Building Soils for Better Crops, Third Edition

Using Fertilizers and Amendments

SARE Outreach
Fred Magdoff and Harold van Es | 2010 | 294 pages
PDF (6.8 MB)

This title is temporarily out of print. We expect to publish an updated edition in the spring/summer of 2021.

There are four main questions 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 21 details the 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.

Table 18.1: Composition of Various Common Amendments and Commercial Fertilizers (%)
 NP2O5K2OCaMgSCl
N Materials
Anhydrous ammonia82      
Aqua ammonia20      
Ammonium nitrate34      
Ammonium sulfate21    24 
Calcium nitrate16  191  
Urea46      
UAN solutions (urea + ammonium nitrate)28–32      
P and N+P Materials
Superphosphate (ordinary) 20 20 12 
Triple superphosphate 46 14 1 
Diammonium phosphate (DAP)1846     
Monoammonium phosphate (MAP)11–1348–52     
K Materials
Potassium chloride (muriate of potash)  60   47
Potassium–magnesium sulfate (“sul-po-mag”)  22 11232
Potassium sulfate  50 1182
Other Materials
Gypsum   23 18 
Limestone, calcitic   25–400.5–3  
Limestone, dolomitic   19–226–13 1
Magnesium sulfate   21114 
Potassium nitrate13 44    
Sulfur     30–99 
Wood ashes 26232  

 

Nutrient Sources: Commercial Fertilizers vs. Organic Materials

There are numerous fertilizers and amendments that are normally used in agriculture (some are listed in table 18.1). 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 are well established. The timing, rate, and uniformity of nutrient application are easy to control when using commercial fertilizers. However, there also are drawbacks to using commercial fertilizers. All of the commonly used N 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. The production of nitrogen fertilizers is also very energy intensive—it’s estimated that N fertilizers account for 25% to 30% of the energy that goes into growing a corn crop. 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 when both are used properly. For example, high rainfall events on a sandy soil soon after ammonium nitrate fertilizer application will probably cause more nitrate loss than if compost had been applied. (On the other hand, high rainfall events on a recently plowed-down alfalfa field may also result in significant nitrate leaching below the zone that roots can reach.) 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. Of course, soils overloaded with either inorganic or organic sources of nutrients can be large sources of pollution. The key to wisely using either commercial fertilizers or organic sources is not applying more nutrients than the crop can use and applying in ways that minimize losses to the environment.

DO ORGANIC NUTRIENT SOURCES REDUCE ENVIRONMENTAL IMPACTS? IT DEPENDS!

It is commonly assumed that the use of organic nutrient sources always results in lower environmental impacts. This is generally true, but only if good management practices are followed. For example, in temperate climates a plowed alfalfa sod releases a lot of organic nitrogen that can easily meet all the needs of the following corn crop. But if the plowing is done too early—for example, in the early fall—much of the organic N is mineralized in the following months when the soil is still warm and then lost through leaching or denitrification over the winter and spring. A study in Sweden compared conventional and organic crop production and found similar nitrate leaching losses. Organic sources like manure may create a problem with nutrient runoff if left on the surface, or with leaching when applied in the fall. So, even when using organic nutrient sources, good agronomic management and careful consideration of environmental impacts are essential.

Organic sources of nutrients have many other good qualities, too. Compared to commercial fertilizers that only “feed the plants,” organic materials also “feed the soil.” They are also sources of soil organic matter, providing food for soil organisms that aid in forming aggregates and humus. Organic sources can provide a more slow-release source of fertility, and the N availability is frequently 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 a particular soil and crop; thus, routine soil testing is important. Poultry manure, for example, has about the same levels of N and P, but plants take up three to five times more N than P. During the composting process a lot of N is commonly lost, making the compost much richer in P relative to N. Thus, applying a large quantity of compost to a soil might supply a crop’s N needs but serve to enrich the soil in unneeded P, creating a greater pollution potential.

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” and “organic.” Both also use limestone and a few other materials. However, most of the commercial fertilizers listed in table 18.1 are not allowed in organic production. In place of sources such as urea, anhydrous ammonia, diammonium phosphate, 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 18.2)

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 on the type of animal, its diet, and how the manure is handled. For cover crops, the N contribution depends on the species, the amount of growth in the spring, and the 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 N and P when your soil already has high P levels.

Selection of Commercial Fertilizer Sources

Table 18.2: Products Used by Organic Growers to Supply Nutrients
 %N%P2O5%K2O
Alfalfa pellets2.70.52.8
Blood meal13.02.0
Bone meal3.020.00.5
Cocoa shells1.01.03.0
Colloidal phosphate18.0
Compost1.00.43.0
Cottonseed meal6.02.02.0
Fish scraps, dried & ground9.07.0
Granite dust5.0
Greensand7.0
Hoof & horn meal11.02.0
Linseed meal5.02.01.0
Rock phosphate30.0
Seaweed, ground1.00.22.0
Soybean meal6.01.44.0
Tankage6.514.5
Values of P2O5 and K2O represent total nutrients present. For fertilizers listed in table 18.1, the numbers are the amount that are readily available.Organic growers also use potassium–magnesium sulfate (“sul-po-mag” or “K-mag”), wood ashes, limestone, and gypsum (listed in table 18.1). Although some use only manure that has been composted, others will use aged manures (see chapter 12). There are also a number of commercial organic products with a variety of trade names.Source: R. Parnes (1990).

