The effect of N fertilisation on the seed yield and on the crude prote
in and amino acid contents of soya was examined in a lysimeter experim
ent set up on Ramann's calcareous sandy roam (CaCO3 2.1 %, humus 1.3 %
, silt 28 %, poorly to moderately supplied with available P and K). Th
e treatments were 0, 40, 80 and 120 kg/ha N in the form of NH4NO3, wit
h 100 kg/ha P2O5 and 120 kg/ha K2O as basic fertiliser. The four N tre
atments were set up in three replications using a total of 12 lysimete
rs, each of which had a soil volume of 4 m(3) and a growing area of 4
m(2). The plant stand was equivalent to a density of 500 000 plants/ha
and the water supply was adjusted to 70 % field water capacity. The m
ajor conclusions can be summarised as follows: 1. The yield of uninocu
lated soya increased parallel to the rate of N fertilisation or the N
supplies of the soil. The N % and crude protein content of the seed wa
s not modified to any great extent by N fertilisation, since the soil
supplies were utilised in forming a yield surplus. The crude protein y
ield per hectare increased with the seed yield and rose by some 70 % a
s the N supplies improved. 2. As a result of the N surplus arising in
the 2nd and 3rd years of the experiment the seed yield was 4-5 t/ha, t
he seed N content was 6-7 %, the crude protein content was 35-40 % and
the crude protein yield was 1.8 t/ha, i.e. three times that obtained
in the 1st year. 3. In contrast to the results of field experiments, p
robably due to the less drastic N surplus, the amino acid content did
not differ significantly in the majority of cases as a function of the
N supply. However, there was a significant rise in the contents of th
e essential amino acids threonine (THR) and methionine (MET) and in th
ose of the non-essential amino acids serine (SER) and aspartic acid (A
SP). This increase was moderate, averaging 10-15 %. 4. The essential a
nd non-essential amino acid contents of soya flour amounted to 10 and
15-16 g/100 g protein, respectively, giving a total value of 25-26 g/1
00 g protein. The total amino acid yield fluctuated between 226 and 37
6 kg/ha, rising roughly parallel with the seed yield. In the first yea
r of the experiment the N surplus was not great enough to cause major
modifications in the crude protein contents or amino acid composition.