Rit Landbúnaðardeildar : B-flokkur - 01.12.1961, Blaðsíða 36

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effects and soil effects cannot be separated, and comparisons of inherent soil productivity at
two or more sites must therefore be done with caution. Consequently this paper attempts only
to relate the experimental findings to two variables: the annual quantities of applied nitrogen
and phosphorus in fertilizers. In this connection it may be pointed out that a relative appraisal
of different soils on the same experimental site is easier by assuming that the climatic effects
are indentical for the soils in question.
In reading through Table 8 one notes for instance the following: The average phosphorus
content of grass from plots that have received no phosphatic fertilizers for 6 to 11 years was
close to 0,20 per cent (ranging from 0.18 to 0.26 per cent). Plots that during this period have
annually received 70 kg P2O5 per hectare yield on the average grass with approximately 60
per cent greater phosphorus content. This high phosphorus content clearly indicates that the
mentioned fertilizer quantity was superfluous for the soils in question.
In the experiments, the results of which are listed in Table 9, nitrogen and phosphorus fertiliza-
tion increased simultaneously for the treatments. As shown by graphs 1 and 3, nitrogen in-
creased while phosphorus reduced the „apparent phosphorus balance" of the soil. In accordance
with these facts, Table 9 demonstrates that increasing nitrogen fertilization counteracts the
reducing effcts of phosphorus fertilizers on the „apparent phosphorus balance"; of 24 single
observations listed, only two show Iower „apparent phosphorus balance“ than 60 per cent.
With comparatively less phosphorus the „apparent phosphorus balance" unquestionably could
have been increased appreciably without detrimental effects on yield.
Comparing results of pot experiments and field trials.
The fertilizer trials in Mitscherlich pots, discussed in part I of this paper, are the first experi-
ments of this kind carried out outdoors in Iceland. At their initiation the direct value of this
experimental technique for practical grass land culture therefore was unknown. Comparisons
of the results now available with those from field trials indicate that the Mitscherlich pots
give different answers which cannot be transferred directly to field conditions The following
points support this conclusion.
1. The growth of the grasses in the pots appeared less vigorous than one ordinarily observes
in fields that receive 150 kg N per hectare. Nitrogen deficiency became apparent at an ab-
normally early stage, and after the cutting of the grass, its growth was negligible.
2. The phosphorus content was very low compared to grass from fields with similar fertilizer
treatments as those of the pots; this is clearly evidenced by the columns No. 2 in Tables 2
and 3 and by the data of Tables 11 and 12. This difference was not explained by the fact that
phosphorus was applied in solution to the pots. Equivalent quantities in granular triple super-
phosphate gave practically identical results as the solution.
3. The „apparent phosphorus balance" of those pots that received 90 kg P2O5 per hectare
ranged from 20 to 30 per cent, whereas this ratio for comparable field plots, referred to in Tables
5 to 7 and Table 9, ranged from approximately 40 to 60 per cent. This means that the grass
in the field took up twice as much phosphorus as the grass in pots with comparable phosphorus
application per hectare.
The information presented above shows that the results of the pot experiments in question
deviated widely from comparable field trials, and that it would be erroneous to extrapolate
the pot results to the field. It is true that the comparisons referred to cover different soils
and climatic environments. Yet the differences in question were so large and clear-cut that
their significance was beyond any doubt. The reasons for the relatively poor performance of
the soil in Mitscherlich pots are unknown. A reasonable guess is that varying soi! temperatures
in a pot placed on a 75 cm high bench and in a soil under field conditions was partly responsible.
Possibly the watering with a cold tap water (6° to 8°C) has also been detrimental.