ORGANIC LETTUCE GROWTH AND NUTRIENT UPTAKE RESPONSE TO LIME, COMPOST AND ROCK PHOSPHATE

Fertilizer recommendations are needed to increase organic vegetable yields. Thus, organic lettuce growth and nutrient uptake was investigated in a randomized block pot experiment with twelve treatments from the factorial structure of three factors: (i) Gafsa phosphate [0 and 200kg phosphorus pentoxide (P2O5) ha(-1)], (ii) compost from source separated municipal organic waste (0, 15, and 30 t ha(-1)) and (iii) limestone [0 and 8 t ha(-1) calcium carbonate (CaCO3) equivalent]. Lettuce yield increased with compost application and a first order interaction between lime and phosphate was clear because lime partially replaced the need for phosphate. This was explained by the effect of liming on P availability in acid soils. Nitrogen (N), phosphorus (P), and potassium (K) accumulation increased in lettuces produced with compost or phosphate but only the accumulation of N was increased with lime. This compost is recommended to increase nutrient availability for organic lettuce whereas the need for phosphate fertilization may decrease with liming.