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Humus formation in shallow pits and soil analysis


The formation of humus derived from human excreta using soil, wood ash and vegetable matter like leaves lies at the centre of the principles being promoted in this work.  Eco-latrines are thought of, more as units which form humus, rather than disposal units for human excreta. There is a change of emphasis. In the case of the Arborloo, humus is formed in shallow pits in preparation for tree planting. In the Fossa alterna humus is formed in shallow pits in preparation for excavation and later mixing with other soils and application to gardens. In the case of urine diversion methods (e.g.Skyloo) humus is formed by first adding dry soil, wood ash and the paper used for anal cleansing to the faeces in the bucket and then adding further soil to this mix once it has been transferred to a “secondary composting site” like a split cement jars or shallow pit. Every attempt, in all three cases, has been made to place the use of soil at centre stage. The soil is at the heart of the recycling process required for humus formation.


The composting process


The composting process, whereby the various ingredients are converted into humus is an art as old as time.  Compost is a mixture of organic materials such as leaves, soil, manure, kitchen scraps and weeds etc which decay into a dark crumbly mass called humus. Micro-organisms in the “compost” need carbon, nitrogen, phosphorus and potassium to make the enzymes that transform matter into the humus that feeds the plants. So a variety of materials are needed. The composting process needs air so that aerobic bacteria can get to work. Moisture is also required - all natural biological processes require moisture. The addition of humus to soil improves the ability of the soil to retain moisture. The humus also lightens and aerates heavy clay soils and also makes light sandy soils more spongy and absorbent. Growing plants gather the minerals of their choice from the soil. When plants are composted at the end of their life, they return these materials to the soil to be dispersed. The nutrients in compost are also needed by soil organisms which in turn provide plants with more nutrients. The more varied the composting materials, the greater the variety of nutrients the compost will contain. Thus when the soil is added to the human excreta to promote the conversion, it is wise to add other materials in addition to the soil like leaves, leaf mould, vegetable matter and even some existing compost which contain living materials. For the same reason the soil that is added should ideally be a living fertile soil and not an inert sand or dust.  The addition of wood ash turns the mix slightly more alkaline which helps the biological process. Wood ash also provides potash. The ash also helps to cover fresh faeces with a layer which reduces surface moisture and odour on the freshly deposited faeces. This process makes the faeces less attractive to flies.


Change in the faeces


When faeces come into contact with soil they slowly change colour from the yellow/brown to a much darker brown. The texture also changes considerably. The slimy material changes into a more crumbly soil like material, possibly because the water in the excreta is absorbed by the soil and the soil takes up space that was hitherto occupied by excreta. The smell also changes considerably from a foul odour to a pleasant aroma. The speed at which these changes take place depend upon the conditions in which the mixture is placed. The change is more rapid as the temperature rises within the range that beneficial bacteria and fungi and also animal life can function. That is within the range of normal ambient temperatures. If a mixture of soil and excreta are placed within a small jam jar for instance, colour changes at the interface between faeces and soil can be noted within days. Depending on the nature of the soils, plugs of faeces will be changed into plugs of soil over a period of a few months.  In demonstrations of the conversion, it helps to place identifying objects (such as a metal keys) into the raw faeces to establish that the “new soil” was indeed faeces at an earlier stage of its life.


How the “new soil” becomes slightly more crumbly and even contains small inorganic particles retaining the properties of the soil thrown into the pit, can possibly be explained by the fact that the base soil added to the cement jars or shallow pit invades the space occupied by the faeces/excreta. This is because the excreta is mainly water. Even the faeces contain 70 - 80% water, so this liquid fraction of the faeces will be slowly absorbed into the soil. Consequently there will be physical invasion of the  space previously occupied by faeces by soil. Only about 30% of the faeces will be solid matter..


