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Nutrient Management: An Introduction
The purchase of plant food is an important matter,
but the use of a [fertilizer] is
not a cure-all, nor will it prove an adequate substitute
for proper soil handling.
J.L. Hills, C.H. Jones, and C. Cutler, 1908
Of the 18 elements needed by plants, only three nitrogen
(N), phosphorus (P), and potassium (K) are commonly deficient in
soils. Deficiencies of other nutrients, such as magnesium, sulfur,
zinc, boron, and manganese, certainly occur, but they are not as
widespread. However, in locations with lots of young minerals that
haven't been weathered much by nature, such as the Dakotas, potassium
deficiencies are less common. Deficiencies of sulfur, magnesium,
and some micronutrients may be more widespread in regions with highly
weathered minerals, such as the southeastern states, or those with
high rainfall, such as portions of the Pacific Northwest.
Environmental concerns have placed more emphasis on
better management of nitrogen and phosphorus over the last few decades.
These nutrients are critical to soil fertility management, but they
are also responsible for widespread environmental problems. Poor
soil and crop management, the overuse of fertilizers, misuse of
manures, sludges and composts, and high animal numbers on limited
land area have contributed to surface and groundwater pollution
in many regions of the U.S. Because both nitrogen and phosphorus
are used in large quantities and their overuse has potential environmental
implications, we'll discuss them together in chapter
17. Other nutrients, cation exchange, soil acidity (low pH)
and liming, and arid and semi-arid region problems with sodium,
alkalinity (high pH), and excess salts are covered in chapter
18.
The ABCs of Nutrient Management
a. Build up and maintain high soil
organic matter levels.
b.Test manures and credit their nutrient content
before applying fertilizers or other amendments.
c. Incorporate manures into the soil quickly, if
possible, to reduce nitrogen volatilization and potential
loss of nutrients in runoff.
d. Test soils regularly to determine the nutrient
status and whether or not manures, fertilizers, or lime
are needed.
e. Balance nutrient inflows and outflows to maintain
optimal levels and allow a little "draw-down"
if nutrient levels get too high.
f. Enhance soil structure and reduce field runoff
by minimizing soil compaction damage.
g. Use forage legumes or legume cover crops to provide
nitrogen to following crops and develop good soil tilth.
h. Use cover crops to tie up nutrients in off-season,
enhance soil structure, and reduce runoff and erosion.
i. Maintain soil pH in the optimal range for the
crops in your rotation.
j. When phosphorus and potassium are very deficient,
broadcast some of the fertilizer to increase the general
soil fertility level, and band apply some as well.
k. To get the most efficient use of the fertilizer
when phosphorus and potassium levels are in the medium range,
consider band application at planting, especially in cool
climates.
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The Bottom Line:
Nutrients and Plant Health,
Pests, Profits, and
the Environment
Management practices are all related. The key is to
visualize them all as whole-farm management, leading you to the
goals of better crop growth and better environmental quality. If
a soil has good tilth, no subsurface compaction, good drainage,
adequate water, and a good supply of organic matter, then plants
should be healthy and have large root systems. A large and healthy
root system enables plants to efficiently take up nutrients and
water from the soil and to use those nutrients to produce higher
yields.
Doing a good job of managing nutrients on the farm
and in individual fields is critical to general plant health and
management of plant pests. Too much available nitrogen in the early
part of the growing season allows small-seeded weeds, with few nutrient
reserves, to get well established. This early jump-start may then
enable them to out-compete crop plants later on. Crops do not grow
properly if nutrients aren't present in sufficient quantities and
in reasonable balance to one another. Plants may be stunted if nutrients
are low, or they may grow too much foliage and not enough fruit
if nitrogen is too plentiful relative to other nutrients. Plants
under nutrient stress, such as too low or too high nitrogen levels,
are not able to emit as much of the natural chemicals that signal
bene-ficial insects when insect pests feed on leaves or fruit. Stalk
rot of corn is aggravated by low potassium levels. On the other
hand, pod rot of peanuts is associated with excess potassium within
the fruiting zone of peanuts (the top 2 to 3 inches of soil). Blossom-end
rot of tomatoes is related to low calcium levels, often brought
on by droughty, or irregular rainfall/irrigation, conditions.
