« AnteriorContinuar »
Bacteria providing nitrogenous food for plants seem to be of three classes. One of these works on the nitrogen contained by the soil humus, and comprises three genera, each of which has an essential function in reducing nitrogen to a form available as plant food. Another class develops symbiotically with the growing plants, swarming in colonies upon the rootlets. Their vital activity oxidizes atmospheric nitrogen. The third class apparently secures the same result without symbiosis.
Efforts were made to inoculate soils with artificial pure cultures of the third class and thus increase the nitrogen content without the aid of manure or mineral fertilizer. While some very successful experiments were made, the percentage of failures was too great for practical purposes. The root tubercle bacteria seem to give the greatest promise of success. All of those which have yet had any practical importance were found exclusively on the roots of legumes. Some cultures of these organisms, known as nitragin, were placed upon the market a few years ago by German experimenters. They were adapted to specific crops only, for it was claimed that each kind of leguminous plant had a special germ which was more successful upon it than any other form. There were so many failures that the manufacture of nitragin was abandoned.
Previous to 1902 the United States department of agriculture inaugurated extensive practical experiments in an effort to find improved methods of soil inoculation. The reasons for the failure of the German pure culture method were worked out. Improved ways of handling and preserving pure cultures were discovered, as well as means of rapidly and enormously increasing them after they were received by the farmer. Great progress was made toward developing an organism effective for all legumes, and the virility of the bacteria was so increased that they fixed over five times as much nitrogen as formerly. When the department could send in perfect condition to any part of the United States "a dry culture, similar to a yeast cake and no larger in size," the nitrogen-fixing bacteria of which could be "multiplied sufficiently to inoculate at least an acre of land," the prospects of an early and complete solution of the nitrogen problem seemed to have a rosy hue indeed.
In spite of the great progress that has been made, however, there has been little to encourage the hope of directly increasing the nitrogen supply of the soil for the wheat crop by means of bacteria. The more practical solution seems to be the indirect one of growing in rotation with wheat leguminous crops aided by artificial cultures. Success in this has been pronounced and practical. In 1904 the United States department of agriculture made a very extensive experiment with artificial inoculation of leguminous crops. About 12,500 tests were made under all sorts of conditions and in almost all of the states in the union; 74 per cent of the tests properly made proved successful.1 Not only was nitrogen thus fixed in available form for subsequent grain crops, but the leguminous crops sometimes yielded five times as much as non-inoculated ones grown under similar conditions, the usual increase ranging from 15 to 35 per cent. One result of the success of the experiment was a demand for cultures far beyond what the department could supply. A great improvement was made in 1906 by abandoning dry cultures for pure liquid cultures hermetically sealed in glass tubes.
Perhaps the best method of distributing and applying the organisms is by inoculating the seed of the legumes used. This way is thoroughly effective and costs but a few cents per bushel of seed treated. One gallon of liquid culture will inoculate 2 bushels of seed. Soil may be inoculated and then distributed as fertilizer would be, or earth may be transferred from a field containing the bacteria. Both of these methods are expensive, less certain of success, and weeds or pests may be transferred with the soil.
The nitrifying bacteria are parasitic plants that penetrate the roots of legumes to obtain food carbohydrates. After the roots are from 2 to 4 weeks old, the bacteria are unable to enter them. It is now known that tubercle formation is not essential to successful inoculation, and that the bacteria may be present in an efficient state in the absence of tubercles.2 Humid soil and a temperature of 60 to 80° F. are most favorable to the growth of soil bacteria, 35" F. being the lowest, and 98°F. the highest temperature at which growth is possible.
1 Bu. of Plant Industry, Bui. 70, p. 41.
2 Yearbook U. S. Dept. Agr., 1904, p. 49.
Denitrifying organisms thrive best in a soil at least slightly organic, and so packed as to exclude the oxygen of the air. The nitrifying bacteria are unable to develop in organic matter, but its presence to some extent is not fatal to them. The presence of nitrogenous substances has a deleterious effect upon the cultures of nitrifying bacteria, which seem to fix atmospheric nitrogen only in the absence of plenty of nitrogen in the soil, consequently little benefit is to be obtained from inoculating soils containing a good supply of nitrogen. Most of the nitrifying germs seem to exist in the first foot of soil, while few, if any, exist at a greater depth than 18 inches. Other bacteria, such as those which change the sulphur and the iron compounds, also exist in the soil.
