The potential for lateral agricultural production to meet the food security needs of the growing population in Tanzania is constrained by a combination of low soil fertility and erratic and unreliable rains. Deforestation, overgrazing and inappropriate cultivation practices are the major causes of land degradation which is a major environmental concern in the country. Lateral agricultural expansion, use of marginal lands, short fallow periods and mechanized conventional tillage systems with little observation of agronomic and structural soil conservation measures lead to soil degradation and declined crop yields. Sound indigenous soil conservation practices such as pitting and mound farming are too localized in a few areas only. Many soil and water conservation programmes have been initiated with limited success as they did not take into consideration the peoples' socio-economic factors and practice. Therefore, in order to protect the soil for sustainable agricultural production, land conservation should be integrated into the normal crop and livestock husbandry practices of the small-holders with emphasis on the land husbandry concept. Agronomic, vegetative and structural measures should be complementing each other in the system. A more participatory, community based holistic approach is suggested and strong coordination of Government efforts encompassing all the various players in environmental protection is recommended. 
  

Introduction 

The mainstay of Tanzania's economy is agriculture, which employs 85% of the population and produces 61% of both the GDP and merchandise exports. Agricultural production is predominantly subsistence, with 3.7 million farm families cultivating 86% of the cultivated land, working small holdings averaging 1 ha per household. It is estimated that only 5% (7 million ha) of the total land area is under cultivation (LRDC, 1987). The productivity of the small-holder farmers is generally low. Increases in production to match the population increase have usually been achieved by expansion of agricultural land as opposed to intensification. As a result, new land is continuously cleared and where there is high land pressure, marginal areas such as steep slopes and river banks which are sensitive to erosion are put to use. However the potential for lateral agricultural expansion, to meet the food security needs of the growing population is constrained by a combination of low soil fertility and erratic and unreliable rainfall. 

More than 60% of the country is classified as semi-arid with 1/3 of the land with a bad prospect of obtaining an annual rainfall of less than 750 mm (Dutt et al, 1981; Gommes and Houssian, 1992). Further to this, nearly throughout the country, potential evapotranspiration exceeds rainfall during more than nine months of the year. 

The soils, as with the rest of the tropical semi-arid zone, are on the whole light, porous and shallow with low moisture holding capacity, making them susceptible to soil erosion. The soil surface is usually exposed because of the limited vegetation cover, allowing a rapid run off from the sporadic conventional down pours and hence encouraging soil erosion [Box 1]. With the increased population pressure, the fallow periods have become shorter for the soils to recover, perpetuating the "soil mining" of nutrients, which leads to a further decline in soil fertility. 
  

Land degradation in Tanzania 

Land degradation is a major environmental concern to sustainable agricultural production in Tanzania. There are many causes of land degradation in the country and some of the important ones are deforestation, overgrazing and inappropriate tillage or cultivation practices. 

Deforestation 

In Tanzania, it is estimated that between 300,000 and 400,000 hectares of forest and woodlands dissappear each year. Sixty per cent of this is accounted for by agricultural expansion and wild fires (De Pauw 1994). Tree felling is also done for the purpose of providing fuel wood for cooking, heating, local brewing and firing bricks and providing building materials such as poles and timber [Box 2]. Bush fires are done for the purpose of hunting, bee harvesting or as part of the traditional natural management in getting the regeneration of new grass for livestock. 

The long term consequences of reduced forest cover include changed rainfall pattern, decline in soil fertility and, increased surface runoff. This leads to severe erosion, seasonal flooding and siltation in the plains, dams and lakes. 

Overgrazing 

The number of livestock has been increasing from 9 million heads of cattle and 5 million oats in the 1970s, to 15,1 million heads of cattle, 10.7 million goats, 3.5 million sheep and 134,186 donkeys (MAC, 1997). Extensive free range grazing of livestock is practiced on communal lands. This practice encourages pastoralists and agro-pastoralists, to move freely to search for suitable pastures. During the dry season there are large movements of cattle herds from northern and western regions (Arusha, Mwanza, Shinyanga and Tabora) to the southern regions hunting for favourable grazing and water sources. Overgrazing depletes the land of its vegetation cover, thus exposing the soil to water and wind erosion. Excessive trampling by the animals destroys the soil structure and reduces infiltration rates. This leads to high run off and hence accelerating soil erosion. The animal stock routes too, lead to gully erosion. 

