Agricultural Meteorology: The Effect of Weather on Crops (1920)
J. Warren Smith, B.S.M.S.

CHAPTER II
AGRICULTURAL METEOROLOGY

Agricultural meteorology may be defined as "meteorology in its relation to agriculture." It considers the vegetation and animal life of the globe, the distribution of food and other crops, and farm operations as affected by climate. It shows the effect of weather on the growth and yield of crops. It treats of the influence of climate and weather on insect activities, the development of plant diseases, and the protection of crops, animal life, and buildings from damaging meteorological phenomena.

72. Conditions for plant growth.—There is an optimum combination of temperature, moisture, and sunshine in which plants make their best growth, and under which the largest yields will be obtained. Food is available to the roots of plants only in a soluble form; it is carried into the plants and converted into vegetable tissue under the influence of solar energy expressed in heat units or calories.

73. Factors must be in correct proportion.—If there is not enough moisture to furnish sufficient soluble plant-food, part of the solar energy is wasted, while on the other hand if there is more food brought to the roots than the solar energy can utilize, the food material is wasted. In the arid districts too little food is available, under natural conditions, for the solar energy, but when, through irrigation, a large amount of food is made available, large crop yields result. Moisture is the controlling factor in these regions. In the highest latitudes, there is generally an excess of moisture and a deficiency of heat. These are the conditions that prevail in much of northem Europe, Alaska, and some high mountain regions. Here the crop yields are largely a question of temperature variations.

74. Requirements vary.—Different plants require unlike proportions of moisture, heat, and sunshine, and most plants require varying amounts for best growth at different stages of development. A few plants require hot arid climates while others reach their best development only in cold moist regions.

75. Critical periods of growth.—Many plants have a certain (frequently short) period during growth when there must be a well-defined combination of certain weather factors to produce large crop yields; others have the ability to stand nearly dormant when unfavorable conditions prevail, but will revive and make an excellent growth when the weather factors are in correct proportion.

76. To determine critical period.—There are three well-defined methods for determining the most critical period of growth of farm crops and the weather factor having the greatest influence in varying the yield: (1) Laboratory experiments; (2) field observations; (3) correlations of weather with past records of crop yields.

77. Laboratory experiments.—One method is to carry out laboratory experiments in which the various factors can be under control. The moisture, temperature, and sunshine are the most important meteorological factors, and by keeping two of these constant and varying the other, its influence can be determined. Or one can be kept constant and the other two varied. Some work of this kind has been done, but the experiments that may and should be made are sufficient to engage the attention of many men. It requires special apparatus and close attention and should be attempted only by trained plant physiologists, or ecologists.

78. Field observations.—With the most carefully arranged details, the conditions that surround plants in the field can hardly be duplicated in the laboratory tests. Hence it is desirable that detailed records be made of all the meteorological factors and the growth and yield of various crops at many different places and covering a period of many years.

79. Observations in Russia.—Russia was the pioneer in the organization of a group of agricultural-meteorological and horticultural-meteorological stations to determine quantitatively the relation of different climatic factors to crop production. The Russian Bureau of Agricultural Meteorology was authorized in 1894 and observations were begun in 1896. In 1912 Russia had observations under way at eighty-one different experiment stations where meteorological records were being kept as near as possible to the test plats.

80. Records in Canada.—Similar records were begun in Canada in 1915 at fourteen different experiment farms where particular attention was given to spring wheat. Some results have been published, but the records should cover several years to make the correlations conclusive.

81. Records in the United States.—A division of Agricultural Meteorology was organized in the United States Weather Bureau early in 1916, and one of the first important things done was to start plans for the inauguration of agricultural meteorological stations at each of the main agricultural experiments stations in each state. The war emergency made it necessary to hold these plans in abeyance, however. It is believed by the chief of this Division that this is one of the most important steps that can be taken in the interest of agriculture in this or any other country. The climate of the United States is so varied, the crops are so diversified and the yield so variable, that a careful record of all the weather factors and the consequent development of the crop plants must be made for several years to determine the critical periods of growth.

82. Value of records.—If it is found, for example, that a light rainfall in May means a small hay crop, other forage crops can be planted. If it has been learned that the crop of winter grains will be reduced by certain weather during the winter, the spring grain area can be increased if these weather conditions prevail. When the water requirements of various crops are determined for different stages of growth, water can be more economically handled in the arid and semi-arid regions of the West. The climate of a district will be more carefully studied and crops planted that are best adapted to the prevailing climate, or where the season is long enough, seeding will be done at such a time as will bring the critical period of growth when the weather factor most affecting it will be nearest its optimum for that crop. There are innumerable ways in which this knowledge can be applied. Much has already been done by experiment stations in this direction but the work lacks the system and correlation that would be obtained under definite direction.

83. Correlation of weather with past records of crop yields.—While records and results are being accumulated by laboratory experiments and field observations, much valuable knowledge can be obtained by a mathematical correlation of accumulated climatic data with records of crop yields during past years. The manner of making these correlations will be explained in Chapter IV and some of the results already obtained will be shown in Chapters VII to IX.

84. Not a new science.—While the term "agricultural meteorology" is new, the importance of studying the relation of weather to crops has been recognized and referred to by agriculturists and meteorologists for many years. Measurements of rainfall were made in Palestine in the first century of the Christian era, and there was a network of rainfall stations in the important rice-growing districts of Korea as early as the middle of the fifteenth century. As this crop needs a large amount of moisture during its growth, it is only reasonable to suppose that the farmers of Korea made a practical use of their knowledge of the distribution of moisture in connection with the growth of this important food crop.