Burning greatly alters the nutrient cycles. Fallowing field and allowing bush to grow is one way of accumulating rapidly cycling nutrients, particularly nitrogen which is often a limiting agricultural nutrient. The burning of the bush releases a flush of nitrogen to the soils that allows a short term increase to productivity. However high temperature burning can volatilize some nutrients and allow those nutrients to leave the system in a way they may not have otherwise. So burning could increase the rate of nutrient loss in some systems. It would depend on the type of burning, and the supply of nutrients in that system.

Burning also adds a lot of black carbon to the soils, which is typically a good thing but also may depend on the soil. Burning can physically alter some types of soil, either through the heat or through the drying process that accompanies burning. Some mineral constituents, for instance allophane in young volcanic soils, can be irreversible broken down by excessive drying. Other soils would react differently.

Then there will also certainly be an effect on the microbial, fungal, etc. communities of the soil. This again will depend on what the community is to begin with. Iʻm not an expert in this arena, but I think most bacteria would be okay, but I would think a lot of fungi might not survive the burning in the topsoil anyway. These any other soil biota have a major effect on the soils, especially regarding nutrient retention and cycling, and although they do seem to recolonize quickly, the composition and relative abundance of the soil biota will almost certainly change with continued burning over time, favoring some species over others.

I agree with Noa, burning has the ability to alter soil nutrient concentrations and forms. Since combustion is an oxidizing process, organic nutrients are converted or removed. This land management practice could also lead to higher rates of soil subsidence. However this all depends on the type of soil. The degree of these influences are dependent upon the intensity of the fire. Quick fires would result in lower temperatures, reduced nutrient conversion, reduced volatilizing nutrients, etc. Long hot fires would result in significant changes.

Thre is a huge literataure on the effects of 'slash and burn' and increasingly on 'slash and char' on soils. I suggest that Nye and Greenalnd 1960 'The soil under shifting cultivation' is a good start point. Much of the litrataure focuses on nutrient effcts, but the burn also affects soil phyical properties and biota.

In Mexico this practice is part of the tropical agroecosystem called "roza-tumba-quema" and this practice has been practiced by centuries (until Mayas). This practice has produced some plant adaptations. But the fertility of the soil is low and people sow just in some tiny spaces, and weeds came out so quickly that campesinos (farmers) has to move to an other space of natural vegetation (tropical forest), starting the process of cutting of and burning. However due to the high density of the population in such regions, the period of rest of the spaces in the tropical forest (called barbecho) has been reduced from 20 to 30 years (30 years ago) to an average 5 or 7 years (today), provoking that the natural fertility of the soil is not recuperated at all, and therefore campesinos has low yield.

I'd be more concerned about the alteration of biological soil properties. Wouldn't it be better to cut the bushes and use the material as green manure instead of burning?

Janet - I agree that the alteration of the biological properties is likely one of the biggest impacts. But whether that is a good or bad thing depends on both the soils and the soil biota. Many indigenous cultures used fire as a way to "purify" the soils, and erase the buildup of pathogens and other plant pests. Organic farmers still do this today, and it is particularly effective against nematodes. Of course as I mentioned in my first post there are a lot of beneficial things you will be killing off too, and what survives and what doesnʻt, and what is around to recolonize afterwards will all affect the future of the soil and crops but not in a way that is easy to predict at all.

As far as using the green manure that is also a situational dependent assessment. If for instance the plant matter is very high in carbon it can actually reduce the availability of soil nutrients as bacteria uptake nutrients to digest the available carbon. In other cases that available carbon can actually foster biological nitrogen fixation from the atmosphere by N-fixing bacteria. Green manure can prevent or enhance soil moisture depending on how it is applied and managed. When you look at traditional mulching practices they are extremely specific, and managed very intensely to produce the desired outcomes. The organic notion today, that simply applying compost is always a good thing, is not necessarily true.

Isn't the main intention of slash and burn to get very fast area for farming and easily available nutrients in the ash to boost crop growth? This method of agriculture is a thing of the past and should be abandoned in favor of sustainable agroecological farming methods for example agroforestry with includes the planting of Gliricidia and other legume tree species using their nitrogen rich leaves as green manure. Some more interesting examples of agroecology are shown in the movie "crops of the future" by Marie-Monique Robin, the French journalist who also produced the very interesting documentaries about Monsanto (The world accorsing to Monsanto) and pesticides (Our daily poison).

