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Organic Source of Nutrients: Some Simple Computations Please

UgandaCompostOptOne of the aspects in promoting more sustainable agriculture for smallholders in developing countries is to promote enhanced nutrient cycling by relying on organic sources of plant nutrients. Many host governments have highly endorsed the concept in the expectations of saving scarce foreign exchange required for importing mineral fertilizers. However, before getting overly committed to organic fertility with potential catastrophic consequences, there may be a need for a reality check on just how much organic material is available relative to that needed for the commercial crops essential for poverty alleviation, as well as the time and energy required to recover the organic nutrients relative to the dietary energy available to undertake the manual labor required to manage the nutrient recovery and energy derived from the potential increased agronomic production obtained from their use. To consider anything less than a commercial crop would most likely inadvertently promote poverty entrapment.

Zero Sum Movement

Except for legumes, blue green algae associated with rice paddies or other cyanobacteria that can fix nitrogen, organic sources of plant nutrients do not represent a new source of fertility. Instead they are simply a movement of a very bulky supply of mineral fertilizer within the confines of a production area in what can only be, at best, a zero sum effort. That is there will be a source that will be losing fertility in order to have a field gaining in fertility. Thus to be effective and sustainable it is necessary to determine the ratio of the area from which the organic nutrients need to be collected to the area over which they will be distributed. Typically, if a commercial grain crop such as rice removes 2/3rd of the mineral nutrients with the grain and this is sold outside the area or consumed, so the nutrients are lost, then, if the fertility recovery process is 100% efficient it will require 3 ha of crop residues to provide sufficient organic nutrients to fertilize 1 ha of future rice fields. How often will smallholder communities have this much land available for every hectare of cropped land? Likewise, if this land is also being cultivated, how will it be fertilized? It is also unlikely the recovery will be 100% efficient thus additional source land will be required to accommodate the inefficiencies in the recovery process.

Similarly, it was estimated in an AIT MSc thesis based in Mindanao, Philippines that it would require four animal units to provide sufficient manure to fertilize 1 ha of maize land. Each animal unit would require 1.25 ha of unimproved communal grazing lands. Thus, for animal manure to be effectively utilized as the primary source of fertility to produce a commercial crop, it would require 5 ha of unimproved communal grazing land for each ha of crop land. Is this really very practical in most smallholder communities?

Supply Of Organic Material

Similarly it is important not to over estimate the supply of organic material. For example, an organic farming project in a banana/plantain production area of Uganda, see photo at top of page, recommended one wheelbarrow of organic material for every square meter of area. How much does this amount to? A wheelbarrow is not a very precise measurement, but a good wheel barrow load of organic material might weigh 30 kg. Thus, for a hectare of organic bananas/plantains it would take 10,000 wheelbarrow loads at 30 kg each for a total of some 300 tons of material. How realistic is this? What plants or ecosystems can generate this amount of organic material and what area would be required? Wouldn’t this be a challenge even for Napier grass, the world’s largest forage producer? Over what distance would this have to be transported? Would some simple evaluations of the material available and computations of distance over which it has to be transported, etc, avoid some unrealistically embarrassing recommendations and projections? Would this be another case of what can be concentrated and demonstrated on small areas quickly losing its validity when extended to a full hectare or farm? Something the smallholders will quickly realize, much to the discredit of the promoters.

It should also be noted that most organic farms in the USA are usually not fully independent in nutrient cycling, but dependent upon neighbors with large animal operations to provide the manure that represents a net import of plant nutrients to the organic farm replacing what was removed with the marketed crop and allowing organic farms to be sustainable. An example would be the large 1000 ha organic vegetable farm outside of Fort Collins, Colorado that depends on a neighboring dairy farm that milks some 1500 cows on only 25 ha with has a major disposal need.

