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Experiments are reported in which a study was made of the absorption of NH4(-) and NO3(-)N and plant growth by cotton seedlings. The seedlings were grown in solution, sand, and soil cultures. The solution cultures were used in a series designed to study the influence of various concentrations of NH4(-) and NO3(-)N on the absorption of these ions. The effect of the age of the seedlings, reaction of the culture solution, and length of the absorption period on the relative amounts of NH4(-) and NO3(-)N removed from the culture solutions were studied. Results of a study of the NH4(-) and NO3(-)N content and the pH value of sap from plants at different ages are included. The results of the investigation may be briefly summarized as follows: NH4(-)N was used in larger amounts than NO3(-)N by the young seedlings until they were from 3 to 5 weeks old. After this age more NO(3)-N was absorbed. The data show that both NH4- and NO3(-)N were absorbed in large amounts when the plants were 4 to 8 weeks old. Both growth and fruiting of plants were largest when both forms of nitrogen were present. NH4(-)N absorption increased as the acidity of the culture solution decreased; the absorption of NO(3)-N was only slightly affected by the reaction of the solution used. The highest total nitrogen absorption usually occurred at pH 6.0. Total nitrogen absorption was greatest when both forms of nitrogen were present. The growth and fruiting of cotton in field plats agreed well with that of the culture solutions. Data are presented which indicate that nitrogen in the seed is available to the sprouts and young seedlings in the NH4 form. A theoretical discussion of the observed absorption phenomenon is included.