Take a closer look at the soil. Most of the essential nutrients required by crops are derived from the soil, meaning that the nutrient status of the crop is directly related to the nutrient levels in the soil. Therefore, diagnosing crop nutrient deficiencies must include an assessment of the soil's nutrient condition.
1. Extraction of available soil nutrients and indicators. Soil nutrients exist in various forms, but only a portion is readily available for plant uptake. Due to differences in soil properties and nutrient characteristics, it's crucial to conduct soil nutrient analysis to determine the availability of nutrients. For more detailed information, refer to the section on Soil Nutrient Indicators in the Nutrient Analysis Guide.
2. The effectiveness of soil nutrients in diagnosis is influenced by many factors. As a result, the total nutrient content in the soil does not always reflect the actual nutrient availability for crops. Factors such as soil texture, pH, temperature, moisture, and microbial activity can all affect nutrient availability. Additionally, different crop species and varieties vary in their ability to absorb and utilize specific nutrient forms. Therefore, other environmental conditions must be considered when evaluating soil nutrient status. For example, soil pH, temperature, water table, and microbial activity all play significant roles in how well plants can absorb and use soil nutrients, affecting their growth and development.
The effect of soil pH on nutrient availability is particularly noticeable. One aspect is the plant’s adaptability to acidic or alkaline conditions. For instance, a plant adapted to acidic soils may show abnormal growth if planted in high-pH soils, even if the overall nutrient levels are sufficient. Another factor is how pH influences nutrient availability and microbial activity. In alkaline soils, the availability of phosphorus, iron, manganese, zinc, boron, and copper decreases, while molybdenum becomes more available. In acidic conditions, most micronutrients except molybdenum become more available, but very low pH can lead to toxicity from iron, manganese, and aluminum. Phosphorus availability also decreases due to fixation by iron and aluminum.
Temperature plays a key role in root nutrient uptake. At low temperatures, the absorption of certain nutrients follows this order: phosphorus > nitrogen > sulfur > magnesium > calcium. This shows that low temperatures have the greatest impact on phosphorus. Sometimes, even if soil phosphorus levels are adequate, plants may show low phosphorus concentrations in tissues due to reduced root uptake.
The groundwater level affects soil moisture and aeration. High water tables or poor drainage can lead to waterlogging, creating reducing conditions in the soil. This can cause accumulation of ferrous iron and sulfides, which are harmful to roots and nutrient uptake, leading to stunted growth and poor development.
Next, take a close look at the plants.
1. Identify where the symptoms first appear. If the symptoms start on the older leaves, it often indicates a deficiency in nitrogen, phosphorus, magnesium, or zinc. If the symptoms appear first on the new growth, it may signal a lack of calcium, iron, boron, or sulfur.
2. Observe whether the old leaves have lesions or if symptoms appear on the new leaves without any damage. If there are no lesions, it could suggest a phosphorus or nitrogen deficiency. If lesions are present, it might indicate a potassium or zinc deficiency. If the symptoms begin on the new leaves, and the tips die easily, it could point to a boron or calcium deficiency. If the terminal buds remain intact, the deficiency might be iron, sulfur, manganese, molybdenum, or copper.
3. Determine the specific nutrient deficiency based on symptoms.
1. Nitrogen: A nitrogen-deficient plant typically starts showing yellowing on the lower leaves, which gradually spreads upward. The roots tend to be pale and thin, with fewer roots than normal.
2. Phosphorus: Plants lacking phosphorus experience stunted growth, with small, narrow leaves that turn dark green and may develop a purplish hue in severe cases.
3. Potassium: A potassium-deficient plant shows yellowing along the edges and between the veins of older leaves, followed by drying, scorching, and necrotic spots.
4. Calcium: Calcium deficiency leads to short plants, stunted roots, and damaged meristems. Severe deficiency causes curled young leaves, difficulty in leaf emergence, and burnt or dead leaf tips and edges.
5. Magnesium: Magnesium deficiency appears first on older leaves, starting with yellowing between the veins, which eventually turns purple, while the veins remain green, forming a clear network pattern.
6. Sulfur: Sulfur-deficient plants are usually smaller, with small, curled, hard, and brittle leaves that fall early. Flowering is delayed, and fruit production is reduced.
7. Iron: Iron deficiency causes yellowing of young leaves, often starting between the veins, leading to chlorosis and impaired photosynthesis.
8. Manganese: Manganese-deficient plants show yellowing and necrotic spots on new leaves, with curled white leaves and malformed fruits in fruit trees.
9. Copper: Copper deficiency results in yellowing and necrotic spots on new leaves, with curly white hairs between the leaves and deformed branches in fruit trees.
10. Zinc: Zinc-deficient plants exhibit yellowing, brown spots, and tissue necrosis on the leaves, with fruit trees developing "lobular disease."
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