Nutrient
Management Goals
- Satisfy crop nutrient requirements for yield
and quality.
- Minimize pest pressure caused by fertility
imbalances.
- Minimize the risk of damage to the environment.
- Minimize the cost of supplying nutrients.
- Use local sources of nutrients whenever
possible.
- Get full nutrient value from fertility sources.
Modified from OMAFRA, 1997
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When plants either don't grow well or are more susceptible
to pests, this affects the economic return. Yield and crop quality
usually are reduced, lowering the amount of money received. There
also may be added costs to control pests that take advantage of
poor nutrient management. In addition, when nutrients are applied
beyond plant needs, it's like throwing money away. And when nitrogen
and phosphorus are lost from the soil by leaching to groundwater
or running into surface water, entire communities may suffer from
poor water quality.
Organic Matter and Nutrient Availability
The best single overall strategy for nutrient management
is to work to enhance soil organic matter levels in soils. This
is especially true for nitrogen and phosphorus. Soil organic matter,
together with any freshly applied residues, are well known sources
of available nitrogen for plants. Mineralization of phosphorus and
sulfur from organic matter is also an important source of these
nutrients. As discussed earlier, organic matter helps hold on to
potassium (K+), calcium (Ca++), and magnesium
(Mg++) ions. It also provides natural chelates that maintain
micronutrients such as zinc, copper, and manganese, in forms that
plants can use.
Essential Nutrients for
Plants |
Element |
Common
Available Form |
Source |
needed in large amounts |
Carbon |
CO2 |
atmosphere |
Oxygen |
O2, H2O |
atmosphere and soil pores |
Hydrogen |
H2O |
water in soil pores |
Nitrogen |
NO3-, NH4+ |
soil |
Phosphorus |
H2PO4-, HPO4-2 |
soil |
Potassium |
K+ |
soil |
Calcium |
Ca+2 |
soil |
Magnesium |
Mg+2 |
soil |
Sulfur |
SO4-2 |
soil |
needed in small amounts |
Iron |
Fe+2, Fe+3 |
soil |
Manganese |
Mn+2 |
soil |
Copper |
Cu+, Cu+2 |
soil |
Zinc |
Zn+2 |
soil |
Boron |
H3BO3 |
soil |
Molybdenum |
MoO4-2 |
soil |
Chlorine |
Cl- |
soil |
Cobalt |
Co+2 |
soil |
Nickel |
Ni+2 |
soil |
Improving Nutrient Cycling on the Farm
For economic and environmental reasons, it makes sense
to utilize nutrient cycles efficiently. Goals should include the
reduction in long-distance nutrient flows, as well as promoting
"true" on-farm cycling. There are a number of strategies
to help farmers reach the goal of better nutrient cycling:
- Reduce unintended losses by promoting water infiltration
and better root health through enhanced management of soil organic
matter and physical properties. The ways in which organic matter
can be built up and maintained include increased additions of
a variety of sources of organic matter plus methods for reducing
losses via tillage and conservation practices.
- Enhance nutrient uptake efficiency by carefully using
fertilizers and amendments. Better placement and synchronizing
application with plant growth both improve efficiency. Sometimes,
changing planting dates or switching to a new crop creates a
better match between the timing of nutrient availability and
crop needs.
- Tap local nutrient sources by seeking local sources
of organic materials, such as leaves or grass clippings from
towns, aquatic weeds harvested from lakes, produce waste from
markets and restaurants, food processing wastes, and clean sewage
sludges (see discussion on sewage sludge in chapter
8). Although some of these do not contribute to true nutrient
cycles, the removal of agriculturally usable nutrients from
the "waste stream" makes sense and helps develop more
environmentally sound nutrient flows.
- Promote consumption of locally produced foods by supporting
local markets as well as returning local food wastes to farmland.
When people purchase locally produced foods there are more possibilities
for true nutrient cycling to occur. Some Community Supported
Agriculture farms, where subscriptions are paid before the start
of the growing season, encourage their members to return produce
waste to the farm for composting, completing a true cycle.
