Which of the following is the least available source of nutrients?

Soil minerals are considered the least available source of nutrients due to their inherent characteristics and the processes involved in nutrient availability. Nutrients found in soil minerals are often bound in a crystalline structure, making them less accessible to plants compared to nutrients from organic matter, fertilizers, or soil microbes.

Organic matter is a critical source of nutrients because it not only contains essential elements but also helps improve soil structure, enhance moisture retention, and support microbial activity, which in turn aids in nutrient cycling. Fertilizers provide readily available nutrients specifically formulated to meet crop needs and can be quickly utilized by plants. Soil microbes play a vital role in breaking down organic matter and converting soil nutrients into forms that plants can easily absorb, thus enhancing nutrient availability.

In contrast, while soil minerals are a significant reservoir of nutrients, their availability is limited until they undergo weathering processes or are transformed by biological or chemical reactions in the soil. This means that even though minerals are fundamental to the nutrient supply, they are not as immediately accessible as other sources, which is why they are regarded as the least available source of nutrients.