Journal of African Earth Sciences, Vol. 9, No. 2, pp. 357-362, 1989 0899-5362/89 $3.00 + 0.00 Printed in Great Britain Pergamon Press pie

Agrogeology in East Africa: the Tanzania-Canada project

W. CHESWORTH*, P. VAN STRAATEN* and J. M. R. SEMOKA**

*University of Guelph, Guelph, Ontario, Canada N1G 2Wl. **Sokoine Agricultual University of Agriculture, Morogoro, Tanzania

Abstract- Fertilizer self sufficiency is a necessary prerequisite for food security in a nation. In promoting this, the role o f the geologist is to find raw materials farmers can use to increase the productivity of their soils. The Tanzania-Canada agrogeology project is the first o f its kind wherein geologists, soil scientists and agronomists are working together to find and test materials of a geological provenance, that small scale farmers can use to improve the food carrying capacity of their land.

I N T R O D U C T I O N

The first agrogeology project to receive ma jo r funding b e g a n J a n u a r y 1, 1985, (Chesworth et al., 1985). It l inks geologists f rom the Depai-tment of Land Resource Science, Universi ty of Guelph, C a n a d a wi th the i r c o u n t e r p a r t s f rom the Geological Survey of Tanzan ia {MADINI) in Dodoma, and wi th soil sc ient i s t s f rom the Sokoine Univers i ty of Agricul ture in Morogoro and from the Uyole Agricul tural Centre nea r Mbeya. The loca t ions of these las t two ins t i tu t ions m a r k the two m a i n a reas s tud i ed b y us , (Fig. 1). The principal sou rce s of f u n d s for the project are the In terna t iona l Deve lopment Resea rch Centre, O t t awa and Madini. The ins t i tu t ions in Morogoro, Uyole and Gue lph con t r ibu te impor tan t ly in t e rms of pe r sonne l and r e sea rch facilities. The first p h a s e of the project ended D e c e m b e r 31, 1987, and a s econd 3 -year p h a s e was funded in 1988.

The overall objective is to advance agr icul tural p roduc t ion in s o u t h e r n Tanzan ia b y us ing local geological r e s o u r c e s to improve chemical and phys ica l a s p e c t s of the farmers ' soils. This involves determining the phys ico-chemica l charac te r i s t i cs of the soils to f ind ou t w h a t specific p rob lems exist; locat ing geological r e s o u r c e s tha t are readily avai- lable and tha t c an be u s e d to amel iorate the problems; us ing the sa id r e sou rce s in pot and field exper iments to t es t their agronomic effectiveness; and finally m o u n t i n g an ex tens ion effort to show local f a rmers the effects of each succes s fu l mate - rial. In the first 3 y e a r s all b u t the las t objective have received se r ious effort and have resu l ted in some no tab le ach ievements . It is still, however , p r e m a t u r e to cons ide r set t ing u p an ex tens ion programme, t h o u g h in order to s t imula te d i scus - s ion amongs t in teres ted par t ies an Agrogeology

newsle t te r w a s founded in 1986 a n d cur ren t ly goes ou t to a b o u t 300 people.

B A C K G R O U N D

A coun t ry t ha t does no t control the m e a n s to ma in t a in the fertility of its soil is essent ia l ly depen- dent on foreign impor t s of fertilizer. The legit imate hope tha t people have to s ecu re the i r food supp ly canno t u l t imate ly be achieved in s u c h a s i tuat ion. In o ther w o r d s food secur i ty d e p e n d s to a s u b s t a n - tial degree on fertilizer self-sufficiency. A brief examina t ion of soil fertility will show tha t fertlllTer self suff iciency itself d e p e n d s largely on the ready availability of the n e c e s s a r y geological raw mate - rials.

