2002. nutrients and environmental quality

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Better Crops/Vol. 86 (2002, No. 2) 15

Scientists have long stated that there is noscientific basis for claims that organicnutrients are superior to inorganic nutri-

ents. Still, there is widespread public percep-

tion that organic agricultural systems are moreenvironmentally friendly and more sustain-able than high-yielding con-ventional farming systems. Infact, long-term studies fromaround the world indicatethat sustained yields and soilproductivity can be achievedwith balanced nutrition,using either or both organic

and mineral fertilizer nutrientsources.

Regardless of the source,plant nutrients are absorbedprimarily as inorganic ions.For example, N is taken up aseither ammonium (NH4

+) ornitrate (NO3

-), phosphorus asHPO42

- or H2PO4-, and potas-

sium (K) as K+. Thus, even ifthe nutrient source is organic, it must undergotransformation so the nutrients it contains canbe released in the inorganic form before cropplants can use them.

Organic matter (OM) has long beenknown to have positive influences on soilstructure, tilth, bulk density, and moisture-holding characteristics. Among its other ben-efits are increases in the soils cation exchange

capacity (CEC) and reduced soil runoff anderosion losses. The humus fraction of OM,along with certain cations, is able to retain sig-nificant amounts of P, and OM can reduce Pfixation under some acid soil conditions.However, continued heavy applications of ani-

mal manure can result in P movement intogroundwater as the manure is mineralized.It should be noted that groundwater NO3

--Nlevels are likely to be elevated long before P

because NO3- is far more mobile in the soilthan is P.

Micronutrients and heavymetals occur naturally invarying amounts in rocks,soils, and water. Severalheavy metals are essentialor beneficial to both plantsand animals, but can be-come toxic if accumulated in

excessive amounts (Table 1).Proper management canreduce or eliminate thebuildup of toxic levels inagricultural soils.

More than 97 percent ofmineral fertilizers manufac-tured in North America aremade from natural sourcessuch as atmospheric gases or

mineral deposits rather than industrial by-products. Cadmium (Cd) is the heavy metal ofmost concern because of the levels found nat-urally in phosphate rock, the principal orefrom which phosphate fertilizers are made.While the long-term use of phosphate fertiliz-ers can result in a gradual buildup of Cd in thesoil, it would take hundreds or even thousandsof years at normal application rates to reach

critical levels. Micronutrient and heavy metalconcentrations in animal manure and sewagesludge are highly variable. At typical userates, loadings are generally higher frombiosolids than from fertilizers.

As with successful crop production,

Nutrients and Environmental Quality

B y C . S . S n y d e r a n d T . W . B r u u l s e m a

When properly managed,

mineral fertilizers and ani-

mal manure can increase

soil productivity, enhance

sustainability, and increase

carbon (C) sequestration.

There are, however, man-

agement challenges andrisks associated with both

nutrient sources relative to

1) nitrogen (N) and phospho-

rus (P) losses to surface and

groundwater; 2) heavy metal

accumulation; 3) pathogen

accumulation; and 4) green-

house gas production.


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nutrient input is essential for sus-tained aquatic productivity, but anexcess of nutrients such as N and Pcan over-stimulate aquatic growth,creating algal blooms. As the plantsdie, bacteria begin to decompose

them, using dissolved oxygen (O2)in the water. When the O2 levelsbecome too low, some aquaticplants and animal life can suffer,and less mobile organisms may die.Concern over this situation hasbeen well publicized for the U.SChesapeake Bay area and the Gulfof Mexico. Nitrogen and P fromcommercial fertilizers have been

blamed as a primary cause ofincreased nutrient flux in such sur-face water bodies.

Scientists recognize that undercurrent crop rotations and manage-ment in the U.S. Midwest, there is no way tokeep N and P out of drainage waters and, ulti-mately, rivers and streams that feed theMississippi River. However, practices thatimprove N use efficiency, such as placement,timing, source, and application rate, can min-imize the impact of nutrient enrichment insurface and groundwater. Also, P manage-ment, as well as other practices such as tillagesystems, can reduce the impact of P (and N)on water quality. For example, research inArkansas showed that vegetative filter stripscan be quite effective in reducing runoff Plosses. Other studies have shown that proper-

ly designed vegetative filter strips havedecreased N and P losses by 40 to 90 percent.Excess NO3

- in groundwater poses apotential threat to humans. In agriculturalsoils, contamination is most likely in humid orirrigated areas where sandy soils overlay shal-low aquifers. In clay or clay loam soils thatdrain slowly, denitrification can significantlyreduce NO3

- and prevent its leaching togroundwater. Large applications of animal

manure can increase the soils total N andinorganic N contents. Nitrate is the primaryend product of manure decomposition andmay pose environmental risks when manure isapplied to provide N rates above cropuse. On well drained or tile drained soils, the

16 Better Crops/Vol. 86 (2002, No. 2)

TABLE 1. Reaction of plants and animals to trace elements.

