The Martian (2015 movie): Why couldn't Matt Damon grow more potatoes using the poop logic he used for the first time?

In the movie The Martian (2015), Matt Damon's character, Mark Watney, uses a combination of Martian soil, Earth soil, and human feces (poop) as fertilizer to grow potatoes. However, there are several reasons why this method might not be sustainable for growing more potatoes over time:

1. Pathogens in Human Feces: While Watney uses his own feces to avoid pathogens from others, fresh feces can still be harmful to plants due to high nitrogen content. This can damage or kill plants if not properly composted12.

2. Lack of Bacterial Activity: Initially, Watney uses freeze-dried feces from his crewmates, which are pathogen-free but lack beneficial bacteria. He mixes his own feces into the soil to introduce bacteria necessary for decomposition and nutrient release14.

3. Martian Soil Challenges: Martian soil lacks essential nutrients for plant growth and is high in salt and perchlorates, which are toxic to most plants. Watney mitigates this by using some Earth soil and creating a controlled environment within his habitat14.

4. Sustainability and Scale: Watney's intensive farming methods are not sustainable for long-term use on Earth due to soil degradation. On Mars, the challenge is compounded by limited resources and the need for continuous water and nutrient supply1.

5. Caloric Needs: Even with successful potato growth, Watney faces significant challenges in meeting his caloric needs solely through potatoes. He requires about 1,500 calories per day, which is difficult to achieve with potatoes alone12.

In summary, while Watney's initial success with growing potatoes using feces as fertilizer is plausible, sustaining this method over time is fraught with challenges related to soil quality, pathogen risks, and nutritional deficiencies.