- Reduce exports of nutrients in farm products by adding
animal enterprises to crop farms (fewer exports, and more reason
to include forage legumes and grasses in rotation). The best
way to both reduce nutrient exports per acre, as well as to
make more use of forage legumes in rotations, is to add an animal
(especially a ruminant) enterprise to a crop farm. Compared
with selling crops, feeding crops to animals and exporting animal
products results in far fewer nutrients leaving the farm. (Keep
in mind that, on the other hand, raising animals with mainly
purchased feed is the best way to overload a farm with nutrients.)
- Bringing animal densities in line with the land base of
the farm can be accomplished by renting or purchasing more
land to grow a higher percentage of animal feeds and for manure
application or by reducing animal numbers.
- Develop local partnerships to balance flows among different
types of farms. As pointed out in chapter
8 when we discussed organic matter management, sometimes
neighboring farmers cooperate with both nutrient management
and crop rotations. This is especially beneficial when a livestock
farmer has too many animals and imports a high percentage of
feed and a neighboring vegetable farm has a need for nutrients
and has an inadequate land base to allow a rotation that includes
a forage legume. By cooperating with nutrient management and
rotations, both farms win, sometimes in ways that were not anticipated
(see "win-win" box). Encouragement and coordination
from an extension agent may help neighboring farmers work out
cooperative agreements. It is more of a challenge as the distances
become greater.
Some livestock farms that are overloaded with nutrients
are finding that composting is an attractive alternative way to
handle manure. During the composting process, volume and weight
are greatly reduced (see chapter 12), resulting
in less material to transport. Organic farmers are always on the
lookout for reasonably priced animal manures and composts. The landscape
industry also uses a fair amount of compost. Local or regional compost
exchanges can help remove nutrients from overburdened animal operations
and place them on nutrient-deficient soils.
Strategies
for Improving Nutrient Cycles
- Reduce unintended losses.
- Enhance nutrient uptake efficiency.
- Tap local nutrient sources.
- Promote consumption of locally produced
foods.
- Reduce exports of nutrients in farm products.
- Bring animal densities in line with the
land base of the farm.
- Develop local partnerships to balance flows
among different types of farms.
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Using Fertilizers and Amendments
There are four main issues when applying nutrients:
- How much is needed?
- What source(s) should be used?
- When should the fertilizer or amendment be applied?
- How should the fertilizer or amendment be applied?
Chapter 19 details use of
soil tests to help you decide how much fertilizer or organic nutrient
sources to apply. Here we will go over how to approach the other
three issues.
Win-Win Cooperation
Cooperation between Maine potato farmers and
their dairy farm neighbors has led to better soil and crop
quality for both types of farms. As potato farmer John Dorman
explains, after some years of cooperating with a dairy farm
on rotations and manure management, soil health "has
really changed more in a few years than I'd have thought possible."
Dairy farmer Bob Fogler feels that the cooperation with the
potato farmer allowed his family to expand the dairy herd.
He notes that "we see fewer pests and better quality
corn. Our forage quality has improved. It's hard to put a
value on it, but forage quality means more milk."
From Hoard's Dairyman, 4/10/99 |
Nutrient Sources: Commercial Fertilizers vs. Organic
Materials
There are numerous fertilizers and amendments normally used
in agriculture (some are listed in table 16.1).
Fertilizers such as urea, triple superphosphate, and muriate of
potash (potassium chloride) are convenient to store and use. They
are also easy to blend to meet nutrient needs in specific fields
and provide predictable effects. Their behavior in soils and the
ready availability of the nutrients is well established. The timing,
rate, and uniformity of nutrient application is easy to control
when using commercial fertilizers. However, there also are drawbacks
to using commercial fertilizers. All of the commonly used nitrogen
materials (those containing urea, ammonia, and ammonium) are acid
forming, and their use in humid regions, where native lime has been
weathered out, requires more frequent lime additions. Also, the
high nutrient solubility can result in salt damage to seedlings
when excess fertilizer is applied close to seeds or plants. Because
nutrients in commercial fertilizers are readily available, under
some circumstances more may leach to groundwater than when using
organic nutrient sources. For example, high rainfall events on a
sandy soil soon after ammonium nitrate fertilizer application will
probably cause more nitrate loss than if a compost had been applied
or a legume cover crop recently incorporated. Likewise, sediments
lost by erosion from fields fertilized with commercial fertilizers
probably will contain more available nutrients than those from fields
fertilized with organic sources, resulting in more severe water
pollution.
