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Under greenhouse conditions the response of maize to added Mo to an alfisol was studied. It was followed and experiment with five treatments of P with three differnt amounts of Mo. which ranged from of = to 125 ppm P and from of 0 to 4 ppm Mo. respectively. It was found that the phosphorus fertilization did not influence on the Mo desortion from this soil. In spite of the variable P concentrations. similar values of soil Mo where extracted by the ammonium oxalate procedure. There was not improvement in the dry-matter production by the application of Mo. Although it was detected an increase in this nutrient from 1.46 to 5.50 ppm in maize. In both Mo treatments. Mo did not attain the toxic level in plant material because the concentrations obtained were lower than 15 ppm Mo.

Fertilidad de suelos, Under laboratory and greenhouse conditions the pattern of a soil which behaves like an alfisol, related to added P was studied. The relation between applied P and that extracted from soil by the methods of Olsen and Bray-Kurtz were determined. With yhe procedure of Burriel-Hernando there were problems related to the development of the blue solour due to the interference of the high content of iron present in the soil extract. In the grrehouse, maize was used like a biological assay. There was a positive response of this crop to P added to the soil (0 to 344 ppm P) since the dry matter production increased according to P treatment. At P lower than 200 ppm of phosphorus added to soil, the content of this nutrient in plant material remained constant, achieving a value of 0.10 % P and deficiency symptons of this nutrient were detected in the leaves of maize. As long as the P fertilization was increased, the content of this nutrient was omproved to 0.20 %. There was also a good concordance among the plant-yield values and those of P extracted from soil by the methods of Bray-Kurtz and Olsen. However it was not possible to delimit the critical concentration for these precedent laboratory proceedures.