Fig. 41. Rolling in mangold seeds on the farm
If you watch an allotment holder who grows onions really well working away at his seed-bed you will see what a beautifully fine tilth he gets. If you try to do the same you will probably fail; his seeds will be up before yours and will grow into healthier plants. Only after long practice will you succeed, and then you will have mastered one of the great mysteries of gardening.
As soon as the plants are up they have to be hoed, and the more often this is done the better. Hoeing has several useful effects on the soil; during summer time some experiments may be made to find out what these are. A piece of ground is wanted that has got no crop on it. Set out three strips each six feet wide and six feet long, leave one entirely alone, hoe the second once a week, and the third three times a week; put labels on so that no mistake can arise. The surface of the untouched plot becomes very compact and glazed in appearance; the other soils look nice and crumbly. Take the temperature of the soils by placing a thermometer into it at various depths—half inch, three inches, and six inches—also take the temperature of the air; enter up the results as in the table, which shows what happened at Harpenden.
Air
Date temperature Soil temperature
Hoed Hoed
Untouched once three times
weekly weekly
1910
June 20th 30 1/2 inch 35 31.5 31.5
3 inches 30.5 29.8 28.8
6 inches 27 26.5 24
June 27th 18 1/2 inch 17.5 17 17
3 inches 16.7 16.3 16.2
6 inches 15.8 15.5 15.6
The thermometer readings are in degrees centigrade.
Remarks. June 20th: Hot sunny day, there had been no rain since June 11th.
June 27th: Cold, cloudy day, several cold, wet days during the past week.
On the cold day there was very little difference between the plots, but on the hot day the hoed plots were cooler than the others. Now only the top inch is touched by the hoe, and so it appears that the layer thus loosened shields the rest of the soil from the sun's heat. If this is the case we ought to find that any other loose material would act in just the same way. We must, therefore, set out a fourth plot alongside the others, cover it with straw or cut grass (a cover like this is called a mulch), and take the temperature there. Some of the results were as follows:—
Air
Date temperature Soil temperature
Hoed plot Mulched plot
1910
Sept. 24th 15 1/2 inch 17.5 12.25
3 inches 12.5 11.75
6 inches 12.25 11.5
Oct. 5th 17 1/2 inch 17 15.5
3 inches 16.7 15
6 inches 15.5 14.5
Remarks. Sept. 24th: Warm day after a rather cold spell. Oct. 5th: After a long spell of dry, warm weather.
The untouched plot had become smothered in weeds and could no longer be used for this experiment. The mulched soil is, however, cooler even than the hoed soil, and our expectation that mulching would keep the soil cool has turned out to be correct.
