Perfecting your orchard’s nutrient balanceNews
By Dean Rainham
Good orchard nutrition for intensively managed apple and pear orchards includes understanding the interaction between nutrients to ensure those applied are most effectively used for fruit production.
Most growers know the importance of applying the right amounts of nutrients to ensure the tree is not lacking any of the essential macro or trace elements. However, some deficiencies are not caused by a shortage of the element, but an imbalance with other nutrients. As we strive for higher yields, these nutrient imbalances tend to show up more.
The importance of obtaining the optimum ratio of nutrients in the soil and plant becomes vital to sustaining high marketable yields. This article will introduce some principles to illustrate the importance nutrient interactions play in achieving the optimum nutritional program for intensive apple and pear production.
Nutrient sufficiency vs nutrient balance
Understanding the relationship between nutrient sufficiency and balance is critical to developing the optimum nutritional program for a high producing pome fruit orchard. Sufficiency is about addressing individual nutrient status, balance is understanding nutrient interactions.
Step one to a balanced nutritional program is to ensure your tree has sufficient amounts of nutrients to complete their vital growth functions. The higher the yields, the more nutrients they will require. Liebig’s ‘Law of Minimum’ states that crop yield/growth is limited by the nutrient or vital plant growth factor that is most limiting, even if all other vital factors/nutrients are adequate.
Identifying those nutrient(s) that are most limiting and then applying sufficient amounts to correct this is fundamental to achieving high yields. This is where soil tests and leaf tests are invaluable.
Ensuring the nutrient concentration is in the ‘adequate’ zone maximises yield potential. Leaf testing is a useful tool to especially pick up nutrients in the marginal zone. Also known as ‘hidden hunger’, this is the pre-clinical deficiency stage where yield is being compromised due to a lack of a nutrient but you cannot yet see the symptoms in the leaf. Potassium is a good example of this.
To sustain a high yielding orchard it’s not enough to just look at nutrient sufficiency. Growers must also apply nutrients in the right balance and this requires an understanding of nutrient interactions.
Mulder’s chart of nutrient interactions illustrates how different nutrients interact with each other. Nutrients can have a stimulation effect where there is an increase in the need for a nutrient by the plant because of the increase in the level of another nutrient. Nutrients can also have an antagonistic effect whereby there’s a decrease in the availability of a nutrient caused by the interaction of another nutrient.
The potassium, magnesium and calcium interaction
These three cations (positively charged ions) are critical in pome fruit production. They compete with each other on the soil exchange sites and also when in soil solution for plant uptake. Too much of one nutrient will suppress the uptake of the others. A balanced supply of these three nutrients is therefore vital. Table 1 illustrates the effect of varying concentrations of potassium on calcium and magnesium uptake by the plant. As the potassium status in the soil increases the uptake of calcium and magnesium decreases.
Once you have achieved sufficient levels of nutrients in the soil you now need to ensure they are in the right ratios to achieve a balanced uptake by the plant. A soil test is an excellent guide to determine balance.
Starting with calcium, this nutrient needs to dominate the soil exchange sites with at least 65 per cent of the sites being occupied. It’s important that sufficient calcium is released into soil solution and available for plant uptake especially during the critical cell division stage, which is six weeks after fruit set. Calcium is required in the fruit for cell wall integrity; to prevent pit and lenticel breakdown; and to grow good, firm fruit.
Next target the potassium/magnesium ratio in the soil which needs to be close to 1:2. If the ratio is low, more potassium will need to be added, whereas if the ratio is high, additional magnesium will be required. Build your fertiliser program accordingly to ensure a good balance of supply based around these optimum ratios.
Potassium is needed in the largest quantities and is needed for fruit size and colour. It is also important in the water relations in the plant and for maintaining the balance of salts. Deficiency symptoms include marginal necrosis and cupping of the leaf. Potassium is very mobile in the plant so deficiency symptoms will be first seen in the older leaves. Beware that yield and quality can be affected by potassium deficiency long before leaf symptoms are seen. So it’s important to monitor this nutrient in the leaf early and take a proactive management approach.
The nitrogen and potassium partnership
Nitrogen is essential for protein and chlorophyll synthesis, however too much nitrogen promotes excessive vigour, delayed maturity and can lead to soft, poorly coloured fruit with storage issues. It is well documented in many crops, including apples, about the positive interaction between nitrogen and potassium.
Potassium ions act as a carrier for nitrate from root to leaf, where proteins are synthesised and photosynthates are metabolised. So with adequate potassium input balancing out the nitrogen input, nitrogen is much more efficiently metabolised to useful components such as amino acids and proteins and so yield potential is optimised and quality maximised. Potassium could be viewed somewhat as an antidote to excessive nitrogen levels.
Leaf nutrient ratios
Leaf testing is an extremely good way to monitor the health of your trees and pick up nutritional imbalances before they affect productivity. Early season leaf testing will allow changes to be made to the nutritional program so productivity can be optimised in the current year. Macro nutrient ratios, especially the nitrogen/potassium and potassium/magnesium balance (Table 2) should be measured early in the season (Nov/Dec) so any adjustments to the nutritional program can still be made and be effective for the current season’s crop.
Achieving nutritional balance in the plant will optimise tree health and maximise yield, fruit colour and size as well as lessen post-harvest storage issues. Trace elements are essential nutrients but only needed in very small quantities. Leaf testing will quickly pick up any trace element imbalance and in most cases can be solved by a single foliar fertiliser application.
OrchardNet® – Nutrition Centre
The real benefit of monitoring the nutritional status of your orchard is to follow the trends over time and then relate this to what fertiliser was actually applied and ultimately how it affected productivity and profitability. Wouldn’t it be useful to have all this information in one place and be able to run nutritional reports?
OrchardNet has a new feature called the Nutrition Centre. This is where you can enter and store all your soil test results, leaf test results, fruitlet test results, fruit test results and nutrient inputs for each block. You can save multiple years’ results and then run reports which show nutrient trends, nutrient inputs over time (ground, fertigation and foliar applications) and where your results fit in relation to the optimum range.
Because it’s all recorded in OrchardNet where you have also recorded all your production and financial information against the same block, it now turns into a very powerful tool to help determine those blocks that are highly productive and profitable and relate this to what role nutrition played in achieving this.
Please have a go for yourself by logging on to the demonstration site:
Nutrition plays a critical role in orchard productivity. Understanding nutrient sufficiency in combination with nutrient balance is crucial to deliver the optimum nutritional program in your orchard.
Using diagnostic tools, such as soil and leaf testing and then correctly interpreting the results is fundamental in delivering a balanced program that can sustain high yields of quality fruit. Soil nutrition is a dynamic environment so monitoring and analysing trends over time allows you to more accurately assess the nutritional status of your orchard and the efficacy of your nutrient programs so more informed refinements can be made.
About the author
Dean Rainham, Horticultural Consultant, AgFirst: [email protected].