Time and Effort

Organic material tends to be very bulky to handle. Normally it will have a nutrient content of no more than 2 or 3% of the dry weight, and considerably less for fresh weight. Thus, it would require some 7 tons of dry material to be accumulated, transported, and distributed to obtain 200 kg of N, P, & K nutrients typically recommended for a commercial crop using mineral fertilizers. While this is substantially less then the 300 t suggested above, it still requires a substantial amount of physical effort and will require considerable amount of time, effort, and energy even if assisted with a bullock cart. Also, when will smallholder actually have the time to undertake this extensive operation? During the dry season when the soils are too hard to easily work?

The amount of time and effort required to utilize organic nutrient materials raised the question what is the Caloric Energy Balance for the manual effort? That is, what is the manual effort and the calories exerted to utilize organic nutrients relative to the calories recovered through the increased yields that result from their use? In developed countries where organic cycling is done under a luxury consumption diet and people are not dependent on organic cycling in their home gardens for their primary substance, it is possible to exert more calories than are recovered and write it off to needed and healthy physical exercise. However, for smallholders in developing countries that are deriving virtually all their substance from the production of the land, there are no surplus calories to exert as there may be a 50% deficit in available calories needed for a full day of diligent field work, and they have to recover more calories from the increased agronomic production than they exert in the organic nutrient recovery, or they could become even more severely under nourished, and either suffer an unhealthy weight loss, or become more lethargic in the afternoons as discussed in the oversights of the Basic Premise, and Options for Undernourished Farmers. An example might be that if a smallholder trying to recover organic nutrients is exerting some 280 calories each hour, then the increase in agronomic production will have to be 102 g of paddy rice, 77 g of maize, 175 g of cassava or 229 g of plantain per hour of effort just to balance the energy being exerted. Anything less would result in a negative energy balance and not be sustainable. It also has to be appreciated that this additional production, if actually obtained, will not be available until the next harvest, perhaps some six or more months in the future. Perhaps this is one reason why organic nutrient cycling in smallholder communities rarely goes beyond the homestead garden, and then more as a waste disposal problem than nutrient recovery effort.

Compost and Green Manure

While both compost making and green manure crops can be easily demonstrated in small plots or on a small scale near a homestead, they are most likely “non-starters” as a major source of fertility for the primary crop production land of smallholder communities. Compost requires additional labor intensive processing rather than simply incorporating crop residues to accumulate the material, compost it, redistribute it, and incorporate it into the soil. Most likely the energy exerted will exceed the energy derived from the additional agronomic yields, to say nothing about the delay between energy exerted and energy recovered. The latter happening only after the next crops are harvested, as mentioned earlier. Secondly there really is just not enough material available to effectively provide organic nutrients for an extended farm area to produce a commercial crop necessary to prevent the farmer from being entrenched in poverty.

Green manure is dependent on the surplus labor that as expressed elsewhere may be more myth than reality. It depends on farmers deliberately delaying basic crop establishment, when they are more likely hard at work on another parcel representing the Lack of Means for timely planting of all fields. Also, farmers working without reliable access to 65 to 80 HP 4-wheel tractors equipped with an array of disc plows have difficulty incorporating dry crop residue from the previous crop in a timely manner resulting in the common practice of burning the residue, will be even more challenged incorporating a fresh green manure crops that cannot be burned..

It also needs to be pointed out that all organic material applied to soils must be fully mineralized to inorganic ions by the soil microbes before being available to the plants. During this mineralization periods what mineral nitrogen is available will be hoarded by the microbes doing the decomposing and not available to the plant. This is well know as “nitrogen immobilization”. Thus once large amounts of organic material is applied to the soil, commonly the plants will turn yellow with nitrogen deficiency for a couple weeks while the mineralization process is completed. After this the plants will mostly recover.

