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Improved Tropical Forage Fertilizer-P Calibration: In support of FRST to conduct state-level soil-fertility correlation and calibration trials for P and/or K

Tropical forages grown for feed is the agricultural commodity occupying the largest land area in Puerto Rico (ELA, 2012; USDA-NASS, 2018). High nutrient extraction rates and crop response to fertilizer-phosphorus (P) have been documented in Puerto Rico (Vicente-Chandler, 1982). Current recommendations are to apply 2,000 lb/acre of complete formulation 15-5-10 (or 100 lb P2O5/ac). Soil test levels are not used to guide P fertilization; rather decisions are made intuitively. For example, in manured soils only fertilizer N as urea is applied.
Current soil test P critical levels are 22 and 16 mg P/kg for Bray 1 and Olsen-P, respectively. The soil test critical levels have not been validated in Puerto Rico and are derived from an extensive literature review (Sotomayor-Ramírez and Martínez, 2019). There is a need to validate current soil test critical levels as well as carry out calibration studies (crop response to fertilizer P at varying soil test levels). We report on a calibration experiment using an improved tropical forage, Brachiaria decumbens in a soil with soil test P in the Low category.

Materials and Methods
An experiment was established in a private farm in Lajas, southwest Puerto Rico. A 0.09 ha field was selected having a history of limited fertilization. The predominant soils were Paso Seco (Fine, mixed, superactive, isohyperthermic Entic Udic Haplusterts) and Palmarejo (Fine, mixed, semiactive, isohyperthermic Typic Haplustults). An area within the field was selected having soil test P (Bray1) concentration ranging from 1.2 to 1.6 mg P/kg. Soil pH ranged from 5.4 to 6.4. The soil within the experiment was mapped as Palmarejo series (USDA-NRCS, 2021). The predominant forage was a 10-year stand of Brachiaria decumbens (Stapf), otherwise known as Signal grass or Urochloa decumbens (Stapf).
The experiment consisted of five treatments with four replicates in randomized complete block design. The fertilizer-P treatments were 0, 60, 120, 180, 240 kg P₂O₅/ha-yr split in two applications at the start of the experiment (17 February 2021) and four months thereafter (24 June 2021). The fertilizer-P source was triple superphosphate. The fertilizer treatments were applied by hand. All plots received complementary nutrients at an annual rate (kg/ha) of 300 N, 300 K2O, 25 micronutrient mix, split in two, on 19 February 2021 and 25 June 2021 (or 9 and 135 days after experiment initiation). Fertilizer-treatments and complementary nutrients were applied after selected forage harvests and hay removal.
Forages were harvested at approximately 42 to 45 d intervals. Nine harvests were scheduled at the selected forage harvest intervals. Forage material in 0.58 m2 quadrants cut and the fresh and dry weight was recorded and calculated. After forage subsampling, the plots were mowed to simulate haylage. Selected forage samples were analyzed for total elemental analysis.
Prior to experiment initiation, soils were sampled from each plot at 0 to 15 and 15 to 30 cm depth intervals. Soils were -re-sampled at 0 to 15 cm depth 98 d after experiment initiation (after the second harvest). Soils were analyzed for Diagnostic Test in Ag-Source Laboratories (Lincoln, NE). Soil test P was analyzed using Bray1 and Mehlich 3 extractants and quantified using ICP.

Cumulative monthly precipitation was 635.02 mm until 4 November 2021 (260 d period). Rainfall was in deficit (-12% less) relative to the 30 year mean for the study period. Forage growth may have been limited by available soil moisture.
Soil test P prior to experiment initiation was 1.8 mg P/kg. Soil test P increased with increasing fertilizer-P addition. The increase in soil test P per fertilizer P added was not necessarily consistent with the magnitude of application, ranging from 1.0 to 2.0 mg P/kg per 100 kg fertilizer-P2O5/ha.
Soil pH was near 5.5, which is close to that in which exchangeable Al can limit yield of most crops. Soil Ca, Mg, K were above suggested critical levels of 6, and 2.5 cmolc/kg, respectively. Soil K was below suggested critical levels of 0.4 cmolc/kg, yet we assume that fertilizer-K application near 300 kg K2O/ha was sufficient to ameliorate any potential deficiency. There was an increase in exchangeable K as a result of fertilization.
Cumulative dry matter forage yields of six harvest after 252 days were 7,595, 7,568, 7,659, 7,233 and 8,417 kg/ha with treatments of 0, 60, 120, 80, and 240 kg P2O5/ha, respectively. There were no significant differences between treatments (ANOVA, p = 0.82). There were significant differences between harvests (ANOVA, p <0.01).
Phosphorus concentration in vegetative tissue was determined in treatments P1 and P5 for samplings 1 and 2. In the first harvest, P concentrations were 1.85 g/kg and 2.73 g/kg for P1 and P5, respectively. In the second harvest P concentrations were 1.75 g/kg and 2.75 g/kg for P1 and P5, respectively. Based on current cumulative dry matter yields, extraction estimates were for specific harvests were 13.29 kg P/ha, and 23.14 kg P/ha, for P1 and P5, respectively.

