| Soils, Crops and Fertilizer Use |
|Chapter 9: Using chemical fertilizers|
NOTE: Appendix D lists the nutrient content of common chemical fertilizers.
Nearly all chemical N fertilizers contain either ammonium (NH4+) or nitrate (NO3-) nitrogen. The nitrate form is quicker acting because it's more immediately mobile (leachable) and reaches the roots sooner if applied to a growing crop. But, remember that ammonium is rather quickly converted to mobile nitrate in warm soils (all of it within 7-10 days).
N fertilizers and soil pH: Most N fertilizers containing ammonium N have a gradual acidifying effect on the soil; this will be covered in detail farther along.
Loss of N by volatilization: All ammonium N fertilizers will release ammonia gas when applied to soils with pH's above 7.0. If applied to the soil surface, significant amounts may be lost to the atmosphere. Urea fertilizer releases ammonia at any pH. Losses can be avoided by placing such fertilizers a few centimeters deep.
Common Nitrogen Fertilizers
Ammonium Nitrate (33-34% N)
• Contains half nitrate N and half ammonium N, so is quicker acting than straight ammonium fertilizers.
• Absorbs moisture and becomes slushy in high humidity; keep bags well sealed.
• Can become explosive if mixed with oil. Releases oxygen when exposed to fire which encourages combustion.
Ammonium Nitrate with Lime (26% N
• Same as above but is coated with dolomitic limestone to neutralize the acid-forming properties of regular ammonium nitrate and to reduce moisture absorption.
Ammonium Sulfate (20-21% N)
• In addition to N, it contains 23% sulfur (or 69% sulfate).
• Good handling and storage properties
Urea (45-46% N)
• The highest-strength solid form of N.
• Its N is initially in the amide form (NH2) but is converted to ammonium in moist warm soils within 1-2 days (a week or two in cooler soils) and then to nitrate by soil bacteria.
• Unlike ammonium N fertilizers, urea is mobile and leachable until its amide N has been converted to ammonium.
• Regardless of soil pH, some N will be lost to the atmosphere as ammonia gas if urea is left on the soil surface. Losses are highest above a soil pH of 7.0 and can reach 35% when urea is broadcast (spread) over grass pastures. Losses are minimal, however, if rainfall or irrigation occur within a few hours after such surface applications.
• Can "burn" (injure) seeds and seedlings if placed too close due to release of free ammonia.
• May sometimes contain excessive amounts of biuret ( toxic to plants) due to faulty manufacturing. Biuret is most toxic when urea is mixed with water and applied foliarly ( sprayed on the leaves).
• Tends to absorb moisture, but not as much as ammonium nitrate.
• Can be fed to ruminants like cattle as a protein source; the rumen bacteria convert the N to protein; BUT urea can be toxic at anything but very low levels and must be fed in combination with certain other feeds. Vinegar is the antidote.
Sodium Nitrate (16% N) (Chilean nitrate)
• Its nitrate N is readily leachable.
• Unlike most ammonium N fertilizers, it has a gradual basic effect on the soil.
• Can easily burn seeds and seedlings because of its very high salt content. (Fertilizer burn is covered farther along)
• Absorbs moisture and can become slushy in high humidity; keep bags well sealed.
• Expensive because of its low nutrient content relative to shipping costs.
Anhydrous Ammonia (82% N)
• Exists as a liquid under pressure and a gas when released into the soil.
• The highest-strength N fertilizer available.
• Must be injected into moist soil about 15 cm deep to avoid ammonia loss.
• Very dangerous; inhalation and facial exposure can cause blindness and fatal lung damage.
• Requires special storage and application equipment.
Aqua Ammonia (21% N)
• Made by dissolving ammonia gas in water. Has strong odor of ammonia. Unlike anhydrous ammonia, it doesn't have to be applied or stored under pressure.
• Should be applied at least 4-5 cm below the soil surface to avoid loss of ammonia.
• Requires special storage and application equipment.
• Releases irritating fumes.
Potassium Nitrate (13-0-44: See under K fertilizers.
Ammonium Phosphate Fertilizers: See under P fertilizers.
Time-Release or Slow-Release N Fertilizers: They're coated with special substances that reduce their solubility and slow down the rate at which soil bacteria convert ammonium to nitrate. Leaching losses are much lower, but they're usually too expensive to be cost effective for farmers.
The phosphorus in most chemical fertilizers comes from reacting rock phosphate with sulfuric, phosphoric, or nitric acids or with anhydrous ammonia.
Water-soluble vs. Citrate-soluble vs. Insoluble P
A chemical fertilizer's P can exist in several forms which should be listed on the label:
Water-soluble P: This type of P is soluble in water and moves quickly out of the granules into the soil. But, that doesn't mean it will be 100 percent available to plants, because it's still subject to the soil's ability to tie up (fix) P. When P fertilizer is placed in a band, hole, or half-circle near the row, it's recommended that at least half the fertilizer's P be water-soluble. When P fertilizer is broadcast on soils below pH 7.0, water solubility isn't important, because soil acidity helps dissolve the P.