Organic sources of nutrients have many other good
qualities too. They usually provide a more slow release source of
fertility and the nitrogen availability is more evenly matched to
the needs of growing plants. Sources like manures or crop residues
commonly contain all the needed nutrients, including the micronutrients
but they may not be present in the proper proportion for soil and
crop needs. These materials are also sources of soil organic matter,
providing food for soil organisms and forming aggregates and humus.
Organic Farming
vs. Organic Nutrient Sources
We've used the term "organic sources"
of nutrients to refer to nutrients contained in crop residues,
manures, and composts. These types of materials are used by
all farmers"conventional" or "organic."
Both also use limestone and a few other materials. However,
most of the commercial fertilizers listed in table 16.1 are
not allowed in organic production. In place of sources such
as urea, anhydrous ammonia, diammonium phosphate (DAP), concentrated
superphosphate, and muriate of potash, organic farmers use
products that come directly from minerals such as greensand,
granite dust, and rock phosphate. Other organic products come
from parts of organisms such as bone meal, fish meal, soybean
meal, and bloodmeal (see table 16.2).
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One of the drawbacks to organic materials is the variable
amounts and uncertain timing of nutrient release for plants to use.
The value of manure as a nutrient source depends upon the type of
animal, its diet, and how the manure is handled. For cover crops,
the nitrogen contribution depends upon the species, amount of growth
in the spring, and weather. Also, manures typically are bulky and
may contain a high percentage of water so considerable work is needed
to apply them per unit of nutrients. The timing of nutrient release
is uncertain, because it depends both on the type of organic materials
used and on the action of soil organisms. Their activities change
with temperature and rainfall. Finally, the relative nutrient concentrations
for a particular manure used may not match soil needs. For example,
manures may contain high amounts of both nitrogen and phosphorus
when your soil already has high phosphorus levels.
Selection of Commercial Fertilizer Sources
There are numerous forms of commercial fertilizers, many given in
table 16.1. When you buy fertilizers in large quantities, the cheapest
source is usually chosen. When you buy bulk blended fertilizer,
you usually don't know what sources were used unless you ask. All
you know is that it's a 10-20-20 (referring to the percent of available
N, P2O5, and K2O), or a 20-10-10,
or an other blend. However, below are a number of examples where
you might not want to apply the cheapest source:
- Although the cheapest nitrogen form is anhydrous ammonia,
the problems with injecting it into a soil with many large stones
or the losses that might occur when injecting it into very moist
clay may call for other nitrogen sources to be used instead.
- If both nitrogen and phosphorus are needed, diammonium phosphate
(DAP) is a good choice because it has approximately the same
cost and phosphorus content as concentrated superphosphate and
also contains 18 percent N.
- Although muriate of potash (potassium chloride) is the cheapest
potassium source, it may not be the best choice under certain
circumstances. If you also need magnesium and don't need to
lime the field, potassium-magnesium sulfate would be a better
choice.
Method and Timing of Application
The timing of fertilizer application is frequently related to the
application method chosen, so in this section we'll go over both
issues together.
Fertilizer Prices
Costs of fertilizers are currently around
24 cents/lb of N, 30 cents/lb of P2O5,
and 19 cents/lb of K2O. Nitrogen prices fluctuate
greatly, while those for phosphorus and potassium have been
more stable. |
Broadcast application, where fertilizer is evenly distributed
over the whole field and then usually incorporated during tillage,
is best used to increase the nutrient level of the bulk of the soil.
