Lessons on Soil

by Edward J. Russell

Fig. 43. Cultivation and mulching reduce the loss of water from soils

These results are so important that some indoor experiments should be made to furnish more proof. Fix up three inverted bell jars with corks and bent tubes as shown in Fig. 43, fill all with dry soil well pressed down, then add water carefully till it appears in the glass tubes. Next day mark with stamp paper the level of liquid in each tube and then leave one jar untouched, carefully cultivate with a penknife every two or three days the top quarter of an inch of the second, and cover the third with a layer of grass. After a week notice again the levels of the liquid and mark with paper; you find that the water has fallen most in the untouched jar, showing that more has been lost from this than from the jars covered with a mulch either of soil or of dry grass.

A slate or flat stone acts like a mulch; if you leave one on the soil for a few days in hot weather and then lift it up on a hot day you will see that the soil underneath is quite moist; you may also find several slugs or other animals that have gone there for the sake of the coolness and the moisture. Plants and trees also keep off the sun's heat and so make the soil cold and moist. Grass land is in summer and autumn, and even in early winter, cooler near the surface than bare land. At Harpenden we found:—

Soil temperature
Date Grass land Bare land
1910
Sept. 24th 1/2 inch 13 17.5
3 inches 12.5 12.5
6 inches 12.5 12.25
Oct. 5th 1/2 inch 17.5 17
3 inches 15 16.7
6 inches 14.5 15.5

Even if the ground is not covered a certain amount of protection is still possible. Trees are often planted round ponds to prevent evaporation of the water. The wind helps to dry the soil very much, and a hedge that shields from the wind not only protects the crop but also keeps the soil moist: a road with high hedges at each side remains wet for a long time after more exposed parts have dried. The effect on the temperature can be well seen on a day when a N.E. wind is blowing. Fix up on a piece of the experimental ground a little hedge made of small pea-stakes or brushwood, and take the soil temperature at one inch depth, both on the windward and on the leeward side. Two results were:—

Temperature at 1 inch depth--sheltered side 15.5
" " " " windward side 14

We have already seen that on the hot day, June 20th, the top half-inch of soil was hotter than the air: the mercury in the thermometer rose directly it was put into the soil. There is nothing very unusual about this; if you touch a piece of iron lying on the soil you find it hotter than the air. Lower down the soil had the same temperature as the air, and still lower it was cooler[1]. The sun's heat travels so slowly into the soil in summer that months pass before it gets far down, but then, as it takes so long to get in, it also takes a long time to get out, and it takes still longer to get either in or out if there is a mulch or if grass is growing.

During the early winter you may notice that the first fall of snow soon melts on the arable land but remains longer on the grass; towards the end of the winter, however, the reverse happens and the snow melts first on the grass. There is no difficulty in explaining this. The arable land is, as we have seen, warmer in autumn and early winter than grass land, and so it melts the snow more rapidly. But during winter the grass land loses its heat more slowly, and therefore it is warmer at the end of the winter than the arable land, hence the snow melts more quickly.

In Chap. V. it was pointed out that dark coloured soils rich in humus are greatly favoured by gardeners and farmers. The value of humus can easily be shown: take a sample of soil from a garden that has for a long time been well manured and another from a field close by—next to it if you can—and find the amounts of moisture present. Two soils at Rothamsted gave the following results:—

Date April 6th May 6th July 6th Oct. 28th
Moisture in dark soil
rich in humus 20.0 18.0 20.7 23.3
Moisture in lighter
soil poor in humus 13.1 11.9 12.0 17.5

Humus, therefore, keeps the water in the soil and saves it from being lost.

Another beneficial effect of hoeing is to keep down weeds. Weeds overcrowd the plant, shut out light, take food and water, and occupy space. Few plants can compete against weeds, some fail very badly in the struggle. Sow two rows of maize two yards apart; keep one well hoed for a yard on each side and leave the other alone to struggle with the weeds that will grow. Fig. 44 shows the result of this experiment at St George's School. At Rothamsted a piece of wheat was left unharvested in 1882, and the plot has not been touched since; the wheat was allowed to shed its seed and to grow up without any attention. Weeds flourished, but the wheat did not; the next year there was but little wheat, and by 1886 only a few plants could be seen, so stunted that one would hardly recognise them. The ground still remains untouched, and is now the dense thicket seen in Fig. 45. Most of our land would become like this if it were neglected for a few years.

Fig. 44 a. The hoed plot, no weeds. Maize cannot compete successfully against weeds

Farmers occasionally leave their ground without a crop for a whole year and cultivate it as often as they can to kill the weeds. This practice is called "fallowing," and is very ancient; it is much less common now that crops like mangolds and swedes are grown, which can, if necessary, be hoed all the summer.

Fig. 44 b. Untouched plot, many weeds

We have already seen (p. 69) that ordinary cultivated plants will not live in a water-logged soil. Wherever there is an excess of water it must be removed before satisfactory results can be obtained. Fig. 46 shows a field of wheat in May where the crop is all but killed and only certain weeds survive on a patch of undrained land that lay wet all the winter. Draining land is difficult and somewhat expensive; trenches are first cut to a proper depth, and drain pipes are laid on the bottom, taking care that there is a gentle slope all the way to the ditch. The rain soaks into the soil and gets into the pipes, for they are not joined together like gas or water pipes, but left with little spaces in between; it then runs out into the ditch. Usually only clay soils need drainage, but occasionally sandy soils do also (see pp. 30, 106). A great deal of drainage was carried out in England between 1840 and 1860, and it led to a marked improvement in agriculture and in country life generally. There is, however, a great deal that wants doing now.