The heat-retaining properties of a soil must always be taken into account in growing grapes. The great preference which many varieties of grapes show for sands, loams, shales and gravels, depends largely upon the greater amount of heat found in such soils. In northern regions it is especially needful that the soil furnish an abundance of bottom heat for the grape. The removal of an excess of moisture is helpful in regulating soil heat; and, other things being equal, a well-drained soil is warmest.
Grapes grow more or less well in any soil adapted to fruit-growing. It is not true, even, that the grape is more particular as to soils than other fruits. But the necessity of having great quantity and high quality of fruit in profitable viticulture makes it very necessary to take their preferences as to soil into strict account.
Air currents are of minor importance compared with the other factors discussed yet are worthy of attention. They are chiefly of importance in grape-growing in the suppression of fungi. It has long been noticed that in regions where there are strong currents of air the dreaded black-rot and the mildew are not nearly so harmful. Winds may be beneficial, too, when they bring warm air, when moisture laden, when they keep frosty air in motion, and possibly they have an effect on some small insects as the leaf-hopper. On the contrary they may be detrimental when too dry, strong or cold. Natural or artificial windbreaks may greatly modify the effects of wind currents though their value is usually overestimated as their benefits are often offset by the undesirable conditions caused.
Lastly, the prevalence or lack of insects and fungi in a region may decide its value for viticulture. In several instances flourishing viticultural industries have been destroyed in this country by insects or fungi, or both. In other regions the present supremacy of commercial grape-growing is almost wholly due to the fact that neither insects nor fungi are seriously troublesome. The advent of spraying and a better knowledge of the life histories of insects and fungi are lessening the importance of the parasite factor in determining the value of a region for grape-growing, but it is still of high importance.