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close this book Soils, Crops and Fertilizer Use
close this folder Chapter 9: Using chemical fertilizers
View the document What are chemical fertilizers?
View the document Are chemical fertilizers appropriate for limited-resource farmers?
View the document An introduction to chemical fertilizers
View the document Common chemical fertilizers and their characteristics
View the document The effect of fertilizers on soil pH
View the document Fertilizer salt index and "burn" potential
View the document Basic application principles for N, P, and K
View the document Fertilizer application methods explained and compared
View the document Troubleshooting faulty fertilizer practices
View the document Getting the most out of fertilizer use: crop management as an integrated system
View the document Understanding fertilizer math

Getting the most out of fertilizer use: crop management as an integrated system

It's true that fertilizer use can be the factor producing the largest yield increase, especially on low-fertility soils. However, it's important to realize that low soil fertility is just one of many limiting factors that can affect crop yields. Many a farmer and extension worker have learned the hard way that chemical fertilizer alone may give disappointing results. As a sole input, chemical fertilizers have a further disadvantage in that they provide none of the additional benefits that organic fertilizers offer (i.e. filth improvement, etc.). In this case, it's particularly important to be sure that other limiting factors are addressed, aside from fertility alone.

The "Package" Approach to Yield Improvement

One essential part of successful farm or garden project management is selecting and implementing an integrated system of complementary practices designed to favor production and control major limiting factors. Trying to overcome several major limiting factors at once usually gives a more impressive yield increase than tackling them one at a time. A well-designed and appropriate "package" of practices will actually lower farmer risk, and the synergistic effect can be remarkable. The results of a farm trial on dryland wheat in in Mexico shown below are a good example:


Yield Increase

Fertilizer only


Irrigation only


Fert. + irrigation


Some Possible Objections to the "Package" Approach

In Two Ears of Corn, the widely-respected book on grassroots ag extension, author Roland Bunch points out the possible disadvantages of "package" technologies that involve more than two or three new practices. He favors purposely limiting the technology for several reasons:

• Most of the successful people-centered extension projects are ones that started slowly and on a small scale. Each new practice increases program complexity in terms of research, training (of extensionists and farmers), supervision, and availability of inputs.

• In order for innovations to achieve long-term success, they must be adopted by a significant portion of farmers (roughly 25-45%). This "critical mass" is vital to assure eventual wide-spread adoption. Simple packages create less confusion and allow more people to be reached.

• Simple packages that involve a minimum of purchased inputs are more likely to be affordable to small farmers. High-input packages are income-biased and favor the wealthier farmers.

• Packages that achieve spectacular yield and income increases may promote even more economic disparity in a community and lead to jealousy and resentment. In addition, limitedresource farmers (and most of us, for that matter) may not use such sudden income increases wisely.

It's important to note that a "package" doesn't have to include purchased inputs that involve considerable capital.

When working with limited-resource farmers or gardening projects, there are a number of low- or no-cost practices whose introduction can precede or accompany the use of chemical (or organic) fertilizer such as:

• More timely and thorough weeding . Better crop and variety selection . Better water management (See Chapter 5) . Better plant spacing and thinning . Erosion control (See Chapter 3) . Fencing . Mulching . Improved recordkeeping


Available Moisture

Crops can't utilize nearly as much fertilizer when moisture is limiting, although low to moderate rates will help improve water use efficiency. For example, in the semi-arid wheat regions of the U.S., fertilizer nitrogen recommendations are often geared to the amount of stored subsoil moisture and outlook for rain.

Drought-resistant crops like sorghum and millet exhibit much less response to fertilizer when grown under low rainfall conditions, compared to when moisture is sufficient for good yields. In fact, fertilizer may not show a profitable return where moisture is seriously limited.

Another moisture-related instance where fertilizer use is unlikely to be cost-effective is recessional agriculture. In this system, crops are planted in the saturated soil of riverbanks and floodplains as water levels recede after the rainy season ends. In this case, applied fertilizer will soon end up in dry soil as moisture levels drop, unless it's feasible to hand-water the crop from the river or other source.

Type of Crop

Given adequate moisture and an appropriate variety, cereal grains, most pulses, most vegetables, bananas, sugarcane, and pastures tend to show more response to fertilizer than coffee, cacao, and most tree crops. Soybeans and peanuts often respond better to residual fertility remaining from applications to previous crops.


Improved varieties, including hybrids, generally give a better response to fertilizer than traditional ones, though this isn't always the case. For example, during the first years of the Puebla Maize Project in Mexico, some of the native varieties consistently outyielded anything the plant breeders developed. In India, on the other hand, an adapted hybrid yielded 4 times as much as the traditional local variety when both were grown under the same package of practices.

Varieties of the same crop can vary greatly in important characteristics aside from fertilizer response such as draught and heat tolerance, resistance to diseases and nematodes. Each of these can also have a big bearing on yield. Be sure that the variety used is well adapted to the area's growing conditions. Be particularly careful with donated seed from U.S. seed companies that is often distributed by relief agencies.

Planting Date: This is an important consideration in areas where delayed, premature, or unseasonal planting increases the likelihood of yield-limiting stresses such as excessive heat or cold, too much or too little moisture, insects, and diseases.

Plant Density (Plant population)

Too low a plant density is a common cause of poor fertilizer response, especially with cereal grains. Where soil fertility is low, farmers tend to plant fewer plants per hectare so that each plant gets a better share of the limited amount of nutrients. Such low plant populations may not be be able to respond well to added nutrients. The reason is that the individual plant's yield-determining factors, such as number of ears per plant, ear size, or seeds per head, may reach their limit at relatively low NPK rates. If so, it may be necessary to increase plant population when fertilizer is introduced in order to obtain more grain yield. Caution is needed here, since varieties vary in their population tolerance, and overly-high plant densities may encourage lodging (tipping over) in cereal crops or use up scarce soil moisture in drought-prone areas.

Soil Limiting Factors: Poor filth, poor drainage, soil compaction, low water-holding capacity, and pH problems all have an adverse effect on fertilizer response.

Weed Control: Weeds not only compete with the crop for sunlight, water, and nutrients, but harbor insects and diseases as well.

Insects, Diseases, and Soil Nematodes: While a well-nourished plant has better resistance to most diseases and pests, they can still wipe out profits if not controlled.

Management Level: The farmer's or project's willingness and ability to implement the minimum level of needed management is a crucial factor. This includes general management skills such as planning and timeliness, as well as essential practices such as weed and insect control where needed.