Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 16

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.

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

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.

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

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.

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).

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.