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.

Figure 4: Relationship between crop growth (or yield) and nutrient soil concentration in the root zone. The maintaining of the minimum optimal nutrient concentration allows for reducing nutrient loss risk (e.g., NO3− leaching) without affecting crop produce and quality. (From Incrocci, Massa and Pardossi, Horticulturae 2017, 3(2), 37)

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).

Figure 5. Effect of pH on nutrient availability, the wider the bar, the greater the nutrient availability. The greatest level of plant nutrient availability is between pH 5.5 to 6.5. (Source: Agriculture Victoria)

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.