Also the soil contains many living organisms, and whilst the bacteria and fungi are unable to move themselves, insects, earth worms and many other forms of micro fauna do move about within the soil and also at the soil/excreta interface. Mites can occupy faecal layers in pits. Slowly a mixing of the ingredients takes place. Also the roots of plants invading the soils close to the interface will bring oxygen and much biological activity into the rhizosphere zone around the roots. Thus just as the soil itself is a biological workhouse, so the interfaces between the soil and excreta is subjected to this activity and this activity invades the excreta as time passes and the conversions of excreta into “new soil” takes place.


Obviously urine is made almost entirely of water and what part does not drain away from a shallow pit will itself be absorbed into the soil which is added - when some of the nitrogen present will be absorbed by the soil. Soil tests of eco-humus reveal however that the increase in nitrogen level is minimal. Where urine leaches further into the surrounding soil, the nutrients will be held and may later become available to trees which are planted there, as in the case of the arborloo.


Humus formation in shallow pits


The unique challenge of the Fossa alterna is to achieve the conversion of human excreta into a safe and valuable humus well within the time it takes for the second pit to fill up. If this can be achieved then a true alternate use of the twin pits can take place indefinitely. Most families of about six persons will fill a 1.2 metre deep pit in 12 month or more with a combination of faeces, urine and paper (anal cleansing material) together with soil, wood ash and leaves. Thus in the family unit, it is essential that the conversion of raw excreta into a pleasant humus must take place within a 12 month period. Against this challenge, one must not forget that in a normal pit latrine, the conversion of excreta into humus takes many years - even up to one decade, depending on the condition of the pit.


The conversion of excreta in such a short span of time becomes possible simply because extra ingredients are added to the pit contents which promote the formation of humus. Also simplicity is of the essence - hence the use of the drop and store method as in the pit latrine. A complicating factor is that urine is added into the pit and this must either be allowed to drain away or better still combine with the soil added and hence improve, albeit minimally,  the nutrient level of the material formed within the pit. In practice much urine will drain away from the pit, but some will be retained in the soil added to the pit. For this reason the volume of soil added must be about equal to the volume of solid excreta added - a small mugful after every deposit of solids. To improve the texture, as well as the nutrient level of the final product - it greatly helps to add plenty of leaves as well.


The ideal combination of pit contents in the Fossa alterna is faeces, urine, fertile soil, wood ash and leaves. It was this combination which was first used in the prototype built and tested in Woodhall Road, Marlborough, Harare in  1999 (see Ecological Sanitation in Zimbabwe. Vol. 1). The excreta occupied about half the volume of the pit (the remaining half being soil and leaves) which had a total occupied capacity of about 212 litres (the pit was quite shallow). A pleasant humus like material was formed within 4 months and excavated by the writer and used in various ways in the garden. Since that time large numbers of Fossa alterna pit have been excavated and their contents recycled.


Factors which will assist the conversion of excreta into humus within shallow pits.


In the standard pit latrine, excreta is added together with all manner of other articles as the pit usually serves as a dumping ground for all sorts of garbage. Most of these “dumped” articles are inorganic waste such as plastic, rags, bottles etc - they are of no value to the composting process. Also whilst wood ash may be added to counteract odour and fly nuisance in a small percentage of cases, soil is rarely if ever added to pit latrines. In the standard pit latrine, a solid mass of excreta builds up which digests very slowly and inefficiently in the deeper pits. These pit latrines are not eco-latrines. The contents will take years to convert into humus, particularly if the pits are lined with bricks or cement rings where contact with the soil is limited and drainage of the liquid fraction is confined to the base, which may become plugged.


Shallow eco-pit latrines must operate in a completely different way. By adding meaningful quantities of soil and wood ash, preferably in combination with liberal quantities of leaves, and by strictly avoiding the addition of non bio-degradeable materials (rags, plastic, bottles  etc), everything changes as far as the conversion of excreta into humus is concerned. The process becomes efficient and “ecological.” That is a major difference between the  processes seen in eco-latrines using shallow pits and the standard “deep” pit latrine.