Cultivation Methods 

Pressure on land has resulted in drastic reduction of fallow periods. In many cases it has been replaced with intensive cultivation - albeit with low use of inputs due to the farmers inability to purchase what is necessarily required. This leads to the "mining of the soils" resulting to rapid soil degradation and reduced crop yields. The effect of population pressure, has also reached marginal lands such as steep slopes which fall easy prey to rapid soil degradation, as rarely do farmers take up appropriate soil conservation measures. 

Shifting cultivation, is still being practised in some parts of the country with the slash and bum method as an example. This practice, encourages deforestation as is the case of Rukwa region where it has been associated with fingermillet growing. The rate of deforestation is also high in the tobacco growing areas [Box 3]. 

Mechanized conventional farming by tractor or oxen, whereby primary tillage is followed by secondary tillage leads to soil degradation especially in the semi-arid areas. The implements used, which include disc and mouldboard ploughs, together with disc harrows, cut, invert and pulverize the soil. If the operation is done frequently, it leads to compacting, crusting, poor seedling emergence, low water retention capacity and consequently reduced yields. In Njombe district in the Southern Highlands, it was observed that the grain yield in maize and wheat dropped with 5 tons/ha and 2 tons/ha respectively in ten years. This was the result of the formation of plough pans (2-10 cm below the surface), formed as a result of continuous convential tillage using 34 disc trailed harrows year in and year out (Shetto and Kwiligwa, 1989). Moreover with the possession of more farm power, farmers tend to increase their cropped area tremendously leading to more woodlands being cleared [Box 4]. 
 

Tillage Practices In Tanzania 

Tillage is defined as the, generally mechanical, manipulation of the soil surface layer to modify soil conditions for crop establishment, growth and yield. The primary objectives of tillage are seedbed preparation, provision of a good medium for plant roots, water infiltration and conservation, erosion control and weed control. Tillage practices in Tanzania are location specific to meet the various requirements as a result of the country's biodiversity. 

Traditional tillage systems 

These are mainly localized soil and water conservation tillage methods. They have been practised for more than 150 years, evolved by the farmers to suite certain environments. These practices include the pitting systems of the matengo in the southern part, mound farming and the noprimary tillage system. The no primary tillage system is mainly practised in the semi-arid regions, where it is popularly known as "Kuberega". This is essentially a dry planting slash and bum system. The method involves slashing the vegetation or stover, leaving it on the ground to dry and burning it to leave a clean seedbed. Sowing is then done directly without disturbing the soil, except for the planting holes which may be made by a digging stick or handhoe. 

Ridge Tillage 

Ridge Tillage is mainly associated with specific crops such as beans, groundnuts, sweet potatoes and cassava. On sloping hillsides, ridges are made across the slope to control erosion. Ordinary ridges are 20-30cm high and are spaced between 60 and 80 cm. In some areas, bigger broad ridges are made which may be 50 cm high and spaced at 1.5cm. They are used for growing beans in the southern highlands or sweet potatoes elsewhere. Ridges are commonly made by using the hand hoe, though in some regions in the Lake zone, draught animals are used. Ridges are used in fertility improvement by incorporating vegetation under the ridge during their construction. Many farmers claim that weeding is easier in ridges too. 

Conventional Flat cultivation 

Flat cultivation is the most commonly practised system in the country. Hand hoes, ox ploughs and tractor drawn ploughs and harrows are used. The system is associated with modem farming where a fine seedbed is prepared. The soils are cut, inverted and pulverized, burying the crop residues underneath. The practice can be disastrous under tropical storms, as soils may cap or crust under the impact of rain drops. This reduces infiltration and increases water run off, thus accelerating soil degradation. In addition to crusting, some soils are compacted, restricting root growth and development. In many cases, flat cultivation does not follow the contours lines and hence increase the risks of erosion. Sometimes, ploughing is done up and down the slope, especially with draught animals as it is claimed to be faster (BACAS, 1996; Shetto and Mkomwa, 1996). 

Conservation Tillage 

Conservation tillage can be defined as a crop planting system that allows minimum disturbance of the soil to allow seeds to be sown, while ensuring the maintenance of crop residue on the surface. 

