Nitrogen Affects Cacao (Theobroma cacao L.) Tree Growth, Macronutrient Demand, and Water Use

Cacao (Theobroma cacao L.) is a globally important crop grown predominantly in agroforestry systems relying on soil nutrients, rainfall, and shade trees. Yields are chronically low. Improving fertilizer management by understanding how nutrients affect cacao growth and reproduction is key to increasing cacao yields. Differential nutrient treatments were given to 9-month-old seedlings growing in 200-L-volume lysimeters in a climate-controlled greenhouse until 40 months of age. The seeds were obtained from the International Cocoa Quarantine Center from a manually self-pollinated CCN-51 clone. There were seven replicate trees per each of five fertilizer treatments, given via fertigation. The treatments all had the same phosphorous (P), potassium (K), and microelement concentrations, with the water supply supplying all other elements; the treatments varied only in nitrogen (N) concentration (10, 25, 50, 75, and 150 mg N/L). The goals were to characterize, in response to N fertilization level: cacao tree growth, leaf N dynamics, uptake of P and K, and water use. N fertigation concentration of 50 to 75 mg N/L resulted in maximum growth and development. Both lower and higher N fertilizer concentrations resulted in significantly reduced growth and water use in comparison. Plant demand for both P and K per unit biomass was double under low N availability compared with higher N treatments. Optimal N uptake values for these young, preproductive, size-constrained trees were equivalent to 98 and 153 kg N/ha/year (calculated for 1100 trees/ha) in consecutive years. This can be compared with current “best practice” in cacao-producing regions where at most, N equivalent to that removed in the fruit, 19 kg N/ha/yr on average, is given. To put this into perspective, trees in the clearly deficient N treatment took up the equivalent of 22 kg N/ha/yr. The conclusion is that much higher N loads than commonly given are needed to promote growth and fruit set and to accommodate the trees’ requirements as they increase in size.