18.5: Using Fertilizers and Amendments

There are four main questions to ask 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.

Nutrient Sources: Commercial Fertilizers Versus Organic Materials

Numerous fertilizers and amendments 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 and leached out, requires more frequent lime additions. The production of nitrogen fertilizers is also very energy intensive; it is estimated that, aside from solar energy that the crop uses, N fertilizers account for 25%–50% of the energy that goes into growing a corn crop. In addition, the high nutrient solubility can result in salt or ammonia damage to seedlings when excess fertilizer is applied close to seeds or plants. Nutrients in commercial fertilizers are readily available and allow for more precise timing with crop uptake, but if managed improperly they may become more readily lost to the environment compared to organic nutrient sources. (On the other hand, high rainfall events on a field with recently plowed-down alfalfa or applied manure may also result in significant nitrate leaching below the root zone.) Slow-release forms of synthetic nitrogen fertilizers such as sulfur- or polymer-coated urea help better match N availability to crop needs. Similarly, adding nitrification and urease inhibitors can facilitate more efficient nitrogen fertilizer use. Organic fertilizers generally contain a significant slow-release portion of N but are of such variable composition that it is difficult to know how much will be released in a given time. Feather meal, a commercially available processed organic fertilizer, is about 12%–13% nitrogen, with most released slowly.

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 following recommendations based on soil tests, not applying more nutrients than the crop can use, and applying in ways and at times that minimize losses to the environment. 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. With grazing animals it can take a very long time because so few nutrients are being exported from the field and farm in animal products. On the other hand, when hay is harvested and sold off the farm, P drawdown can happen more rapidly.

Table 18.2 Composition of Various Common Amendments and Commercial Fertilizers (%)

	N	P2O5	K2O	Ca	Mg	S	Cl
N materials
Anhydrous ammonia	82
Aqua ammonia	20
Ammonium nitrate	34
Ammonium sulfate	21					24
Calcium nitrate	16			19	1
Urea	46
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)	18	46
Monoammonium phosphate (MAP)	11–13	48–52
K materials
Potassium chloride (muriate of potash)			60				47
Potassium–magnesium sulfate (“K-Mag”)			22		11	23	2
Potassium sulfate			50		1	18	2
Other materials
Gypsum				23		18
Limestone, calcitic				25–40	0.5–3
Limestone, dolomitic				19–22	6–13		1
Magnesium sulfate				2	11	14
Potassium nitrate	13		44
Sulfur (elemental S, gypsum, ammonium sulfate)						30–99
Wood ashes		2	6	23	2

Are Organic Nutrient Sources Better for Soil and the Environment than Synthetic Fertilizers? The Answer is Complicated!

It is recommended to include organic nutrient sources as part of a nutrient management program to sustain soil health because they feed the plants while also better supporting soil biological functions. But on many farms commercial fertilizers are required to achieve good yields. Due to the structure of agriculture with associated nutrient flows (especially exports from grain production areas) and to the current inefficiencies in cropping systems, commercial fertilizers remain essential to feeding a growing global population. In fact, completely eliminating commercial fertilizers would not only cause a breakdown of the global food system, it would also negatively affect soil health. Inadequate nutrition of crops would reduce carbon capture from the atmosphere, biomass production and yields, and thereby also fresh carbon and nutrient supplies for the soil. Additional nutrients are critical to building organic matter in depleted soils (every ton of new carbon stored in the soil requires about 200 pounds of additional nitrogen and 30 pounds of phosphorus). Therefore, although organic matter is critical to building soil health, commercial fertilizers may be needed to achieve our goals.

At the global scale, commercial fertilizers are still critical to meeting the demands of our growing population until better practices (cover crops, better rotations, decreased tillage, integrating animal and plant agriculture, cooperating with nearby farms and towns, etc.) are used to lessen nutrient flows off the farm and until farms obtain more nutrients from local sources (legumes, leaves, composts, collected kitchen wastes, manures, clean sludges [biosolids]).

Regarding environmental losses, it is commonly assumed that the use of organic nutrient sources always results in lower impacts. This is only true if good management practices are followed. A study in Sweden compared conventional and organic crop production and found similar nitrate leaching losses. For example, in temperate climates a plowed alfalfa sod or a large manure application releases a lot of inorganic nitrogen that can easily meet all the needs of the following corn crop. However, if alfalfa is plowed 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 can be lost through leaching or denitrification over the winter and spring. In this case N losses might be as high as when N fertilizer is applied too early. Organic sources may also create a problem with nutrient runoff if left on the surface, or with leaching when applied out of sync with plant uptake. While using organic nutrient sources has greater benefits for soil health than commercial fertilizers, the environmental impacts in both cases are best addressed through good agronomic management including 4R practices and careful consideration of environmental impacts.

