4. Tuber Yield of Potato. In analyzing the combined varian ce, it was observed that the effect of green manures and
nitrogen fertilizers at 1 % and their interactions at 5 % on the tuber yield were significant. Comparing the mean of
treatments interactions showed that clover treatments with 100% and 75 % of recommended nitrogen had a
significant difference from the without green manure & N100 % (control) at 1 % level; and they increased tuber yield
per m
-2
by 44 % and 33%, respectively; in comparison with the control (Fig. 4). Moreover, among other treatments
that didn’t have significant difference from control, the treatments of vetch with 100 % and 75% of recommended
nitrogen and canola with 100 % of recommended nitrogen were superior in comparison with others and they
increased tuber yield per m
-2
by 26.5, 23 and 15 percent, respectively. Therefore, clover legume plant with 75% of
recommended nitrogen was the best treatment because it have increased the tuber yield by 33 % in comparison with
the without green manure & N100 %; and moreover, it have decreased nitrogen fertilizer consumption by 25 %. The
results of the attribute correlation table show that tuber yield had a positive correlation with the attributes of the stem
& stolon number and plant height which were highly significant.
The results show that, firstly, green manure factor had a significant effect on all three measured attributes. The green
manures for increasing vegetative growth of potato plants are caused that the height of plant and also stem number per
15
plant increase and therefore, by increasing the produced matters at the sources, matters transfer to the sinks increases,
and because of this, the accumulation of nutritional reserves and so tuber yield per m
-2
increases. Secondly, among of
the green manures, the legume plants of vetch and clover were noticeably superior in comparison with others; because
the legume plants, due to the bacteria existing in their roots, produce nitrogen; and moreover, given their soft tissues,
they decompose rapidly in the soil; and therefore, by contri buting to the soil fertility, are caused increasing in plant
growth and tuber yield. Thirdly, it is observed that clover treatment with 75 % of nitrogen, regarding the effect on tuber
yield in unit area, in addition to be considered as the superior treatment, it was caused decreasing in nitrogen
consumption by 25 %; so it can be regarded as the best treatment.