Phosphoric acid is supplied in the form of phosphates of lime, iron, and alumina. Some of the materials used are bone phosphate (phosphate of lime), raw bone, bone meal, steamed bone, bone black or animal charcoal, bone ash, South Carolina, Florida, Canada and Tennessee rock phosphates, iron phosphate, a by-product of the manufacture of iron phosphatic iron ores, and superphosphates (a product made by treating some of the foregoing elements with acid to make readily available or soluble phosphoric acid).

Fertilizers should be purchased on a per pound basis for the amounts of the elements actually contained. The fertilizer that costs the most per ton may be the cheapest. The following table shows the relative amounts of the elements in various combinations and is used in checking up guarantees:

To convert the guarantee of Ammonia Nitrogen Nitrate of soda Bone phosphate Phosphoric acid Muriate of potash Actual potash Sulphate of potash Actual potash into an equivalent of:

Nitrogen.

Ammonia.

Nitrogen.

Phosphoric acid { Bone phosphate.

Actual potash.

Muriate of potash.

Actual potash Sulphate of potash.

Multiply by.

0.8235

1.214

0.1647

0.458

2.183

0.632

1.583

0.54

1.85

It is often best to buy the elements singly, and with a guarantee as to the contents of available plant food. There are ways of manipulating mixtures so as to puzzle the buyer, and at the same time permit the seller to keep within the law. In buying combined fertilizers the cost of mixing, handling, and advertising must always be added and paid for by the buyer.

In order to maintain and build up virgin fertility and soil condition a grower should be continually adding stores of vegetable matter and such mineral or organic elements as will probably be depleted first. The latter can be applied in forms that are available at once or in less soluble forms where the usable fertility becomes available gradually.

In the countries of Europe where the best results are obtained, and in this country where growers have given the most careful attention to the business, the highest importance is placed on the use of the decaying root systems of strong growing plants for fertilizing material and for keeping the soil in good condition. Sir John Lawes, the eminent British farmer and experimenter, said: "It is the physical condition of the soil, its permeability to roots, its power of absorbing and radiating heat that is of more importance than its, strictly speaking, chemical composition.'

An interesting article on the value of green manures is given by R. W. Thatcher in "Bulletin 32" of the Washington Agricultural Experiment Station. It follows:

Soil is a mixture of inorganic material - i.e., rocks or mineral substance broken down into more or less fine particles - with organic matter - i. e., decayed or decaying material which has once been living vegetable or animal tissue. The inorganic particles usually compose the greater portion of the soil (95 per cent. to 98 per cent. of sandy or gravelly soils, 80 to 95 per cent. of loams and clayey soils, less than 80 per cent. of muck or peaty soils) and supply the necessary mineral elements of plant food. The organic portion of the soil furnishes the supply of nitrogen, a very necessary element of plant food, without which no crop can make any growth; supplies also the other elements which were taken up in the growth of the plant or animal and which by their decay are returned to the soil in a form readily available for plants; helps to render the mineral elements of the soil available by the action upon the inorganic matter of the acids produced in the decay of organic matter; affects very beneficially the physical properties of the soil, increasing its ease of tilth, moisture holding capacity, capacity to absorb heat, and decreasing the tendency to 'puddle' when wet or 'bake' when dry. Too much organic matter usually results in a soil which dries out very rapidly and which is likely to be 'sour' from the excess of organic acids which it contains. Very few, if any, well-drained lands contain an excess of organic matter, however.

It is apparent, therefore, that the maintenance of a proper balance between the organic and the inorganic or mineral portions of the soil is one of the first essentials to fertility and to proper physical condition of the soil. Many of the so-called 'wornout' soils have only had their store of organic matter depleted by improper methods of cropping and can be restored to fertile condition by the plowing under of some additional supply of vegetation to decay.

