The effect of slash and burn agriculture on plant nutrients in the Tertiary Region of Central Amazonia ()

SUMMARY Foram estudadas moduicacoes na compos1cao quimica do solo na camada superior do solo (ato 20 em) em latossolos amarelos (85% da area total) P solos hidromorficos (cerca de 1% da area total), an tes e depois da queima da mata derrubada. Foram obtidos os seguintes resultados : 1 - A fertilidade em condicoes naturais e baixa nos tipos de solo considerado; ~ - Durante a queimada, apreciavel teor de nutri­entes e perdido sob a forma de substâncias volateis ou sob a forma de particulas; 3 - Uma grande quantidade de nutrientes, e rapL damente liberada e depositada sobre o solo apos a queimada; 4 - Grandes quantidades de nutrientes, tempora. riamente disponiveis sao perdidas por lixivia. cao uma vez que a capacidade de absorcao das plantas supera a disponibilidade de nutrientes; 5 - Uma fixacao solida reduzira a disponibilidade atual de nutriente de forma notavel; 6 - O retorno de nutrientes pela queimada nao a capaz de garantir a longo termo a agricultura tropical; 7 - A correcao do pH e o uso de fertilizantes

Tropical agricultura and recent cattle breeding in Central Amazonia are primarily based on slash and burn.Clearing and burning the cnmax forest is understood to be a con-( * ) -Instituto Nacional de Pesquisas da Amazônia.
of Central Amazonia < t> W. L. F. BRINKMANN(•J J. C. DO NASCIMENTo(• •) venient and economic method to bring a natural forest area of relatively low economic value to a higher state of yield.
Without any doubt, the ecological equilibrium of such areas will be changed, as the impact of atmospheric controls as solar radiation, air temperatura, air humidity, wind, etc. come into play with a modified efficiency (Brinkmann andVieira, 1971, Brinkmann andGóes Ribeiro, 1971}.A lthough labor is quite inexpensive, th<;: extremely high costs for NPK-fertilizer in C entral Amazônia out the profits down and partia!ly block the development of an intensive land-use.With this in view, the available plant nutrient::. in the soils after burning are quantitatively anci qualitatively of considerable interest in agricultura !planning and development.

MATERIAL AND METHODS
In 1970, a tropical rain forest site was se lected for the establishment of a cacao experimental scheme near Km 30 of the Manaus-ltacoatiara Road (fig.1).
The SEMA (Serviço Experimental em Manaus) manages an area of about 30 ha, a donation of the state government.
Actually, the site was covered by a terra fi rme rain forest typical for the "chapadas" of the Tertiary formations along the Road AM-1 O.The area was floristically inventorized in 1965 (Rodrigues, 1967).The dominant tree species of the 137 000 ha forest inventory, counting only trees with a stem-diameter above 25 em .,. ,.
According to Takeuchi (1961) more than 40% of ali trees above 10 em in diameter (DBH) belonged to the families Leguminosae Letythi• daceae and Sapotaceae.
The canopy height of the "chapada" forest ranged from 25 m to 35 m and different canopy stratCl were partially well developed.Epiphytes, Bromeliaceae and Orehidaeeae were abundant, trailing lianas quite common.Various palms like Astrocaryum munbaca, Syagrus inajai, Bactris sp et.ai. and saplings formed the substratum.A dense eommunity of eountless seedlings, stemless palms and herbs covered the ground.Normally the litter layer was well developed, but in some plaees hardly a litter layer oeeured.
The soils along the Manaus-ltaeoatiara Road (fig. 1) were inventorized by a research group of the Instituto de Pesquisas e Experimentação Agropecuária do Norte in Belém, state of Pará (IPEAN, 1969).Nearly 85% of the soils mapped were yellow were latosols ranging over a textura!speetrum from very heavy to light.The physical properties of these soils were good, while the chemical properties were quite unfavourable as far as the agricultura!potentials of these soils were concerneél.
This in mind, the slash and burn system was tested whether the release of plant nutrients bound to the standing crop is able to provide a soficiently high and permanent nutrient supply for the growth of various tropical crops, such as maniok, bananas, beans, black pepper, cacao, et.ai.

56-
In late May 1970, an area of 26 ha at the experimental site (fig.1) was subdivided into 7 equal-sized plots of approximately 4 ha eaeh.The terra firme rain forest was cut down and soil samples were collected in 20 em depth of the soil profiles (fig.2).Ten subsamples at a time were thoroughly mixed to one prime soil sample, which was taken to the laboratory to be analyzed.While the soil samples No. 1 to No. 6 were colleeted from heavy to very heavy latosols, soil sample No. 7 was taken from a hydromorphic soil near a small rain forest stream (tab. 1) .
