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comparing "organic" to "conventional"

Posted by rweil on 21 Feb 2013 at 04:06 GMT

This paper presents interesting results that are in line with other reports in the literature on nutritional quality factors. As a soil scientist and ecologist I wonder about the possible soil-related mechanisms for these results.

This study is essentially an unreplicated comparison between two nearby farms...not between "organic farms' in general and "conventional farms" in general. Therefore it is important to understand the differences and commonalities of the two farms and their soils. Unfortunately only a single soil analysis is given and no information is supplied about how this soil was sampled. Therefore, I wonder if the soil analyzed was from one of the farms or the other – or if it was a mixture meant to represent an average for the two farms.
The content of soil organic matter in particular caught my eye as it is extremely low such as might occur in exposed subsoil on a badly eroded site. Even a poorly managed Latosol would have perhaps 10 times higher OM content than the 2.6 g/kg reported. If the reported value is correct, the serious stresses would be occurring on the “conventional” farm. We are told that the fruits are larger and have a higher water content – but are they also more numerous (i.e. is the yield higher on a fresh or dry weight basis)?

Since at least one of the farms applied 10 t/ha manure (nutrient content unspecified) plus an unspecified amount of plant organic material annually, the soil organic matter level (and accompanying CEC and nutrient reserves, as well as microbial biomass) can be expected to be much higher than in the soil of the other farm. The manure and bagasse amendments can be expected to add much more potassium than the meager 80 kg/ha K2O added on the “conventional” farm. Potassium is known to promote the production of sugars and anti-oxidants in plants. The same could be said for copper in the Bordeaux mixture… increased copper supply promote antioxidant synthesis. Specifically copper promotes production of (copper-bound) superoxide dismutase. Tomato production with only 80 kg/ha K2O supplied can be expected be K limited. The soil analysis (0.2 cmolc as K/dm) does not seems to reflect a history of producing tomato (or sugar cane) with 80 kg/ha K2O annual application. One has to wonder also about the extremely high 400 kg/ha annual P2O5 application resulting in only a medium level of P.
A final note on soil fertility aspects: the application of 10 mt/ha of manure plus a “legume cocktail” green manure crop could be expected to deliver at least as much nitrogen tot eh crop as the rather modest application of 100 kg /ha of N as soluble fertilizer, so the equal amounts of chlorophyll in the leaves is not surprising.

I encourage future research to pay closer attention to the soils involved and examine some of the specific soil differences mentioned instead of attempting to be over-broad in characterizing the general management system as meeting “organic” requirements or not.

No competing interests declared.

RE: comparing "organic" to "conventional"

aoverbay replied to rweil on 24 Feb 2013 at 13:39 GMT

Very insightful. I found the most concerning aspect of this research was the media's drive to generalize results from a study that has not merit at least in that aspect. Soil amendments are very important. The comment I reply to is well written and spot on.

No competing interests declared.