A growing crop requi res a var ie ty of e lements and c o m p o u n d s for its well being. For m o s t of these , the soil ac t s a s an effective reservoir, and with the except ion of ni trogen, ca rbon dioxide, and water , the nutr i t ional needs of the p lan t are provided b y geological mater ia ls . On weather ing, p r imary minera l s b r e a k down to p roduce s e c o n d a r y ones and in the p roces s m a n y nu t r i en t e lements are m a d e available to the b iosphere . A fract ion is t aken up by p lan t s and the ba lance , t h o u g h it m a y be s to red for a while on the su r f aces of c lay minerals , is eventual ly lost to the s y s t e m b y leaching. This m e a n s tha t even if a crop is not r emoved f rom the land, there is a s teady, na t u r a l loss of soil fertility over time. Eventua l ly the soil, a t leas t in h u m i d cl imates, will become acid and relatively infertile. This s tage h a s b e e n r eached on m a n y of the oldest, u n d i s t u r b e d l a n d s c a p e s of the world, m o s t of which are to be found in the tropical and sub- t ropical coun t r i e s of the developing world.

W h e n c rops are r emoved from the soil th is t r end towards an acid, infertile end point, is increased b y roughly a h u n d r e d fold, and if the fa rmer did

357

358 W. CnESWORTI-I, P.VAN STRAATEN and J.M.R. SEMOKA

nothing about it, the food-carrying capacity of the land would be rapidly exhausted. Consequently fertilizers, including manures , composts and other wastes, are used to replace nutr ient elements removed by cropping or lost by leaching. In addi- tion, techniques such as crop rotation, intercrop- ping and fallowing, allow the land to recover, and, when one of the crops Is a legume, provide a biological means of fixing atmospheric nitrogen and thereby lessen the farmer's dependency on costly nitrogen fertfllTers.

The salient fact in all of this is that for all but the nitrogen fertilizers, geological raw materials are necessary. Since nitrogen can be managed biologi- cally, the development of fertilizer self sufficiency then depends on the geologists finding appropriate raw materials. What materials are in fact appro- priate will depend upon what problems the target- ed population of farmers face. Consequently any agrogeology project mus t start by examining the farmers' problems.

SOIL RELATED PROBLEMS IN SOUTHEILN TANZANIA

These will be illustrated by reference to the soils of the Rungwe massif near Mbeya in southern Tanzania. Mount Rungwe is situated at a triple junct ion of r r r type of the Westem Rift (Burke and Whiteman, 1973; Fig. 1). It has been active since

De~ Sea T m ~ m l

late Tertiary t imes and the principal products have been alkali basalts and phonolite-trachyte ash (Fig. 2). The surrounding rocks are quartzo- feldspathic gneisses, pelites and amphibolites of the early Proterozoic Ubendian supergroup. Since the most productive soils of the region are develo- ped on the volcanic parent materials it is these that are stressed in what follows.

Both massive basalts and ash evolve on weather- ing towards refractory combinations of SiO=, A1203, FezO 3 and 1-120 (Fig. 31. This trend towards geo- chemical matur i ty is paralleled by a trend toward mineralogical matur i ty expressed by assemblages dominated by kaolinite, goethite and quartz. Most of the alkali basalt shown on figure 3 has reached this late stage of weathering as has some of the ash. But fortunately for the farmers of the region most of the ash is still at an earlier stage of weathering and is inherently more fertile than the softs over- lying basalt.

Where the materials have reached the more advanced stages of weathering the agriculturalist encounters three principal kinds of problem. First, the intense leaching decreases the available plant nutr ients in the soft and the latter becomes pro- gressively more infertile. Second, nutr ient cations are replaced by protons and soft pH decreases to a point where acidity related problems arise, notably Al and Mn toxicities. Last, the clay fraction be- comes dominated by aluminal and ferrol bonded surfaces and these with the free Al released at low pH fix the important plant nutr ient P so that it becomes unavailable to most crops.

@umenga- K ~

ressioe uhum

~ A C T I V E

1.

Field test areas 1 Mbeya 2 Morogoro

l-uan0wa.

Fig. I. Location of the two s tudy areas in Tanzania, within the framework of recent tectonics. Note that the Mbeya

s tudy area is centred on Mount Rungwe, an active rrr triple point (Burke and Whlteman, 1973].

