8 BULLETIN 27, PORTO RICO EXPERIMENT STATION. 
TESTS WITH FERTILIZERS. 
Cultural practices that have been in general use in Porto Rico for 
yams do not include fertilization, and the fertility of the soil is sel- 
dom considered when the choice of a location is made. The prevail- 
ing belief is that soils suitable in texture are sufficiently rich for yam 
production, and a satisfactory crop can be expected from soil which 
is properly prepared. The home garden in Porto Rico is generally 
made near the house to prevent petty stealing. This land has been 
used for many years and would be expected to yield good returns. 
The station has tested both stable manure and chemical fertilizers 
to determine their effects on different varieties of yams. The soil 
where the experiments were made is a level tract of heavy, dark, 
clay loam built by washings from the hills of the interior. In all 
parts of the field, except where in recent years a slight excess of 
humus and vegetable matter has been deposited on the surface, it is 
even in quality. Neither the surface soil nor the subsoil drains well, 
and ridges on which the crop was planted were built high enough to 
prevent any injury by subsoil water. The land was plowed deeply, 
and the ridges were built with plow and shovel preparatory to plant- 
ing. When finished the oval ridges were 6 feet apart and about 1J 
feet high, and the soil was of good texture throughout. Just before 
planting the fertilizer was applied to the shallow furrow which had 
been made in the top of each ridge and thoroughly mixed with the 
soil. The stable manure was worked deeply into the center of the 
ridges and two applications of nitrate of soda were made near the 
surface after the plants started growth. Tests with chemical fertiliz- 
ers were continued over four years with the Potato and Agua va- 
rieties, and for three years with the Barbados and Guinea varieties. 
From 12 to 30 plants of each variety were grown yearly in each plat, 
the number varying in different years according to the size of the 
plat. Plants of the Potato variety were placed 1 foot apart, while 
the Agua, Barbados, and Guinea varieties were each planted 1\ feet 
apart. The basic formula of the fertilizer used was 5 per cent 
nitrogen, 8 per cent phosphorus, and 12 per cent potash, and the fer- 
tilizer was applied at the rate of 3,000 pounds to the acre. Plantings 
were always made as soon as the spring rains occurred, which was in 
late March or early April. The crop was not dug until the following 
February or March, so that the roots might remain in the ground 
until planting time to insure first-class seed. 
The table following shows the average yield per hill of the differ- 
ent varieties used in these tests in the fertilized and check plats for 
all the harvests while the work was in progress. 
