Abstract
The objective of this study was to test for appropriate quantity of fertilizer (NPK 12-11-18) required to boost the production of cassava (variety TME 419) in the forest zone of Cameroon. The trial was conducted at the experimental farm of the Institute of Agricultural Research for Development (IRAD) in Mbalmayo. The experimental design was completely randomized block design in three replicates. The treatments were as follows: T0 (Control (No input)); T1 (200 kg.ha-1); T2 (300 kg.ha-1); and T3 (400 kgha-1). The data were analyzed using the IBM SPSS Statistics 20 Software. The results showed that chemical fertilizer improves the growth (number of leaves, plant height, and petiole length) and yield (number of marketable root, yield) of cassava mainly because it enhances soil nutrient availability. Thus, NPK-fertilizer significantly (p<0.001) increased petiole length by 20.71, 27.83, and 31.62% for T1, T2, and T3, respectively at 5 months after planting (MAP). The highest plant height and number of leaves at 4 and 5 MAP was observed with T3 followed by T2 and T1. Similarly, the highest root yield (43.33±10.97 t.ha-1) was observed with treatment T3. Hence, the fertilizer application rate of 400 kgha-1 of NPK 12-11-18 appears most appropriate for intensification of cassava production in the humid forest agro ecological zone of Cameroon. Key words: Cassava, growth, NPK, root yield, soil fertility.  
Highlights
Cassava (Manihot esculenta Crantz) is a food crop grown in all agro-ecological zones of Cameroon for its leaves and tuberous roots (Mouafor et al, 2016; Temegne et al, 2016; Fonji et al, 2017; Njukeng et al, 2017)
The objective of this study was to investigate the appropriate amount of NPK fertilizer required to boost cassava production in the humid forest agro ecological zone of Cameroon, which is characterized by poor ferralitic soils
3 months after planting (MAP), the number of leaves was significantly the highest at T3; it was followed by T2, T1; T0 showed the lowest number of leaves
Summary
Cassava (Manihot esculenta Crantz) is a food crop grown in all agro-ecological zones of Cameroon for its leaves and tuberous roots (Mouafor et al, 2016; Temegne et al, 2016; Fonji et al, 2017; Njukeng et al, 2017). Cassava is still considered a soil-depleting crop (Ngome et al, 2013) and its production on degraded soils of Cameroon requires fertilizer use. Large areas of sub-Saharan soils including Cameroon are affected by various types of degradation including soil fertility decline (Ngome et al, 2011). Nutrient balances are largely negative and symptoms of nutrient deficiencies are widespread (Temegne et al, 2015). This phenomenon is exacerbated by intense cultivation (Nambiro, 2008), poor residue management (Ngome, 2006) and poor access to mineral fertilizer due to high unit cost and irregular supplies.
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