Buckwheat and Phosphorus

I once grew buckwheat in town between our sidewalk and the street. It got a lot of looks from passersby and one lady even knocked on my door to ask about it. I planted it with the naïve hope that it would help control the established field bindweed (this was back when I was an engineer, not an agronomist). It didn’t, but it is a useful crop. Buckwheat provides flowers and nectar for insects, competes with (most) weeds, and grows a quick soil cover. It’s also promoted as a good crop for making soil phosphorus available. I’ve wondered about that last benefit. Is there credible evidence for this?

Rows of buckwheat
Buckwheat used to protect a grass seed crop in the Columbia Basin of Washington State, but does buckwheat supply phosphorus to the grass crop or to the following crop? Photo by Andrew McGuire

I’m not the only one who has wondered about this: “Reports supporting folklore beliefs that buckwheat can significantly contribute solubilized phosphorus (P) from sparingly soluble soil P to subsequent crops remain anecdotal” (Teboh and Franzen 2011). This brings up a series of questions. Does buckwheat extract P from the soil better than other crops? What P is being made more available, and how much more available is it? Can it do this in unfertilized low-P soils? Does this ability benefit the following crop or intercropped species? There’s not as much research as I thought, but let’s take a look.

Buckwheat does not do well in low-P soils

Research has found that in low P soils (low soil-P test results) buckwheat does not do any better than other crops. Schelfhout et al. (2018) found no difference between buckwheat and four other crops in P removal from low-P soils but found that buckwheat removed more P than three other crops in soils with moderate or high P levels. Maximum P removal by buckwheat was in high-P soils. (See also Schiemenz and Eichler-Löbermann, 2010) It appears that buckwheat can mobilize P in soils with high total P, but not in soils with low available P (Hallama et al., 2019).

Teboh and Franzen (2011) found that buckwheat took P from a less available pool and moved it to a more available pool. And it did this much better than wheat. While this may not seem useful, remember that up to 80% of the total P in a high-P soil may still be unavailable to plants. Moving more P to a more available pool could supply more P to the following crop than having it all remain in the more unavailable pool. While possibly beneficial, this is not exactly making P available from the soil mining of minerals.

Numbered vials holding pureed buckwheat
Juiced biomass from several cover crops. #4 is from buckwheat and stands out visually. I am not sure why. Photo by Andrew McGuire

Buckwheat effect on yield of following crop

The benefit to following crops would come when the P in the buckwheat biomass is released to the soil after decomposition. An increase in soil test P after buckwheat is sometimes seen (Boglaienko et al., 2014) but not always (Teboh and Franzen, 2011; Possinger et al., 2013; Hallama et al., 2019). In low-P soils, the timing and rate of decomposition of the buckwheat biomass probably plays a part in how the following crop responds (Hallama et al., 2019). In no-till systems, the lack of mixing of buckwheat residues in the soil would limit the P-related benefits to following crops, at least in the short-term.

Whatever its effect on soil test P levels, research has not observed many benefits to following crops. A meta-analysis of 25 published studies found that crop yields did not benefit from being grown after buckwheat (Hallama et al., 2019). The authors note, “Whether or not buckwheat’s potential could be improved with other main crops, in a mixture with other cover crop species, or if its beneficial effects were limited by the low biomass observed in the studies included in the meta-analysis, warrants further investigation” There were no differences in the results from organic vs. conventional management.

Buckwheat with P is not like legumes with N

The quote above mentions buckwheat in mixtures. While the mixture of legumes with non-legumes in low nitrogen soils has long been known to provide benefits to the non-legume, mainly nitrogen related, researchers have not observed the same with buckwheat and P. Lopes et al. (2021) did not observe any transfer of P from buckwheat to other plants in a crop mixture. Regarding a role of mycorrhizal fungi, Hallama et al. (2019) note “There is no conclusive evidence that AMF-colonized plants are able to take P from soil sources that cannot be accessed by the roots themselves; rather, they increase the soil volume from which the same P pools can be acquired.” Again, the P does not become available to other plants until it is released after decomposition.

Buckwheat is a P hog

One possible reason buckwheat has been associated with phosphorus is that it is a luxury consumer of P (Schelfhout et al., 2018), a P bioaccumulator. Luxury consumption of a nutrient by a plant is the nutrient uptake that does not result in additional biomass production. This might make buckwheat a better P scavenger in soils with moderate to high P levels than other crops. Also, contrary to “low nutrient needs” claims for many cover crop species, and applicable to many of them, buckwheat responds to both N and P fertilization (Inamullah et al., 2012)

Wilting buckwheat plants
Buckwheat wilts in afternoon heat, even in wet soils, but it recovers overnight. Photo by Andrew McGuire

Buckwheat does well with rock phosphate

Graph of P content sampled from monoculture and mixed culture plots
The P content of shoots and roots of buckwheat and maize plants grown with three P sources: no added P (NaP), Phosphate Rock (PR), and Monocalcium Phosphate (McP). From Lopes et al. (2021), licensed under a Creative Common attribution-type BY in Scientia Agricola.

Organic growers have reported most observations of buckwheat’s P-supplying abilities (Tebow and Franzen, 2011). Given buckwheat’s poor performance in low P soils, the best explanation for these observations is that buckwheat is utilizing P from the rock phosphate that organic farmers use in place of P fertilizers. This is what Lopes et al. (2021) found in low-P soils amended with rock phosphate: greater growth from buckwheat than other crops. This use of buckwheat with rock phosphate is probably its best use in terms of making P more available to following crops.

So, while cover crops in general can keep more P available to following crops (Soltangheisi et al., 2020), there is little evidence for buckwheat being better at this than other species. This does not mean that buckwheat doesn’t provide benefits. A good stand of buckwheat can suppress weeds, feed bees and other insects, and provide a quick soil cover to control erosion. Given that controlling erosion is often the most important issue in soil P management, buckwheat’s quick cover is its most reliable P-related benefit.