Did you read my posts at all?? I coming at this from a soil science perspective, and like I said these things are all situationally dependent. Burning can be managed very well, as can green manure. Isnʻt biochar all the range in agroecology, or permaculture, or biodynamics, or whatever you want to call the latest organic farming fad? That is just a form of burning. You CANNOT generalize that green manure is good and burning is bad. You cannot even generalize that nitrogen is good or bad. It all depends on the situation...the management of the technique and the soil that it is being practices on. Iʻm very aware of agroecological methods, and Iʻm equally wary of them because of all the greenwashing that comes along with it. Iʻm not saying there are not great methods out there, Iʻm only saying that, like most everything that has anything to do with ecology, it will be situationally dependent.

Hoi, calm down! Sure I read your post and I did not generalize green manure as the all problem solving final solution. It is just one of many possible ways of sustainable farming while burning is not sustainable in the long run. Of course it all depends on the local conditions. Burning in areas with a vegetation depending on fire for reproduction is not bad. It occurs also naturally and can benefit biodiversity. However, slash and burn is still often applied in areas with naturally unsuited soils, where it works well for a few years but then results in depleted and impoverished soils that have to be given up. In the long run land demand for fresh, "virgin" soils is large or even increasing with this way of farming. Therefore, this method should be replaced with sustainable farming methods.

In many cases I agree with you, but burning is not necessarily unsustainable, and shouldnʻt necessarily be replaced, and in well managed cases can be more sustainable than manuring methods. So I have to disagree when you generalize again that burning should necessarily be obsoleted and replaced with other methods.

Noa and Janet you guys have spoken excellently well assessing the question from biota and soil science perspective. i feel alternating the various methods could be beneficial (slash and burn, green manuring and other land management practice) to the soil and appropriate soil testing should be carried out before anyone of this procedure are carried out

Bush burning  distorts the ecological balance. The ensuing temperature differential also  destroys vital fauna and microbial activities that are necessary for sustained fertility and aeration. Improper burning may release insidious dioxins to the environment. Whereas,  the globe is concerned about the consequences of climate change, bush burning destroys sequestered carbon and releases carbon dioxide (a greenhouse gas) to the atmosphere. This compounds the effects. The ash produced from burning alters the pH & electric conductivity of soils. This in turn determines the mobility and availability of elements (in this case, nutrients) for plant

Bush burning  distorts the ecological balance. The ensuing temperature differential also  destroys vital fauna and microbial activities that are necessary for sustained fertility and aeration. Improper burning may release insidious dioxins to the environment. Whereas,  the globe is concerned about the consequences of climate change, bush burning destroys sequestered carbon and releases carbon dioxide (a greenhouse gas) to the atmosphere. This compounds the effects. The ash produced from burning alters the pH & electric conductivity of soils. This in turn determines the mobility and availability of elements (in this case, nutrients) for plant

Bush burning  distorts the ecological balance. The ensuing temperature differential also  destroys vital fauna and microbial activities that are necessary for sustained fertility and aeration. Improper burning may release insidious dioxins to the environment. Whereas,  the globe is concerned about the consequences of climate change, bush burning destroys sequestered carbon and releases carbon dioxide (a greenhouse gas) to the atmosphere. This compounds the effects. The ash produced from burning alters the pH & electric conductivity of soils. This in turn determines the mobility and availability of elements (in this case, nutrients) for plant

Bush burning  distorts the ecological balance. The ensuing temperature differential also  destroys vital fauna and microbial activities that are necessary for sustained fertility and aeration. Improper burning may release insidious dioxins to the environment. Whereas,  the globe is concerned about the consequences of climate change, bush burning destroys sequestered carbon and releases carbon dioxide (a greenhouse gas) to the atmosphere. This compounds the effects. The ash produced from burning alters the pH & electric conductivity of soils. This in turn determines the mobility and availability of elements (in this case, nutrients) for plant

Bush burning  distorts the ecological balance. The ensuing temperature differential also  destroys vital fauna and microbial activities that are necessary for sustained fertility and aeration. Improper burning may release insidious dioxins to the environment. Whereas,  the globe is concerned about the consequences of climate change, bush burning destroys sequestered carbon and releases carbon dioxide (a greenhouse gas) to the atmosphere. This compounds the effects. The ash produced from burning alters the pH & electric conductivity of soils. This in turn determines the mobility and availability of elements (in this case, nutrients) for plant