Traditional Soil and Water Conservation Practices 

Despite modernizing agriculture with conventional tillage methods, some farmers in Tanzania, practice what can be described as "indigenous" systems of crop production. These systems, some of which have survived the test of time, indicate that, conservation tillage was probably among the major preoccupation of farmers in ensuring sustainable crop production. 

The Matengo Pit Cultivation System 

The Matengo pit cultivation system, locally known as "ngoro" has been practised for many centuries by the inhabitants of the Matengo highlands in southern Tanzania. The practice main attribute, is soil and water conservation as well as fertility restoration by refuse decomposition [Box 5]. Most of the pits, are on steep slopes ranging from 10-60%. The pits control runoff while conserving moisture simultaneously, as rain water collected in the pits percolates into the soil slowly. The incorporation of crop residues into the soil improves soil fertility. During weeding, all the weeds are thrown into the pit, so are the crop residues at the end of the season. These are left to decompose and form compost. 

The Ufipa Mound Cultivation System 

The Ufipa mound cultivation system, "Ntumba" is a traditional subsistence farming method practised by the Wafipa in southwest Tanzania. Mounds are prepared by heaping soil and grass from an area of about one square metre, ensuring that the grass is covered completely. A leguminous crop is planted randomly on the mounds which are 40–60 cm high and 50-60 cm in diameter. In the following rainy season, the mounds are flattened throughout the field and the main crop, which is fingermillet, or maize is grown. Again this is an in situ composting for fertility management. 

The Iraqw intensive Farming System 

This is a well functioning and sustainable agricultural system developed over a period of some 200 years by local farmers of the Iraqw tribe in northern Tanzania. Today, both crop production and cattle rearing are organized in a detailed intricate farming system which has with-stood many of political changes. In this hilly area, all the crop residues are left in the fields and incorporated into ridges. Manure from the stall fed cattle, is used to further maintain the fertility of the leached granite soils. Soil erosion is controlled by terracing which is done by moving soil with a hand hoe from higher parts of the fields to lower parts. On the edges of the terraces, fodder for the cattle is cropped, which is supplemented by grass from fallow fields. Other soil conservation measures such as trash lines and cut off drains are used to slow down surface run off and to increase infiltration (Thomton, 1980). 

Minimum Tillage 

Minimum tillage (apart from the hand no-primary tillage) is mainly practised in large scale mechanized wheat farming in northern Tanzania. In this system, there is little disturbance of the soil, as the amount of soil inverted during cultivation is minimized. Crop residues left on the surface cushion rain drop impact and reduce water movement and hence soil erosion. As water runoff and evaporation are reduced, water penetration is improved, while crop residues and roots build up in the long term improving soil structure. Cultivation in the area, is mainly done by tractor drawn tined implements. Herbicides are often used to control weeds. Use is made of air seeders in planting as they work better in trashy fields with their sweep like tined furrow openers up front. The need for minimum tillage implements in animal traction based tanning systems has started to be felt as exemplified by the cotton farmers in Chunya district in the southern highlands [Box 6]. 

Animal drawn ripper tines have only been recently introduced in northern Tanzania by the Land Management Project (LAMP). Also an animal drawn ripper planter developed by SEAZ Agricultural Equipment of Mbeya is currently under test and farmers in Chunya and Shinyanga have shown some interest. 

Status of soil Conservation Practices 

The ability of the soil to support a good crop and increased production depends on the soil type and how the soil is treated and managed. Inappropriate land use activities often cause changes in the ground cover and soil physical and chemical conditions, which in turn contribute to accelerated erosion and declined soil fertility. Therefore, in order to improve the soil productivity, appropriate land management measures should be developed, in order to sustain agricultural production. These measures should include nutrient input supply and recycling, soil structure improvement, increased water infiltration, erosion control and organic matter maintenance. In Tanzania, a number of soil conservation measures have been undertaken since time immemorial. These measures include agronomic, vegetative, and structural or mechanical. In many cases, with the exception of structural measures, they have been practised by the farmers voluntarily in some localised areas in the country and they are popularly known as traditional or indigenous methods. 

