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That there are disturbing elements in regard to this relation in arable soils is not to be denied, but it seems probable that these can be determined and that the true relation of the soil to heat can be established, enabling us, from the temperature of one, or at most two depths of the soil, to estimate very closely that for the whole, thus making it as easy to keep a record of the temperature of the soil as it is that of the air.
The relation of the soil to heat is, practically, to a certain marked extent, under our control, and we make constant use of this fact. The better we understand this relation and the more we know about it, the more intelligently can we perform many of our important farming operations. For early vegetables or fruit one would hardly select a northern exposure in preference to others, but would expect with a south or south-eastern exposure that his vegetables would ripen at least one or two weeks earlier.
Thorough drainage and cultivation modifies the temperature of the soil greatly. Prof. Johnson says in “How Crops Feed,” page 195, “the capacity of water for heat is much greater than that of the soil.
Lime-sand and quartz-sand are the slowest of all the ingredients of the soil to suffer changes of temperature when exposed to a given source of heat. Now water is nine times slower than quartz in being affected by changes of temperature, and as the en ire surface of the wet soil is water, which is, besides, a nearly perfect non-conductor of heat, we can understand that external warmth must affect it slowly.' Again: “Thorough drainage by loosening the soil and causing a rapid removal from below of the surplus water has a most decided influence, especially in spring time, in warming the soil and bringing it into a suitable condition for the support of vegetation."
To give another instance of our knowledge of, and important control over, this relation : Some of the vine-growers in France and Germany found that their grapes ripened earlier and better on soils covered with fragments of quartz and slate, and in some places they have the practice of spreading these substances, charcoal and other absorbents of heat over the ground to hasten the
ripening of their grapes. In some districts they also have the practice of training their vines along, and only a few inches from, the ground, thus utilizing the surface heat in another way for ripening their grapes, and at the same time giving material protection against the occurrence of early frosts.
Practically, what value have some of these modifying causes with our climate and soils? They must control in great part the chemical and biological changes of the soil, and, directly or indirectly, the kind and amount of crop raised.
. If a man wanted to raise oranges, or lemons, or pine-apples in Boston, it would not be necessary for him to send to Florida for soil to grow them in, but he must provide a greenhouse where the climate, especially the temperature, will be under his control. A successful florist or gardener must understand how to manage his heat above all things, as on this alone depends in great part his success. He must have different houses and hot-beds and cold frames where different temperatures prevail; some of his plants must have "bottom heat," while others must be put up close to the glass and away from “bottom heat,” according to
" the habits of the plant and the development he wishes it to attain. This is of vastly more importance than whether he gives the plant this kind of commercial fertilizer or the other.
An opportunity to study some of these questions practically was presented on the Farm. There was a small meadow at one end of the place upon which water was always standing over 'most of the surface. It was intended to drain this and seed it
What effect would thorough drainage have upon the temperature of the soil? A spot was selected where the soil was completely saturated with water, but with no water actually standing upon the surface, except when the spongy sod was pressed by the foot or hand, when water would ooze up. The grass was not above an inch high and by no means shading the ground. Soil thermometers, which had previously been compared with our others, were put in on August 16th, and three daily readings were taken for a month, directly after the other instruments were read.
down in grass.
About a week after the thermometers were put in, a ditch was dug on either side of the meadow (and thermometers) for drainage, but they hardly lowered the water inside the meadow at all, so that the surface around the thermometers continued to be saturated throughout the month. The results of the observations on the temperature of the soil, as compared with those obtained in the cotton field, are interesting, and confirm what has been mentioned as one of the advantages of well-drained land.
Only the mean weekly readings are compared with those in the cotton field, for obvious reasons. Generally the temperature of the meadow was notably warmer in the morning than the cotton field, except at a depth of three inches, which was generally somewhat colder, occasioned possibly by the greater amount of water evaporated from the surface of the ground. At 1. P. M. and at 7 P. M. the soil in the meadow was notably colder than in the cotton field. This in itself gives some interesting results.
Comparing the relation of these soils to heat, it seems to be indicated that the fine Granville county tobacco soil (a gray sand), which would seem to be warmest of all, was in reality the coldest at a depth of three inches and lower; the meadow comes next, and the cotton field seems to be, on the whole, considerably warmer than either of the others. It still seems to be indicated, however, that, of the three, the meadow would be the slowest to warm up in the spring. This will be further looked into in 1887.
A record has been kept through the growing season of the amount of sunshine the crop has received. The apparatus used was very similar to one described in the report of the New York Experiment Station for 1885. It consists of a small wooden box, painted black inside, with a small, short focus lense in one end. Inside the box is a movable frame on which is put every night a piece of sensitive silver paper (the kind used for printing photographs) exactly in the focus of the lens. In front of the lens, about twelve inches away, is a glass globe (a roundbottom flask will do) painted black inside. When the sun is shining this blackball reflects a small spot of light into the lens, which is brought to a focus on the silver paper where it makes a black dot. On a clear day, as the sun moves across the heavens the spot of light moves around the ball, and the image moves on the silver paper, giving a continuous black line. When the sun is obscured by clouds, or its light cut off from the ball, a break occurs in the record, the time and duration of which are indicated by a scale from which can also be read off the number of hours and minutes of sunshine for the day. An apparatus, in every way as good as this, could doubtless be made for five dollars, or less. As Dr. Sturtevant says, such an apparatus records about 10 to 15 per cent. less sunshine than actually occurs, as it will not record for some little time after the sun rises, the time depending probably upon the amount of moisture in the air, which absorbs most of the heat and chemical rays of the light. The apparatus is so simple, inexpensive and easy to make, that it would seem that more might be used throughout the country, and even in this State, as the record would be of value. The instrument could be easily put together and the record kept at any of the colleges or any of the signal stations, and the results so obtained would form a valuable record for agricultural science and probably practice.