Nutrient removal in Blackcurrant harvest

A question came up recently about how variable nutrient removal is in blackcurrants, by variety.

Some years back, Geoff Langford and Bronwyn Braithwaite did the tedious research required to parse out how much nutrient is contained in blackcurrant roots, shoots, leaves and fruit.  The work involved growing Ben Ard, Ben Rua and Magnus for three years, digging up whole plants, weighing their components, and then sending them to the lab for whole nutrient analsysis.  The poster that is a testament to their dedication still hangs in our offices.

In a mature block, leaves, roots and shoots remain in the block after pruning and leaf drop, so the only component of the plant where the nutrients are exported (and therefore need replacement from soil reserves and fertilizer) is the fruit.  In a growing block, we might make an estimate of shoot and root growth requirements as the plants are still getting bigger, which could increase the net N uptake by about 15kg/ha/year.

At year 3, yield on Ard was 10.3 tonnes, Rua was at 10.88 tonnes, and Magnus was at 3.6 tonnes.  Based on this data, we can calculate the nutrient content of a tonne of fruit.

Nutrients (kg) contained in 1 tonne of fruit:

A table with the calculated nutrient removal at various yields is provided at the end of this article.

We can look at Ben Ard nutrient removal in a graph:

There are small differences between varieties in the fruit nutrients at a given yield—Ard at 5 tonne yield removes 12kg of N and 16 kg of K, while Rua at the same yield removes 10kgN and 13.5kg K.  However, the main driver of nutrient removal is yield.

Still, even at a high yield, the actual differences in nutrient removal are relatively small.  At 15 tonnes, Ard removes 36 kg of N and Rua removes 29 kg N.  Many Canterbury soils will actually supply this much nitrogen by mineralization over the course of the growing season.

Grain crops are a whole different league to fruit crops, where the nutrient removals are much larger.  A 10 tonne wheat crop removes 230 kgN/ha and 57 kgK/ha, and if the straw is removed (7 tonne/ha), then another 50 kgN and 120 kgK is removed (https://www.yara.co.nz/crop-nutrition/wheat/how-to-increase-wheat-yield/).

Appendix: Table showing nutrient removal in fruit of three varieties at various yield points.

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