Abstract
Most of the phosphorus (P) used in fertilizers comes from phosphate rocks, a rare natural resource that must be preserved. One option is to better optimize the efficiency of P use by plants. For several years, controlled release phosphate fertilizers have been developed with polymers-based coatings to reduce the interactions between P and soil minerals and increase the synergy between P release and plant needs. However, their production cost remains a major obstacle to their use. Moreover, most of the polymers used are not degradable in the soil. The objective of my thesis is (1) to use biopolymers derived from vegetal biomass waste to prepare coating formulations (2) to test these coatings on triple superphosphate (TSP) granules, to characterize them and to study their dissolution in water, (3) to evaluate their effect on the properties of a Mediterranean field crop soil. The different coated fertilizers formulated based on lignin extracted from olive pomace, polysaccharides (carrageenan, sodium carboxymethyl, sodium alginate), and plasticizers (polyethylene glycol, glycerol) showed a slower release of P in water, compared to uncoated fertilizers. The rate of P release depends on the thickness of the coating, its elasticity, its elongation, its hydrophobicity and its hygroscopy. Of all the formulations tested, coated fertilizers based on lignin and lignin-carrageenan had the slowest P release rate. Special attention was paid to these two formulations to study their effect on soil properties. These coated fertilizers, when dissolved in soil, slightly acidify the soil and have a P migration distance of 14 mm in diameter. Uncoated fertilizers further acidify the soil and have a larger P migration zone (21 mm in diameter) after the same 28-day period. However, after 28 days, the TSP coating did not affect the availability of P in the soil. However, coated fertilizers tend to increase the microbial P concentration in the soil.