Optimum nutrient levels and the effect of pH
Adding more nutrient than needed is unwise. It will increase costs and will not increase returns. It can even reduce yield. Adding more nutrient than the plant needs does not increase yield and at high levels may even decrease yield quantity and quality. Any unused nutrients add to the risk of leaching which can have negative environmental effects.
Figure 4 from Incrocci and others shows the relationship between crop growth, nutrient loss risk and nutrient in the root zone for a nutrient such as nitrogen.
The pH conditions of the soil affect nutrient availability – and at either extreme (too acidic or too basic) plant growth will suffer even if soil nutrient levels are optimum (Figure 5).
Optimum pH will vary for different crops however it should generally be between 5.5 and 6.5.
If the soil is too acidic (low pH) plant uptake of Magnesium (Mg), Nitrates, Potassium (K), Sulphur (S), Molybdenum (Mo), and Calcium (Ca) will be limited even if there are adequate levels in the soil.
If the soil is too alkaline or basic (high pH) plant uptake of Manganese (Mn), Zinc (Zn), Iron (Fe), and Phosphate (P) will be limited despite adequate amounts present in the soil.
Where nutrients have the same biochemical uptake pathway (Phosphate, Iron, Calcium and Aluminium), their availability curves overlap, with the notched areas indicating uptake of one nutrient will reduce uptake of the other.