Vegetable production in the Philippines is one of the prime aspects of the country’s agriculture. Aside from being a major source of nutrition for Filipinos, it is also a major source of livelihood for our farmers.

Providing higher economic returns, developing new export markets for high value crop and livestock commodities, and improving farmers’ income are among the Department of Agriculture’s main priorities. However, for vegetables, the high cost of production inputs, such as fertilizers, is a major impediment to improved productivity.

cabbage field

It has been observed further that vegetable farmers apply high levels of fertilizers as a normal practice without the benefit of soil test and knowledge as to the nutrient status of the soil. Not only is this a waste of resource, but this practice also tends to accumulate nutrients in the soil which has detrimental effects on the general environment. It is not sustainable over any length of time.

In order to correct and improve on the said farming practice, Dr. Anabella B. Tulin and Ms. Marciana B. Galambao of the Visayas State University (VSU) set up two fertilizer trials in a cabbage farm located in Cabintan, Ormoc City, Leyte whose soil is characterized as a phosphorus-deficient acid soil. The study, “Enhancing the Yield of Cabbage in P Deficient Acid Soil through Site Specific Nutrient Management and Nutrient Balance Studies” won the silver award during the 24th National Research Symposium (NRS).

The study

Barangay Cabintan is located 18 km northeast of Ormoc City with an elevation of around 900 m above sea level. Common land uses in the area include vegetable cropping (sweet peppers, cabbage, eggplant, tomatoes), and corn production. The rest of the area is allowed to fallow or left to wild shrubs and forest tree species. The soil of Cabintan is derived from volcanic deposits and the abundance of strongly-binding amorphous material means that phosphorus-fixation capacity is high in this type of soil. While phosphorus may be present in the soil, it is rendered unavailable to plants by this characteristic.

The objectives of the study is to determine the most sustainable soil treatments in terms of increasing the yield of cabbage while minimally impacting the environment, and to establish through nutrient balance assessments the best levels of nitrogen (N), phosphorus (P), and potassium (K) fertilizer application for profitable cabbage production in P-deficient soil.

To address this concern, two fertilizer trials were conducted in P-deficient acid soils in farmer’s fields planted to cabbage in order to establish and verify the most sustainable treatments in terms of increasing the yield of cabbage with least harm to the environment. The partial nutrient budgets of NPK in the soil were also quantified. In both trials, the farmer’s practice of applying fertilizers to cabbage were used as one of the treatments (control) followed by alternative treatments that were based on the soil test results and on the application of site-specific nutrient management (SSNM) tools.

Treatments include for the wet season are: T1, 444-93-142 kg/ha N-P2O5-K2O (as the farmer’s practice), and the following alternative treatments: T2, 44-55-142 kg/ha N-P2O5-K2O; T3, 88 -110- 60 kg/ha N-P2O5-K2O; and T4, 66 -82.5 -45 kg/ha N-P2O5-K2O. For the dry season, the treatments include: T1, 355 -258 -396 kg/ha N-P2O5-K2O (as the farmer’s practice); T2, 120-120-90 kg/ha N-P2O5-K2O; T3, 60-60 -30 kg/ha N-P2O5-K2O; and T4, 90-90-60 kg/ha N-P2O5-K2O.

The findings reveal that reducing fertilizer inputs from 444-93-142 kg/ha N-P2O5-K2O to 88-110-60 kg/ha N-P2O5-K2O during the wet season and from 355-258-396 kg/ha N-P2O5-K2O to 90-90-60 kg/ha N-P2O5-K2O during the dry season gives sustainable yields of cabbage, reduces the cost of fertilizer inputs, and improves the balance of nutrients. These treatments not only reduced the fertilizer inputs, but also increased the yield of cabbage from an average of only 5 tons per hectare to about 12 tons per hectare.

Observations and Findings

The researchers believe that there is need to assess the current soil fertility status in soils used for vegetable production, quantifying the rates of nutrient removal from these systems (mass balances), and developing strategies for matching nutrient inputs to crop and soil requirement through judicious and integrated application of inorganic and organic fertilizers.

The use of organic fertilizers is recommended but there is a need to quantify the availability and types of materials, evaluate the treatment and stabilization technologies, and determine how organic fertilizers can be applied in conjunction with inorganic fertilizers to optimize productivity and profitability. Dissemination and training activities are also required to promote the outcomes and maximize the benefits of proper soil fertilization to growers.

Dr. Tulin reiterated that crop production can be increased without any increase in the negative environmental impact associated with agriculture. Greatly enhanced efficiency in the use of nitrogen, phosphorus and water, as well as the adoption of integrated pest management minimizes and can even do without the need for toxic pesticides. The study of Tulin and Galambao further noted that nothing is wrong with fertilizers biophysically or environmentally when properly used.

The usual recommended rates for fertilizers assume that soil fertility is low. However, it is not appropriate to apply these to continuously cultivated soils as such soils increase in residual fertility over time (Yoo and Jung, 1991). When the accumulation of some nutrient elements in the soil is already evident, it is time to re-evaluate soil fertility.

Macronutrients are the most important elements considered by most vegetable growers. Plants absorb these elements at different rates and according to growth stages. If there is an excess of soil nutrients, the surplus will not be taken by plants anymore but could be lost by denitrification and/or by leaching for N, and fixation for K and P (Saito, 1991). Thus, an assessment of the amount of available NPK, as well as the level of micronutrients, through soil testing is recommended.