Various observations made in the field of several sites where Fossa alternae are being tested reveal the following.


1. Add plenty of soil, ash and leaves


The ratio of soil/ash/leaves etc to excreta can be increased by adding compost or leaves to the base of a pit before it is put to use. The addition of a layer of leaves or compost to the base of a pit will help to increase drainage as the raw excreta is less able to seal off the fresh soil interface as it will sit on a more spongy aerated material filled with biological ingredients. Here the conversion of faeces can take place more quickly. Also the ratio of soil/ash to excreta can be increased further by adding more fertile soil /leaves once every few months to the pit. The higher the ratio of additional materials (fertile soil/ ash/leaves/ etc), the better the conversion will be and the higher the quality of the final product will be.  Thus it pays handsomely to add a good mix of additional material to the pit in addition to excreta alone. Once the pit is nearly full and the superstructure moved, the pit contents are topped up with soil to increase the soil/excreta ratio further. It also helps to ram in extra soil and leaves into the pit contents with a pole prior to topping up the pit contents with soil. This will raise the percentage content of soil and also help to mix the ingredients. Also the addition of leaves placed on top of the final soil layer may help to reduce “caking” of the soil exposed to harsh sun and rain. Some clay like soils added on top of the organic layer may actually act like a seal, cutting off the processes below from the air above. Soil added to these shallow pits should remain loose and crumbly and allow air to pass and micro fauna to survive. 


2. Add fertile topsoil if possible.


It  is best to add fertile topsoil to the pit, as this will have a higher contents of living organisms, and lead to a more fertile humus being formed, compared to adding poor soil. However it is not always possible to find good topsoil near an eco-latrine. What ever the soil added, the final resulting soil will be considerably improved in terms of nutrient levels. However the texture of the eco-soil will resemble the type of soil added. Sandy soils will produce an eco-soil which has a sandy texture, grey soils turn into darker grey soils. Clay soils will result in a soil which is more clay like. Crumbly soils with good texture make the best eco-humus. The best eco-soils are made by adding a fertile soil in combination wood ash and plenty of leaves. The leaves convert into leaf mould - and this is an excellent material for improving soil texture and also nutrient levels in the final product.


There is also merit in returning some of the eco-soil which has been dug out of the pit back into the system.  In some cases this may be the only good soil available. Soils dug out of newly excavated pits may be low in fertility and many soils taken from the rural areas are often quite infertile. The proportion of humus and other biological components of the soil decreases with depth beneath the surface. There is a good case for combining this less fertile soil with excreta in the shallow pit, thus improving texture and nutrient level of even almost barren soils (see later). Soils added to eco-pits must be dry and collected during the dry season and bagged and stored in a dry place, especially during the rains. 


3. Good pit drainage


This is essential in the eco-pit. A proportion of the urine added will be absorbed into the soil which is added from above and a proportion will drain through the side walls or the base or sides of the pit. Humus formation does not take place in flooded pits or even pits which are wet or full of liquid excreta. Thus the use of washing water should be limited. A vent pipe helps to take some of the moisture away from the pit by removing humid air held in the pit. It is true that a vent will allow some rain water to enter, but this will be minimal. It should also be noted that the capacity for seepage is reduced the deeper down and more compact the soil becomes. Thus eco-pits are best dug shallow - about one metre, 1.2 m  or 1.5m maximum.   Improved drainage characteristics are seen in shallow pits which are only partly lined with bricks or cement. The best drainage characteristics are seen in pits fitted with a ring beam.


There is a dilemma on pit linings. If the pit is fully lined with bricks (apart from the base) the drainage potential of the pit is reduced. Fully lined pits have the advantage that they make the pit very stable and excavation may be carried out repeatedly. Partly lined pits are less stable but provide a larger surface area of soil available for drainage. Also the area of soil in proximity to excreta is increased in partly lined pits which also help the conversion process. This is because microorganisms in the soil can work directly in the excreta and also because root hairs in the soil can invade the pit contents.


