1 Jan., 1902.} QUEENSLAND AGRICULTURAL JOURNAL. 19 
lower organisins, take this nitrogen from the air and combine it with oxygen, 
making thereby a substance which is suitable as plant food. So far as is now 
known, only the leguminous plants possess this property. 
From the above it follows that nitrogen may exist in such a state or 
condition that, though it may be present in abundant quantities, plants cannot 
make use of it. In effect, much may be said of potash and phosphoric acid. 
As these two plant-food elements exist in the soil naturally, they are of little 
use to growing plants. Plant food must be soluble in the water of the soil, in 
order to be available as plant food. Now, the natural soil may contain enough 
potash and phosphoric acid to grow a hundred crops, as shown by chemical 
analysis, yet fail to grow one. This is because this potash and phosphoric acid 
are locked up in the soil in rock particles which are insoluble in water ; hence the 
fertiliser cannot take such form as to be useful as plant food. 
The importance of this latter point is that legumes or clovers cannot 
assimilate nitrogen gas unless certain quantities of potash and phosphoric acid 
are present in the soil in ayailable form. As legumes store up nitrogen, 
they form vegetable growth—the nitrogen is a part of this growth, and the 
quantity of this nitrogen stored depends directly upon the quantity of this 
growth. But this vegetable growth cannot be made without the proper amounts 
of potash and phosphoric acid to accompany the nitrogen. Hence, to grow 
fertiliser nitrogen through the use of legumes, potash and phosphoric acid are 
part of needful soil preparation, quite as much as the seed. 
It may be well to look up here the relative quantities of potash and 
phosphoric acid needed. The following table shows the pounds of potash and 
phosphoric acid required for every 100 Ib. of nitrogen stored up by the 
legumes. 
Every 100 lb. of nitrogen requires : 
Potash. Phosphoric Acid. 
Lb. Lb. 
Clover ys ay “en 84: tg 21 
Cow peas... ay 00 69 oe 22 
Alfalfa or lucerne ... ae 88 2, 17 
Beggar weed os sc 69 are 26 
Tt will be observed at once that the potash greatly exceeds the phosphoric 
acid, whereas, as a matter of fact, commercial fertilisers commonly show 
‘uite the opposite proportions. It is true that phosphoric acid tends to take 
forms in the soil which place it beyond the reach of growing plants. At the 
same time, equal parts of phosphoric acid and potash certainly restore the 
balance between the two elements, and even more than restore it. 
There is another point to touch upon here. It is quite impossible for 
plants to take up all the plant food applied to the soil ; indeed, the best 
authorities practically agree that 50 per cent. of efficiency is fairly good work. 
This means that to obtain 100 Ib. of fertiliser nitrogen from the air the follow- 
ing quantities of potash and phosphoric acid must be used :— 
Potash. Phosphoric Acid. 
Lb. Lb. 
OO ape vom wn 9 UGH me A? 
Cow peas... onc LOS vod 44, 
Alfalfa or lucerne ... ox: WUIASS aes 34 
Beggar weed oc PLoS ox) 52 
The point arises, will it pay to use such quantities of plant food merely to 
grow fertiliser nitrogen—considering also that the phosphoric acid is made equal 
to the potash. The following table explains. Potash and phosphoric acid cost 
the farmer about 5 cents (23d.) per lb., nitrogen 14 cents (7d.) The first 