Mbeya

_JPRONOLITE- ~-- ) TRACHY'rE

TANZANIA

-"~,Zk-L-~w~ ~, ALKALI BASALT N~yasa" 0 20km ~ _ _ ~ _ ~ - -

Fig. 2. Ash and basalt, parent mater ia ls of the softs of the Rungwe centred region.

Agrogeology in East Africa : the Tanzania-Canada project 359

CaO+MgO

30

20

Koalinite / ~'r,~ary~'~a~'_L+ - \ \1o Goeth.e / oua.,

Plagioclase, 2:1 sheet silicates, allophane SiO=+AI203+Fez03 goethite, kaolinite

CaO+MgO

0 Plagioclase / ~ , ~ ' ~ + allophane ~ ,~ - Nepheline

Na=O+K.~O ~: / =,, w =,~ z , . ,< o x 100% 20% 10

SiO=+AI203+F~O3

Fig. 3. W e a t h e r i n g t r e n d s of soi l p a r e n t m a t e r i a l s of vo lcan ic p r o v e n a n c e in M b e y a region . A: a lka l i b a s a l t , B: phono l i t i c a s h , s h o w i n g . p r i m a r y m i n e r a l o g y a n d m i n e r a l o g y o f s e c o n d a r y m a t e r i a l s

in t e r m s of t he 3 w e a t h e r i n g s t a g e s of C h e s w o r t h (1977).

In summary, the chemical problems are all rela- t edby the dynamics of weathering and the tenden- cy of the softs to evolve towards the residual system of weathering. The interrelated problems are:

a. low pH b. low fertility c. AI and Mn toxicities d. P-flxation e. Cu deficiency. The extent to which these problems are inter-

related can be shown in terms of pH and pe conditions in the soft environment, pH measure- ments were performed at Uyole Agricultural Centre and provided by J_~. Kamasho, principal soft scien- tist there. Est imates of pe were made by-a techni- que modified from Chesworth and Macias (1985). Areasonable assessment of the range of conditions is shown in figure 4 which indicates that a high solubility of at least three, potentially toxic ele- ments can be expected. In addition the field label- led high solubility is the field of maximum leaching of nutr ient cations, so low fertilites are the rule, and it Is also where P-fixation problems are great- est.

In addition two physical problems are encounter- ed in the region and are related respectively to erosion of soil, and to availability of water. The volcanic ashes especially, are prone to sheet and gully erosion and corrective practices are not well developed. Consequently topsoil constantly winds up in the bottom of river and s t ream valleys.

Water problems generally increase from south to north since the climatic regime varies from humid to arid in that direction. However, even in the more humid areas problems arise because of the sea- sonality of the rainfall and because many of the soils are on materials a n d / o r slopes that tend to drain rapidly.

pe

15

10

5

0

-5

-10

0

t i I I

_ ~ Water oxidised

High._. ,r,, solubility " ' ,

a " " i .01 ppm'~Mn __~ d \ ,'

I \', " '~. / "~', h \ I lj.Ol ppm Fo"

- ~ i~ !

.o, i- "-~ solubility

Water reduced

I l i t i i

4 8 12 pH

Flg. 4. The box abed shows conditions of weathering ha the Mbeya region ha terms of pe and pH. Part or all of the box represents conditions ha which Mn, Fe and AI may be mobfllsed, producing toxicity problems and problems of phosphate fixation. The box is also si tuated ha a portion of the diagram where most cation nutr ients are highly soluble and therefore readily leached ha humid climates. This gives rise to fertility problems. Solubility contours are drawn for the following conditions: 250C, 100 Kpa total pressure, 2000 mgL total CO 2 as HCO 8- and 2000 mg/L total S as SO4-%

(modtfled from Hem, 1972).