However, the Arborloo, being a latrine on the move will require only a ring beam protecting the pit. The latrine structure is light and portable. This also applies to the Fossa alterna in almost  every case. So in practice the ring beam method serves this method too.  The decision to line partly or wholly may depend on soil type. In very loose sandy soils a full pit lining is recommended and may be essential. But in such soils the drainage characteristics will be good. Harder or clay like  soils, which drain less efficiently, will not need full linings, especially if the superstructures are light.


The “ring beam” concept is an excellent option in most situations. If well made in concrete it can provide good service for many years in combination with a concrete latrine slab. If the superstructures are light, as they usually are with eco-latrines, it will provide good support in almost every situation and protect against pit erosion caused by flooding. Concrete “ring” beams are now being tested even in sand veld conditions. It is possible that after a few years the wall lining of an unlined pit will change shape and erode. But then all the parts of these simple eco-latrines are portable. A new site can be chosen. A single ring beam uses a quarter 50 kg bag of cement to make. Two concrete ring beams, a concrete slab and a sturdy pedestal can be made with a single 50 kg bag of cement with enough left over to make a low cost vent pipe.


4. Observe rate of filling


The rate of filling is also an important factor The aim should be to fill one of the twin pits of the Fossa alterna in about one year or more and to change pits once a year so that the various ingredients have a minimum of 12 months  to fully convert to humus. This will be longer than is normally required and provides a good safety margin.  The extra time will also allow full drainage even of pits which may have been partly flooded. Excavation is best carried out during the dry season and even better towards the end of the hot dry season which in Zimbabwe will be between September and November. Pits in which humus is being formed are best covered with a wooden lid (or tin or asbestos), especially during periods of heavy rain. This avoids pit flooding which does not help the composting process at all.  


The filling rate will depend on the number of users, the size of the pit and the volume of additional materials (soil etc) added to the pit. The addition of an equal volume of soil/ash to the excreta does not mean that the filling rate of the pit is doubled. In fact faeces holds between 70 - 80% of its volume in water. The liquid fraction which constitutes almost the entire bulk of excreta will slowly be absorbed into the soil and the solid fraction that remains converted by the soil organisms. Bulky leaves, which should be added generously to eco-pits soon condense in volume considerably once they have composted. They may only occupy one quarter or even less of their original volume. The importance of adding leaves cannot be overemphasised.


If the rate of filling is high where the number of users is high, the double pit system may not be adequate. If for instance the pit fills in 6 months due to overuse, this will upset the balance required between pit filling and pit content decomposition. Under these conditions, normally a large family or family with lodgers or a communal system, the Fossa alterna concept will only work if extra pits are dug and protected.  It will be necessary to dig and protect at least one extra pit and possibly two extra pits. The portable structure is then rotated around the pits in sequence. What is essential is that each pit is allowed at least 12 months to compost before it is dug out.


5. Spread out pit contents


In dry conditions, piles of excreta (made less mobile by the addition of soil/ash) will build up directly beneath the squat hole or pedestal into turrets (called “turretting”). Under these conditions it is desirable for the owner to use a pole passed through the squat hole or pedestal to  try to level off the pit contents during the filling time. Where central mounds of excreta form, added soil/ash tends to fall to the sides of the mound. Good distribution of soil/ash within the excreta is then not easy. The flattening out of the pit contents is an important part of routine management of eco-latrines using shallow pits. It is desirable to add some water down the pit from time to time to keep the pit contents moist.  


6. Prepare for excavation


Ease of excavation of the humus will be improved considerably if it is just humus which is extracted and not all sorts of other ingredients that are often added to pit latrines. Thus the owners should be encouraged NOT to add rags, bottles, plastic bags or plastic sheets, cloth or cloth rags, wire, rubber articles, glass, string, fishing line etc to the pit, as these do not compost at all, and will have to be removed if the humus is to be retrieved. This makes the excavation process tedious and unpleasant. The addition of leaves which makes the final product more crumbly, also helps the final excavation.


