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close this book Soils, Crops and Fertilizer Use
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Organic vs. chemical fertilizers: which are best?

There's no one right answer to this question. Both organic and chemical fertilizers have their appropriate uses in small small farmer agriculture in the Third World. Both have their pros and cons. They can also be used together. What's beat for a given situation depends on many factors such as the farmer's circumstances, type of crop, area involved, and the availability and costs of organic and chemical fertilizers. First, let's go over the pros and cons of each type:

Possible Advantages of Organic Fertilizers

Organics like compost and manure are generally free or very low cost for most farmers.

Organic fertilizers take relatively little skill to use properly.

Plant- or animal-derived organics like compost or manure usually contain significant amounts of micronutrients in addition to macronutrients such as N, P, and K.

Plant- or animal-derived organics like manure not only supply plant nutrients but also organic matter which improves soil physical condition, stimulates beneficial soil microorganisms, and provides all the other benefits covered in Chapter 2.

Much of the nitrogen and phosphorus in organics is in a slow-release, organic form. This is a plus for nitrogen which is susceptible to leaching losses when supplied by chemical fertilizers. (However, as opposed to well-rotted manure, fresh manure has much of its N in the quick-release inorganic (mineral) form.

The phosphorus in organic fertilizers is less prone to soil tie-up than that from chemical fertilizers, making it more available to plants.

Possible Disadvantages of Organic Fertilizers

Most plant derived organics like compost and manures are low-strength fertilizers; this means very large amounts (i.e. 3-8 kg/sq. meter or 30,000-80,000 kg/ha per crop) must be applied to supply enough nutrients for crop growth and to add enough humus to benefit soil physical condition. Most small farmers aren't likely to have enough organic fertilizer to cover all their crop land adequately.

NOTE: Some animal-derived organics like blood and fish meal approach the nutrient content of some chemical fertilizers but are usually too expensive to be cost-effective or aren't available locally.

The exact nutrient content of most organics like compost or manure varies a lot.

It takes a good deal of labor to apply most organics or to make compost because of the large amounts needed.

Possible Advantages of Chemical Fertilizers

They are high-analysis nutrient sources. For example, 50 kg of 10-5-10 chemical fertilizer hen about the same NPK content as 1000 kg of typical manure. This means much less labor is needed to apply an equal amount of nutrients.

Unlike most organics, the nutrient content of chemical fertilizers can be verified from the label. If the farmer has her soil tested, she can usually buy the chemical fertilizers that will supply what's needed.

Possible Disadvantages of Chemical Fertilizers

Although chemical fertilizers are very cost-effective when applied correctly in conjunction with overall good management, they do cost money. As a rough figure, a farmer relying totally on chemical fertilizers would have to spend about $75-S125 (U.S.) per hectare per crop (based on unsubsidized prices). Trying to rely entirely on chemical fertilizers isn't feasible for most limited-resource farmers, many of whom have no access to reasonable ag credit.

Most Third World countries must import chemical fertilizers which can be a drain on their balance of payments. Even if they manufacture their own, the fertilizer plants arc heavily dependent on foreign inputs.

Compared to organics, the proper application of chemical fertilizers takes considerably more skill as far as dosage calculations, timing, placement, and application methods. Many farmers are not using them efficiently.

Some Other Issues

Effect on Nutritional Value: Most researchers agree that when vegetables are grown on low-fertility soils and are underfertilized, their vitamin contents are abnormally low, especially in the case of vitamin C and carotene (the latter is converted to vitamin A in the body). Host research bodies, including the National Cancer Institute, now believe that vitamin C and carotene help prevent several types of cancer. (Pre-formed vitamin A [retinol] is found only in animal products like liver but hasn't been shown to be as effective an anti-carcinogen as the carotene found in dark green, orange, or yellow vegetables and fruits.) Both chemical and organic fertilizers can increase overall vitamin content (especially that of vitamin C and carotene) when applied to vegetables (especially leafy greens like pak choy and amaranth) grown on low-fertility soils.

Until recently, most studies showed no nutritional differences between organically and chemically fertilized crops. Now, new research (including more accurate analysis techniques) has shown that overly high (as opposed to normal rates) of chemical N fertilizer may markedly lower the vitamin C content of leafy vegetables like lettuce and Chinese cabbage. Heavy applications of compost and well-rotted manure don't have this effect, because they release N slowly; however, excessive rates of fresh manure are likely to have an effect similar to chemical N fertilizers. (This research is summarized in the American Soc. of Agronomy Special Publication 46: Organic Farming.)

