Rit Landbúnaðardeildar : B-flokkur - 01.12.1961, Síða 34

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The effect of the time of fertilizer application
on the utilization of phosphorus
By glancing through column No. 5 of Table 3, one notices the large seasonal variation ex-
pressed by the figures. The highest figures are observed for the summer of 1956, but the lowest
ones for 1959. The high fertilizer utilization in 1956 remains unexplained, while the late appli-
cation in 1959 (see page 4) probably was at least partially responsible for the relatively
small fertilizer phosphorus uptake. Approximately 30 per cent less fertilizer phosphorus was
taken up by the grass in 1959 than for 1958 and 1960. This calls attention to the fact that
an early spring phosphorus application is desirable, since it is important that the grasses
have access to sufficient available phosphorus when they resume growth.
II. PHOSPHORUS UPTAKE BY GRASS IN FIELD TRIALS.
For the past five years the total phosphorus content has been determined in many grass
samples from field trials at four experiment stations in Iceland, Akureyri, Reykhólar, Sáms-
stadir and Skriduklaustur. These analyses together with yield data permit the calculation of
the amount of phosphorus taken up by the grass crop. However, these analyses were not carried
out primarily for this purpose, and consequently the data presented in this paper are some-
what fragmentary and discontinuous. Yet, the available data are quite voluminous and may
yield useful information.
The „apparent phosphorus balance“ of the soil, a definition.
In order to simplify the presentation and appraisal of the data, a particular per cent ratio,
named the „apparent phosphorus balance“, is used in Tables 4 to 9. This ratio states the per
cent of total annual phosphorus uptake by grass (the above ground parts), as compared to
the amount of phosphorus annually applied to the soil. If for instance the amount taken up is
equal to that applied, the „apparent phosphorus balance" is 100 per cent. Obviously this ratio
is influenced by several factors, such as the fertility of the soil, the rate of phosphorus appli-
cation, and climatic conditions. For one single year it would be of limited value. If, however, it
is determined for several seasons for plots with unchanged annual fertilizer applications, the
ensuing picture is more likely to assume practical significance.
The effect of nitrogen fertilization on the phosphorus
uptake by grass.
Table 4 and graph 1 show the large effect of nitrogen on phosphorus uptake Table 10 de-
monstrates that for a constant phosphorus application the phosphorus content of the grass
remains almost constant for nitrogen applications ranging from zero to 120 kg N per hectare;
hence the total uptake of phosphorus was almost proportional to the yield. The „apparent
phosphorus balance“ increased on the average from about 40 per cent on the zero N plots
to approximately 80 per cent on the 120 N plots. Consequently more phosphorus should
gradually accumulate on the zero N than on 120 N plots (see Table 13). Yet, after 7 years
of unchanged annual fertilizer treatments, the grass on the latter plots took up 56 and 132
per cent more phosphorus than on the zero plots, for Akureyri and Skriduklaustur respectively.
The effect of phosphorus fertilization on the phosphorus
uptake by grass.
Tables 5, 6 and 7 and graph 2 present the available data relating to the effect of phosphoru*
fertilization on the phosphorus uptake. In each one of the experiments the rate of application