In late August 1970 the ehaotic mass of out down forest had dried up sufficiently and was set on frre.The soil samples were collected on the 13th, the 148th and the 290th day after burning (fig.2).
The sampling procedures were the same as those mentioned above.

RESULTS AND DISCUSSION
The hydrogen-ion eoncentration (pH).caleium and magnesium, potassium, phosphorus and aluminum were determined in yellow latosols and hydromorphic soils at the experimental site (fig.1) near Km 30 of the Manaus-ltacoatiara Road (AM-10) before and after burning was applied The soil analyses earried out before burning showed the following natural soil eonditions : The soil pH (pH/H20l of both soil groups was very low (range: pH 3 .6 to pH 4 .1) .When crops as cacao.black pepper, et.ai. are introduced to such soils the soil pH has to be corrected by means of liming.Limestone i s not available in Central Amazônia, which makes the costs for sue h treatments excessive.
The calcium and magnesium concentrations in both soil groups are Jow (range : O. 20 ME% to 0.50 ME%).Both ions, however, are accumulated in the standing crop.Calei um i s often immobilized in trees in the torm of cristais of calcium oxalate, as calcium phosphate or forms salts with various organic acids.Magnesium is the only mineral constituent in the clorophyll molecule.As large parts of the calcium present in the plant tissue are located in older Jeaves, a conciderable amount of the ion must be exspected to be accumu:ated in the litter layer.The yearly calcium and magnesium return to the soil of a terra firme rain forest near Km 64 of the Road AM-10 was calculated with 17 Kg/ha for calcium and 12 Kg/ha for magnesium (Kiinge and Rodrigues, 1968).Soluble calcium and magnesium also returned to the forest floor in stemflow and throughfall (Brinkmann and Santos, 1971).
Both soil groups, the yellow latosols and the hydromorphic solls are extremely low in potassium (range: 0.04 ME% to 0.08 ME%).Bound to the standi ng crop potassium occurs as soluble inorganic salts and is highly mobile.The potassium return to the forest floor of a "chapada" forest near Km 64 of the Manaus-•ltacoatiara Road was in the order of 12 Kh.ha• 1 • year• 1 (Kiinge and Rodrigues.1968) .
Soil analyses data point out, that the topsoil layer of the yellow Jatosols and the hydromorphic soils at the experimental site are low in extractable calcium, magnes ium an<.l potassium.Likewise , this holds for the cation exchange capacity (tab.2) .
The amount of total phosphorus (range : 0.49 ME% to 0.69 ME%) was at very Jow concentration for both soil groups studied (tab.1).Phosphorus in plants is l inked into organic combinations in highly oxidized form.During growth, phosphorus is accumulated in the seeds and fruits and consequently available in the standing crop.The phosphorus return to the soil observed at a terra firme rain forest near Km 64 of the Road AM-10 was in the order of 2 Kg.ha• 1 • year• 1 (Kiinge and Rodrigues, 1968) .A conciderable amount of phosphorus was present in stemflow and throughfall of a simil ar cl imax forest at Ducke Forest Reserve, Km 26 of the Manaus-ltacoatiara Road (Brinkmann and Santo's, 1972).
Actually a larger portion of soluble aluminum (range: 1. 7 M E% to 2.1 ME%) is present in the acid yellow Jatosols at the experimental site, while the hydromorphic soils are relatively Jow in aluminum (tab.1).The yellow Jatosols are rich in clay content (about 60% to 95% or the total granulometric composition) .SiOz and Ab03 are reported to be in the order of 29 g to 37 gf 100 g air-dried soil and 24 g to 33 g/ 100 g air-dried soil, respectively.Aluminum concen• trations increasf:d with depth in ali soil profiles studied (IPEAN, 1969) .
Organic matter and nitrogen compounds concentrate in the top-soil layer.
Carbon and nitrogen content was quite high tor the yellow latosols, but Jow for the hydro• morphic soils (tab.2) .While the clay contem in both soil groups increased with depth, the organic matter decreased progressively.A very tight nutrient cycle is typical for the terra fir• me rain forest in Central Amazonia (Brinkmann and Santos, 1971, Santos et .ai., 1971, Stark, 1971) .Temporarily nutrients are stored in the litter layer, but the principie source of nutrients ls the stock accumulated in/ on the standing crop.In fact, fire is a means to make these nutrient deposits rapidly available for tropical ag riculture .