Researchers have known for many years that excessive rates of chemical N fertilizer or fresh manure may produce excessive nitrate levels in leaf vegetables like spinach. In plants, nitrates are converted to nitrites which are toxic in themselves, but can also be further changed into nitrosamines which are strongly linked to stomach cancer. Compost and well-rotted manure release their N slowly enough to avoid this problem.

Do Chemical Fertilizers "Poison" the Soil? Organic advocates often claim that chemical fertilizers destroy beneficial soil life as well as good filth (workability), eventually ruining productivity. However long-term studies haven't confirmed this. The fact is that soil decline (aside from erosion) is directly linked to a drop in soil humus, which occurs when soils are continually cropped without making large and regular applications of organic matter. Chemical fertilizers themselves don't speed up the loss of humus, but may actually slow it down, since higher yields produce more crop residues that can be returned to the soil. On the other hand, using organics in sufficient amounts can automatically assure that humus level is maintained and possibly increased.

Overuse of chemical nitrogen fertilizer has been linked to nitrate contamination of rivers, lakes, and wells due to leaching and runoff of the excess N. However, fresh manure can also release large amounts of nitrates and may cause similar problems when large quantities are applied or stockpiled.

Likewise, phosphorus runoff from farmland may promote excessive algal growth in lakes and reservoirs leading to oxygen depletion and fish kill. However, both chemical fertilizers and animal manure will produce surface runoff of P, especially when applied to sloping fields without being worked in thoroughy.

The Energy Cost of Chemical Fertilizers: Nearly all the nitrogen in chemical fertilizers is derived from ammonia gas which is formed by combining hydrogen (made by burning natural gas) with nitrogen gas taken from the atmosphere. When critics of chemical fertilizers say that they are too petroleum dependent for their manufacture, they're referring to this process. Surprisingly though, only about 3 percent of the natural gas in the U.S. is used in making N fertilizer. One study showed that if the amount of natural gas needed to heat an average Midwest home were converted into N fertilizer, it would produce enough extra maize to feed 275 people for a year!

However, as we'll see, there's much that can be done about the faulty application practices and poor management that waste a good deal of the chemical fertilizer used worldwide.


Now that we've covered the pros and cons of organic and chemical fertilizers, here are the main factors that should govern a farmer's choice:

• Given their low cost, simplicity of use, and multiple soil benefits, the use of organics should be strongly encouraged, especially for limited resource farmers. However, chemical fertilizers may be the only present feasible alternative where organics are in short supply, are of poor nutrient value (i.e. poorly stored manure), or where labor is inadequate to handle and apply them. Given the current state of suitable organic technologies, there are many Third World areas where it's still not possible to become totally reliant on organic fertilizers for all crops.

• Size of field and type of crop: As with chemical fertilizers, organics are beneficial to all crops when applied properly. However, since many small farmers aren't likely to have enough organic fertilizer to cover all their land, they're usually better off using what's available on their smaller plots which are usually used for vegetables. This will enable them to apply a high enough rate to supply a beneficial amount of nutrients and organic matter. If enough is available, it can also be applied to the larger fields which are typically devoted to staple cereals and pulses (grain legumes) such as maize, sorghum, and cowpeas.

NOTE: One type of organic fertilizer that is often feasible for larger fields is green manure which is covered later on in this chapter.

• Where organics are in short supply, field crops will often benefit from chemical fertilizers, though cost may be prohibitive where credit isn't available. Where organics are available or are of poor nutrient value, a good case can be made for using chemical fertilizers on small plots, too, such as vegetables, at least as a temporary measure. On small areas, the cost of chemical fertilizer, which is roughly about 1 U.S. cent per sq. meter (mid-1980's prices), becomes more reasonable.

• Organic and chemical fertilizers often work very well together. For example, chemical fertilizers can be used to supplement animal manure if it is low in nutrient value or if supplies are limited. Chemical fertilizers can also be used to supply specific nutrients when available organics are unable to do so. A good example would be the use of ammonium sulfate fertilizer as a nitrogen sidedressing on vegetables in cases where the only available organic fertilizer is poorly-stored manure that is low in N. LowP soils may require the addition of a chemical fertilizer, such as superphosphate, in conjunction with organic fertilizers. Likewise, low-nutrient organic soil conditioners, such as rice hulls and sawdust, can be used along with chemical fertilizers to improve both the physical condition and the fertility of clayey soils. In addition, organic fertilizers help reduce the tie-up of chemical fertilizer phosphorus.

In summary, both organic and chemical fertilizers have their appropriate uses. Many farmers may find that both have a place on their farms, but they will usually be best off trying to maximize the use of organics.