Organic sources of nutrients have many other good qualities. Compared to commercial fertilizers that only “feed the plants,” organic materials also “feed the soil,” increasing biological activity by providing soil organisms with sources of energy as well as nutrients. Aggregates and humus are formed as organisms use the added organic materials. Organic sources can provide a more slow-release source of fertility, and the N availability better coincides with 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. Applying it based on N needs of plants will therefore load the soil with unneeded P, increasing the pollution potential of any runoff. A lot of N is commonly lost during the composting process, making the compost much richer in P relative to N. Thus, applying a large quantity of compost to a soil that has sufficient P might supply a crop’s N needs but enriches the soil in unneeded P, creating a greater pollution potential.

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 manure handling and application. For cover crops, the N contribution depends on the species, the amount of growth in the spring and the weather. In addition, manures typically are bulky and may contain a high percentage of water, so considerable effort is required 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 the action of soil organisms. Their activities change with temperature and rainfall. Finally, the relative nutrient concentrations for a particular manure 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.

Understanding The Terms: Organic Farming Versus Organic Nutrient Sources

For some, there is confusion around the term “organic.” We have used the term “organic sources” of nutrients to refer to nutrients contained in crop residues, manures and composts—i.e., the nutrients are applied in organic forms. All farmers, “conventional” and “organic,” use these types of materials. Both also use limestone and a few other materials. However, most of the commercial fertilizers listed in Table 18.2 are not allowed in organic production because they are synthetically derived. 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 blood meal (see Table 18.3). Finally, to make matters more confusing, many countries, especially in Europe, label products as “bio” or “biological” when they are grown using organic practices.

Selection of Commercial Fertilizer Sources

There are numerous forms of commercial fertilizers given in Table 18.2. 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 is 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 or dry sandy soil 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.

The choice of fertilizer should be based on the nutrient needs of the crop and their availability in the soil (ideally determined by a soil test). However, the availability of the right fertilizer source may depend on the region. In countries with sophisticated agricultural supply infrastructures (like North America and Europe), farmers have a lot of choices of fertilizer materials and blends that match their needs. But in many developing countries fertilizer markets are underdeveloped and products are more expensive due to high transportation costs. (A 2011 study found fertilizer prices in Sub-Saharan Africa to be four times higher than in Europe). This limits the choices of fertilizer materials, and oftentimes farmers use only one or two fertilizer types (like DAP) without knowing the true crop needs.

Table 18.3 Products Used by Organic Growers to Supply Nutrients

	% N	% P2O5	% K2O
Alfalfa pellets	2.7	0.5	2.8
Blood meal	13	2	—
Bone meal	3	20	0.5
Cocoa shells	1	1	3
Colloidal phosphate	—	18	—
Compost	1	0.4	3
Cottonseed meal	6	2	2
Fish scraps, dried and ground	9	7	—
Granite dust	—	—	5
Greensand	—	—	7
Hoof and horn meal	11	2	—
Linseed meal	5	2	1
Rock phosphate	—	30	—
Seaweed, ground	1	0.2	2
Soybean meal	6	1.4	4
Tankage	6.5	14.5	—
Feather meal	11-13	—	—
Notes: 1. Values of P2O5 and K2O represent total nutrients present. For fertilizers listed in Table 18.2, the numbers are the amount that are readily available. 2. Organic growers also use potassium magnesium sulfate (“sul-po-mag” or “K-mag”), wood ashes, limestone and gypsum (listed in Table 18.2). 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. (See materials listed by the Organic Materials Review Institute (OMRI) at www.omri.org.) Source: R. Parnes (1990)

Method and Timing of Application

Fertilizer application timing and application methods are frequently related, so in this section both will be reviewed together.

Broadcast fertilizer application is evenly distributed over the whole field using a spin applicator (for granules) or sprayer (for liquids). If using plow or harrow tillage, it would usually be incorporated during tillage. Broadcasting 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. When using no-till, nutrients tend to be more stratified and care should be taken to lessen potential runoff that would be enriched in phosphorus—routine cover cropping will especially help. Broadcasting (with or without incorporation) usually occurs 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 over the soil surface.)