7. Best at family level


Because good management lies at the heart of an efficient eco-latrine, the unit should ideally be used at the family level. Six persons is an ideal number. Communal latrines pits may have to absorb considerably more urine and the regular addition of soil/ash etc may not take place. These factors must be taken into consideration and do not favour efficient humus formation. In the case of communal latrines, if the addition of soil/ash is not regular, an attendant must add larger volumes (a sack full) of fertile soil/leaf letter etc from time to time, say every month or two to the pit contents. This will also have the effect of raising the soil ratio and helping to absorb excess moisture derived from urine. However eco-san does not favour the use of communal latrines.


When the pit of the Fossa alterna is nearly full, the slab and structure are moved over to the second pit.  The “full pit” is then topped up with a generous layer (100 mm+ ) of good soil, leaves etc  and left to digest and compact for about one year. As indicated earlier, it helps a great deal to ram in extra soil with a pole at this stage to increase the soil/excreta ratio. Ramming in extra soil is known to hasten the composting process as it helps to distribute the soil with the excreta, especially in the upper layers. A note should be taken of the date of pit change. Aim for a one year cycle.  “Testing” of the ingredients can be done with a “dip stick” - a steel rod plunged in to the mass. After extraction from the mass the rod  can be sniffed to see the state of conversion. A sweet smelling aroma will be recognisable when the process is complete. The materials directly under the squat hole or pedestal will be more compact and have a higher proportion of excreta than the material to the side which may be looser and have a higher content of soil etc.


8. Use second pit of Fossa alterna


Since leaves, compost and fertile soil are desirable additives to the pit, it makes good sense to the use the second pit of the Fossa alterna as a “composting pit” during the period when the first pit is being used and filling up for the first time. An excellent leaf mould can be made in the second pit by adding leaves followed by thin layers of soil and adding water. Keep adding leaves as the leaves in the pit compact. At the end of the year a most valuable leaf mould can be dug out which will add great fertility to the garden in general. The second pit can also be planted with vegetables flowers or even comfrey. Thus once the second pit has been dug (which should be at the same time as the first pit is dug) all effort should be made to gather fertile soil, leaves, grass, weeds, compost, etc to add to the pit. Soil and vegetation should be layered to make a good healthy humus. This humus  will be valuable and a good source of fertile soil to add to the pit filling with excreta. It will also be a good soil to cover over the composting pit once nearly full.


9. Learn about composting - it pays


It is very wise to learn the art of composting at the same time as the learning the method of managing the eco-latrines. Thus a knowledge of the worlds of organic farming and eco-sanitation can be learned together.


These various factors also apply to shallow pits used for tree planting (with the Arborloo ). But the conversion of excreta into humus is not so critical, as the pit contents of the Arborloo are never handled. However trees will grow far better in humus compared to poorly converted excreta. The fact that trees do grow in conventional pits is probably due to the fact that the tree is planted many months or even years after the pit is abandoned. Trees do also germinate naturally in abandoned latrine pits, possibly in the layers of soil or compost that accumulate there.


For the newly initiated to the eco-san principle, it makes good sense to learn all about compost making, the value of leaf mould and the advantages of the use of humus and compost in general garden practice. Leaf mould and compost can be made in heaps, bins, brick surrounds, half drums, bags and other containers. Sites of leaf accumulation under trees or hedges can be found in the back garden or homestead. Old compost heaps can be located and used. It pays to look around and find such pockets of fertile soil. Even in barren environments, such pockets may be found if eagerly looked for.