Relying on Farm Animals – The Mobile Composters

Perhaps the most effective means of promoting nutrient cycling in smallholder communities is to rely on the herbivore animals to graze crop stubble, consume stacks of straw or other crop residues. At its very basic level composting and feeding crop residues to animals represent the same biochemical process, and, if looking at large scale compost making, will utilize the same material. That is the microbial breakdown of the organic material reducing the carbon thus the volume of the final material and concentrating the plant nutrients in the final product of processed compost or manure. For compost this is the soil microbes while for the animals it would be the ruminant or similar stomach microbes. However, the animals will actually go out an accumulate the material, quickly process it with their internal body heat expediting the process, and then redistribute it near where it is need. Perhaps the animals should be considered as mobile compostors Also, while you have to exert considerable time and energy in making and managing compost, the animals actually derive energy from the material. Thus feeding the animals will have some economic return associated with the value of the animals. Based on some recent discussion on a Lao based forum considering the value of water buffalo no longer extensively needed for draft, this could be estimated at about US$2/kg live weight gained or retained, if during the dry season when farm animals typically lose weight. However, it has to be noted that this a poor quality fodder that should be supplemented with feed concentrates, but this rarely happens for smallholders.

Feeding crop residues to animals is actually fairly common in smallholder communities. In Egypt the nomadic Bedouins will contract with ConsumingStrawfarmers in the main crop lands of the Nile Delta for their migrating herds of sheep and goats to graze on crop stubble, while the summer fodder is largely wheat straw, making the straw of equal value with the grain. Similarly, in Ethiopia animals will graze stubble or consume stacks of straw (photo). Since this is not manufacturing nutrients, the best that might be said is that feeding crop residues to animals is converting crop stubble from a form that is normally burned for lack of power to expediently incorporate it into the soil, to a form that is easily and normally incorporated into the soil. With the limited resources normally available to smallholders to manage their land and incorporate crop residues and the need to exert less energy than they derive from the land, this might be the best prospects for sustainable organic nutrient recovery.

Nitrogen Fixation

As mentioned at the beginning of this page legumes and blue-green algae in paddy soils do fix atmospheric nitrogen to Ammonium (NH3) according to the chemical equation:

N2 + 8H+ + 8e- → 2NH3 + H2.

This will commonly contribute some 30 to 50 Kg N/year, but representing only a fraction of the typically 100 kg/ha of N usually recommended for commercial crop production. However, nitrogen fixation is not a “Free Lunch” as the conversion represent some 8 high energy valances shifts. as noted by the 8e- in the above equation. The only place this energy can come from is plant’s photosynthesis. Since there is no economic return to the blue-green algae, or from a leguminous “green manure” crop the diversion of photosynthesis to nitrogen fixation may not be a problem. However, in the case of grain legumes it will be at the expense of plant vegetative development and ultimately from the bean yield. Thus, it is always possible to get a higher bean yield with chemical fertilizer than relying on nitrogen fixation. It is estimated that it takes 10 kg of photosynthate to fix 1 kg of Nitrogen. Also, it has to be appreciated that when N fertilizer is applied to legumes the nitrogen fixation will quickly stop as noted by the nodules changing color from pink, indicating active fixation, to green indicating no fixation. Why would the plant waste energy for N fixation if readily provided? When this happens the rhizobium bacteria shift from having a symbiotic association with the plant to be parasitic to the plant. At this point the plant has shifted from being a legume to being basically a grain. Also since the plant is exerting this amount of energy to fix nitrogen it tends to hoard it and not release it until the plant matures and dies back, which typically is at the end of the crop season. Thus rarely will fixed nitrogen be available during the season when it is fixed, but only the following season provided it does not get converted to NO3- and leached out of the soil. The best option might be the blue green algae in paddies that will be shaded as the rice plant reaches full canopy forcing the die back that could releasing the N just as the rice plant needs it the most, but it will not really be sufficient to fully compensate for top-dressing with mineral fertilizers.


