EXPERIMENTS WITH ALFALFA AT YUMA 9 
per acre. Figuring alfalfa seed conservatively at 12 cents per pound, 
it can readily be seen that the increase in this case gave a handsome 
profit on an outlay of $5.25 for fertilizer. In addition to this there 
was a marked increase in the hay crop before and after harvesting 
the seed. 
SOURCES OF SUPPLY OF ACID PHOSPHATE 
As the use of fertilizers is relatively new to many of the farmers on 
the Yuma project, there has been a considerable demand for infor- 
mation as to what acid phosphate is, how it is made, and whether 
it may have any injurious effects on the soil. For this reason these 
matters are here briefly discussed. 
So far as plant growth is concerned, the essential element in acid 
phosphate is phosphorus. Alfalfa is generally regarded as being 
especially heavy in its demands for this element, and it is therefore 
not surprising that after several years of heavy crop production the 
addition of phosphoric acid should prove beneficial. 
Phosphorus in nature occurs in bones and rocks. At present 
the supply for fertilizers comes largely from phosphate rocks under 
the name of acid phosphate. 
Acid phosphate is the name generally applied to the product made 
by treating rock phosphate with sulphuric acid. These rocks have 
been known in Florida, Tennessee, and South Carolina for some 
time. More recently large beds have been found in Montana and 
other Western States. The phosphorus in the rock is practically 
insoluble and is very slowly available to plants. By treating the 
rock with sulphuric acid, the solubility of the phosphorie acid is in- 
creased so that it becomes more readily available to the plant. 
In the manufacture of acid phosphate, the phosphate rocks are 
finely ground and mixed with sulphuric acid. The mixed mass is 
allowed to cool and harden, after which it is ground again. In order 
to avoid the presence of free acid in the acid phosphate the quantity 
of acid used 1s somewhat less than that needed to dissolve the phos- 
phate completely, depending upon the composition of the rock. 
Roughly, about half a ton of sulphuric acid is used in making 1 ton 
of acid phosphate. There are various grades of acid phosphate on 
the market, the grade being indicated by the percentage of phos- 
phoric acid. The most common commercial grade contains 16 per 
cent phosphoric acid. By further treatment of the low-grade acid 
ae es high-grade phosphates known as triple acid phosphates, 
aving approximately 44 per cent phosphoric acid, are obtained. 
In some cases these high-grade acid phosphates have caused a 
temporary depressing effect on plant growth, owing to the free acid, 
but such effects are not often serious and are not permanent. An 
opinion has been more or less prevalent among farmers that the 
continued use of acid phosphate is likely to prove injurious to the 
soil by making it acid. This is probably on account of the name, 
but as a matter of fact acid phosphate properly manufactured con- 
tains no free acid, and tests have shown a decrease rather than an 
increase in soil acidity through its long-continued use in the Hast. 
Since this practice has not resulted in permanent injury there is no 
apparent reason why there should be any hesitancy in applying it. 
