Nutrient Cycling in Thin Soil

Question 1

A farmer cuts down a small patch of rainforest and plants cassava. The first two years the cassava grows big and green, but by the fourth year the plants are small and their leaves turn yellow. Which reason best explains why the farm stops being productive?

Accepted Answer

The rainforest soil is only a thin layer with few nutrients, and most nutrients were stored in the living trees and their fallen leaves; once the trees were removed, the source of new nutrients disappeared, so the soil ran out of nutrients after a few years.

Answer

The farmer never added any chemical fertilizer, so the natural minerals in the soil were used up quickly.

Answer

Heavy rain in the area washed the nutrients away from the field, leaving the soil empty.

Answer

Without the tree roots, the soil became hard and compact, preventing water and nutrients from reaching the cassava roots.

Question 2

A farmer clears a small piece of rainforest to grow vegetables. The first two years the garden grows well, but after a few more years the plants look weak and yields drop. Which explanation best tells why the garden is no longer productive?

Accepted Answer

When the trees were cut down, the living roots and their partner fungi that stored most of the nutrients disappeared, so the few nutrients left in the soil were quickly used up or washed away, leaving little for the vegetables.

Answer

The soil already had plenty of nutrients, but the farmer did not water the plants enough.

Answer

Removing the trees changed the weather, making the area too cold for the vegetables to grow.

Answer

The farmer chose the wrong type of vegetable seeds, so the plants could not use the soil nutrients.

Question 3

A scientist studies a section of rainforest where the soil is only about 3 cm deep and looks light and sandy. Still, the area is covered with tall trees and thick vines. Which explanation best describes how the plants obtain the nutrients they need?

Accepted Answer

Most nutrients are constantly recycled: fallen leaves and wood are broken down by microbes, releasing nutrients that the tree roots and fungi absorb right away, so the soil itself stays low in nutrients.

Answer

Heavy rain carries mineral nutrients from the mountains and deposits them into the thin soil, providing everything the trees need.

Answer

During the rainy season the trees store all the nutrients they will ever need in their roots, so they no longer rely on soil nutrients.

Answer

The thin soil actually contains a hidden store of rich minerals that the scientist’s measurement missed.

Try three questions

A farmer cuts down a small patch of rainforest and plants cassava. The first two years the cassava grows big and green, but by the fourth year the plants are small and their leaves turn yellow. Which reason best explains why the farm stops being productive?

A farmer clears a small piece of rainforest to grow vegetables. The first two years the garden grows well, but after a few more years the plants look weak and yields drop. Which explanation best tells why the garden is no longer productive?

A scientist studies a section of rainforest where the soil is only about 3 cm deep and looks light and sandy. Still, the area is covered with tall trees and thick vines. Which explanation best describes how the plants obtain the nutrients they need?

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