36(1 W. CHESWORTH, P.VAN S'IRAATEN arid J.M.R. SEMOKA

GEOIX)GICAL C O [ , T N ' I ' E ~ U I t F - , 8

The geological field crew discovered and investi- gated several local materials of use in ameliorating soils affected by problems summarised in the foregoing section. Specifically these are: (a) liming materials principally from igneous (carbonatite) sources, (b) phosphatic rocks of igneous and sedimentary provenance, (c) zeolites from altered volcaniclastic r.ocks, and (d) scoria, tephra and ash from explosive vents.

The carbonati tes of the region, Precambrian and Cretaceous in age (van Straaten, 1987, 1989), are chemically and mineralogically little different from traditional liming materials. However, unlike the latter, they may be rich in potentially toxic ele- ments, especially rare earths. This is a hazard we intend to monitor over the next three years. Several other materials we have investigated are capable of increasing pH in soil. These include the more readily weatherable volcanic rocks (ash for exam- ple) and zeolite. In fact, zeolites added to pot trials at the University of Guelph ultimately produced a pH of 9, a situation as undesirable as the original high acidity. The fact is that carbonatite remains the best option.

Increasing pH to a minimum of about 5.5 is sufficient to prevent the common elemental toxici- ties, as well as increasing base saturation. It thus serves also to increase the nutrient s ta tus of the soll especially in terms of Ca and Mg. However P generally t u m s out to be the limiting nutrient of a geological provenance, in short supply. Ahd again, the main source of P is to be found in carbonatite, two at least of which the geological crew has shown to contain enough apatite to have agronomic po- tential. In addition, minor lacustrine phosphorites occur east of Mbeya, though reserves are not great. The carbonati te apatite deposits and in particular the CaCO 3 free residual apatite deposits of Panda, Ngualla, Mballzi and the Songwe Scarp have the highest potential (van Straaten, 1987). Unfortuna- tely the carbonati te apati tes tend to be hydroxy- fluorapatites which are less soluble than carbo- nate fluorapatites. Consequently, we have investi- gated the possibility of adding zeolites, a common alteration product of the volcaniclastics of the re- gion, to the carbonati te apati tes in an attempt to produce an ion-exchange reaction (Lai and Eberl, 1986) that could result in increase in phosphate release in the system. Our greenhouse trials along these lines have produced an increase of 6-fold in the breakdown of apatite compared with a zeolite- free system (Chesworth et al., 1987}. Other techni- ques, including composting and partia} acidula- tion, could also be used to make the apatite more reactive. Even so, Josepha t Kamasho ( pers. comm.

1988) at Uyole has shown that on the more acid soils, with pH of 5 or less. the apatite-rich weather- ing res iduum of the Panda Hill carbonati te west of Mbeya, can be used directly, without modification, to give results comparable to commercial P-fertili- sers such as TSP. The natural apati tes seem to be a more appropriate P-fertfliser than the commer- cial product since the chemical P-fertilizers are rapid release material and what is not fixed by AI and Fe, tends to be leached and lost in the softs that we encounter in this part of the humid tropics. As Leonardos et al. (1987} point out, the commercial fertilizers were largely developed for the very diffe- rent, generally high CEC softs of the northern hemisphere. A slow release material such as natu- ral apatite is a better amendment for high leaching environments, at least where the soils are acid.

Kamasho and Singh (1987) also showed that some of the ash derived softs in the northern part of the Rungwe massif are Cu-deficient. We have since shown that this condition is probably inhe- rited from the parent material, namely ash from the Rungwe and Ngozi centres. New analyses show that the ash contains generally less than 2 ppm Cu. There is the possibility of using malachite and azurite from a nearby red beds deposit in Iringa region as a soil supplement. Alternatively some of the local scorias can be used in this regard.

Turning to the physical problems encountered in the region, erosion can be countered by contour ploughing, by ridging or by terracing. Furthermore rock mulches, employing the virtually ubiquitous ashes or scorias, can help by preventing splash or sheet erosion.