It is especially useful to build phosphorus and potassium when they
are very deficient. Broadcasting for incorporation is usually done
in the fall or in spring just before tillage. Broadcasting on top
of a growing crop, called topdress, is commonly used to apply
nitrogen, especially to crops that occupy the entire soil surface,
such as wheat or a grass hay crop. [Amendments used in large quantities,
like lime and gypsum, are also broadcast prior to incorporation
into the soil.]
There are various types of localized placement of
fertilizer. Banding small amounts of fertilizer to the side
and below the seed at planting is a common application method. It
is especially useful for row crops grown in cool soil conditions,
such as early in the season, on soils with high amounts of surface
residues, with no-till management, or on wet soils. It is also useful
for soils that test low-to-medium in phosphorus and potassium (or
even higher). Banding fertilizer at planting, usually called starter
fertilizer, may be a good idea even in warmer climates when
planting early. It still might be cool enough to slow root growth
and release of nutrients from organic matter. Including nitrogen
in the band appears to help roots use fertilizer phosphorus more
efficiently. Starter fertilizer for very low fertility soils frequently
contains other nutrients, such as sulfur, zinc, boron, or manganese.
Splitting nitrogen applications is a good management
practice especially on sandy soils, where nitrate is easily lost
by leaching, or on heavy loams and clays, where it is easily lost
by denitrification. Some nitrogen is applied before planting or
in the band as starter fertilizer, and the rest is applied as a
sidedress or topdress during the growing season. Sometimes,
split applications of potassium are recommended for very sandy soils
with low organic matter, especially if there has been enough rainfall
to cause potassium to leach into the subsoil. Unfortunately, relying
on sidedressing nitrogen can increase risk of reduced yields if
the weather is too wet to apply the fertilizer (and you haven't
put on enough preplant or as starter) or too dry following an application.
Then the fertilizer stays on the surface instead of washing into
the root zone.
Once the soil nutrient status is optimal, try to balance
farm nutrient inflows and outflows. When nutrient levels, especially
phosphorus, are in the high or very high range, stop application
and try to "draw down" soil test levels.
Oxide vs. Elemental Forms?
When talking about using fertilizer phosphate
or potash, the oxide form is usually assumed. This is used
in all recommendations and when you buy fertilizer. When you
put 100 lbs. of potash per acre, you actually applied 100
lbs. of K2O that's the equivalent of 83 lbs. of
elemental potassium. Of course, you're not really using K2O
but rather something like muriate of potash (KCl). It's the
same idea for phosphate and 100 lbs. of P2O5
per acre is the same as 44 lbs. of P and you're really using
fertilizers like concentrated superphosphate (that contains
a form of calcium phosphate) or ammonium phosphate. |
Tillage and Fertility Management: To Incorporate or Not?
With systems that provide some tillage, such as moldboard plow and
harrow, disk harrow alone, chisel plow, zone-till, and ridge-till,
it is possible to incorporate fertilizers and amendments. However,
when using no-till production systems, it is not possible to mix
materials into the soil to uniformly raise the fertility level in
that portion of the soil where roots are especially active.
The advantages of incorporating fertilizers and amendments
are numerous. Significant quantities of ammonia may be lost by volatilization
when the most commonly used solid nitrogen fertilizer, urea, is
left on the soil surface. Also, nutrients remaining on the surface
after application are much more likely to be lost in runoff during
rain events. Although the amount of runoff is usually lower with
reduced tillage systems than with conventional tillage, the concentration
of nutrients in the runoff may be quite a bit higher.
Soil Tests
Soil tests, one of the key nutrient management tools,
are discussed in detail in chapter 19. |
If you are thinking about changing from conventional
tillage to no-till or other forms of reduced tillage, you might
consider incorporating needed lime, phosphate, and potash, as well
as manures and other organic residues, before making the switch.
It's the last chance to easily change the fertility of the top 8
or 9 inches of soil.
Sources
Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA).
1997. Nutrient Management. Best Management Practices Series.
Available from the Ontario Federation of Agriculture, Toronto, Ontario
(Canada).
Parnes, R. 1990. Fertile Soil: A Grower's Guide
to Organic and Inorganic Fertilizers. agAccess, Davis, CA. Soil
Tests Soil tests, one of the key nutrient management tools, are
discussed in detail in chapter 19.
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