Potatoes in Canada

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Adding biochar to soil can reduce potato yields

Studies have shown adding biochar to soil can improve soil fertility, increase nutrient utilization in plants, improve soil water-holding capacity, increase crop yield and reduce emission of greenhouse gases.

However, if you are a potato farmer, your joy may be short-lived. According to research from Aarhus University in Denmark, biochar and potatoes do not go together very well, especially if you’re aiming to save water.

Caixia Liu, a PhD student in the university’s department of agroecology, investigated the effects of adding biochar produced from wood on potato growth, yield, nutrient uptake and water utilization when three other factors were also taken into consideration: irrigation methods; phosphorous fertilization; and inoculation with a certain class of beneficial fungi. Liu’s aim was to investigate the interactions between biochar and the fungi on the growth of potatoes.

Biochar and AM-fungi do not go well together
Potatoes are rather sensitive to drought and phosphorous deficiency because of their relatively small root system. It would be easy to merely irrigate and fertilize in plentiful amounts, but since both phosphorous and water are limited resources, it is important to use them optimally.

Earlier studies at Aarhus University have shown water can be saved by irrigating alternately on each side of the potato ridge and letting the other side remain dry – the so-called alternating partial root zone drying irrigation method.

It is also possible to save phosphorous. In the course of her PhD studies, Liu found inoculation of potatoes with the beneficial arbuscular mycorrhizal fungi (AM fungi) can improve potatoes' utilization of phosphorous, make utilization of water more efficient, and increase potato yield in crops that are stressed due to drought or phosphorous deficiency.

The question was what would happen when biochar and inoculation with AM fungi are combined. Would there be a double win? The answer was no.

Liu conducted a series of studies with various combinations of irrigation (either full irrigation or alternating partial root zone dehydration), phosphorous fertilization (none or 0.11 milligrams of phosphorus per gram of soil, or mg P/g soil), inoculation with AM fungi (inoculation or no inoculation) and addition of biochar (addition or no addition).

Biochar inhibited the growth of potatoes
If the crop is irrigated fully, the soil is given no phosphorous at all, and the potatoes are not inoculated with AM fungi, then addition of biochar can increase potato yield. This was the only case in which this was true; in all other instances, addition of biochar to the soil had the opposite effect.

The negative effect on potato growth was especially pronounced when phosphorous was added, alternating partial root zone drying irrigation was used, and the potatoes were inoculated with AM fungi. Addition of biochar inhibited the growth and vigour of young potato plants. Some of the young potato plants even died.

“I would recommend that the farmer refrains from adding biochar produced on the basis of wood to an AM system, where the soil has been fertilized with phosphorous or if the soil is prone to drought. Biochar remains in the soil for a long time so there is no going back,” Liu said, in a press release.

October 5, 2016
By Aarhus University