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Nutrient management for pome fruit

Quality Management

The key to a targeted approach to nutrient management is understanding your soil, leaf composition and nutrient levels in fruit pre-harvest.  

Nutrient management is a key component of high-quality fruit production. The human analogy is that ‘we are what we eat’. The same applies to plants and particularly apples and pears. The goal is to produce high dry matter, crunchy, high-colour fruit. Unlike a vegetable grower, where it’s all about leafy green yield, the apple grower must manage the nutritional balance to ensure the fruit are of high eating quality. 

As with many Future Orchards® orchard walks, during the November 2023 spring walks the AgFirst presenters returned to the nutrition component of the production jigsaw. Prior to the walks, soil samples were taken, and during the walks, leaf tests. The presenters were also able to observe the crop and talk to the growers about their historical fruit outcomes. 

Sampling, timing and frequency 

There are a range of nutrient analysis techniques that offer insight into plant nutrient composition and, therefore, block performance and crop quality. In combination, these tests help quantify nutrient levels through the different stages of the plant nutrient cycle, and how these nutrients exist in the soil, plant and are then removed as crop.  

Best practice is to incorporate various testing techniques into orchard management, to better understand your block-specific nutrient budgets. These tests include:  

  • soil tests 
  • leaf tests 
  • fruitlet and fruit tests 
  • fertiliser/nutrient application history 
  • crop outcomes.  

Soil testing 

Soil testing best practice is to test each block (or distinct soil type) at least every two years. This helps growers understand the soil nutrient balance, as well as the implications of the soil test results and how these translate through to block performance.  

Figure 1: Leaf magnesium deficiency.

In soil, the relationship between nutrients, their ratios and the soil pH result all have a bearing on nutrient availability and uptake.  

Fertiliser input history provides background and validates recommendations, especially when aiming to alter nutrient ratios, maintain/optimise available nutrients, or fix identified deficiencies. It can also highlight historical nutrient applications that may have contributed to elevated levels.  

Leaf testing 

Best practice leaf testing involves seasonal testing of each distinct block, with a test taken in mid-November followed by a secondary test in late January (pre-harvest).  

The mid-November leaf test highlights any nutrient deficiencies the plant has experienced over spring. The aim of this test is to capture this information before visual symptoms occur and the damage has been done. Once a nutrient deficiency can be diagnosed through visual leaf symptoms, the plant potential has already been compromised.  

Leaf tests should then be retaken in late January. These will show if changes made to block-specific nutrient plans have achieved the desired results. This test also guides any post-harvest nutrient requirements. If you are targeting high yields, the second test will validate that nutrient levels are changing at the correct rates throughout the season and remaining within the target ‘zone’ – outlined in Table 2.    

Paired leaf tests (tests taken from two separate areas of a block) could be carried out if a grower was trying to narrow the focus of a block-specific issue. For example, one of the 10 case study blocks analysed had a long history of poor fruit colour at harvest. Both ends of the block were tested, both showed elevated nitrogen levels. Both the N:P ratio and the N:K ratio were above optimum due to the elevated nitrogen levels. The identified N% was likely to remain above the desired ‘pre-harvest leaf test’ parameters (mentioned in Table 2) and is understood to be a key cause of the crop’s inability to colour at harvest.  

Table 1: Paired test from one of 10 case study orchards. Leaf test showing only macronutrients, highlighting the high N levels in the leaf in mid-November.

Pre-harvest fruit samples could also be taken to show the internal nutrient content of the fruit and highlight specific block challenges. Examples of this include: 

  • N:K ratios and, therefore, the impacts on ability of fruit colouring at harvest  
  • Mg:K ratios in the fruit at harvest can confirm fruit quality issues 
  • Calcium levels can highlight any concerns around calcium-related storage disorders, allowing for changes to the post-harvest management of the fruit.  

