Nutrient management using best practice monitoring

Mid November saw the Future Orchards® spring walks on orchards across Australia. Nutrient management was the primary focus for this series with key messages including:

• Frequency of nutrient monitoring including timing and response
• The value of monitoring over time
• Using nutrient input history and test results to form the basis of a nutrient management plan.

To make accurate, data led adjustments to nutrient management strategies with a drive towards optimising block performance, growers should utilise multiple data sources including block history, soil tests, leaf tests and trends over time, as well as a visual analysis of the block. These metrics help growers understand their blocks and make informed, well timed management decisions, that optimise plant uptake, crop quality and production outcomes.

The following case study shows a variety of trends between the different sources of information including:

• Direct correlations between soil and leaf tests
• Counteracting results between the 2 testing mechanisms (requiring further testing)
• Relationships between macro and micronutrients influencing plant uptake and block performance.

Soil tests

Soil tests are crucial to understanding the plant available nutrients within the soil profile. Figure 1 shows the soil sample results for this case study block, taken in May 2023.

Figure 1 Soil test results for case study block, May 2023

Key features of these soil results include: a low soil pH, low Calcium (Ca), high Potassium (K), high Magnesium (Mg) base saturation per cent, and high Phosphorus-mehlich (P). Although Nitrate levels measured in the soil are generally considered unreliable due to the ‘point in time’ nature of the test, results for this block were also below the optimum range.

Leaf tests

Best practise leaf sampling timing is one in mid-November, with a second sample taken in late January/early February.

At each orchard walk block, a standard dry ash leaf sample was taken as a follow on from the winter soil tests. This testing programme is used to outline both macro and micronutrient levels in the plant, as well as monitoring any changes you may have already made to your block specific nutrient programme.

Figure 2 Leaf test results for case study block, November 2023

Key insights from the case study blocks mid-November leaf test include:

• Leaf Nitrogen of 2.69 per cent, at the upper end of the optimal range.
• Leaf Phosphorus levels at the lower end of the optimum range which is strange with such good levels of P in the soil.
• Good leaf potassium levels (also high in the soil)
• Very low leaf calcium per cent
• Very high Iron and Boron levels.
• Magnesium levels are OK however we expect these to drop as the fruit mature.
• Sulphur is at the upper end of optimum.
• Zinc (Zn) levels are low, recommend foliar application of a safe form of Zn

Nutrient ratios are of importance when looking at leaf Nitrogen levels.

• The N:K ratio is optimum.
• The N:P ratio is above optimum. Nitrogen levels are relatively high; however, phosphorus is at the lower end of optimum.

Key Insights

To close the loop on soil and leaf test results, it’s important to look at input history. This helps with understanding whether nutrient inputs have influenced results, and if the current management plan is working, or if changes need to be made.

Table 1 Case Study 1 – Nutrient application history by season (2022, 2023)

Despite the low Nitrate results shown in the soil tests, leaf Nitrogen is at the upper end of the range for this case study block.  Over the last couple of seasons Nitrogen inputs have averaged 127kgs/ha, with visually high leaf condition in the block. This combined with the knowledge that the block struggles to finish high-grade fruit colour suggest that the Nitrogen input should be reduced to 70kg/ha per year (closer to the current calculated crop removal rate). The timing of N application will important, ensuring it is completed while the plant is still active – applying Nitrogen to the soil very late autumn is likely too late for pre-winter uptake into the tree.

Reducing Nitrogen inputs on this block would ideally balance out the currently high N:P ratio, aiming to hold N at the lower end of the mid-November optimum range.

Although Phosphorus (P) soil levels appeared high, Leaf P is at the lower end of optimum, which seems strange considering the blocks input history of 30kg/ha per season. These results combined with block input history warrant further testing before making changes to the current nutrient management programme.

Potassium (K) Base saturation per cent tested high, with leaf tests also showing levels at the upper end of optimum. Thus, the K applications should stop for a couple of years, keeping in mind that excess Potassium effects how the plant uptakes both calcium and magnesium.

In reaction to the soil tests, the grower has applied lime to increase pH to an optimum of 6.4.

Following the trend from the soil test taken May 2023, the case study block was also deficient in leaf calcium, with only 0.77 per cent concentration. To ensure fruit have sufficient calcium for good cell strength and long term storage, a good foliar or fertigation calcium program should be applied over the remainder of the growing season.

The block also showed high leaf Boron levels of 51mg/kg. Elevated levels of Boron have been known to result in phytotoxicity, as well as having an antagonistic relationship with Calcium. Currently Boron is being applied each season in the form of ‘Organi bor’. This should be dropped from this blocks nutrient management programme, in response to the most recent leaf test results.

Interestingly, 6 of the 8 future orchard blocks showed leaf calcium deficiencies. Calcium is primarily taken up through the roots and is relatively immobile moving through the plant via the xylem. Well-timed foliar applications are required throughout the season to supplement cell strength.  Varieties known for their storage disorders (such as Kanzi® and Jazz™) require a diligent foliar Ca programme to support the fruit through the entire growing season.

Once changes are made to a nutrient management strategy these should be monitored. Secondary leaf testing in late January/early February not only guides post-harvest nutrient management but measures nutrient use (and therefore removal) throughout the season. The next step to optimise this grower’s nutrient management strategy will be to take these preharvest leaf tests, which will guide postharvest nutrient requirements prior to leaf senescence – setting the block up for the 2025 growing season.

This is a good case study on the benefit of good annual soil and plant tissue testing. We have aimed to empower growers to debate the issues with their agronomists so that as a team, nutrient inputs can be applied cost effectively to maximise the crop.