Soil is an excellent converter


The use of the Fossa alterna in particular demonstrates that the soil is an excellent converter of human excreta into “humus.”Also that a combination of human excreta, soil leaves and wood ash when processed under the right conditions makes an excellent “humus” which is rich in nutrients. A valuable product is made just by combining soil and excreta, but it is better still if leaves and wood ash are added. The addition of wood ash to the mixture assists by acting as an absorbent of moisture, increasing the potash component and also making the reaction slightly more alkaline, which may help the biological process. The addition of other vegetable matter such as leaves, compost, organic kitchen scraps helps also by improving, not only the final fertility of the humus but also its physical characteristics. Nutrient levels are improved and also the crumbly texture of the final product. These are important characteristics of the soil.  Plants grow best in soil which is fertile, holds water and is living.


For those families who are using the Fossa alterna, it will take two years to get a result - ie to dig out humus from the pit. This is a year to fill and a year to make humus. The educational process can be accelerated by demonstrating the conversion of excreta (particularly faeces) in jars when the process is complete within 4 months. It is important for people to witness first hand the miraculous conversion of excreta into humus. One of Nature’s marvels.


Nutrient levels in extracted Fossa alterna “humus” - an analysis of soil  


After one year of composting in a shallow pit a mix of human excreta, soil, ash and leaves will completely change into a pleasant humus. However this ideal mix may rarely be achieved in practice. It is most likely that any poor surrounding soil will be added rather than a mix of soil, ash and leaves. Even in the trials carried out by the writer, it is rare indeed that the ideal mix is added, the norm being soil alone with very few nutrients. Thus the results below are a result of the combination, not of the ideal mix, but of the mix which has resulted in practice - just soil and excreta. The results reveal however, that even when just soil and excreta are added and allowed to compost, the nutrient level in the resulting humus is very high, and infinitely higher than either the soil that went into the pit or the surrounding soil.


The figures below show the pH and levels of Nitrogen (ppm - after incubation), Phosphorus, (ppm) and also Potassium, Calcium and Magnesium (ME/100gms.) in ten samples of the Fossa alterna taken from the Friend Foundation, Harare. Later these figures are compared to various naturally occurring soils in Zimbabwe (in the Harare area) and also to samples of humus excavated jars used to process faeces derived from the urine diverting Skyloo.  


Examples of Fossa alterna soils


Note all the sample below are a mix of human excreta and soil only. In these early experiments the ideal mix of soil wood ash and leaves were not added. Also very often poor soils were added rather than richer more crumbly soils. Thus the humus formed was a very basic type and by comparison with what is possible, is of a lower quality especially in terms of soil texture.


The final quality of the eco-humus is much related to the type of soil put in. This means that the final product is superior in every way if a humus like rich crumbly soil is added. However in none of the samples  analysed was this regime followed. Partly because of the experimental aspects, but partly because adding poor soil is closer to the norm. However in time it is hoped that the users of the Fossa alterna  will realise the befit of adding good soil in combination with wood ash and a generous helping of leaves too. Time will tell.





Soil source                                                      pH            N            P            K             Ca            Mg      


Fossa alterna Sample 1.                              6.5            269            317            1.59            20.77            11.28

Sample 2.                                                        6.1            246            330            4.64            5.53            5.41

Sample 3.                                                        6.6            174            374            3.74            8.59            5.74

Sample 4.                                                        6.2            222            422            2.22            3.60            3.57

Sample 5.                                                        6.5            319            196            3.26            13.70            7.26

Sample 6.                                                        7.7            316            242            3.84            9.96            3.42

Sample 7.                                                        7.6            355            258            7.14            8.97            6.26

Sample 8.                                                        6.9            305            230            6.65            12.00            10.32

Sample 9.                                                        7.7            354            257            9.18            9.26            3.46

Sample 10                                                       6.3            197            299            2.94            26.64            4.77


Mean value (Fossa alterna)                                   6.75            275            292            4.51            11.89            6.14


Examples of Naturally occurring top soils


Soil source                                                      pH            N            P            K             Ca            Mg      

Harare (Tynwald 1.)                                        6.1            32            68            1.59            6.42            4.02

