Phosphorus Availability and Carrot Growth as Influenced by Phosphate Rock Management under Acid Soils in Murang’a and Tharaka Nithi Counties

Soils from Central Highlands of Kenya are characterized by low phosphorus (P) availability with crops exhibiting P deficiency symptoms. The objective was therefore to determine the P sorption characteristics of the soils and to assess their standard P requirements in order to recommend the P fertilizer required for different crops in the area. Besides, the study aimed at assessing the efficiency of lemon and pineapple juices and the concentration and time needed to release more than 50% of available P from phosphate rock (PR), and; the effect of different types of PR management on carrot yields, nutrient uptake, and P-use efficiency. In the first study, four different soils (Humic andosols, Orthic acrisols, Humic nitisols and Rhodic nitisols) from the Central Highlands of Kenya were used for the study. Phosphorus sorption was determined using the Batch Equilibrium method, data fitted in the Langmuir isotherm model and relationships between the P adsorption maximum (Smax) and soil properties determined by simple regression. The standard phosphorus requirement (SPR) of the soils was determined from P sorption curves at soil solutions of 0.05 to 0.2 mg L-1. About 46 to 81% of P added to the soils was adsorbed. The sorption maxima (Smax) of P were similar for the Humic andosols and Humic nitisols and were significantly higher than in Rhodic nitisols and Orthic acrisols. Gibbsite, kaolinite, calcium and iron were identified as the main factors influencing Smax in the soils. In the second and third experiments, a laboratory incubation trial to assess efficiency of organic juices to solubilize PR and field trials to assess efficiency of dissolved PR on crop growth and yield were set up at two sites with Humic Andosols and Orthic Acrisols over two seasons in Kenya. Lemon and pineapple juice and distilled water at volumes 100, 200, 300, 400 and 500 mLs were used in dissolution of 100 g of PR. In a Randomized Complete Block Design, replicated three times, eight treatments were compared. Lemon juice was effective in solubilizing PR, releasing 63% of the total P applied into available P, compared to 11% for pineapple juice and 6% for water. The combined application of compost and PR dissolved in lemon juice at planting significantly increased P and K uptake, P use efficiency and carrot yields which was comparable to the use of triple superphosphate and compost. Soils from Murang’a and Tharaka Nithi counties in the central highlands of Kenya fixed large amounts of phosphorus and interventions will be required to reduce phosphorus fixation and enhance its availability to crops. However, the differences in soils’ phosphorus fixing capacity and their standard phosphorus requirement indicate that policy interventions should be soil specific. The study concludes that the dissolution of phosphate rock with lemon juice at a ratio of 1:5 phosphate rock to lemon juice and its combined application (immediately after dissolution) with compost at planting improves nutrient uptake, phosphorus use efficiency and crop yields. It is recommended that further studies be done to explore the possibility of using citrus peels or other acidic organic materials to enhance the solubility of phosphate rock, and to assess their practical feasibility and the economic advantage(s) in the large scale production of high value crops.