Re: Avery/Benbrook Debate and the Tropics


I was hoping someone would ask these questions, because they are very
pertinent to the discussion.  I hope you don't mind that I post my answer
with your questions on SANET.

>I read with great interest in what you wrote about developing nations. 
>I have some questions that I hope are not too stupid. I am trying to 
>learn more about the soil and soils quality in different regions. If 
>these farmers are planting in savannah, isn't the soil quality fairly 
>high in fertility? Why, instead of using chemical fertilizers, can't some 
>inexpensive soil testing be done, and more natural methods of retaining 
>soil quality, such as the use of compost and mulches be used? If they are 
>clearing 10-20 ha, are they raising any livestock on this land. What is 
>being doine with all the animal manure, plus all the cleared *debris* 
>(for lack of a better word)? There is some pretty marginal land in the US 
>that is managed for gardens year after rear, by the above methods. 
>My lack of knowledge may be very evident with these questions. I feel 
>that the way livestock is being raised here leaves a lot to be desired, 
>and hate to see our less than sustainable methods being exported 
>exclusively to third world nations. If you have any books or other 
>references that you would recommend I read, please tell me. I guess I 
>mainly don't understand why these people would be having to abandon their 
>land after three years from lack of fertility, when there is knowledge 
>that I would think would prevent that.
>Thank you for your time and expertise.
>Ann Wells, DVM 

The Savannas of West and Central Africa are largely characterized by soils
with low cation exchange capacity, due to coarse texture in many places and
a clay fraction dominated by kaolinite, which is considered a low activitity
clay.  There are exceptions, of course, such as the more fertile loess soils
of Northern Nigeria.  That means that the soil is able to hold a limited
amount of nutrients relative to more fertile soils.  Hence organic matter
plays a very crucial role in plant nutrition since it also is a store of
nutrients and in low CEC soils, comprises a large percentage of exchange
capacity.  During the cropping phase, tillage results in a rapid breakdown
of organic matter through microbial action.  This benefits the first crops,
which are able to utilize the nutrients released from this decomposition.
But with successive seasons the organic matter and thus nutrients in the
soil decline and crop productivity declines as well.  Once the land is
abandoned to natural fallow, the organic matter is restored with the natural

Your question about compost and mulches is very pertinent, because they can
be used to maintain productivity.  The problem is where to obtain sufficient
quantity of organic matter to accomplish this on a field scale.
Undoubtedly, better use could be made of the organic resources they have,
but for most crop farmers, this would not serve for much beyond the kitchen
gardens.  You have to understand that in much of sub-Saharan Africa, you do
not have mixed livestock/crop farming, like we know it in Europe and North
America.  Traditionally, livestock and crop farming tend to be separated
along ethnic lines, with tribes such as the Fulani and Massai living as
herdmen concentrating on cattle rearing, while other groups being sedentary
crop farmers.  Although Africa is changing, the distinctions generally hold.
Crop farmers often have a few small livestock, such as chickens and goats,
but these roam freely or are tethered in the field, so that collection of
manure is not convenient, and at any rate would not be sufficient to sustain

The Western solution to restoring organic matter was to introduce leguminous
cover crops as green manures.  That has generally not been adopted by
low-resource farmers.  Farmers generally are not willing to plant a crop
simply to restore the soil, with no other benefit.  To me, it makes sense
that so long as they don't have traction and must rely on manual labor for
tillage it will not be an option.  Recently, there has been some interest in
the slash mulch, a modified cover crop system with velvet bean that has been
developed in Central America.  Velvet bean is an annual legume which
produces a lot of N-rich vegetation.  The problem is that velvet bean can
become a very competitive weed and may not fit in cropping systems where
there are two crops a year.

One of the most promising alternatives is alley cropping, where fast-growing
leguminous trees are planted in rows about 4-5 meters apart and are pruned
regularly to provide nitrogen rich mulch for crops which are grown between
the rows of trees.  There has been a lot of research on this during the last
10 years and most of it has been positive.  One of the first trials was
established in 1978 in Nigeria and continues to support a crop of maize and
a crop of cowpeas each year.  It is still in the testing phase on farm in
Africa and adoption is limited.  It is being adopted by some farmers in
Haiti and apparently in the Philippines and Indonesia.  Although there are
some problems to work out, I believe that it is one of the most promising
alternatives for sustaining crop yields of low resource farmers.

References on tropical soils:

Sanchez, P.A.  1976.  Properties and Management of Soils of the Tropics.
Wiley-Interscience, NY.

Kowal, J.M. and A.K. Kassam.  1978.  Agricultural Ecology of Savanna: A
study of West Africa.  Clarendon Press, Oxford.

Jones, M.J. and A. Wild.  Soils of the West African Savanna.  Commonwealth
Agricultural Bureau, Commonwealth Bureau of Soils Tech. Comm. No. 55
Dennis A. Shannon
Department of Agronomy and Soils
202 Funchess Hall
Auburn University, Alabama  36849-5412

Telephone: 334-844-3963
Facsimile: 334-844-3945

E-mail: dshannon@ag.auburn.edu