Any farm crop which is grown for the sole purpose of plowing it under to increase the supply of organic matter in the soil is known as a 'green manure.' Green manures affect the soil beneficially in many ways. Some of the possible benefits are: (1) The addition of vegetable matter or 'humus,' with its attendant beneficial effect upon the physical and chemical properties of the soil. (2) Increasing the nitrogen content of the soil by fixation of nitrogen of the air, when leguminous crops are used as the green manure. (3) Using surplus available plant food which might otherwise be lost. (4) Plant food from lower depths may be brought nearer to the surface and made available for subsequent crops.

The kind of crop which may best be used as a green manure depends upon which one or more of these beneficial effects is most desired. If the addition of humus, or an increased supply of decaying vegetation, is the only necessity, then any rank-growing farm crop may be used. The more succulent or juicy plants are best, as they decay much more quickly and are more easily incorporated in the soil. If, however, the supply of nitrogen in the soil is small and its increase is either the chief necessity or a desirable addition to the increased humus content, then some leguminous crop must be used, as no other farm crop has the power of utilizing atmospheric nitrogen or of returning to the soil any essential element of fertility which it did not draw from it. If it is desired to bring up from below some of the mineral plant food which is present in deeper layers of soil, then a deep-rooting crop should be used.

The legumes, or leguminous crops, are a group of plants which are characterized by growing their seed in pods and by having peculiar knots or nodules on their roots. These nodules are formed by the action of a certain group of bacteria, immense numbers of which are found in each nodule, which have the peculiar property of being able to use the gaseous nitrogen of the air for their own growth, and supplying this element as they die and decay to the host plant on whose roots they are located. Included in this group are alfalfa, all the clovers, vetches, peas, beans, etc. No other group of plants or animals, so far as is now known, is thus able to make use of atmospheric nitrogen. Legumes may grow in soils which are rich in available nitrogen without the presence of the nodule-producing bacteria, deriving their nitrogen supply directly from the soil as do other crops, but have the distinctive power of being able to flourish in soils poor in nitrogen if the proper bacteria are present to grow upon their roots and supply them with nitrogen from the air, and when so grown to increase the supply of soil nitrogen when plowed under as green manures."

The potato growers of the Jersey Islands use the following mixture at the rate of one ton per acre in addition to animal manures, etc.:

1000 pounds super-phosphate 600 " sulphate of ammonia 300 " sulphate of potash 100 lime dust or sulphate of lime 2000 pounds.

William D. Hurd states that in Maine "the most common and popular commercial fertilizer used for potatoes has been the one analyzing 4 per cent. ammonia, 6 per cent. available phosphoric acid, 10 percent, potash, and costing $37 to $39 a ton. The following materials for a home mixture would duplicate the above formula:

150 pounds nitrate soda (15 per cent, nitrogen). 800 " tankage (7 per cent. ammonia, 15 per cent. total, 10 per cent. available phosphoric acid). 300 " acid phosphate (15 per cent. phosphoric acid).

400 " sulphate potash.

1650 pounds (equivalent of a ton).

The above material can be purchased f.o.b. Boston at the following prices: Nitrate of soda $50 per ton, tankage $27 per ton, acid phosphate $16 per ton, sulphate potash $48 per ton.

The cost of this mixture would be as follows:

150 pounds nitrate soda at $50......

$ 3.75

800 " tankage at $27.......

10.80

300 " acid phosphate at $16.....

2.40

400 " sulphate potash at $48.....

9.60

Cost of materials f.o.b................................

$26.55

Labor of noixing, waste, aacking, etc.....................

75

Total............

$27.50

To this must be added the cost of freight. At most Maine points this would not be over $2 to $2.50 for the above quantity. A saving then of about 25 per cent. can be made by home mixing.

Many Aroostook County growers have abandoned the 4-6-10 fertilizer and are now using one analyzing 5 per cent. ammonia, 8 per cent. available phosphoric acid, and 7 per cent. potash.

This sells at $41 to $42 a ton in Aroostook County. Such a mixture can be made from the following materials, using the same grade as in the 4-6-10 mixture given above:

200 pounds nitrate soda. 980 " tankage. 400 " acid phosphate. 280 " sulphate potash.

1860 pounds (equivalent to a ton).

Cost f.o.b. Boston, $28.15. Cost of mixing, same as before. Freight extra."