Citrate-soluble P: This type of P isn't soluble in water but will dissolve in a weak acid solution. Heat-treated rock phosphate contains largely citrate-soluble P which is usable only in acidic soils.
Insoluble P: This type of P isn't soluble in water or a weak acid solution, so it has very limited availability to plants. Most of the P in raw rock phosphate is insoluble and only very slowly available, even in acid soils.
Common Phosphorus Fertilizers
Single Superphosphate (16-22% P2O5, 8-12% S): A common P fertilizer and also a good sulfur source. About 78% of its P is water soluble (see above). Made from rock phosphate and sulfuric acid.
Triple or Concentrated Superphosphate (42-48% P2O5): Has much more P than single super but only 1-3% sulfur. About 84% of its P is water soluble. Made from rock phosphate and phosphoric acid.
Ammonium Phosphate Fertilizers
There are 3 classes, all with 100% water-soluble P:
• Mono-ammonium phosphate (11-48-0, 12-61-0): Tends to work better than all-ammonium phosphate on alkaline soils. Low in sulfur. Less likely to cause burning than DAP.
• Di-ammonium phosphate (16-48-0, 18-46-0, 21-53-0): A good P source but can injure seeds or seedlings due to ammonia release if placed too close.
• Ammonium Phosphate sulfate (16-20-0, 13-39-0): Both are also good sources of sulfur (915% S in 16-20-0, 7% S in 13-39-0).
Miscellaneous NP and NPK Fertilizers: 20-20-0, 14-14-14, 12-24-12, etc.
Heat-treated Rock Phosphates: These vary a lot in P content and are made by heat treating rock phosphate which greatly increases its low availability. Its P isn't watersoluble but is citrate-soluble (see above) and will slowly become available in acid soils when broadcast. It may be a cheap P source in areas with phosphate deposits but is only recommended for acid soils or where organic matter is very high. It should be in a finelyground form and be applied by broadcasting to promote the release of its P through soil reaction. It doesn't become available quickly enough to be used as the sole source of added P for short-term annual crops like maize. Much higher rates are needed than for more available forms. Where mycorrhizae soil fungi are abundant (see Chapter 1), they increase the availability of rock phosphate to plant roots.
Raw rock phosphate: See Chapter 8.
Basic Slag (8-25% P2O5) A by-product of steel making. About 60-90% of its P is citrate soluble, so it's best used on acid soils, much like heat-treated rock phosphate. It has a gradual basic effect on soils.
The most common K fertilizers are:
• Potassium chloride (muriate of Potash): Contains about 60%-62% K2O
• Potassium sulfate: Contains about 48-50% K2O and 18% S.
• Potassium nitrate (13-0-44).
• NPK fertilizers like 10-20-10, etc.
NOTE: Tobacco, potatoes, and sweet potatoes are sensitive to high amounts of chlorides which affect crop quality. In this case, potassium chloride should be avoided or minimized.
SECONDARY NUTRIENT FERTILIZERS (Calcium, Magnesium, Sulfur)
Calcium and Magnesium
Even acid soils have enough calcium for most crops. Where liming is needed and magnesium is also deficient, dolomitic limestone (a mixture of calcium and magnesium carbonates) should be used. Liming with calcium only can also provoke a Mg deficiency. Gypsum has no effect on soil pH and is often used to supply calcium to crops with high needs, such as peanuts, without raising the pH.
Magnesium sulfate (epsom salts; 9-11% Mg) and potassium magnesium sulfate (11% Mg) are other sources and have no effect on soil pH. The Mg content of fertilizers is often expressed in terms of magnesium oxide (MgO); the conversion is: Mg x 1.66 = MgO MgO x 0.6 - Mg
Some common fertilizers are good S sources like single superphosphate (8-12% S), ammonium sulfate (23-24% S), 16-20-0 (9-15% S), and potassium sulfate (17% S). Usually, the higher the NPK content of the fertilizer, the lower the S content (i.e. triple superphosphate contains only 1-3% S).
Sulfur deficiencies are on the increase in non-industrial areas, due to the growing use of high-analysis fertilizers with lower S contents. It's usually a good idea to include a sulfurbearing fertilizer in a fertilizer program, especially on acid, sandy soils. Organic fertilizers are a good source of S. Appendix D lists the S content of chemical fertilizers.
The S content of fertilizers is often expressed in terms of SO4 (sulfate). The conversion is: S x 3 = SO4
Some NP and NPK fertilizers may have added amounts of micronutrients (check the label) but usually too little to correct deficiencies. If a meaningful amount of a micronutrient is present, it may be indicated by a fourth number in the fertilizer formula, referring to it.
Separate micronutrient fertilizers like copper sulfate, ferrous sulfate (iron), zinc sulfate, manganese sulfate, and borax can be used for soil or foliage (leaf) application. Remember that soil tie-up of added manganese and iron is often a problem on deficient soils (see Chapter 6).
Micronutrient chelates: Specially synthesized forms of micronutrients called chelates are available and used where soil tie-up problems are serious. A chelate has a special molecular structure that protects the micronutrient from being tied up.
Some fungicides like Maneb (containing manganese) and Zineb (containing zinc) can supply these micronutrients in conjunction with a disease control program.