Harare (Tynwald 2.)                                         5.5            27            5            0.29            10.23            4.11

Harare (Marlborough Vlei)                             5.1            72            30            0.99            22.88            18.06

Harare (Epworth 1)                                          4.0            18            9            0.08            1.46            0.32

Harare (Woodhall Rd)                         6.2            27            32            0.63            9.68            2.30

Ruwa (Knuth Farm 1. veld)                            7.5            30            30            0.12            3.79            0.56

Ruwa (Knuth farm 2. pit soil)             5.1            14            23            0.01            1.12            0.48    

Ruwa (Knuth farm 3.- garden)                         6.7            96            143            0.73            15.23            1.96

Harare (Epworth 2)                                          4.1            23            54            0.07            1.72            0.50


Mean value (local soils)                           5.5            38            44            0.49            8.05            3.58




Mean value (local soils)                           5.5            38            44            0.49            8.05            3.58

Mean value (Fossa alterna)                                   6.75            275            292            4.51            11.89            6.14

Mean value (composting jar-Skyloo)            6.72            232            297            3.06            32.22            12.06  


These figures show clearly how human excreta when mixed with topsoil can produce a product with significantly more nutrients than most naturally occurring soils. Enough nutrients in fact to mix in equal proportions with existing topsoils to enhance the final product sufficiently to make viable vegetable production possible without further fertilisation.


However because there will be a variation in the texture and nutrient level of both Fossa alterna humus and the soil which is mixed to it the final result can only be judged by the growth of plants in the medium. For instance, if very poor soil has been added to the Fossa pit with no other ingredients and the soil to which the Fossa soil is added is very sandy and poor, the final soil, while significantly improved may not contain the full range of nutrients required by the plants. In such cases, observations can be made on the plants as they grow. Also there is a variation in requirements. Spinach, for instance, copes better with low levels of nutrients compared to Rape. Where there is doubt, the addition of liquid plant foods including those derived from urine can enhance the growth of vegetables even further.


Enhancement of deposited soil


The quality of the humus derived from Fossa alterna pits varies depending on what extra ingredients are added to the pit in addition to excreta (urine and faeces). The texture, nutrient levels and water holding capacity, for instance, are improved if fertile topsoil and leaves are added in addition to the excreta. The texture of the excavated humus is similar to the soil added to the pit. However even when poor soil is added alone significant improvements can be achieved in nutrient levels as the results below show.


Example 1.


Soil source                                                      pH            N            P            K             Ca            Mg      

Soil added to FA pit            (cemetery topsoil)            4.9            50            13            0.18            2.95            0.78

Humus removed from Fossa alterna                       

Sample 1                                                         6.2       222      422      2.22     3.60     3.57

Sample 2.                                                        7.7            316            242            3.84            9.96            3.42


Example 2.


Soil source                                                      pH            N            P            K             Ca            Mg      

Soil added to FA pit (kennels site..pit soil)            5.5            27            5            0.29            10.23            4.11

Humus removed from Fossa alterna

Sample 1.                                                        7.6            355            258            7.14            8.97            6.26

Sample 2.                                                        6.9            305            230            6.65            12.00            10.32

Sample 3.                                                        7.7            354            257            9.18            9.26            3.46


Urine inclusion and urine diverting


In one experiment a urine diverting slab was placed over the pit. The resulting soil formed was compared to soil formed in a pit where both urine and faeces were added.


Soil source                                                      pH            N            P            K             Ca            Mg      


Soil added to FA pit                                      5.5            27            5            0.29            10.23            4.11

Humus removed from Fossa alterna

Sample 1. (Urine inclusion)                                7.6            355            258            7.14            8.97            6.26

Sample 2. (Urine diverting)                                6.9            305            230            6.65            12.00            10.32


Note that levels of NPK are slightly elevated in the eco-humus in which urine and faeces are added compared to those in which urine is diverted elsewhere.


