Building Soils for Better Crops, p213

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Building Soils for Better Crops, 3rd Edition
Sustainable Agriculture Research and Education (SARE).

The publication can be downloaded from SARE’s web site.
Sustainable Agriculture Research and Education – Publications – Building Soils for Better Crops, 3rd Edition.
To obtain paper copies, contact SARE.

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* Front Cover.
* Contents.
* About the Authors.
* About SARE.
* Preface.
* Introduction.
* 1.Healthy Soils.
* 2.Organic Matter: What It Is and Why It’s So Important.
* 3.Amount of Organic Matter in Soils.
* 4.The Living Soil.
* 5.Soil Particles, Water, and Air.
* 6.Soil Degradation: Erosion, Compaction, and Contamination.
* 7.Nutrient Cycles and Flows..
* 8.Soil Health, Plant Health, and Pests.
* 9.Managing for High-Quality Soils: Organic Matter, Soil Physical Condition, Nutrient Availability.
* 10.Cover Crops.
* 11.Crop Rotations.
* 12.Animal Manures for Increasing Organic Matter and Supplying Nutrients.
* 13.Making and Using Composts.
* 14.Reducing Erosion and Runoff..
* 15.Preventing and Lessening Compaction.
* 16.Reducing Tillage.
* 17.Managing Water: Irrigation and Drainage.
* 18.Nutrient Management: An Introduction.
* 19.Management of Nitrogen and Phosphorus.
* 20.Other Fertility Issues: Nutrients, CEC, Acidity, and Alkalinity.
* 21.Getting the Most from Routine Soil Tests.
* 22.How Good Are Your Soils? Field and Laboratory Evaluation of Soil Health.
* 23.Putting It All Together.
* Glossary.
* Resources.
* Index.
* Back Cover.

Chapter 19
Management of Nitrogen and Phosphorus
. . . an economical use of fertilizers requires that they merely supplement the natural supply in the soil,
and that the latter should furnish the larger part of the soil material used by the crop.
—T.L. LYON AND E.O. FIPPIN , 1909

Both nitrogen and phosphorus are needed by plants
in large amounts, and both can cause environmental
harm when present in excess. They are discussed
together in this chapter because we don’t want to do a
good job of managing one and, at the same time, do a
poor job with the other. Nitrogen losses are a serious
economic concern for farmers; if not managed properly,
a large fraction (as much as half in some cases) of
applied N fertilizer can be lost instead of used by crops.
Environmental concerns with N include the leaching
of soil nitrate to groundwater; excess N in runoff; and
losses of nitrous oxide, a potent greenhouse gas. For P,
the main concerns are losses to freshwater bodies.
High-nitrate groundwater is a health hazard to
infants and young animals because it decreases the
blood’s ability to transport oxygen. In addition, nitrate
stimulates the growth of algae and aquatic plants just
as it stimulates the growth of agricultural plants. The
growth of plants in many brackish estuaries and saltwater
environments is believed to be limited by a lack
of N. So, when nitrate leaches through soil, or runs off
the surface and is discharged into streams, eventually
reaching water bodies like the Gulf of Mexico or the
Chesapeake Bay, undesirable microorganisms flourish.
In addition, the algal blooms that result from excess
N and P cloud water, blocking sunlight to important
underwater grasses that are home to numerous species
of young fish, crabs, and other bottom dwellers. The
greatest concern, however, is the dieback of the algae
and other aquatic plants. These plants settle on the bottom
of the affected estuaries, and their decomposition
consumes dissolved oxygen in the water. The result is
an extended area of very low oxygen concentrations in
which fish and other aquatic animals cannot live. This is
a serious concern in many estuaries around the world.
Denitrification is a microbial process that occurs
primarily in surface layers when soils are saturated with
water. Soil bacteria convert nitrate to both nitrous oxide
(N2O) and N2. While N2 (two atoms of nitrogen bonded
together) is the most abundant gas in the atmosphere
Photo by Dennis Nolan

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