Journal of Agricultural Research 
Vol. IX, No. 7 
186 
CHANGES IN THE NITROGEN CONTENT OF SOILS FOLLOWING THE 
ADDITION OF BLOOD OR OTHER ORGANIC SUBSTANCES 1 
In the studies on the changes in the nitrogen content of soils following 
the addition of nitrogenous materials, a representative sample of soil 
was secured from each field or plot to be studied by making a large 
number of borings to a depth of 12 inches. The soil was then thoroughly 
mixed and passed through a clean sieve to remove gravel, roots, etc., 
and held in a closed container until the moisture content could be deter¬ 
mined. A sufficient quantity of the soil was then weighed out to make 
1 kgm. of dry soil, the desired quantity of the substance to be studied 
was added, thoroughly mixed with the soil, and the moisture content 
brought up to the optimum. The incubation was carried out in 1-quart 
Mason jars held at a temperature of 28° C., the optimum moisture content 
being maintained at all times. When green plants or other substances 
were added, they were first passed through a grinder which rendered 
them sufficiently fine to make it possible to secure a uniform distribution 
in the soil. With the exception of dried blood, the moisture was deter¬ 
mined in all materials used, and the addition based on the dry weight. 
The ammonia was determined by extracting the soil with 10 per cent 
hydrochloric acid, rendering an aliquot part of the filtrate alkaline with 
sodium hydroxid, and then distilling. The reason for using a 10 per cent 
acid will be discussed at length in a second paper, which will deal espe¬ 
cially with the determination of ammonia in semiarid soils. The nitrates 
were determined by the aluminium-reduction method, which gave very 
satisfactory results when the reduction was allowed to take place for 24 
hours, and an acid trap used to prevent any loss of nitrogen during the 
reduction period. All determinations were made in duplicate, and, in 
case there was not a close agreement, additional analyses were made. 
The rate at which ammonia and nitrates are formed from green plant 
material is controlled in a large measure by the maturity of the plants, 
and in order to secure a fair comparison of different plants it is necessary 
that they be taken at the same stage of maturity. All plants used in 
the following experiment were therefore selected at the flowering stage, 
unless otherwise stated in the text. 
The total nitrogen determinations were all made in triplicate by the 
Kjeldahl-Gunning-Jodblauer process. 
On April 8, 1914, soil samples were taken from three plots in the Citrus 
Experiment Station grove and the transformation of nitrogen, following 
the addition of blood or other nitrifiable materials, studied. The am¬ 
monia and nitrates were determined at 7-day intervals for a period of 6 
weeks. At the end of the incubation period the total nitrogen was 
determined, and the gain or loss of nitrogen during the incubation period 
calculated from the original nitrogen content of the soil and the nitrogen 
added in the dried blood or plant substances. 
1 In all ammonification or nitrification studies in which dried blood or other nitrogenous materials 
were used, the figures given represent the gain in ammonia or nitrates over the amount found in the 
control samples, which received water only. 