Agronomic Measures 

Agronomic measures are those biological or cultural practices such as mixed cropping, crop rotations, contour cultivation, mulching and manuring that promote soil and water conservation. They are generally of short duration, are repeated routinely each season and are usually associated with annual crops. They do not lead to changes in the soil profile (Thomas, 1997). The effectiveness of agronomic measures arises from the control of rain splash erosion, the improvement of soil structure and fertility and the reduction of runoff rate and volume. 
 

Biological Practices 

Crop rotation 

This entails growing of different crops in sequence. Crops vary in their nutrient demands, susceptibility to pests and ability to control erosion. Good crop rotations facilitate the conservation and addition of humus, restoration of soil structure and fertility, control of erosion and reduction of pests and diseases. Rotations that are done are mainly between grains and legumes especially for maize and beans. The method is mainly practised in the highlands. Sometimes fallow is included in the rotations. 

Inter-cropping / Mixed cropping 

Inter-cropping is the growing of two or more crops simultaneous in the same field in the same season. When there is no distinct row arrangement, the practice is known as mixed cropping. Both inter and mixed cropping are widely practiced in the country. In most cases grains and leguminous crops are mixed. The fast growing legumes provide soil cover early in the season, shielding the soil form the impact of raindrops. Leguminous crops, fix nitrogen too and thus help to maintain fertility. Mixed cropping has an added advantage of spreading risks. Mixed cropping is widely practiced in Tanzania. Other practices like relay cropping, alley cropping and strip cropping are rarely used. 

Cultural Practices 

Most of the cultural practices in the country are used both for erosion control and fertility enhancement through in situ composting of biomass. 

Ridging 

As described earlier, ridges control erosion when they are made across the slope and improve fertility through the incorporated vegetation in ridge construction. They are commonly found in the highlands and the semi-arid regions in central and lake zone regions. 

Pitting 

The system controls erosion and conserves moisture while improving soil fertility by the composting of the buried crop residues and grasses. It is mainly practised in the Matengo highlands in southern Tanzania. 

Mound farming 

The practice is mainly for fertility improvement through in situ composting of the buried vegetation. The system is mainly found in south west Tanzania. 

Trash lines 

This is a widely practised system on sloping hillsides with maize stover used as trash to make the lines. Trash lines help in slowing down the flow of runoff and trapping eroded soil. The technique is used both for erosion control and fertility improvement. 

Contour farming 

This is a system where all field operations such as ploughing, harrowing, planting and weeding are done along the contour, rather than up and down. It is a common system in the country which is used for erosion control. 
  

Vegetative Measures 

These are techniques which involve the use of perennial crops, grasses, shrubs or trees. They include agroforestry technologies. They are of long duration and sometimes lead to changes in slope profile. The system is more prominent in the coffee-banana farming systems in Kilimanjaro and Kagera regions. 

Structural Measures 

These are permanent features formed from earth, stones or mansory, designed to protect the soil from uncontrolled runoff and erosion. Mechanical conservation structures have not been popular with farmers as they were mainly associated with colonialists who imposed the practice on them. With time many farmers are now realizing their importance. The most common conservation structures which are promoted and used, are the contour bunds, bench terraces and cut-off drains. 

Contour bunds 

This is the most popular system promoted by the Soil Conservation and Land Use Planning Unit of the Ministry of Agriculture and Cooperatives. Contour bunds are earth banks, 1.5-2m wide, laid across the slope, approximately along the contour line. In most cases they are constructed using the fanya chini process [Box 7]. Contour bunds are very effective in reducing runoff and in conserving moisture and are suitable for slopes of I- 1 2%. Napier grass, guatemala, elephant grass or sometimes leguminous trees such as Leucaena spp. are planted on the bunds to stabilize them. The fodder is used for feeding the animals. 

Bench terraces 

Bench terracing is a means of converting a sleep slope (up to 55⁰) into a series of level steps formed on the contour and separated by embankments. Bench terraces control erosion by reducing the slope of the cultivated land and increasing the infiltration of water. Many farmers in the coffee growing areas are practicing bench terracing. However they are labour intensive and reduce the net area available for cropping, which might be a disincentive in adopting the practice. 

Cut off drains 

These are graded channels constructed across a slope. They are designed to intercept surface runoff and convey it safely to an outlet such as a waterway. A cut off drain is always found along the upper boundary of a farm. The system is being encouraged by the Soil Conservation and land Use Planning Unit in areas with steep slopes. 
  