After fire was applied to the cut down forest, the soil analyses (tab. 1) showed the following results : Calcium and magnesium concentrations (soil sampl ing 11) increased significantly for the yellow latosols (400%) and the hydromorphic soils (900 %), respectively.This holds as well for the soil pH which increased slightly (about 0.6 pH) for the yellow latosols and considerably (2. 8 pH ! ) for the hydromorphic soi ls .Calcium and magnesium were released from the burr.tlitter layer and forest leftovers.The great amount of available calcium and mgnesium in the hydromorphic soils were due to a richer litter Jayer present before burning.The decay rates of the litter Jayer are much lower for the hydromorphic soils than those observed on the yellow latosols.During the first period of measurements (soil sampling I to soil sampling 11), rainfall at the experimental site was in the order of 4. 2 1 .m-2 • day-1, while between burning and soil sampling 11 rainfall decreased to 2.2 1 .m• 2 • day-1 , i. e. the leaching of soluble calcium and magnesium compounds out of the top-soil layer was limited.
In the second period of measuremenls (soil sampling 11 to soil samplin 111) rainfall was at about 5. 1 1. m-2 • day-1 • The rapidly avai l able soluble calcium and magnesium compounds especially from the burnt litter were easi ly washed through the upper 20 em of the soil profiles.
Calei um and magnesium concentrations decreased to 100% for the yellow latosols and 100% for the hydromorphic soils, respectively, as reported by the analytical data of soil sampling 111.The steep decline of calcium and magnesium leveis in the hidromorphic soils are due to a higher percolation capacity of the top-soil layer of this particular soil group (only 8% to 10% clay content in the top-soil) .
In the third period of measurements (soil sampling 111 to soil sampling IV) rainfall was in the order of 15.0 1.m• 2 .day•',i.e. available leaching water surpassed about 7 times the amount of water available at the período burning to soil sampling 11, and 3 times that of the period soil sampling 11 to soil sampl ing 111.
While soluble calcium and magnesium compounds of the burnt litter had been washed Lhrough the top-soil iayer in the second period of measurements (fig.2), the great amount o f water brought about the leaching of solub!E: calcium and magnesium constituents from the forest leftovers as partly deteriorated trunks, branches and remainders of burnt fruits and cinders.
During ~he third period of measurements (fig.2) soil moisture is at field capac ity (rain season), i. e. the percolation rates are considerably reduced.Consequently soluble caleium and magnesium compounds remain longer a time in the top-soil layer .The temporary accumulation of both constituents is recorded by an recurrent increase in calcium and magnesium cor.centrations to about 300% for the yellow latosols and 400 % for the hydromorphic soils.Coincidently the soil pH increased in consequence of higher calcium and magnesium 58 -leveis.In comparison to the soil pH under natural forest conditions the pH values for both soil groups were at about O. 5 pH higher than the initially observed soil pH.
Evidently the burnt forest leftovers as partially destroyed trunks, branches and fruits are permanently leached.
In the course of time, weathering and the continuous utilization of such nutrient deposits by photosynthetic and non-photosynthetic organisms will reduce the available stock to a very minimum.On the other hand, a new source of nutrient supply is not available, as the amount of litter produced by tropical crops is unsufficient and the nutrients stored in the fruits are taken away (Brinkmann, 1971).
Potassium in the standing crop occurs in the form of soluble inorganic salts .After burning, the initially measured concentrations of potassium already extremely low in the yellow latosols and the hydromorphic soils under natural forest conditions increased 31 times and 160 times, respectively.As potassium is highly mobile, the high concentrations shortly after burning decreased rapidly and at the end of the second period of measurements (soils sampling 111) only a small departure of potassium concentrations from the initially measured values was reported for both soil groups.Actually at the end of the third period of measurements (fig.2) potassium concentrations present in the yellow latosols and the hydromorphic soils were at the same leveis as analized for soil samples taken from a natural rain forest site.
As referred to natural forest conditions total phosphorus was low in both soil groups.After burning an unsignificant increase in phosphorus was observed in the yellow latosols.The availability of phosphorus in the soil is large ly determined by soil pH, and the presence of soluble iron, manganese and aluminum.In the presence of these elements the fixation of phosphorus in lhe soil will rapidly occur and phosphorus is kept unsoluble and not any more available for plant growth .Phosphorus is one of the nutrient contrais of the tight nutrient cycle of the tropical rain forest on the terra firme uplands in Central Amazonia (Brinkmann and Santos, 1972).As the natural nutrient cycle at the experimenta!site was destroyed <D   phosphorus must be understood as one of the most important limiting factors in slash and burn agriculture.The hydromorphic soils, however, are considerably low in alum inum, iron and manganese in the to-soil layer.A ftet burning (soil sampling 11), the phosphorus content surpassed about 4 times the values measured before burning.At the end of the third period of measurements (fig.2) the phosphorus content of the top-soil layer of the hydromorphic soils was only slightly higher than that of the analyses of the first series of soil samples (tab.1).
