Competition Between Crop Plants and Weed Plants
One of the main competitions between crop plants and weed plants is indeed for space and nutrients. While both space and nutrients are essential for the growth and development of plants, the availability of these resources can significantly impact the success of crop plants in outcompeting weed plants.

Space and Nutrients as Key Resources
- Space: Space is a crucial resource that directly affects the ability of plants to access sunlight, water, and nutrients. Crop plants need adequate space to grow and spread their roots, allowing them to efficiently absorb essential resources. Weed plants often compete for space with crop plants, leading to reduced growth and yield.
- Nutrients: Nutrients such as nitrogen, phosphorus, and potassium are essential for plant growth and development. Crop plants require a sufficient supply of nutrients to support their metabolic processes and produce a healthy yield. However, weed plants can also utilize these nutrients, leading to competition and potential nutrient deficiencies in crop plants.

Why Nutrients Were Not Included in the Given Options
The options provided (space, water, light, air) likely focus on the most limiting factors that directly impact plant growth and competition. While nutrients are undoubtedly crucial, they are often considered secondary to factors such as space and light in the context of plant competition. Additionally, nutrients can be replenished through fertilization practices, whereas factors like space and light are fixed resources that cannot be easily modified.
In conclusion, while space and nutrients play a significant role in the competition between crop plants and weed plants, the options provided likely prioritize the most immediate and critical resources that influence plant growth and success. By understanding and managing these key resources effectively, farmers can optimize crop production and minimize the impact of weed competition.