Need, Time and Application of Fertilizers.—Nearly all wheat land that is under continual cultivation, even if crop rotation is practiced, yields larger returns when fertilizers are properly used. In each individual case local conditions and the economic position of the wheat grower must determine to what extent it is advantageous to fertilize. Each farmer must, in a large measure, learn by experience whether the application of a certain fertilizer is profitable under his circumstances. It is now well known that yield does not increase in proportion to the amount of plant food applied, and that the increase in straw is greater than that in grain. In determining the value of such application, it must, of course, be remembered that more than one crop is benefited. The wheat crop may be increased either by direct fertilizing or by the residual effect of fertilizers applied to other crops in the rotation. The composition and condition of the soil determine the relative importance of different fertilizing constituents. Phosphoric acid used alone generally increases the yield of wheat grown anywhere on the glacial drift area of the United States.1 Either nitrogen or potash applied alone does not seem to increase the yield greatly, while the application of both with phosphoric acid gives the greatest gain. A fertilizer that can usually be found on the market is one containing 4 per cent each of ammonia and potash 1 Hunt, Cereals in Amer. (1904), p. 75
and 12 per cent of available phosphoric acid. By applying from 250 to 500 pounds of this commercial fertilizer per acre the best general results are obtained. If land has been quite exhausted by continuous wheat growing, the proportion of nitrogen and potash should be greater. Commercial fertilizers are best applied by means of an apparatus made for this purpose and attached to the wheat drill. They may also be broadcast just in front of the drill. In the case of winter wheat, most of the nitrogen is often applied early in the spring so as to prevent loss through drainage during the winter.
Kinds of Fertilizers.—These naturally fall into two classes, barnyard manure and commercial fertilizers. It is only on farms where the supply of manure is not adequate to preserve a high state of soil fertility that commercial fertilizers are economical. In general farming the former has the greatest relative value on account of individual or combined physical, chemical and bacterial influences not yet fully understood. Considering equal weights, however, most commercial fertilizers contain more plant food than manure does. It is also claimed that when applied to wheat they will produce larger returns. Nevertheless, the lower cost of farm manure always makes its use more profitable than that of other fertilizers. Where both are
used, it is most profitable to apply the manure to crops grown in the rotation, such as maize, and to apply the commercial fertilizers directly to the wheat crop.
Manure.—It is claimed that the soils of China have been in continuous cultivation for more than 4,000 years without falling off in productiveness, and that the continued soil fertility is due to the utilization of all animal manures and of sewage. During the eleventh century in France, stable manure was unknown as a fertilizer, though flocks of sheep were used for this purpose. Stable manures were utilized in the medieval husbandry of England, and they have been used to great advantage in France and Germany for over a century.
In America manure has always been utilized as a fertilizer by progressive farmers, but it has also been looked upon as a farm nuisance. It has been charged with producing dog fennel and various other weeds, and with "poisoning" the soil. In parts of Oregon and South Dakota it has been burned, sometimes for fuel. It has been hauled into ravines in California, into the creek in Oklahoma, into a hole in the ground or to the side of the field in Kansas, to the roadside in Missouri, to great piles in North Dakota and Idaho, and to the river in the Mohawk valley.1 It is estimated that the farmers of the United States annually lose over $7,000,000 by permitting barnyard manure to go to waste. As the fertilizing value of the manure annually produced by the farm animals of the United States is calculated at over two billion dollars, it must, however, be very generally utilized, a fact which does not excuse the foolish and useless waste. The fertilizing value of the average amount produced yearly is estimated for each horse at $27, for each head of cattle $19, for each hog $12, and for each sheep $2. The amounts of fertilizing constituents in the manure stand in direct relation to those in the food of the animal, and have a ratio to them varying in value from one-half to unity.
Experiments have shown that equal weights of fresh and of rotted manure have equal crop-producing powers. As 60 per cent of the weight is lost in the rotting process, manure should be used in fresh condition. "Barnyard manure contains all the
1 Yearbook U. S. Dept. Agr., 1902, p. 529; Industrial Commission 10:clxxxviii. (