Leaf test targets 

Different stages of the season have different leaf test targets. These targets are aimed at optimising plant health and crop quality. As an example, nitrogen should be around 2.5–2.7 per cent at the mid-November leaf test, but by late January, nitrogen should have dropped to around 2.0 per cent to support fruit pigmentation at harvest. Late January leaf nitrogen levels will guide post-harvest application requirements, such as the decision to make a post-harvest nitrogen application to support bud quality and return bloom.   

Table 2: Desired leaf sample nutrient ranges. Source: AgFirst

It is important to consider the influence from the following factors when making decisions on block-specific nutrient management programs: 

  • Seasonal crop loads (nutrient removal) 
  • Block vigour status 
  • Local nutrient delivery sources 
  • Natural composition of the soil bedrock (K, P, Ca, Mg) 
  • Soil organic matter, compost application or a legume dominated sward (N) 
  • Rainfall, ground water and irrigation composition (N, Ca, P) 
  • Historical nutrient inputs. 

Crop removal rates 

Crop nutrient removal in apples has been widely studied by analysing the nutrient composition of the fruit at harvest. Although this varies depending on the variety, generally, the numbers in Table 3 can be used to understand block-specific nutrient budgets.  

Table 3: Fruit removal rates (kg/ha).

To ensure understanding of Table 3, a 100t per ha crop removes 85 kg of nitrogen and 120kg of potassium. For the trace elements, the removals are 100g of iron and 400g of zinc. These are extremely low – an important aspect to understand. 

Soil CEC balance 

The cation exchange capacity (CEC) determines the number of cation binding sites within the soil profile. The more positive the charge, the more sites available. Sand tends to have a low CEC with few available sites. A little-and-often approach to fertiliser application (potentially fertigation) would be a good option for low CEC soils.  

Compost applications or specific management strategies to improve the organic matter in low CEC soils is essential.  

Case Study 1 

Winter soil test analysis 

Table 4: Winter soil test Case Study Block 1.

The low CEC in the soil sample suggests that there are limited cation exchange sites within this soil. The high base saturation (100 per cent) means that that available sites within the tested profile are full. The organic matter content in the soil is at optimum.  

As the pH levels are below optimum in this soil, lime applications are required to increase them.  

The tests show a high phosphorus and high magnesium base saturation (BS%). Due to the antagonistic relationship between magnesium and calcium, the high magnesium base saturation percentage means that despite optimum calcium levels, a foliar calcium program is likely to be required.  

There is no need for maintenance phosphorus or magnesium applications based on the winter soil test.  

The nitrogen and potassium levels are good, suggesting only a maintenance application is required to support crop removal rates and loss of nutrient through the soil profile (leaching). This soil test measures nitrogen as nitrate-N (No3-N). This sample is considered ‘point in time’. The nature of the nitrogen cycle means No3-N is highly variable due to its high solubility and response to temperature, soil type and pH levels.  

Leaf test analysis 

To validate plant uptake and seasonal nutrient requirements, the grower from Case Study 1 took mid-November leaf tests.  

Table 5: Mid-November leaf test Case Study Block 1.

Despite what appeared to be optimum soil nitrogen levels, the November leaf tests show leaf nitrogen is low at 2.27 per cent. This is reflective of the points mentioned previously regarding the nature of the nitrogen cycle.  

Leaf calcium is very low. This correlates to the winter soil tests which showed a high magnesium base saturation percentage and potential for limited calcium uptake. A calcium foliar program will be of importance to maintain good cell wall strength and give the fruit good storageability. 

Magnesium levels were at the lower end of our target range, which is also surprising considering the high magnesium base saturation levels in the soil. We would expect magnesium levels to drop further as the fruit matures, especially in varieties known to have a higher draw on magnesium, such as Fuji. Foliar sprays are known to be effective in improving magnesium deficiencies throughout the growing season, and applications should be a priority on this block. 

Manganese is also below the optimum; a maintenance foliar program may also be required to support plant health. Iron levels are lowish; however, these are not at a concerning level. The N:K ratio is optimum. This is likely due to the lower leaf nitrogen levels as potassium is comfortably inside the optimum range.  