Mixes of Fossa alterna humus and other soils


Enhancement of nutrients by combining with poor soils


Fossa alterna humus, resulting from a mix of faeces, urine, paper and soil (and preferably leaves) can be mixed with less fertile soils to make a planting medium in which vegetables grow well without additional “fertilisers” being required


The following figures show how the Fossa humus humus can elevate the nutrient levels of very poor soil when a 50/50 mix is made between barren soil and eco-humus.





Example 1.

Soil source                                                      pH            N            P            K             Ca            Mg      

1. Knuth soil                                       6.1            32            68            1.59            6.42            4.02

2. FA orchard                                                 6.6            174            374            3.74            8.59            5.74

Result of 50/50 mix mix .                                   6.2            91            337            1.45            4.58            2.29


Example 2.

1. Knuth soil                                       5.5            27            5            0.29            10.23            4.11

2. FA (1a site)                                                  6.5            319            196            3.26            13.70            7.26

Result of mix .                                               6.4            91            247            0.88            3.05            2.49


Example 3. (33/33/33 mix)

1. Fossa alterna soil (FA1a site)                  6.5            319            196            3.26            13.70            7.26

2. Woodhall Rd soil                                                6.2            27            32            0.63            9.68            2.30

3. Leaf mould                                     7.4            540            266            9.00            291            12.90

Result of mix                                                  6.8            331            294            2.10            12.14            5.85


Example 4. (50/50)


1. Fossa alterna (FA long)                            6.3            197            299            2.94            26.64            4.77

2. Epworth                                                      4.1            23            54            0.07            1.72            0.50

Result of mix                                                  6.4            78            356            1.01            15.75            1.78


What all these results show clearly is the significant level of nutrients held in human excreta. It is sufficient to change a worthless soil into one on which vegetables can be grown, if needs be, without further application of nutrients. Further work describes how the eco-humus is best used in agriculture and also the considerable value of urine when used as a liquid feed for plants.


What these experiments expose is the very considerable value of nutrients which are daily taken to waste, both in standard sewered sanitation and also pit sanitation. In Zimbabwe alone, half a million Blair VIP Latrines accumulate approximately 250 000 cubic metres of human waste per year which remains unused. Had the use of eco-latrines of the type described here this book been introduced and adopted years ago, the land would surely have been more fertile. 


Leaf mould formed in second pit during first year.


During the first year of operation, the second pit, which must be built at the time of the first pit, can be left empty and covered with a wooden lid. However it is wise to take advantage of the second pit during the first year of operation of the Fossa alterna.  One of the best methods is to make compost or leaf mould within this pit for the first year of operation. At a site in Epworth close to Harare, gathered leaves were emptied into the pit interspersed with thin layers of the local topsoil. Water was added from time to time. After 12 months the leaf mould was excavated and proved to be much richer in nutrient than the original soil. In fact plants grew in this leaf mould far better than in the original soil. The second pit acted like a pit composter and was well worth  the simple effort involved of adding leaves, soil and water. The following table shows the increase of nutrient levels in the leaf mould in the second pit compared to the local topsoil


The figures below show the pH and levels of Nitrogen (after incubation), Phosphorus, (ppm) and also Potassium, Calcium and Magnesium (ME/100gms.) in the leaf mould formed in the second pit of a Fossa alterna compared to the surrounding topsoil which was added together with local leaves.




Soil source                                                      pH            N            P            K             Ca            Mg      


Local topsoil (Epworth)                                4.1            23            54            0.07            1.72            0.50

Leaf mould from second pit                    7.7            81            130            1.86            9.31            1.88


These various results show clearly that the Fossa alterna is not only an effect toilet system but also a simple and effective unit for making both  rich humus for enhancing poor soils, and leaf mould for improving garden fertility. By combining all these advantages with its low cost and relative ease of use and management, the Fossa alterna may have much potential in the future development of low cost sanitation throughout many parts of the developing world.