5. Soil: Soil  is the outermost layer of the surface of the earth in which plants grow. Soil with proper humidity and the right balance of all the minerals and nutrients is one of the essential factors instrumental in plant growth. The type of soil and the quality and the nutrients required in it vary according to the plant species. The right pH balance, which measures the alkalinity or acidity of the soil and presence of certain chemicals, is also instrumental in the growth of plants. pH influences availability of certain nutrients. For most plant life, the most favorable pH value is between 6 - 6.8. The soil serves the needs of the plant by providing: water, air, nutrients and stability. The ability of a soil to provide these services may be evaluated by key soil attributes as shown in Table 3)

Table 3: Soil attributes relevant to plants

 

Interesting Facts:

• Photoperiodism is responsible for the geographical distribution of many plants worldwide.
• Bulbing in onions is primarily controlled by photoperiod - day length - as the days become longer, plants begin to bulb.