154 NATURE STUDY AND AGRICULTURE 
caying humus produces nitrogen compounds, and that supply 
is now cut off, so there is a shortage. (d) The gases from 
decaying humus help to dissolve the mineral matter of the 
soil and thus make it available for plant growth. Such 
action is now diminished. (e) Soil bacteria that help to 
replenish the supply of plant foods now fail to thrive. 
PROJECTS 
1. Kinds of soil. — If several kinds of soil are found on the different 
farms of the vicinity, examine them carefully and note the difference. 
Try to name each'kind. Find out from the owners the advantages and 
disadvantages of each kind. 
2. Samples of soils. — Collect samples of as great a variety of soils 
as possible, preserving them in small bottles. They should include: 
clay, sand, loam, gravel, etc., and should be taken at or near the surface, 
9 to 12 inches below surface, and from the subsoil still deeper. Each 
should be labeled to show kind of soil and exactly where it was taken. 
3. Soil and subsoil. —TIf{ you see a post-hole dug, or a cellar or a 
well, notice the depth of the soil and the character of the subsoil. You 
may also make this observation at embankments along the road, but 
the cuts usually occur ‘on hills where the soil may be shallower and the 
subsoil different from that on the general: level. Ask a good farmer 
which is better, a clay subsoil or a sand or gravel subsoil, and why it is 
better. ; 
4. How much humus ? — Take a sample of very poor, hard soil and 
another sample of soil that is very rich and mellow, and after drying 
them thoroughly weigh an equal quantity of each and put it in separate 
flower pots. Then set the pots directly on the hot coals in a stove, and 
after the soil is thoroughly burned weigh it again. Some of the differ- 
ence in weight may be due to further loss of moisture, but if the samples 
were well dried in the beginning, most of the loss will be due to the burn- 
ing of the humus. A simpler method is to put half a pound of rich, dry 
earth on a coal shovel and place that on a good bed of coals. As the 
humus burns away we can see it glow. 
5. Humus and moisture. — Take some soil that is rich in humus and 
some very poor, sandy soil and let the samples get thoroughly air dry. 
