Key takeaways
- Blueberries can take up NO3-N.
- Provided pH is acceptable, blueberries perform well over a range of NH4-N to NO3-N ratios.
- NH4-N acidifies the root zone and NO3-N alkalinises it, affecting root health and nutrient availability.
- When formulating a fertigation strategy, consider the ratio of NH4-N to NO3-N and root-zone pH, in addition to nitrogen application rates.
Is it true that ammonium nitrogen is superior to nitrate nitrogen for fertilising Southern highbush blueberries?
Blueberry literature commonly states that blueberries prefer ammonium nitrogen (NH4-N) over nitrate nitrogen (NO3-N). Claims include that the plants don’t absorb NO3-N and that NH4-N produces better vegetative growth and yields.
However, technical advisers report good results across wide-ranging ratios of NH4-N to NO3-N, so there seems to be a disconnect between experiments and reality. Or is there?
Nitrates do not harm growth
A 1995 paper by Donald Merhaut and co-author Rebecca Darnell is frequently cited as evidence of preferential NH4-N uptake by Southern highbush blueberries.
In their study, the plants were fertilised with NH4-NO3 for 25 weeks before receiving a once-off drench of either (NH4)2SO4 or NaNO3. Nitrogen uptake was measured at intervals for 48 hours after the drench, when the experiment terminated.
While it is true that the blueberries took up about twice as much NH4-N as NO3-N and translocated about four times as much NH4-N as NO3-N to their shoots, the results cannot be interpreted as saying anything about the effect of different nitrogen forms on plant performance.
As part of his doctoral research, Merhaut compared fertilisation of Southern highbush blueberries with NH4-N and NO3-N, finding that plants fertilised with NO3-N had significantly greater total stem length, and leaf, stem, root, and total plant dry weight than plants fertilised with NH4-N.
On the other hand, leaf and stem nitrogen concentrations were significantly lower in the NO3-N-fertilised plants compared with the NH4-N-fertilised plants. Therefore, greater nitrogen uptake and accumulation did not translate into more growth.
In his dissertation, Merhault comments that fertilising with either NH4-N or NO3-N does not affect carbon assimilation or plant growth. But there is an important caveat.
Nitrogen form and pH
Whereas most people agree that blueberries need acidic conditions (pH below 5.5), the reasons for the plants’ predisposition aren’t clear. One view is that high pH reduces iron availability and uptake. Researchers have also shown that high pH impacts manganese uptake.
Although pH does affect nutrient availability, it isn’t the only reason blueberries struggle at higher pH levels. An Israeli study, published by Guy Tamir and colleagues, examined the roots of Southern highbush blueberries grown at pH 4.5 or 7.5 and with different calcium levels for 12 weeks.
As expected, the researchers report reduced iron and manganese uptake and reduced shoot levels of iron, manganese, and phosphate at pH 7.5, independent of calcium levels. But they also observed root damage within three days at pH 7.5, which was exacerbated by 100 mg per litre of calcium over the 12-week study.
The plants grown at pH 4.5 did not suffer root damage. They had more than twice the chlorophyll levels and accumulated nearly 15 times as much biomass as the plants grown at pH 7.5.
In other work, Tamir and colleagues demonstrated that Southern highbush blueberry plants fertilised with 100% NH4-N performed the same as plants fertilised with 25% NH4-N:75% NO3-N plus sulphuric acid. The medium pH (5.2 and 5.5) was not significantly different between these two treatments, but was significantly higher (6.3) for the treatment with 25% NH4-N:75% NO3-N and no sulphuric acid.
The researchers concluded that the main benefit of NH4-N over NO3-N is that the former acidifies the growth medium. It follows that the ratio of NH4-N to NO3-N can be manipulated to help manage pH across systems with varying water quality.
Nitrates and potassium?
Blueberry growers in Peru are including nitrates in their fertigation programmes not for the nitrogen but because they believe it promotes the absorption of cations.
Marco Valdivia-Huaman and colleagues compared fertigation with a mix of NH4-N and NO3-N to fertigation with 100% NH4-N in Southern highbush blueberries in Peru. The plants receiving nitrates started on a ratio of 70% NH4-N:30% NO3-N in March, transitioned to 30% NH4-N:70% NO3-N in May, and to 50% NH4-N:50% NO3-N at the end of July.
The researchers found no significant difference in shoot length or berry numbers between 100% NH4-N and the mix of NH4-N and NO3-N. But the mix advanced ripening by several weeks, increased berry mass and diameter, and boosted yields by 22%.
Inclusion of nitrates was also associated with higher leaf magnesium levels. Unfortunately, the one thing the nitrates didn’t do was increase potassium levels, at least not in leaves, which were the only plant organs analysed for potassium.
However, a study by Tamir and colleagues did observe significantly lower leaf potassium levels in rabbiteye blueberries fertigated with 100% NH4-N than in plants fertigated with 33% NH4-N. The authors concluded that reduced potassium wasn’t limiting growth. Fruit were not assessed in these trials, which focused on pH, root development, and biomass.
More research is needed on the relationship between nitrogen form, potassium uptake, yields, and fruit quality. One question is whether leaf potassium levels accurately reflect the availability of potassium for fruit development.
Nonetheless, the results of research on nitrogen forms seem clear: optimal blueberry fertigation incorporates both NH4-N and NO3-N. By manipulating the ratio of these forms, growers have an additional tool to manage conditions in the root zone.
References and resources
Merhaut DJ and Darnell RL. 1995. Ammonium and nitrate accumulation in containerized Southern highbush blueberry plants. HortScience 30(7) pp1378–1381.
Merhaut DJ and Darnell RL. 1996. Vegetative growth and nitrogen|carbon partitioning in blueberry as influenced by nitrogen fertilization. Journal of the American Society for Horticultural Science 121(5) pp875–879.
Tamir G, Afik G, Zilkah S, Dai N and Bar-Tal A. 2021. The use of increasing proportions of N-NH4+ among the total applied inorganic N to improve acidification and the nutritional status and performance of blueberry plants in soilless culture. Scientia Horticulturae 276 p109754. Paywall.
Tamir G, Zilkah S, Dai N, Shawahna R, Cohen S and Bar-Tal A. 2021. Combined effects of CaCO3 and the proportion of N-NH4+ among the total applied inorganic N on the growth and mineral uptake of rabbiteye blueberry. Journal of Soil Science and Plant Nutrition 21(1) pp35. Paywall.
Tamir G, Zeng Q, Eli D, Zilkah S, Bar-Tal A and Dai N. 2023. Combined effects of alkaline pH and high Ca concentration on root morphology, cell-wall polysaccharide concentrations and blueberry plant performance. Frontiers in Agronomy 5 p1121448.
Valdivia-Huaman MA, Leon-Chang DP and Bryla DR. 2024. Applying nitrate in addition to ammonium nitrogen fertilizer increases flowering and fruit production in ‘Ventura’ Southern highbush blueberry. In XIII International Vaccinium Symposium 1440 pp483–488. Paywall.
Acknowledgements
Thanks to the following persons for sharing their expertise:
Craig Fulton. International Technical Manager at Mountain Blue.
Dr Eugenie-Lien Louw. General Manager at Firstfruits and Technical Manager at Wasi Berries.