Policies on Soil Conservation and Implementation 

Pre-Independence Era 

As observed earlier, soil conservation efforts in Tanzania started long ago, some 200 years back. These were voluntarily practised by a number of tribes in different parts of the country. Biological practices such as fallowing, mixed cropping and crop rotation were common. Cultural practices such as ridging, pitting and mounding were also practised though not as extensively as the biological ones. However with the increase in population, pressures on the land started to be felt. Fallow became shorter and in some cases disappeared all together, being replaced with continuous cultivation of the same plots. Encroachment of marginal areas such as steep slopes and riverbanks became rampant. Deforestation both for lateral agricultural expansion and for fuel wood to meet the domestic energy requirements increased. This, together with overgrazing, as a result of increasing livestock numbers, left most areas bare. Modernization of fanning, which was accompanied by conventional tillage methods both by tractors and oxen exposed the soil to hazard of erosion. Thus, agronomic measures alone could no longer conserve the soil which resulted in massive land degradation. This lead to the colonial government instituting country wide soil conservation measures which were in line with the global environmental developments on erosion (Box 8). 

However most of these soil conservation measures became very unpopular with the farming community as they encouraged coercion instead of persuasion. The measures were mainly mechanical erosion works while traditional and agronomic/biological practices were completely neglected. Forced labour was used to implement the conservation measures and sometimes they were used as a punishment for poll tax evasion or disobeying local chiefs. This created a negative attitude towards the schemes. Too many rules were imposed which lead to many people being fined and imprisoned for contravening them. Also they were discriminatory applying to subsistence farmers only while favouring settler and commercial farmers with resources. No wonder with the coming of independence in 1961, the whole system collapsed, orders, restrictions and regulations concerning soil conservation broke down. 
  

Post Independence Era 

The collapse of the soil conservation system, coupled with an increase in livestock and human population lead to accelerated erosion and severe land degradation in some parts of the country such as Kondoa district in central Tanzania in the late 1960s. Consequently the Government initiated some steps to curb the situation from 1968 onwards [Box 9]. 

With the assistance of international donor organizations, the Government is now engaged in a number of projects in order to check further land degradation and to conserve the natural resources. 

Numerous ministerial departments are involved in these programmes. They include: 

• The Department of Environment and the National Environmental Management Council in the Vice Presidents Office.
• The Soil Conservation and Land Use Planning Unit and the National Soil Services in the Ministry of Agriculture and Co-operatives.
• The Department of Land Survey and Mapping, the National Land use Planning Commission and the Department of Land Administration in the Ministry of Agriculture.
• The Forestry division in the Ministry of Tourism and Natural Resources.

However most of these land conservation programmes tend to be unisectoral in nature and implementation because of the poor co-ordination and weak inter-ministerial linkages. This has sometimes lead to low adoption of the practices and little impact in the farming communities. 
 

Conclusion and Recommendations 

Appropriate tillage methods and practices are essential in conserving the soil for sustainable agricultural production. Traditional or indigenous methods play an important role in soil and water conservation in Tanzania. However they are labour intensive and difficult to mechanize thus limiting the cropped area. 

• Agronomic / Biological practices should be complemented with structural measures in the extensively practiced conventional flat cultivation system as risks for soil degradation are high. As with mechanical power, serious consideration should be put on the development of animal traction based conservation systems as the use of draught animals is increasing in the country.
• Soil conservation measures should be integrated into the normal croplivestock husbandry practices in the fanning systems, as benefits of such measures are usually of long term. Emphasis should be put on the land husbandry concept where care and improvement of land resources come first, and control of degradation is part of the caring and improvement process.
• Participatory approaches are necessary in ensuring the adoption of the technologies, as top-down approaches have failed even when coercive legislative laws were imposed. In the process, extension and training, both of village extension workers and farmers should be emphasized, with inclusion of soil conservation messages in the normal extension package.
• Land tunure and inheritance rights should be reviewed so as to reduce the "free for all" attitude. This will increase personal responsibility on the land encouraging investment on conservation activities.
• Inter-ministerial linkages should be strengthened so as to co-ordinate and concentrate efforts of the various key players including Government departments, International Organizations, NG0s and the farmers.

References

^{Senior Agricultural Engineering Research Officer, MARTI Uyole, PO Box 400, Mbeya, Tanzania.}