An international berry specialist shares his views on how blueberry growers should approach nitrogen fertilisation.
Agronomist Jorge Duarte is the founder and CEO of Hortitool Consulting, a company specialising in small fruits. Duarte has more than 20 years of experience as a technical adviser to berry producers in Europe, North Africa, the Caucasus, and India. In this interview, he discusses nitrogen fertilisation for blueberries.
Q. What is your opinion on ammonia versus nitrate nitrogen?
There are two schools — the North American and the South American — where we learn to grow blueberries. The North American school mostly grew Northern highbush blueberries, which are calcifuge plants with low nitrogen requirements. So, this school was focused on ammonia [NH4-N] as a nitrogen source.
But when the Chileans started researching blueberries 40 years ago, they showed that blueberries can take up and use nitrates [NO3-N]. And as blueberries have been planted all over the world, we’ve found out that certain genetics allow you to work with more nitrates.
We’ve seen the benefits of having both ammonia and nitrate nitrogen, as proved by recent studies from China. Nitrates are good for restraining growth and promoting elongation, while ammonia is good for increasing leaf area.
In general, a berry crop should receive 90% nitrate and 10% ammonium. In blueberries, this oscillates. The ratio for vegetative growth would usually be 75% ammonium and 25% nitrate, changing to 60% ammonium and 40% nitrate during the generative stage.
When you are in production, nitrogen levels can drop, and you have 50% ammonium and 50% nitrate. But it depends on your objective.
I always look at water quality — it’s crucial. You can’t have 75% ammonium and 25% nitrate if your water quality doesn’t balance the cations and anions. Depending on water quality, more ammonia can help to acidify the water, or, if bicarbonates are low, I would use more nitrates.
Q. Do you think published sufficiency ranges are useful?
Most of the ranges for blueberries, especially Southern highbush, come from American universities. And laboratories sometimes use these instead of making reference ranges based on the source of their samples.
Norms must represent the universe of the varieties and growing systems that a grower has. Otherwise, how accurate can the values be?
For example, some of the limits for aluminium are from universities in the United States. But I see accumulation of higher levels in acidic soils, and there is no damage to the plants.
Boron is another example. We see levels that are two or three times higher than the upper limit, but the plants don’t have tip burns, distorted leaf margins, or leaf drop.
So, you need to create your own reference within your farm based on your varieties and climate. Or, if South Africa has groups that research their own varieties, they can invest in developing references for those varieties.
Q. What technology should growers use to manage fertigation?
If they are growing in substrate, they need a collection point for drainage, so they can track drip and drain daily to adjust their irrigation. There are now sensors that can track this automatically.
Then growers should sample the substrate, water, and the drip and drain every two to four weeks. And they should do leaf analysis monthly or sap analysis every two weeks. This shows them the trends of nutrient flux and the nutrient stocks the plants are building.
With drip and drain, both the plant and the substrate are absorbing nutrients, so we also need leaf and sap analysis. Leaf analysis gives you dry matter, which is historical data, and sap analysis shows you the plant’s demand at the moment the sample was taken.
You must measure the vapour pressure deficit to interpret the other data. It’s a mistake to focus entirely on nutrition without looking at climate stress.
I know all this is expensive and requires more technical people on the grower’s team. But growers must become more technical. Growers who don’t have this technology aren’t going to be high performers because they don’t have data to make decisions.
Q. Which common mistakes do growers make when applying nitrogen?
In my experience, common mistakes are starting with really high total nitrogen and ignoring the balance between cations and anions.
It’s chemically impossible to have 75% ammonium and 25% nitrate unless you balance the ammonium with anions. So, start with 60% nitrate and 40% ammonia, and then you can always reduce the total nitrogen throughout your cycle.
If you are a grower who produces for a local market, you can play with nitrogen levels because you’re focused on yields. In the case of South Africa, you must manage your nitrogen carefully because you need firmer fruit and better shelf-life.
Lower nitrogen levels also reduce disease and pest problems because sucking insects are attracted to turgid, luxurious tissues. It’s been observed that dropping the total nitrogen lowers the impact of aphids and thrips.
Q. What other advice do you have for South African growers?
Most growers want a recipe. But plant nutrition is not just about numbers. It’s a dynamic between what we give and what the plants take up. We need to measure the plants’ demand.
Plants want stability. We need to give them stability by managing irrigation and nutrition. Irrigation is at the heart of a farm. Reducing the investment in the irrigation system to save capex is not acceptable if growers want high performance.
Growers must also understand that they’re working with living organisms. So, they need to be flexible and know the limitations of their system. If they take a fixed approach, they will sometimes have results that don’t match their economic goals.
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Image of Jorge Duarte courtesy of Hortitool Consulting.
For more about Hortitool, visit their website.