There are various methods of applying localized placement of fertilizer. Liquid nitrogen is often injected into the soil in bands because it reduces the potential for losses. Banding smaller amounts of fertilizer to the side and below the seed (usually two inches away) at planting is also 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. While liquid starter fertilizer applied along with the seed at planting has proven successful in no-till planting of small grains, nitrogen rates need to be matched to soil type and planter type, and to row and seed spacing to avoid salt or ammonia damage.

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 can be applied before planting or in the starter fertilizer band, and the rest can be applied as a sidedress or topdress during the growing season. In almost all situations sidedressing a good portion of needed N fertilizer is recommended for efficient use. However, this can increase the risk of reduced yields if the weather is too wet to apply the fertilizer (and you haven’t put on enough N in a preplant or starter application) or is too dry following an application. In the latter case the fertilizer stays on the surface instead of washing into the root zone. Although unusual nationally, recommendations for split K applications are made for very sandy soils with low organic matter, such as on Georgia’s coastal plain, especially if there has been enough rainfall to cause K to leach into the subsoil. Almost all commercial vegetable farmers use irrigation and can easily apply fertilizer through the watering system during the season (this is called “fertigation”). This is especially attractive with drip irrigation, which allows spoon feeding of the crop to maximize nutrient uptake efficiencies. Fertigation of agronomic row crops is common in some regions, frequently by center pivot systems.

Crop Value, Fertilizer Cost And Fertilizer Rates

Most agronomic crops grown on large acreages are worth around $400–$1,000 per acre, and the fertilizer used may represent 25% of non-land growing costs. So, if a corn farmer uses 100 pounds of N that’s not needed (at about $40), that may represent 5% or more of gross income. Add in some unneeded P and K and the implications for lost net revenue become clear. 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 of no fertilizer, 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 while some of the P and K might be available to crops in future years, the $14,000 they spent on fertilizer was mostly 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, fertilizer costs are dwarfed by other costs, such as hand labor. A waste of $70 per acre in unneeded nutrients for these crops would cause a minimal economic penalty, assuming you maintain a reasonable balance between nutrients, but there may be environmental and crop quality reasons against applying too much fertilizer. In general, relative nutrient expenses are greatest for the low-value crops, but these are also grown on the most extensive acres where cumulatively they have the biggest environmental impacts.

Fertilizer Grade: Oxide Versus Elemental Forms

When talking or reading about fertilizer P or K, the oxide forms are used. They are also 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. In fact, their use is codified in state regulations in the United States and by national regulations in Canada. 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 are really not using K₂O but are rather using something like muriate of potash (KCl). It’s similar with 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 not in actual amounts of elemental P or K you purchase or apply.

Tillage and Fertility Management: To Incorporate or Not?

It is possible to incorporate fertilizers and amendments with systems that provide some tillage, such as moldboard plow and harrow, disk harrow alone, chisel plow, zone/strip-till and ridge-till. However, when using pure no-till production systems, it is not possible to mix fertilizer materials into the soil to uniformly raise the fertility level in that portion of the soil where roots are especially active. However, surface-applied fertilizers in no-till systems usually work their way down to the upper part of the root zone.

When broadcasting fertilizer without incorporation, as occurs with no-till, there are potential losses that can occur. For example, significant quantities of ammonia may be lost by volatilization when the most commonly used solid N fertilizer, urea, is left on the soil surface. Thus if rainfall isn’t going to occur very soon after application, another solid source of N fertilizer or a liquid fertilizer should be used. 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. This makes using cover crops as a routine management practice even more important. Over time, using no-till and cover crops, rainfall infiltration rates tend to increase, lessening runoff.

A special concern exists with heavy clay soils that develop continuous macropores from cracks and biological activity (especially deep-burrowing earthworms). Although this is generally good for the health of the soil and crop growth, it can also pose concerns with fertilizer and manure applications when the soils have subsurface (tile) drainage. When materials applied on the soil surface are not incorporated, nutrients can readily enter the macropores with heavy rains, move rapidly to the tile lines and then discharge into waterways.

Soil Tests

Routine soil tests, one of the key nutrient management tools, are discussed in detail in Chapter 21. For newer soil health tests see Chapter 23.

If you are thinking about changing from conventional tillage to no-till or other forms of reduced tillage, incorporate needed lime, phosphate and potash, as well as manures and other organic residues, before making the switch. It’s your last chance to easily change the fertility of the top 8 or 9 inches of soil.