The principal use of rock mulch however is in water conservation. Here we are attempting to transfer a technology developed over the last 200 years in the Canary Islands (Chesworth et a l , 1983) to East Africa. The details were provided by Fernandez Caldas and Tejedor Salguero (1986) at the First African Conference on Agrogeology held at Zamba. Malawi. Soil, either t ransported or in place, is covered by a layer of scoria that prevents the capillary uprise of water and the rebycu t s down evaporative losses. The first extensive field appli- cation of this technique in Africa is currently being made as part of the final year of the first phase of our project in Tanzania.

Table 1 presents a brief summary of the four major locally available geological materials now being tested by agronomists in the Mbeya region.

U N F ~ S ~ D B ~ I N E S S

Three years is much too short a time to perform adequate tests of agronomic materials. For one thing, the residual effects of our slow release amendments need to be monitored before any real

Agrogeology in East Africa: the Tanzania-Canada project

Table 1. List of agricultural raw materials discovered by project scientists in the Mbeya region and a summary of soil-related problems these materials are being used to ameliorate.

Material Provenance Uses

Apatite Residual soils over carbonatite intmsives and fenites, Lacustrine deposits

Major source of slow release phosphate in acid soils.

Carbonate Carbonatite intmsives, Limestone

Liming materials used to correct problems of acidity and associated toxicities.

Zeolites Volcaniclastic- sedimentary beds in lacustrine deposits

Useful in conserving N, and in releasing P from apatite by coupled reaction. Raises pH.

Scoria Explosive vents in & around the Rungwe massif

Used as mulching material to conserve soil water and provide slow release nutrients and micronutrients. Raises pH.

Malachite/ azudte

Red beds Cu deposit between Mbeya and Iringa at Kigugwe

Useful in correcting Cu deficiencies in wheat area of the Poroto mountains.

361

economic comparisons can be made with commer- cially available fertilisers. Consequent lythe econo- mic analysis mus t wait until the second phase. For the same reason (insufficient testing time) no extension effort can yet be made to alert local farmers of the possibilities. At the end of 6 years, we figure we will be in a position to do this.

Finally, the long term monitoring of growth trials that have used the various local materials, is necessary in order to determine whether any ad- verse effects are likely. For example the carbonati- tes are rare earth rich and whereas these elements are supposedly relatively immobile in the weather- ing zone, more analyses will be needed to find out how available to crops they are, and therefore how likely they are to get into the h u m a n diet.

THE FUTURE OF AGROGEOLOGY

The main tenance of soil fertility is one of the most important under takings we owe posterity. We can accomplish this aim only by a thorough knowledge of the physico-chemical environment of the soil and by a clear unders tanding of the reactions that take place when we ameliorate the soil by means of fertilisers, composts, ma nu r e s or other additives.

Geologists and geochemists have a role to play here firstly in investigating the mineral-chemical

basis of soll fertility and secondly in providing raw materials tha t the agronomist can use to improve the growing system.

In the context of third world development, the agrogeologist should alm to find materials that the sman scale farmer, upon whose shoulders m u c h depends in feeding the poorer parts of the world, (Park and Jackson, 1985) can use and develop locally. Food security in the developing world largely depends upon self-sufficiency in fertiliser raw materials. Such self sufficiency is often possible on a local scale. Here fertilisers of a local geological provenance can be used in conjunction with nu- trient conservation measures involving manures , composts, and other wastes, as well as with bio- logical measures such as intercropping, alley crop- ping and crop rotations.

At Guelph, we are now planning agmgeology programmes for others parts of the developing world. Specifically, with the financial assistance of IDRC, we have visited Ethiopia, Bolivia and Colombia and elaborated research proposals with local geologists and soil scientists, What we find is that the experience we are building up in Tanzania has a m u c h wider applicability than the confines of that one country. Furthermore, the more we exa- mine the problem of fertiliser availability the more we realize that even the poorest countries are likely

