Plants need nutrients to grow which they absorb from the soil via the plant’s root system.

• Fertilizers provide the major nutrients (nitrogen, phosphorus and potassium and important secondary elements) that plants need.

Answer:

Introduction:
Fertilizers are substances that are added to the soil or plants to provide essential nutrients for their growth and development. They are commonly used in agriculture to improve soil fertility and obtain higher yields of crops. However, not all nutrients necessary for plant growth are available in fertilizers.

Explanation:
Among the given options, iron (Fe) is the nutrient that is usually not available in fertilizers.

Iron (Fe):
Iron is an essential micronutrient required by plants for various physiological processes, including chlorophyll synthesis, respiration, and enzyme activation. It plays a crucial role in the transfer of energy and the metabolism of nitrogen. However, iron is often present in the soil in an insoluble form, making it less available for plant uptake.

Why is iron usually not available in fertilizers?
1. Insufficient concentration: Iron is required by plants in small amounts, and its concentration in fertilizers is usually insufficient to meet the plant's requirements.
2. Complexation: Iron can form complexes with other compounds in the soil, making it less available for plant uptake.
3. Low solubility: Iron compounds have low solubility in water, which restricts their availability to plants.

How to address iron deficiency in plants?
To address iron deficiency in plants, various strategies can be employed:
1. Iron chelates: Iron chelates, such as iron ethylenediaminetetraacetic acid (EDTA), are commonly used to supply available iron to plants. These chelates enhance the solubility and availability of iron in the soil.
2. Foliar application: Iron can be supplied to plants through foliar application. Iron-containing compounds are sprayed on the foliage, allowing direct absorption by the leaves.
3. Soil amendments: Adding organic matter or substances such as sulfur or organic acids to the soil can help improve iron availability by reducing its insoluble forms.
4. Soil pH adjustment: Iron availability is influenced by soil pH. Acidic soils (pH below 7) favor iron solubility and uptake by plants. Adjusting soil pH can improve iron availability.

Conclusion:
While fertilizers are valuable for providing essential nutrients to plants, they may not contain all the necessary nutrients. Iron, an essential micronutrient, is usually not available in fertilizers due to its low concentration, complexation, and low solubility in water. Iron deficiency in plants can be addressed through the use of iron chelates, foliar application, soil amendments, and soil pH adjustment.