B. decumbens did not respond to fertilizer-P application as was expected. B. decumbens can overcome P deficiency in soils with low P without fertilization through various strategies (Nanamori, et al. 2004; Bonfim et al. 2004)..
Under P deficiency, the source-to-sink ratio has a great effect on carbon export from source leaves. Since carbon metabolism is known to be affected by the P status in plant tissue, Brachiaria increased carbon and root biomass allocation under stress while maintaining P allocation to the shoot (Halsted and Lynch, 1996). Another mechanism used by Brachiaria for acquiring larger amounts of P from soils with low P is the vigorous root system and the increase of root:shoot ratio. A lower P concentration in leaves may indicate that the plant uses P more efficiently to sustain active metabolism for dry matter production (Nanamori et al. 2004).
Rao (2001) indicate that the most prominent root characteristic in grass is the high root length that results in a large root surface area and a high ratio of root surface to shoot dry weight. These parameters greatly enhance P acquisition by roots and supply to the shoots. A finely divided and rapidly developing root system provides better access to less mobile soil nutrients, such as P.

Bonfim, E., Freire, F., Santos, M., Silva, T., & Freire, M. (2004). Níveis críticos de fósforo para Brachiaria brizantha e suas relacöes com características físicas e químicasa em solos de Pernambuco. Revista Brasileira de Ciëncia do Solo, 281-288.
ELA-ORIL (Estado Libre Asociado de Puerto Rico, Oficina para la Reglamentación de la Industria Lechera). 2012. Informe Anual, Año Fiscal 2010-2011. San Juan, Puerto Rico. 76 p.
Halsted, M., & Lynch, J. (1996). Phosphorus responses of C3 and C4 species. Journal of Experimental Botany, 497-505.
Nanamori, M., Shinano, T., Wasaki, J., Yamamura, T., Idupulapati, M., & Osaki, M. (2004). Low phosphorus tolerance mechanisms: Phosphorus recycling and photosynthate partitioning in the tropical forage grass, Brachiaria hybrid cultivar mulato compared with rice. Plant & Cell Phisiology, 460-469.
Rao, I. (2001). Adapting Tropical Forages to Low-Fertility Soils. Proceedings of the XIX International Grassland Congress (pp. 247-254). Piracicaba: Brazilian Society of Animal Husbandry.
Sotomayor-Ramírez and G. Martínez. 2019. Fertilizer recommendations guide for some of the most common crops in the Caribbean. Fertilizer recommendations guide for some of the most common crops in the Caribbean. Final Project Report under Cooperative Agreement no. 68-F352-16-501 between U.S. Department of Agriculture, Natural Resources Conservation Service (USDA-NRCS) and the University of Puerto Rico College of Agricultural Sciences, Mayagüez, PR. Ten chapters.
USDA-NASS, 2018. https://www.nass.usda.gov/Statistics_by_State/Puerto_Rico/index.php.
USDA-NRCS. 2021. Web Soil Survey. Available at: https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm.
Vicente-Chandler, J., F. Abruña, R. Caro-Costas and S. Silva. 1983. Producción y utilización intensiva de las forrajeras en Puerto Rico. UPR-RUM-AES. Bulletin 271. 226 pp.

Release Date
Spatial / Geographical Coverage Area
POINT (-67.0944 18.028268)
Ag Data Commons
Spatial / Geographical Coverage Location
Tai South Farm, Lajas, southwest Puerto Rico
Temporal Coverage
February 17, 2021 to November 1, 2021
Contact Name
Sotomayor, David
Contact Email
Public Access Level