THE FARMER'S MAGAZINE. 



113 



TOP-DRESSINGS— THEIR APPLICATION AND EFFECT. 



" Nitrate of soda is the only artificial manure which 

 can safely be used as a top-dressing at any time : the 

 nitrogen, being in the form of nitric acid, is not 

 volatile, as it is when in the form of ammonia. When 

 guano or sulphate of ammonia is used for a top- 

 di'essing, it should be sifted very fine, and sown 

 before or during rain." So Mr. Lawes advises us in 

 " Rendle's Directory ;" and, undoubtedly, it is sound 

 wisdom to follow his teaching in such a matter. But 

 observe the theory upon which the recommendation, or 

 rather instruction, is founded : it is, tliat the nitrogen, 

 whether in the acid or alkaline combination, benefits 

 plants through the soil, and that the portion of 

 ammonia wbich exhales into the air around the stems 

 and leaves of vegetation is wasted. By changing our 

 theory, we get a very different rule for praetice. 

 Thus, Pi'ofessor Buckman, in his " Essay on the Roots 

 of the Wheat-plant," inclines to the use of such top- 

 dressings for wheat as are rich in ammonia, and of 

 such a consistency, or in such a condition, as to be 

 capable of giving off this gas equally and abundantly. 

 And the theory upon whicli he builds (though cau- 

 tiously and with due diffidence) is, that the stem and 

 flag of the plant respire ammonia. Hence, one reason 

 why " the surrounding the wheat-plant with ammoni- 

 cal manures at the season of its most vigorous growth 

 may he productive of benefit." "While we look over 

 a wheat-field," he says, " on a fine sunny day as 

 summer advances, and see the dazzling dancing in the 

 atmosphere a few feet over the plants, which is caused 

 by the evaporation of water through the cellular 

 system of the leaves, we may know that the crop 

 is pumping up its food from the soil ; but as this 

 is just the time for liberating ammonia also from 

 the soil, there is, I think, reason to believe that the 

 atmosphere charged with carbonic acid and ammonia 



is at this very time being eagerly respired 



Wheat in its growing history is completed in a few 

 weeks, although the production of strong pfints for 

 growth occupies, in the v/inter varieties, many months: 

 and it is just as the new growth commences that 

 ammoniacal manures are so beneficial, as the nitrogen 

 therefrom has to be eliminated in a short time; and as 

 the leaves are small and upright, if we suppose 

 ammonia to be respired by them, they will require 

 a quicker and more constant and greater discharge of 

 this gas, commencing too at a certain time, than 

 is the case with a plant with large leaves which 

 grow without any serious interruption from their birth, 

 and whose under-surface is the only inhaling one, and 

 which is 90 arranged as to insure the due but more 

 gradual and more certain performance of this office 

 without loss." 



Let us now turn to other authoi'ities on the subject 

 Mr. Haxton, wlio gives the following directions for 

 top-dressing wheat : "Guano is more liable to be 



affected in its results by the weather at the time 

 of sowing, than nitrate of soda; and, indeed, its 

 success depends in a gi-eat measure on being well 

 washed into the soil when applied as a top-dressing. 

 Nitrate of soda, on the other hand, is not volatile, and 

 is so easily dissolved that the dew melts it sufficiently for 

 the plants to suck it in both by leaf and root. By 

 mixing the two together, the result, according to the 

 chemical law of catalysis will, in all likelihood, be that 

 the guano is rendered more soluble, while the solubility 

 of the nitrate is in some measui-e lessened ; and even 

 laying aside the operation of the law, the mere affinity 

 of the nitrate for moistui-e would, of itself, tend also to 

 dissolve any minute particles of the guano in contact 

 with it, and both these pass into the circulation of the 

 plant, even although little or no rain should fall." 

 Guano is adapted for early dressing, and nitrate of 

 soda for later dressing in spring ; the former being less 

 soluble, less active to begin with than the latter, 

 and lasting longer in its effect. If it is found necessary 

 to top-dress wheat late in spring, the manure should be 

 as soluble as possible, and the nitrate surpasses all 

 artificials in this property. 



Mr. Hannam tells us (in the same Cyclopaedia), 

 that in all manures which are designed for application 

 in autumn and winter, the ammonia or nitrogen should 

 be supplied in a form that will as far as possible 

 prevent it operating too quickly : " for example, it 

 would be preferable to supply the nitrogen in union 

 with other organic matter, as in rape-dust, bones, 

 woollen-rags partially decomposed, blood, guano mixed 

 with charcoal and gypsum, &c., rather than in the 

 shape of salts of ammonia, which are so speedily 

 soluble." All winter dressings of artificial manure 

 should be mixed in the soil. All mixtures, in which 

 the ammonia exists in a saline form, should be applied 

 as a spring top-dressing upon the crop, if possible? 

 in wet weather. All mixtures containing organic 

 matter rich in ammonia, as rape-dust or guano, 

 if applied with spring-crops, should be harrowed 

 in immediately ; and if applied as a top-dressing^ 

 should be used only in wet weather. Spring top- 

 dressings, containing organic matter, should be applied 

 earlier than those purely saline. 



These rules are based upon a consideration of the 

 degrees of solubility and lasting or evanescent action 

 of difierent manures, and also of the requirements of 

 various crops at different stages of their growth : the 

 inference for practice being the same as that arising 

 from a consideration of the volatility of the manures^ 

 while accepting the theory that the ammonia or nitro- 

 gen feeds plants by their roots. 



Yet the practice of applying highly volatile am- 

 moniacal manure upon the surface of land upon which 

 a crop is growing, which may follow as a deduction 

 from the theory that plants inhale ammonia by their 