Vegetables planted without fertilization will not attain profitable yield and this forces farmers to apply high levels of fertilizers which tend to accumulate in the soil over long periods of time. Usually the farmers apply fertilizers without the benefit of soil tests and they, therefore, are not aware of the nutrient loading and imbalances in the environment and their adverse effects on our fragile soil resources. “Thus, one way to minimize these problems is to develop an effective and holistic site-specific nutrient management to enhance vegetable production in the province of Leyte,” concluded Tulin.

The results from the soil survey described by Tulin et al. (2010) were used to develop the summary of nutrient status of vegetable soils in order to determine which soil chemical properties were limiting factors to the production system described by the farmer. In determining the nutrient status of the soils in Cabintan, five composite soil samples were obtained from different farmers’ fields that have been continuously grown to vegetables as representative of the site.

The soil survey results indicate that the vegetable soils in Cabintan are typically low in soil pH, high in percent organic carbon and total N, low in P and CEC, and are from low to high in K. The low P confirmed the volcanic nature of Cabintan soils which have very high P-fixing capacity. The very high amounts of percent organic carbon and total N could be attributed to the application of high levels of chicken manure in most vegetable soils in Cabintan.

Conclusions and recommendations

Sustainable soil fertility and nutrient management entails an understanding of the soil fertility status, matching inputs to crop requirements, and monitoring to ensure nutrients do not accumulate or diminish over time. Farmers’ better understanding of the application of fertilizer based on crop need and soil analysis can lead to effective and efficient allocation of capital for farm inputs.

The researchers provided scientific evidence on the importance of using the appropriate tools and methods in assessing the fertility status of the soil and in designing alternative fertilizer treatments that will provide the sufficient amounts of nutrients needed by the crop. With the conduct of proper soil tests and nutrient balance determinations, the amount of fertilizers applied by the farmers can be greatly reduced to more economical and sustainable levels.

The findings further indicate the importance of soil test in establishing holistic site-specific nutrient management for cabbage production, especially in coming up with the best treatment and input combinations for profitable cabbage production. The observed increase in the yield of cabbage indicates that the recommended levels of NPK fertilizers used in the project/ experiment do work.

The significant findings and conclusions that can be drawn from the study were as follows:

The nutrient status of phosphorus-deficient acid soil in Cabintan was determined from the soil test results and participatory assessment surveys. The findings gave proof about the importance of applying appropriate and more environmentally-suitable levels of NPK fertilizers to improve the growth and yield of cabbage in the P-deficient acid soils of Cabintan, Ormoc City.

Among the different treatments used, a fertilizer level of 88-110-60 kg/ha N – P2O5 – K2O was found to be more productive and sustainable for cabbage production in either the wet or rainy seasons. This gave a yield of about 12 tons/ha and entailed a fertilizer cost of no more than P16,000/ha which is equivalent to 40% of what the farmer normally spends.

During the dry season, a fertilizer level of 90-90-60 kg/ha N – P2O5 – K2O was found to be most sustainable for cabbage production. This gave a yield of about 12 tons/ha at a fertilizer cost of P6,123.00 which is only 20% of what the farmer normally shells out. Relatively higher levels of fertilizers are needed during the wet season as compared to the dry season.

Site-specific nutrient management practices are guided by the philosophy of matching fertilizer application to the nutrient requirements of the crop and the fertility status of the soil based on soil test results and nutrient mass balance.

Farmers can be taught to understand the fertility status of the soil, as the basis for proper fertilizer management in their area, along with nutrient application, uptake, removal and mass balances in vegetable production.

The most suitable levels of fertilizers based on soil test results and nutrient balance studies for increasing the yield of cabbage were established. The information generated in the project was shared with other farmers through a techno forum conducted at Cabintan, Ormoc City. Dr. Tulin further recommended that the said technology be verified in other sites in Leyte that grow vegetables and those that grow root crops, which is another important source of food in the province. ###

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For more information, please contact Dr. Anabella B. Tulin of the Visayas State University at telephone no. (053) 563-7229 or email: belle24@yahoo.com

References:
1. Saito, M. 1990. Mineralization of soil organic nitrogen and its movement in an Andisol under field conditions. Bulletin of the Tohoku Natl. Agric. Exp. Stn. 82: 63-76. (In Japanese with English summary).
2. Tulin , AB, Quinones CM. Rallos R, Mercado, A Jr, Salvani J, Lapoot C, Justo V, Dorahy C . 2010. Evidence-based nutrient management strategy in identifying fertility status and soil constraints for vegetable production in Southern Philippines. In Gilkes, RG, Prakongkep N, editors. Proceedings of the 19th World Congress of Soil Science; Soil Solutions for a Changing World, 2010 Aug 1-6. Brisbane, Australia: IUSS, pp. 168-171.
3. Yoo, Sun-Ho and Jung Yeong-Sang. 1991. Soil management for sustainable agriculture in Korea. College of Agriculture and Life Science, Seoul National University, Korea.

Source: Patrick Raymund A. Lesaca AND DR. Anabella B. Tulin, BAR Digest April-June 2013 Issue (Vol. 15 No.2)