It is recommended to include organic fertilizer sources as part of a nutrient management program to sustain soil health, but on many farms additional commercial fertilizers are still needed to achieve good yields. On the global scale, until better practices (use of cover crops, better rotations, decreased tillage, and integrating animal and plant agriculture, etc.) are used on farms, commercial fertilizers are still needed to meet the demands of our growing population. There are numerous forms of commercial fertilizers, many given in table 18.1. When you buy fertilizers in large quantities, you usually choose the cheapest source. 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 or a 20-10-10 (both referring to the percent of available N, P2O5, and K2O) or another blend. However, below are a number of examples of situations in which you might not want to apply the cheapest source:

  • Although the cheapest N form is anhydrous ammonia, the problems with injecting it into a soil with many large stones or the losses that might occur if you inject it into very moist clay may call for other N sources to be used instead.
  • If both N and P are needed, diammonium phosphate (DAP) is a good choice because it has approximately the same cost and P content as concentrated superphosphate and also contains 18% N.
  • Although muriate of potash (potassium chloride) is the cheapest K 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 practices together.

Broadcast application, in which 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 P and K when they are very deficient. Broadcasting with incorporation is usually done in the fall or in spring just before tillage. Broadcasting on top of a growing crop, called topdressing, is commonly used to apply N, 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 methods of applying 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—early in the season, for example—on soils with high amounts of surface residues, with no-till management, or on wet soils that are slow to warm in the spring. It is also useful for soils that test low to medium (or even higher) in P and K. Band placement of fertilizer near the seed 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 N as part of the starter fertilizer appears to help roots use fertilizer P more efficiently, perhaps because N stimulates root growth. Starter fertilizer for soils very low in fertility frequently contains other nutrients, such as sulfur, zinc, boron, or manganese.

Splitting N 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 can be lost by denitrification. Some N 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. Although unusual, sometimes split applications of K are recommended for very sandy soils with low organic matter, especially if there has been enough rainfall to cause K to leach into the subsoil. Unfortunately, relying on sidedressing N can increase the 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 for the fertilizer to come into contact with roots. 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 P, are in the high or very high range, stop application and try to maintain or “draw down” soil test levels. It usually takes years of cropping without adding P to lower soil test P appreciably.

CROP VALUE, FERTILIZER COSTS, AND FERTILIZER RATES

The cost of N fertilizer is directly tied to energy costs, because so much energy is used for its manufacture and transport. The costs of other fertilizers are less sensitive to fluctuating energy prices but have been increasing, nevertheless. Use of fertilizers has increased worldwide, and dwindling global reserves combined with the increase in fuel and other input costs to manufacture them have recently led to large price increases.

Most agronomic crops grown on large acreages are worth around $400 to $1,000 per acre, and the fertilizer used may represent 30% to 40% of out-of-pocket growing costs. So, if you use 100 pounds of N you don’t need, that’s perhaps around $65/ acre and may represent 10% or more of your gross income. Some years ago, one of the authors of this book worked with two brothers who operated a dairy farm in northern Vermont that had high soil test levels of N, P, and K. Despite his recommendation that no fertilizer was needed, the normal practice was followed, and N, P, and K fertilizer worth $70 per acre (in 1980s prices) was applied to their 200 acres of corn. The yields on 40-foot-wide, no-fertilizer strips that they left in each field were the same as where fertilizer had been applied, so the $14,000 they spent for fertilizer was wasted.

When growing fruit or vegetable crops—worth thousands of dollars per acre—fertilizers represent about 1% of the value of the crop and 2% of the costs. But when growing specialty crops (medicinal herbs, certain organic vegetables for direct marketing) worth over $10,000 per acre, the cost of fertilizer is dwarfed by other costs, such as hand labor. A waste of $65/acre in unneeded nutrients for these crops would cause a minimal economic penalty—assuming you maintain a reasonable balance between nutrients—but there may also be environmental reasons against applying too much fertilizer.

FERTILIZER GRADE: OXIDE VS. ELEMENTAL FORMS?

When talking or reading about fertilizer P or K, the oxide form is usually assumed. This is used in all recommendations and when you buy fertilizer. The terms “phosphate” (P2O5) and “potash” (K2O) have been used for so long to refer to phosphorus and potassium in fertilizers, it is likely that they will be with us indefinitely—even if they are confusing. When you apply 100 pounds of potash per acre, you actually apply 100 pounds of K2O—the equivalent of 83 pounds of elemental potassium. Of course, you’re really using not K2O but rather something like muriate of potash (KCl). A similar thing is true of phosphate—100 pounds of P2O5 per acre is the same as 44 pounds of P—and you’re really using fertilizers like concentrated superphosphate (that contains a form of calcium phosphate) or ammonium phosphate. However, in your day-to-day dealing with fertilizers you need to think in terms of nitrogen, phosphate, and potash and don’t worry about the actual amount of elemental P or K you purchase or apply.

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.

SOIL TESTS

Soil tests, one of the key nutrient management tools, are discussed in detail in chapter 21.

The advantages of incorporating fertilizers and amendments are numerous. Significant quantities of ammonia may be lost by volatilization when the most commonly used solid N 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.

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.