermore, the regression analysis revealed that corn yield is directly and positively affected by N used in production.Thus, using fertilizer as a proxy for nutrient loss due to soil loss is beneficial for maintaining and improving production.Corn yield increases as the amount of N used in production increases.The study also revealed that farmers used 47.74 kg of N per ha/cropping at a cost of PhP 110.56/kg.Farmers' N use has not yet reached the recommended rate of fertilizer to be used per hectare.Knowing that the area is experiencing soil loss, farmers should increase fertilizer to boost production.For the total N used by the farmers, the cost of N per hectare/cropping was approximately Php 5,278.13/ha/cropping.Therefore, the study concluded that soil loss is very costly.This is very costly to the farmers especially because the majority of them have an income below the poverty threshold thus, it discourages farmers to apply fertilizer in their farms.Moreover, farmers may practice other farming systems such as contour farming systems to help improve soil quality, control soil erosion, and increase income.They can use also manure as a replacement for the use of synthetic fertilizer to improve soil quality for sustainable production and improve the quality of the environment.

Table 3 .
Estimated soil loss in BMRB for 2015 and 2020