Before fi r e was set on the cut down forest, the aluminum content in the yellow latosols was quite high, whi!e that of the hydromorphic soil wr.s about 3 times lower (tab.1) .Afte:r burning, the alumi num levei in the yellow latosols decreased by a facto r 2, whi I e the hydromorphic soil was without aluminum after burning (soil samplin 11), but afterl the third period of measurements (soil sampli ng IV) the initially observed aluminum levei were reached again.The decrease of aluminum concentrations in the top-soi I layer, i .e. the primary root-zone of various tropical crops has a positive effect on the availability of phosphorus for plant growth, which is of some import~mce when fertilizers are applied.

C ONCLUSIONS
After burning the cut down forest, the soil pH, calcium and magnesium content and potassium increased considerably in the top-soil layer of the yellow latosols, while phosphorus did not depart from the pre-fire levei and alumi num decreased.At the end of the third period of rpeasurements (fig.2), only calei um and magnesium content and the concentration of aluminum were favourable in the top-soil byer of the latosols, while phosphorus showed up with the same concentration levei measured before burning and potassium content decreased heavily because its high mobility due to increasing rainfall (15.0 1.m• 2 .day• 1 ) in the third period of measurements.
As a matter of hct, burning the cut down forest wm re lease forest bound plant nutrients to the soil.But, only a part of the rel eased nutrients will be avai lable for plant growth .A considerable part of the liberated nutrients co -will be blown up into the air and spread over a wide area in the form of volatiles and particles.These nutrients, however, will be lost at the clearing .but will contribute to the tight nutri• ent cycle of nearby forest stands as atmospheric fall-out o r wash-out.A considerabl e amount of plêtnt nutrients will be leached through the main root-zone without enabling the ;;rops te use the excessive mass of availablc ;:lant nutrients as ti1e uptake capacity of the crops is limited.In the cou;•se of time, the fixation of plant nutnents will occur and tho availability of nutrients will be seriously reauced .Aditionally soii structure of the topsoil layer will be affected and temporary soll sterilization occur, which is favourable to agriculture as far as fungai and insect attack on seed and seedlings are concerned, but very unfavourable to nitrogen production and micorrhizal associations which play an important part in lfie nutrient uptake of plants.As a matter of fact, fire 1s a means of unpropriate exploitation of the trop1cal envi:-onmenl, es pecially when long-term agricultura!schemes for the tlevelopment of Central Amazon1a are proposed .
In slash and burn agriculture the soil pH remain&~ well below the favourable range fo.important crops.As far as the cacao pilot scheme at the SEMA-site (fig. 1) was concerned, soil pH had to be corrected by heavy liming.CaMg (C03h was applied to the yellow latosols in the order of magnitude of 1200 Kg/ ha .Best growing conditions are obtained, when the soil pH is stabilized in the range of pH 6. O to pH 7 .O. The increase of the soil pH has an favourable effect on microbial life and the availability of various plant nutrients to the crops.At the SEMA-site a plot of 1100 cgcao trees/a was treated with NPK-fertilizer as follows : Nitrogen (urea/46% N) 65 Kg/ ha, Phosphorus (superphosphate/45o/oP) 200 Kg/ ha and Potassium CK20/ 60 %K) 73 Kg/ ha .lt must be emphasized that a heavy treatment with fertil izers is needed to grow perennial crops as cacao.etc. effectively on the yellow latosols along the Manaus-ltacoati ara Road .The short outburst of plant nutrients after burning only enables the growlng of some basic crops as maniok, bananas, etc .for at least a couple of years.According to IPEAN (1969) , the yellow latosols cover about 85% of the total area along the Road AM-10.As shown above, the agricultura( development of these areas is dependent on the use of fertilizer especially phosphates in considerable amounts.
The hydromorphic soil group is limited to only one per cent of the total area of the Tertiary uplands along the Manaus-ltacoatiara Road.Compared with the nutrient conditions of the yellow latosols after the cut down forest was set on fire, increased phosphorus concentrations and low alumi num content in the topso il layer were favourable, but the availability of calcium and magnesium as well as potassium were worse.Soil fertility after burning is such, that cleared areas on hydromorphic soils may be used for a period of 2 or 3 years for the growing of maniok.pineapple and some other basic crops.After the short growing season these areas will be abandoned and rain forest successions will take over the cleared plots.
As a matter of fact, the profit obtained from tropical agriculture along the Manausltacoatiara Road and areas of a similar environment will be tlependent on a favourable exposure of the crops to the impact of the atmospheric controls as solar radiation, air temperature.ai r humidity and wind, i. e. a particular shaddow-system for a particular crop, and the intensive soil treatment with fertilizers after correction of the soil pH by means of liming.Aditionally Lhe application of herbicitles, insecticides and fungicides is essential for the suppression of r o o t competion and the avoidance of plant deseases.