In the period 1947-52 several fertilizer field-trials on potatoes were carried on,most of them at Centinela Experiment Station. Centinela has highly organic soil and a rainfall of about 1.500 milimeters.
Rate of fertilization.-important increases of yields are produced up to a rate of 100, 200, 300 (Kg. per hectare of N, P2O5, K2O). Over these rates increments are rather small and erratic.
Comparison between several phosphates.-According to their effect on the yield the tested phosphates can be ranked as follows:
1. Melón, Pelícano, Superphosphate, Red Guano.
2. Bone flour.
3. Fosforite, (rock phosphate) Phosphal.
Melón, Pelícano and Phosphal are fused phosphates. All the phosphates tested gave measurable increases in yield.
Comparison between organic and mineral fertilizers.-Using the same amounts of nutrients, organic fertilizers always had a better effect than mineral fertilizers.

1 Ingenieros Agronómos de la Sección Suelos y Sección Fitotecnia, respectivamente, del Departamento de Investigaciones Agrícolas.