Case Study 2 

Winter soil test analysis 

Table 6: Winter soil test Case Study Block 2.

The soil CEC is within the optimal range, with a high base saturation of 99.2 per cent. Due to the high base saturation, a little-and-often approach to nutrient application will be required.  

The soil pH is only just below optimum; increasing it to 6.5 would be desirable. Nitrate–nitrogen levels are low. Any nitrogen inputs should be confirmed by history, spring tree vigour status (visual observations) and mid-November leaf tests.  

Soil phosphorus levels are high, particularly in the topsoil. Potassium levels are high but within optimum range in the soil; however, the base saturation is above optimum at 7.0 per cent. Calcium, magnesium and sodium levels are all within optimum range. The soil cation imbalance in this soil could prove challenging for varieties affected by calcium deficiencies. As a result, based on these winter soil tests, a foliar calcium and magnesium program will be required to offset the high potassium base saturation in the soil.  

The soil tests suggest a maintenance program of nitrogen, potassium and magnesium, to replace crop removal, should be applied.   

Mid-November leaf tests 

Table 7: Mid-November leaf test Case Study Block 2.

The mid-November leaf test showed that leaf nitrogen was below optimum. This could be addressed with foliar nitrogen applications; however, to maximise fruit colour, corrective action should be made post-harvest to support return bloom and fruit set.  

Although phosphorus soil levels appear high, the leaf phosphorus levels are low. This result seems strange when looking at Case Study 2’s soil test. As mentioned previously, further testing should be done before making changes to the current nutrient management program. 

Of note, the N:P ratio is high due to the low phosphorus levels in the leaf. Correcting this ratio is likely to have a positive impact on fruit colour. 

Despite good soil calcium levels, plant uptake is limited, with testing showing very low leaf calcium. Getting regular calcium into the plant should be a high priority for the Case Study 2 grower. This deficiency is likely attributed to the high potassium base saturation percentage in the soil (creating a cation imbalance). For this same reason, foliar magnesium applications are recommended, although leaf tests are currently showing optimum levels. Because leaf potassium is within optimum and nitrogen is low, the N:K ratio is balanced.  

Zinc, a trace element, is also at low levels. A foliar application (using a safe form of zinc) is recommended.  

Common trends and outcomes 

Both case studies showed the impact of nutrient relationships and cation balancing between the winter soil test and the mid-November leaf test. The balance of cations in your soils is an important attribute of nutrient management. The antagonistic cation relationship between calcium, magnesium and potassium cations has a huge bearing on the way nutrients are taken up into the plant.

Soil calcium levels in both case studies were within the optimum range; however, both leaf tests showed a calcium deficiency. This can be attributed to the soil’s high magnesium base saturation percentage in Case Study 1 and the high potassium base saturation percentage in Case Study 2.

Both case studies had mid-November leaf tests that showed potassium to be within the optimum range. Both also had low leaf nitrogen levels. Because of this, the N:K ratio was balanced, which should support good fruit colouring at harvest. If any changes were to be made to adjust the nitrogen content of the leaf, the N:K ratio should be watched closely.

The phosphorus levels in both soil tests were well above optimum in both case study blocks.

Both case studies had soil pH levels slightly below optimum. This can influence the levels of plant available nutrients, and adjustments should be made to increase the pH in both blocks.

It is important to note, that both case studies also had high total base saturations, so a little-and-often approach to nutrient application is recommended to make changes to the nutritional composition of the soil and therefore uptake into the plant.

Conclusion 

Successful nutrient management is achieved through understanding your block-specific nutrient requirements (test, test, test), and responding to these results with a drive to optimise crop outcomes (plant health, yield outputs and fruit quality).  

To do this accurately, build a block history, understand your nutrient budget ‘ins and outs’, and test at the different stages of the nutrient cycle to see if any changes should be made to the current nutrient management plan.  

The benefits of understanding your soil, leaf composition and nutrient levels in fruit pre-harvest, allow for a refined and targeted approach to nutrient management.  

 

This article was first published in the Autumn 2024 edition of AFG.

 

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