This retarding of the loss of the rainfall for a short time after rains occurred was apparent during the entire season. While nothing was done in this experiment to determine the effect of such management on the soluble salts, enough has been done elsewhere to warrant the conclusion that a system of surface culture during the earlier part of the summer sets free a considerable amount of plant food, i. e., renders it soluble in the soil water. Growing plants on the soil during the latter part of the summer prevents the rapid loss of the rainfall that must. contain large quantities of this plant food in solution, until the plants have an opportunity to take it up in large part and build it into plant tissue, which is eventually returned to the soil and trees. Within a short time after each rain these conditions were reversed. That is, the plots with the best cover of rapidly growing plants soon showed the least amount of moisture present. Transpiration of the moisture through the plants and evaporation from the soil is more rapid than evaporation alone from the uncovered plots. This removal of the soil moisture, together with the locking up of soluble plant food, rendering it, for the time, unavailable to the trees, is of great importance. It naturally follows that if these two effects do occur the growth of the trees will be checked and they will mature earlier than they otherwise would do.

While our experiment left no doubt about the removal of the moisture by growing crops, and while there can be no doubt but that a considerable amount of plant food must have been taken out of the soil by the growing plants, that would otherwise have been available to the trees, we are not sure just how much growth was retarded and just how much hardier and better prepared the trees were for winter as a result of the use of cover crops. A considerable number of what might be called accidental experiments, and a few others more or less directly planned, have left little doubt but that trees in orchards so handled are less likely to winter injury.

Table II shows the relative amounts of moisture in the various plots for the dates given. The effect of the rapidly growing crops on the moisture content of the soil is clearly shown. This effect continued up until frosts came, when they were gradually reversed, owing to the mulching effect of the crops that fell down on the soil. Those crops that had made the largest growth and removed the largest amount of moisture during the growing season, now retain the most. This condition continues until the crops that lived through the winter start into growth, when these again begin to remove large quantities of moisture from the soil.

TABLE III.

Effect of Growing Crops in Springtime on Soil Moisture.

It goes without saying that the loss of moisture from the soil at the beginning of the growing season may be very injurious, and that such crops should be plowed under and cultivation begun as early as possible.

Table III shows the effect of these crops for the dates given. Of course, this is much later than they should be allowed to grow in practice.

All the figures in the tables herein given are the average of the first and second foot of soil. These determinations were always made separately and invariably showed some difference in moisture content between the two depths. However, it was impossible to detect any constant differences that were traceable to the cover crops directly. The effects heretofore mentioned were nearly always evident in the surface foot first, then in due time transmitted to the second.

TEMPERATURE.

The observations of the temperature of the soil in the various plots perhaps proved the most instructive, because very little similar work has been done and none, so far as I know, so extensively carried out. The mean daily temperature as well as the weather conditions at the exact time of taking the soil temperature were noted. Owing to delay in securing temperature coils, they were not placed in position until October 8, 1904. No direct relationship between moisture content and temperature of the soil was evident, except as to depth of freezing, as we shall see later. The driest plots and those with the least protection were the ones that froze deepest.

The second season the coils were placed July 19, but in both seasons the differences in temperature in the various plots were slight during the growing period, but varying always and directly with the weather conditions and the thoroughness of the cover. That is to say, when readings were taken on bright, sunny days the check plot and poorly covered ones always showed the highest temperature, and on cold, cloudy days, the highest temperature.

This simply means that the temperature of the well-covered plots was more uniform and the greater and more sudden the changes in the air temperature, the more marked does this uniformity become. The most striking illustration of the cover crop effects on temperature will be seen in the depth of freezing during the severe cold of winter.

The figures in table IV show clearly the effect of clear and cloudy weather on soil temperature. Those for Januray 11, 1906, show temperature of plots under six inches of snow. The snow disappeared last from oats, corn and buckwheat, in the order named, and in every case those plots showed a uniformly higher temperature while the snow remained on them.

Cow Peas
Field Peas

Vetch
Rape

Check

Oats

Corn

Rye

Buckwheat

M. D. T.

TABLE IV.

Effect of Clear and Cloudy Weather on Soil Temperature.

As was said before these differences of temperature in the soil became more marked as the air temperature became more extreme, and the results of the observation on the depth of freezing were perhaps the most striking of the entire experiment, and the differences between the crops were here more pronounced and persistent than in either the moisture determinations or the other temperature readings.

In making these observations, measurements were made in two places for each plot and the average taken. Several measurements were taken at different times and in every case the relative differences were the same for the same crops, the same season. Slight variations were seen for some crops the different years, due to the better growth and better cover produced by some crops one year and the poorer growth and cover produced by others.

Table V shows the depth of freezing in the various plots, at the coldest period of each winter. The gray color shows depth of freezing for February 12, 1905, and the black for February 10, 1906.

During the first season the rainfall was not large and all the plots went into winter relatively dry. Throughout the entire season of growth, moisture determinations showed rape to be removing the greatest amount of moisture, and when it was killed by frosts, it gave poor protection from further loss by evaporation. This plot was frozen 18 inches deep, which is the greatest depth recorded.

The difference between the rape plot and the check plot, which was frozen to a depth of 16 inches, is probably due to the drying effect of rape. Field peas give the next greatest depth of freezing, and it will be remembered that this crop was killed early in the season, leaving very little cover. Both oats and corn made a good growth and cover and the soil under them in each case has frozen to a depth of 10 inches. Rye made an extremely dense, close stand

and good growth, and the soil froze only to a depth of 8 inches. The vetch made a good growth, covering the soil with a dense matting of vegetation, which gave by far the best protection from freezing, the depth here being only 6 inches, or just one foot less than the deepest.

In the observations of February 10, 1906, a number of differences are noted not in keeping with those of February 12, 1905, but these are easily explained. The copious rains in the fall kept the moisture content high in all plots. The check plot is frozen deepest (12 inches). The field peas were again destroyed and this plot gave the next deepest freezing (9 inches). Oats and rye both made a very poor growth and cover, and consequently have