Lessons from the past: installment 1 Nitrogen

A Sustainable-Farming-Fund project with the catchy title “Best Practice Protocols to improve profitability and sustainability in blackcurrant production” ran from 2012-2015.  Project looked at various grower practices and how they correlated with yield.

The goal to tease out what production practices make the biggest contribution to high yields.  Research involved plant nutrition, pruning, pest and disease control.

I’ve been re-reading the results and find several angles interesting.  This is the first in a series of articles about those lessons learned.

Nitrogen

Nitrogen is the most important nutrient that could limit blackcurrant growth and yield, but usually it doesn’t.

Plants get N from both fertilizer and N released from soil organic matter (including clover fixation).  On some soils, the N released by mineralization is quite substantial, but it also varies a lot between blocks.  Soil-supplied N over the roughly 30 study sites ranged from <50 kgN/ha/year to 200 kgN/ha/year, with most in the 70-110kg/ha/year range.

For blackcurrants, the relationship between yield and N supply (counting both fert and soil N) is not at all strong.   On most low yielding sites, N deficiency was not the reason for low yield.  Also, sites with surplus N didn’t yield better.  

Even when researchers took both the “surplus N” sites and the low yielding sites out of the data set, there was still only a loose correlation between yield and N supply, with N supply accounting for only 44% of the yield variation.  Clearly, having enough N to support plant growth is important, but is far from the main factor that determines yield.

How much N is enough?

One way to look at this question is to measure how much N is inside a 3 year old Ben Ard bush, yielding 10 tonnes/ha (typical industry spacing of 8333 plants/ha).  The answer is about 60kgN/ha in the plant—roots, shoots and leaves.  Another 24 gets removed in the fruit.  Add that up and it’s about 85 kg/ha of N.

Theoretically, the N in the plant body stays after harvest, and even prunings and fallen leaves return their nutrients to the soil.  Each year the bush needs enough N to grow more shoots and new leaves, as well as fruit. Some of the needed N it takes from reserves within its body and others it gets from the soil.  A bit of math, a realization that our biological systems do leak some N, and extrapolation from the weak N yield curve gives us the basis of the standard recommendation that growing the bush and fruit for a 10 tonne crop requires 95-100 kgN/ha.

Nitrogen timing

In general, it did not matter to yield whether fertilizer N was applied 100% in autumn or 100% in spring, or split evenly between the seasons.  In a perennial crop, significant quantities of N are stored in the plant body, which helps to smooth out supply to plants when the soil supply is less.

Of course, there are times of the season when N supply is critical to setting the plant up for high yields.  If N is limiting in summer, the new shoot growth stops early and there are fewer potential buds that can differentiate into flower buds in autumn.  If N is limiting during flower bud differentiation (late summer), the quality of the flower buds forming is lower (fewer flowers per strig).  “Tired looking” (yellower) leaves post harvest can indicate that the plant would benefit extra post-harvest N.  

After a wet winter, available soil N could have been lost from leaching, and cold spring soils aren’t conducive to mineralization.  In this case, spring N can be beneficial to support spring bush growth and flowering.

There are practical reasons to consider different N timings as well.  Fertilizer spreaders can damage wet spring soils, so for that reason autumn is logistically a better time to apply.  But remember, if fertilizer is applied in late summer, soil moisture must be good enough for the plants to take it up.

The next article will review lessons on soil health.

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