Currently there is a major promotion, particularly from USAID, for soybeans to be produced by smallholder in Africa both as a potential oil crop and for its nitrogen fixing potential to improve soil fertility. This needs to be done with care. The biggest reason is that the nitrogen fixing rhizobium for Soybeans is a specific strain, Rhizobium japonica. Thus, while virtually all other legumes common grown to the tropics will readily cross inoculate with the native rhizobium in the soil soybeans will not. Thus when introduced they require inoculation with the specific rhizobium and should only be promoted with a clear indication that they need to be inoculated and assurance that viable source of inoculum is readily available even in the remote communities where it is being proposed. This substantially adds to the logistic complexity of the introduction, stainability, and expansion of soybean production once the facilitation effort and external assistance ends. This also means the emphasis has to be on dry formulations of inoculum and not moist formulations that will need refrigeration that could be difficult to sustain in remote areas. However, most of the promotions for soybeans introductions including some of the manual being prepared to assist with the introduction fail to mention this and thus the promise of soil improvement from soybean production is reduced to zero. At this point the farmers who have proceeded to produce soybeans with the expectation of soil improvement, should be entitled to compensation for their investment and would have a reasonable court case that should be quickly settled instead of contested.

For many years research efforts including at IITA have attempted to develop promiscuous lines of soybeans that will inoculate with native rhizobium, but while some progress has been obtained commercial lines continues to remain mostly in the future.

Also, the promotion of soybeans as an oil crop needs to be reviewed. Yes, soybeans has 20% oil of medium quality as an edible oil, but physical extraction will always leave 10% oil in the cake. Thus physical extraction will only recover half the oil. To recover all the oil requires using a hexane extraction process, but there are no hexane extraction facilities in Sub-Sahara Africa except perhaps in South Africa. Thus soybeans are virtually used exclusive full-fat. Thus soybeans should not be promoted as an oil crop but as a good industrial crop for animal feed and limited human consumption other than for the corn/soy blend (80% corn/20% soy) used for refugee rations and representing a highly captive audience.


There is a current fad in the rural development effort to encourage applying bio-char to the soil with the expectation of improving soil moisture content and nutrient retention, this in addition to removing excess CO2 from the atmosphere as a greenhouse gas, possibly expecting developing countries to compensate the environment for the excessive use of fossil fuel by countries like the USA, Europe and now India and China. However, this needs to proceed with extreme caution. As with composting it is unlike to have enough charable material for an entire smallholder community or other recommendation domain, and most of the material may have a higher priority in the community such as firewood for cooking. This is really promoted from some rather questionable research studies in which only the highest application level, that was expected to be excessive, showed a statistically significant measurable but not necessarily substantial difference. This might also be limited to some inherently very poor soils with most soils showing no response. The result is an application recommendation of some 10 t/ha of char, which, because in making char 70% of the material is volatilized, requires >30t/ha of burnable material. Is that available without doing some major deforestation and consuming considerable firewood needed for cooking and other domestic requirements? Thus while it is possible to make demonstration plots (photo) it cannot be realistically extended across a smallholder community, and could be environmentally detrimental.IMG_1041-Web

Bio-char is also considered as an additional source of soil organic matter. However, this also needs to be carefully reviewed. While, yes, bio-char retains many of the carbon ring structure associated with soil organic compounds, it is more a mineral form of carbon than an organic form and does not have all the exchange sites true soil organic matter contains nor does it decompose and mineralize and release plant nutrients or aid in soil aggregation as the humic and fulvic acids normally associated with soil organic matter does. Instead is simply sits inertly in the soil for potentially a couple millenniums. This is the same material archaeologists use for carbon dating of ancient sites. Again proceed with considerable care and hopefully the fad will shortly fade away.


While there is little environmental harm in promoting enhanced organic nutrient recovery, smallholders will very quickly recognize the limited application. This could affect the overall creditability of the promoters. It might be more creditable to concentrate on other more realistic approaches to more sustainable agriculture, than continuing to promote ideas that are well outside the operational resource base for the farmers to seriously consider and can predictably be rejected with a little forethought and some quick calculations. In promoting organic fertilizer utilization for smallholders it is necessary to again recognize the limited supply of organic material as well as the limited resources farmers may have to implement the suggestions, and the limited prospects to recover the manual energy exerted. It might be best to when possible allow the animals to to as much of the nutrient recovery as possible.

Last Revised: 12 Nov. 2014.

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