CHAMBERS'S INFORMATION FOR THE PEOPLE. 



plants, and also an important influence on the 

 practices of agriculture. 



THE METEOROLOGY OF AGRICULTURE. 



The proportions of oxygen and nitrogen in the 

 atmosphere are very uniform at all places on the 

 earth, at all heights above its surface, and at all 

 seasons of the year. Dry air contains about 77 

 parts of nitrogen, 23 of oxygen, and one 2cooth part 

 of carbonic acid by weight The atmosphere also 

 contains minute quantities of ammoniacal vapour, 

 which furnishes a supply of nitrogen to plants. 

 Besides these, the vapour of water is always a 

 component of the atmosphere, though it varies 

 more in quantity than any of the others. The 

 higher the temperature of the air, the more 

 moisture can it hold in an elastic state. (See 

 METEOROLOGY.) 



There are good grounds for believing that the 

 amount of ammonia in the air is directly as the 

 quantity of moisture it contains. The supply is 

 larger in hot and moist weather than in cold and 

 dry. This supposition is so far supported by the 

 fact, that it takes more manure to grow a plant in 

 spring than in summer. 



In illustration of this principle, we have only to 

 bear in mind that market-gardeners who force 

 plants early in the season, require to dress their 

 land with the richest manures. Phosphoric 

 manures have no effect on turnips which are 

 sown early in the season, though they are well 

 known to have a very beneficial influence when 

 applied to turnips sown in June. The vivi- 

 fying influences of a high temperature have no 

 doubt much to do in giving plants greater vigour 

 to abstract ammonia from the soil ; but when 

 all the facts are fully considered, they indicate 

 that the leaves of plants have not only greater 

 powers of absorbing ammonia directly from the 

 atmosphere in warm than in cold weather, but 

 that there is more for them to absorb. There is 

 a numerous array of agricultural facts which 

 appear to admit of no other explanation. 



The theoretical dictum which has been put for- 

 ward by English writers, ' phosphates for turnips, 

 and ammonia for corn,' has led to much misconcep- 

 tion regarding the facts involved. If it be neces- 

 sary to draw distinctions on this head with respect 

 to the requirements of annual plants always 

 bearing in mind the qualifications we have made 

 it is pretty near the truth to say, ' ammonia for 

 spring, and phosphates for summer? 



Keeping in view the principles that have been 

 so far illustrated, we shall, in approaching the 

 more practical subjects, now touch upon the 



PHYSICAL CONDITION OF SOILS. 



This is one of the most important elements that 

 determine the natural distribution of plants over 

 the surface of the earth. To maintain plants in 

 a healthy growing state, they must obtain a certain 

 supply of moisture from the soil. Plants require 

 different amounts of moisture from this source, as 

 some evaporate a larger quantity of water from 

 their leaves than others. The capacity which 

 soils have of retaining the rains which fall, or 

 of absorbing moisture from the atmosphere, has 

 thus a great influence on their fertility. 



In a state of nature, we find plants that require 

 little moisture for their healthy growth take pos- 



516 



session of barren sands ; while those that require 

 a larger amount, fix upon soils which are more 

 absorbent or retentive of moisture. 



The agriculturist must often endeavour, how- 

 ever, to make all varieties of soil fitted for grow- 

 ing the particular crops that are most in demand. 

 As his art advances, and his resources become 

 more numerous, the mechanical condition of soils 

 is more under his command, and he strives to 

 give the different varieties such treatment as will 

 fit them for growing all kinds of plants, irrespec- 

 tive of their physical characters. 



Tillage, or other means of disintegrating and 

 pulverising the soil, has the effect of rendering it 

 more absorbent of moisture. It thus so far com- 

 pensates for a larger fall of rain. Tillage, however, 

 is more particularly useful as a means of putting 

 annual plants in possession of the soil, and of 

 causing the organic matter which it contains to 

 decay. Many perennial plants are not benefited 

 by tillage, and most are independent of it Thus, 

 all our ameliorating crops, such as grasses and 

 clovers, receive no cultivation, and produce as 

 much vegetable matter as those that do. 



This fact is rendered still more apparent by 

 viewing what takes place in the forest, where the 

 roots of the trees obtain a greater supply of 

 moisture from the shade which the leaves afford. 

 The shade of the forest is the natural substitute 

 for the absorption of moisture that takes place in 

 a well-tilled field. 



Agriculturists classify the particular physical 

 conditions of soils under the following general 

 heads : sandy, gravelly, clayey, chalky, alluvial, 

 and loamy. 



Sandy soils, containing a large percentage of 

 silicious matter, are often unfruitful, owing to the 

 shallow-rooted plants being readily scorched dur- 

 ing warm weather. Many of our sand-downs in 

 the vicinity of the sea are of this character, and 

 hence they are more profitable when allowed to 

 remain covered by the grasses which are natural to 

 them. These soils cannot absorb much moisture 

 from the air, nor retain what falls as rain. Sandy 

 soils are most productive in moist seasons or in 

 moist climates. Sir Humphry Davy found that 

 the coarsest and most barren soils, when dried at 

 212, absorbed least moisture in a given time. 



Gravelly soils derive their name from resting 

 on gravel. They are usually sandy in their char- 

 acter, and contain pebbles and boulders ; but 

 being liable to be scorched by droughts, their 

 produce is variable. 



Sandy loams contain a larger proportion of 

 vegetable matter than the above, and thus being 

 more absorbent of moisture, are better fitted for 

 maintaining plants in healthy conditions. Sandy 

 loams are benefited more than any other class by 

 tillage, which increases their absorbent powers. 

 These qualities fit them especially for turnip- 

 husbandry. 



Loamy soils are of great variety, for the term 

 loam is one of the most indefinite in agricultural 

 language. Loam may be generally described as a 

 mixture of sand, clay, and vegetable mould, moder- 

 ately cohesive, less tenacious than clay, and more 

 so than sand. These qualities render them suited 

 for all kinds of crops, which are raised with greater 

 certainty than upon any others. 



Clayey soils, if they contain little vegetable 

 matter, are the most difficult class to cultivate.. 



