Lime is especially valuable for plant food and also to make other kinds of plant food available. Crops often fail in soil where there is plenty of plant food, because it is not in the form that the plants can use. Lime hastens the decay of vegetable matter, sweetens sour soil, and greatly improves the texture of clay soils. Besides this it counteracts magnesium in the soil and destroys its bad effects. But lime may not be applied carelessly, because, although some plants cannot live without it, some require a very small quantity. (It is, therefore, wise to send a sample of the soil, to write what we want to grow, to the nearest Government experiment station, who will probably suggest what could be done with it, to make it productive.) The right degree of warmth is another plant necessity. The best temperature for plants generally is 86° of the ordinary thermometer (30° Centigrade). This, however, depends upon the plant. The “best temperature” varies with the species and variety. Usually, if the soil is hotter than that, growth stops, and if the greater heat is kept up, the plant dies. When the temperature is lowered, growth ceases before freezing point is reached. Some few plants may be frozen without injury if they are allowed to thaw slowly, but most of them are easily killed by the frost. Too great heat or too much cold acts the same as lack of water; the heat causes too great evaporation, the cold prevents the roots taking in the water. Shingles stuck in the ground on the sunny side will serve to protect young plants from sun and rain, while cool soil may be had by using the shady places, or by sheltering the ground with flat sheds the roof boards of which have open spaces between them as wide as the boards. This is done in some southern tobacco fields. It is the law of nature that living cells must have a constant supply of oxygen, that is why a tar wash sometimes kills plants by cutting off the air supply. In the same way, too wet soil or too hard a crust smothers the roots and the plants die. The surface soil should be kept loose and sufficiently dry, so that the air can circulate. If this be prevented, the soil becomes hard and sour and unfit to feed plants. It is of the greatest importance to keep the soil open and loose by proper tillage, so as to make plants healthy and vigorous.