Vol. IX. No. 221. 



THE ?AGRICULTURAL NEWS. 



323 



study resulted in thu discovery of the nitrogen-fixing 

 organism, Azotobacter, which without the aid of living 

 plants, and in the presence of an ailequate su])ply ot 

 organic matter, forms a means of directly adding nitrogen 

 to the soil. In connexion with this, experiments at 

 Rothamsted have shown that, while soils containing 

 little organic matter experience a small addition, if any, 

 to their nitrogen content through direct fixation, those 

 jiDSsessing an adequate amount <if ergaiiic matter may 

 have nearly as much as 100 ft. of nitrogen per acre 

 added to them, yearly. 



In face of the dependence which the fertility of 

 soils was found to .show on the presence of micro- 

 organisms, it was hard t<i explain why the exjiosiire of 

 soil to conditions, such as heat and various poisons^ 

 which would kill these organisms, always resulted in an 

 increase of its fertility. It was not until the recent 

 investigations made by Drs. Russell and Hutchinson at 

 Rothamsted showed that the soil contains organisms 

 (protozoa) much larger than bacteria and, cuiiously 

 enough, closely related to the white corpuscles in the 

 blood which enable the body to tight disease; these 

 organisms feed on the bacteria in the soil. The investig- 

 ations in connexion with them were given attention 

 recently in the Agricultural Neius (Vol. IX, ]). '-M), 

 and it only remains to say that they showed that the 

 effect of the exposure of the soil to moderate heat, 

 or to certain poisons, was to kill these protozoa, and 

 thus to give the best chance for the increase in numbers 

 of the bacteria, in this way enhancing the rate of 

 nitrogen fixation. 



The conclusion reached after these considerations 

 is that the cause of the fertility of the soil cannot be 

 looked for in one fixctor alone. Several circumstances 

 subscribe to it, and the absence, or insufficient presence, 

 of any one of these will prevent the others from exert- 

 ing their proper effect. The object of investigations 

 in connexion with the productivity of soil is to ascertain 

 the nature of these factors, and to find means of bringing 

 them under proper control. 



The Financial Times of June 9, 1910, contains a letter 

 written with the object of contradicting reports, that have been 

 put into circulation, to the etfect that the trees of Ceara ruliUer 

 (Manihot Glaziovii) only live for a short period, and that they 

 are not sufficiently hardy to withstand untoward conditions for 

 any length of time. Figures are given to show that, in 

 German East Africa, an average of about i-oz. of wet rublier 

 per tree was obtained from plants three years old, in a dry 

 season, in a di.strict having a rainfall of .56 inches in the 

 year. In older plantations, trees from thirteen to nineteen 

 years old were still producing latex in large quantities, and 

 even plants that had been attacked liy white ants, and had 

 broken down, grew again and formed new trees. 



CONCRETE POSTS ON ESTATES. 



In reply to a request from the Imperial Commissioner of 

 Agriculture, Mr. A. St. G. Spooner, of Bendals estate, Antigua, 

 has kindly supplied information arising from his experience 

 in the making of reinforced concrete posts for use on estates, 

 and relating to the utility of such posts as regards the pur- 

 poses for which they are required. 



After describing his original, unaided attempts to make 

 such posts, Mr. Spooner draws attention to the fact that the 

 Pettyjohn Company, of Terre Haute, Indiana, U.S.A., makes 

 a portable moulding frame for the purpose of manufacturing 

 them. This frame is placed on a board, and the concrete 

 mixture is rammed into it; while it is being filled the rein- 

 forcing strips are set in the mixture — one about |-inch from 

 the bottom of the mould, and the other at the .same distance 

 from the top of it. When the mould has been filled with 

 material, which is tightly rammed, the top of this is smoothed 

 off with a trowel, the pins which held the reinforcing material 

 are pulled out, and the whole mould is lifted away from the 

 concrete by means of handles at each end. The posts are left 

 undisturbed for a whole day, at least; a better time is two 

 dajs: they are then stacked end^dse, covered with trash and 

 allowed to remain moist for a week or two, until they have 

 hardened thoroughly. The posts that are being made at 

 present by Mr. Spooner are 6 feet long, and measure -5 x 4J 

 inches at one end, and 5x3 inches at the other. 



The mixture used in making the concrete is 4 parts of 

 coarse grit sand, clean and sharp, to 1 part of .^tlas cement. 

 Good posts can also be made with mixtures of finely broken 

 stone (i-inch cube), sea sand and cement, in the proportion 

 4:2 : 1 ; but the appearance of these is not as good as that of 

 the others. In the former mixture, '2\\ lb. of cement is 

 required for each post: this is mixed \vith 1,805 cubic inches 

 of sand, loosely measured in a box. The mixture occupies 

 1,.350 cubic inches, when rammed into the mould. British 

 cement is probably better for the purpose than American 

 cement, but the latter is cheaper. 



Mr. Spooner gives figures showing the cost of making 

 one post, with cement at 13s. a barrel; these are as follows: — 



Pence. 

 Cement 8-66 



Sand (say) 0-50 



Labour (by contract) 1'75 



Reinforcing material (about) 2 '50 



Total cost 13-41 



Each post thus costs about is. l|d.; this would be reduced 

 by IJjrf. if cement was admitted free of duty in Antigua. The 

 labour required for making the posts is provided by two men 

 and a boy, and the output per day is from thirty to thirty- 

 six posts. The mould, however, could turn out about 100 

 posts per day, if it was kept supplied by the mixture. 



The reinforcing material consists of steel strips having 

 considerable tensile strength, 1^ inches wide and -f^- inch 

 thick, embedded in the posts as described above. The freight 

 of these and the waste in cutting increase their cost, and 

 Mr. Spooner suggests that a substitute may be found for them 

 in the shape of four pieces of stout steel fencing wire, not less 

 than Xo. 8 B.W.G., with the ends turned over to prevent 

 slipping; these would be placed at the corners of each post 

 about finch from the outside. 



In erecting the posts, experience has shown that the 

 best method is, first of all, to make taut between the strain- 

 ing posts the wire which they will have to carry, and then 

 to fasten it to them, by passing a short length of wire 

 through the holes left in the post, and twisting the ends 

 tightly round the fence wire, by means of a key. 