Essential or beneficial to: Potentially toxic to:

Element Plants Animals Plants Animals

Arsenic No Yes Yes

Cadmium No No Yes Yes

Chromium No Yes Yes DU

Cobalt Yes3 Yes Yes Yes

Copper Yes Yes Yes Yes2

Lead No No Yes Yes

Mercury No No DU1 Yes

Molybdenum Yes Yes DU Yes2

Nickel No3 Yes Yes Yes

Selenium Yes Yes Yes Yes (4 ppm)

Zinc Yes Yes DU DU

Source: Webber and Singh, 1995.1

DU = Data on critical limits unavailable.2Toxic to ruminants (cattle and sheep) at 5 to 20 parts per million

(ppm).3Other sources consider Cobalt to be nonessential and Nickel to

be essential (PPI Soil Fertility Manual).

potential for NO3- leaching goes up as manure

rates increase.

Risks from PathogensAnimal wastes contain intestinal bacte-

ria, many of which present substantial humanhealth risks if ingested through drinkingwater. One of the bacteria of most concern isE. coli O157:H7. The U.S. Centers for DiseaseControl (CDC) has estimated that there areabout 70,000 cases of infection and morethan 60 deaths each year caused by this par-ticular bacterial strain. While animal manureis not the only source for this pathogen, it is

a contributor.Viruses have also been reported inmanure. Those found in poultry litter may pre-sent a bigger environmental problem thanbacteria. A number of antibiotics, growth hor-mones, and disinfectants are used to protectanimal health and to improve production. Theconsequences and fate of these substanceshave not been thoroughly assessed.

Air QualityOdors from ammonia (NH3) and othergases are released from animal productionfacilities, in waste storage lagoons, stockpiles,composted wastes, and in excreta by grazinganimals. Odor-causing gases can arise from


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the decay of organic substances in theabsence of O2. Besides the offensive odorsfrom gases such as hydrogen sulfide (H2S),some of the gases can cause health problems.

High NH3 concentrations can be detrimentalto birds and to farm laborers as well.Atmospheric loss of NH3 from N fertilizers isconsiderable. Such losses can be minimizedby proper N management.

Increased emissions of greenhouse gases(GHGs) are thought to increase the potentialfor global warming. Carbon dioxide (CO2),methane (CH4), and nitrous oxide (N2O) arethe three most important GHGs associatedwith agriculture. Carbon dioxide emissions inagriculture come from the burning of fossilfuel and the decomposition of OM and cropresidues. Methane comes primarily from rumi-nant animals, livestock manure, wetlands, andrice production. Any N source is subject todenitrification reactions in the soil, so allforms of N, organic and inorganic, contributeto N2O emissions. Proper management mini-

mizes the effect.Agriculture has an opportunity to signifi-cantly reduce CO2 emissions by sequesteringmore C in the soil. One 35-year study showedthat fertilization increased the amount of corn-derived C while maintaining native soil C(Table 2). Fertilization and crop rotationincreased both crop yields and soil organicmatter levels.

Former U.S. Assistant Secretary of

Agriculture, Dr. C.E. Hess, stated that agri-culture has a great opportunity to help miti-gate climate change by stashing CO2 as C insoil and vegetation. Practices requiring goodagricultural husbandry, which should beimplemented anyway, can be quite effective

for sequestering C. For cropland, thesepractices include building soil OM levels,improving soil fertility, and growing more foodon less land.

Role of Nutrients in Stabilizing C in

Soil OMSound nutrient management aids the

capture of atmospheric CO2, improves photo-synthesis, enhances O2 release to the atmos-phere, and increases soil C. Several studieshave shown the correlation between N fertil-ization and C sequestration. Long-termresearch has also shown that soil organic Nand C levels are highest when conservationtillage is combined with rotations of high

residue crops and adequate, balanced fertilityto increase crop yields.

SummaryMineral fertilizers and animal manure are

valuable nutrient sources for crop productionto meet the worlds food and fiber demands.Fertilizers are more predictable and thus moremanageable nutrient sources. Improperlymanaged, any source can potentially pollutethe soil, water, and air.

The growing challenge for agriculture isto find ways to increase crop yields andimprove nutrient use efficiency while stabiliz-ing nutrients...not removed in harvestedcrops...in crop residues and, ultimately, OM inthe soil. Nutrient management must be site-specific and cost effective to protect the via-bility of North American agriculture. At the

same time it must also include considerationsfor the protection of our soil, water, and airresources. That means protection from surfacerunoff, leaching, and gaseous emissions.

There is no reason crop productionsystems based on sound nutrient man-agement cannot sustain optimum yieldproduction while also protecting theenvironment.

Dr. Snyder is PPI Southeast Director, located inConway, Arkansas; e-mail: [email protected]. Dr.

Bruulsema is PPI Eastern Canada and Northeast

U.S. Director, located in Guelph, Ontario; e-mail:

[email protected].

Better Crops/Vol. 86 (2002, No. 2) 17

Fertilization Total Corn-derived Native

N-P2O5-K2O, C C C

lb/A tons/A

0-0-0 36 5 31

115-70-30 40 9 31

Source: Gregorich and Drury, 1996.

TABLE 2. Total organic C, corn-derived C, andnative C found in fertilized andunfertilized corn plots, using 13C

2002. nutrients and environmental quality

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