(2) Humus of decaying animals and plants.

(3) Humus

2、( 1)?

Change: Not all topsoil is fertile.

(2)?

Change: Poor soil can be improved by adding humus.

(3)?

(4)?

Change: Topsoil can provide rich nutrients for plant growth.

3.( 1) Required materials: plastic bags, earthworms, soil, vegetable leaves and fruit peels.

Production process:

① Two plastic bags contain the same amount of soil, vegetable leaves and fruit peels. One of them is also full of earthworms.

② Sprinkle a little water evenly in the two plastic bags, and gently tighten the plastic bag mouth.

③ Put two plastic bags in the dark.

(2) Omission

(3) Omission

(4) First, earthworms move in the soil, which makes the soil sparse and makes air and water penetrate deeper into the soil, which is beneficial to plant growth and can improve the soil. Second, earthworms can improve soil fertility by eating rotten organic matter and a lot of soil particles.

Second, 1, humus black

2. Omission

3. The fallen leaves slowly decay and eventually become humus.

4. Soil fertility