362 W. CHESWORTH, P.VAN STRAATEN and J.M.R. SEMOKA

to h a v e i n d i g e n o u s r e s o u r c e s t h a t t h e i r f a r m e r s a n d f o r e s t e r s c a n u s e to i m p r o v e t h e i r softs. F o r th i s r e a s o n , we t h i n k it m a k e s a g r ea t dea l of s e n s e for t he geological s u r v e y s of deve lop ing c o u n t r i e s to se t u p ag rogeo logy s e c t i o n s to w o r k h a n d in glove wi th a g r i c u l t u r a l i s t s a n d fo re s t e r s , to pro- d u c e i n v e n t o r i e s of ava i l ab le m a t e r i a l s , to deve lop t e c h n i q u e s o f soft a n d w a t e r c o n s e r v a t i o n , a n d to d e s i g n n a t i o n a l ag rogeo logy p r o g r a m m e s t h a t wig l e s s e n t h e d e p e n d e n c y of t h e t h i r d wor ld on fer t i - l i se rs f r o m the i n d u s t r i a l i s e d n a t i o n s . S e c u r i t y of food s u p p l y d e p e n d s o n effor ts of t h i s k ind .

Burke, K., and Whiteman, A.J. 1973. Uplift, rifting and the break-up of Africa: p. 735-755 in Implications of Continental Drift to the Earth Sc/ences v. 2, ed. by Tarling, D.H. and Runcorn, S.K., Academic Press, N.Y., 1184 p.

Chesworth, W. 1977. Weathering stages of the common igneous rocks, index minerals and mineral assembla- ges at the surface of the earth. Jour. Soil ScL 28, 490- 497.

Chesworth, W. 1982. Late Cenozoic geology and the second oldest profession. Geoscience Canada, 9, 54- 61.

Chesworth, W., Macias Vazquez, F., Aquaye, D., and Thompson, E. 1983. Agricultural alchemy: stones into bread. Episodes 6, 3-7.

Chesworth, W., and Macias Vazquez, F. 1985. pe, pH and Ix)dzolisation. Amer. J. ScL 28B 128-146.

Chesworth, W., van Straaten, P., Semoka, J. and

Mchihiyo, E. 1985. Agrogeology in Tanzania. Episodes 8, 257-258.

Chesworth, W., van Straaten, P., Smith, P., Sadura, S. 1987. Solubility of apatite in clay and zeolite bearing systems in application to agriculture. Applied Clay Science 2, 291-297.

Fernandez Caldas, E., and Tejedor Salguero, M.L. 1986. Use of rock mulches in the agriculture of the Canary Islands, Spain. Proceedings of 1st African Conference on Agrogeology, Zomba, Malawl (In press).

Hem, J.D. 1972. Chemical factors that influence the availability of iron and manganese in aqueous systems. Geol. Soc. Amer. Sp. Papers 140, 17.

van Straaten, P. 1987. Mineral resources potential of carbonati tes in SW-Tanzania (abs.) 14th Colloq. of African Geology, Berlin, 1987, CIFEG occ. publ. 1987/ 12 p. 175-6.

van Straaten, P. 1989. Nature and s t ructura l relation- ships of carbonati tes from Southwest and West Tanzania. In: CarbonaUtes - Genesis and Evolution. edited by K. Bell, p. 177-199, Unwin Hyman, London.

Kamasho. J.A. and Singh, B.R. 1982. Available copper and zinc s ta tus of some Tanzania volcanic ash soils: A case study. Pedo/ogie ][XXll (2}, 209-224.

LaJ, 2Mng-Ming, and Eberl. D.D. 1986. Controlled and renewable release of phosphorus in soils from mixtures of phosphate rock and NH4exchanged clinoptilolite. Zeolttes 6. 129-132.

Leonardos, O.H.. Fyfe, W.S. and Kronberg, B.I. 1987. The use of ground rocks in laterite systems: an impro- vement to the use of conventional soluble fertilizers. Chem. Geol. 60, 361-370.

Park. P., and Jackson, T. 1985. Lands of plenty, lands of scarcity. Agricultural policy and peasant farmers in Zimbabwe and Tanzania. OXFAM, Oxford, U.K. 22 p.