Types of Fertilizers used for Crop Production

This article throws light upon the five main types of fertilizers used for crop production. The types are: 1. Nitrogenous Fertilizers 2. Phosphatic Fertilizers 3. Potassic Fertilizers 4. Blended Fertilizers 5. Liquid Fertilizers.

Type # 1. Nitrogenous Fertilizers:

Nitrogenous fertilizers are those fertilizers that are sold for their nitrogen content.

Nitrogenous fertilizers can be classified into four classes namely:

(i) Nitrate nitrogen e.g. NaNO₃^– 16% N; Ca (NO₃)₂^– 15.5% N

(ii) Ammonium containing N fertilizers e.g. Ammonium sulphate,

(NH₄)₂SO₄—20% N; Ammonium chloride—24% N; Anhydrous ammonia—82% N

(i) Both ammonium and nitrate containing fertilizers e.g. Ammonium nitrate (NH₄NO₃)—33% N; Calcium ammonium nitrate (CAN)—20% N.

(ii) Amide fertilizers e.g. Urea—46% N; Calcium Cyanamide (CaCN₂)—21% N.

However, the most common and important N fertilizers are ammonium sulphate and urea which are used for crop production.

Characteristics:

Many different chemical and physical forms of nitrogen (N) fertilizers exist. Some of the more common fertilizer nitrogen sources are given here. The nitrogen in most farm-grade fertilizers is readily available. Some fertilizers, such as “turf-grade” fertilizers, release nitrogen very slowly. Plants can use nitrogen in one of two forms: ammonium nitrogen (NH₄⁺) or nitrate nitrogen (NO₃^–).

Ammonium nitrogen (NH₄⁺) carries a positive charge and is adsorbed into soil particles. In this chemical form, leaching of nitrogen does not occur; however, NH₄⁺ is changed to the NO₃^– form by bacteria. Complete conversion from NH₄⁺ to NO₃^– occurs within about a month of application.

Nitrate nitrogen (NO₃^–) carries a negative charge and is not adsorbed onto soil particles; it is free to be leached from the soil. Nitrate nitrogen also can be lost to the atmosphere through de-nitrification when soils become water saturated. Products called nitrification inhibitors can inhibit the conversion of N from the non-mobile ammonium form to the very mobile nitrate form. This can reduce the loss of N.

This effect is greatest under the following conditions: N applied long before crop uptake; N applied to very coarse textured soils, especially when significant rain is expected before crop uptake; and N applied to poorly drained soils, again especially when significant rain is expected before crop uptake.

Generally, even though the products may work perfectly, there is less benefit for N applied at planting or at side dressing time.

A long-term effect of all ammonium based nitrogen fertilizers is to lower soil pH. Anhydrous ammonia, urea, di-ammonium phosphate, and nitrogen solutions, when first applied, greatly but temporarily increase soil pH in the zone of application.

Ammonia is released and can “burn” germinating seeds or seedling roots in the area of fertilizer placement. In the eventual conversion of NH₄⁺ to NO₃^–, however, an acid residue is formed.

The nitrogen in urea is completely water-soluble. Upon application, urea nitrogen changes rapidly to NH₄^–N. Urea nitrogen therefore is readily available to plants on application to the soil. Urea presents another problem, in that when it is surface applied, significant quantities of nitrogen as ammonia may be lost through volatilization.

These losses happen very rapidly, with most occurring within the first day or two following application and can account for over one-third of the urea N being lost within a week after application. Losses are accelerated by warm moist soils, high pH, and surface organic matter.

Losses are higher on low cation exchange capacity (CEC) or sandy soils than on soils with a high CEC, heavy clay content, or a high organic matter content. Thus, urea or nitrogen solutions (which are approximately 50 per cent urea) should be incorporated into the soil by mechanical mixing or by water movement. Light tillage or one-half inch of rain usually is adequate.

Research also has shown that volatilization losses from nitrogen solution can be reduced significantly by dribbling the solution in a band on the surface, rather than spraying it over the entire soil surface. This can be accomplished by using drop tubes on a conventional sprayer. Urease inhibitors can also be used to effectively reduce volatilization from surface application.

These are only effective on urea-containing fertilizers and will only provide a benefit if the fertilizer is not incorporated immediately by tillage or rainfall. Urea and urea- blended fertilizers are not recommended as starter fertilizers because of possible ammonia toxicity to germinating seeds, which results in reduced plant stand.

Some Important Steps for Management Practices of Nitrogen:

i. Application of nitrogen fertilizer shall be limited to that amount necessary to meet projected crop plant needs.

ii. Application of nitrogen fertilizer shall be timed to consider as closely as possible to the period of maximum crop plant uptake.

iii. Application of nitrogen fertilizer shall be by a method designed to deliver nitrogen to the area of maximum crop plant uptake.

iv. Application of irrigation water to meet crop needs shall be managed to minimize nitrogen loss by leaching and runoff.

v. The application of irrigation water shall be timed to minimize nitrogen loss by leaching runoff.

vi. The operator shall use tillage practices that maximize water and nitrogen uptake by crop plants.

Managing Nitrogen from Manure:

Many of the same best management practices suggested for nitrogen fertilizer are relevant to manure when it is used as an N fertilizer source:

i. Apply manure close to the time of crop need.

ii. Take care to ensure uniform spread patterns.

iii. Use the soil nitrate test to guide decisions about rates for additional N. This test is more useful in manured fields than non-manured fields and can reliably indicate N availability from current or past manure applications.

iv. Use the corn stalk nitrate test to assess N availability for the past season. This test is the only one that can diagnose when N availability exceeded crop needs and can also diagnose when N availability was too low or just right.

The biggest challenge to users of manure as an N fertilizer source is determining the amount of manure to apply to provide a target ate of N. Organic nitrogen ranges from 5 to 90 per cent of the total N in manure.

Only a fraction of that N is available in the year of manure application and some organic nitrogen is never released to plants. The rest of the N in manure is likely to be lost through volatilization if it is applied on the surface.

Organic N in manure can act as a slow-release N source, providing benefit in some situations. Poultry litter (90 per cent organic N) is a superior N source for fall application on pastures when compared with commercial fertilizers such as ammonium nitrate.

If conditions are good for fall forage growth, they will also be good for N release from the litter. However, in dry conditions, litter N will stay in a form that is more likely to contribute to spring growth than the more mobile ammonium nitrate.

The following best management practices specific to manure can increase the dependability and value of manure as a nitrogen source:

i. Obtain a reliable estimate (lab analysis) of the nutrient content of the manure, including separate estimates of organic and total N.

ii. Inject low-organic-N manure sources below the soil surface when compatible with the cropping system and conservation plan. Examples of this type of manure include lagoon (shallow body of water separated from a larger body of water by barrier) effluent and liquid slurry. This practice greatly increases the reliability of manure as a source of N.

iii. Calibrate manure spreaders to ensure that you are able to apply your target rate of manure.

iv. Work with a nutrient management planner to calculate the N fertilizer value of the manure.

v. Remember that manure is also an excellent source of phosphorus, potassium, sulphur, zinc and other nutrients. Be sure to credit manure for the full nutrient value to your crop.

For producers with lagoon systems, timing decisions involve balancing two risks:

i. The risk of N loss before uptake by the crop.

ii. The risk of lagoon overflow.

In these systems it is essential to have forage land available to expand the window of opportunity for manure application.

(v) Promoting Efficient Uptake:

Sound general crop management practices are essential to produce crops that are capable of efficient uptake of N fertilizer.

The following best management practices fall within this category:

i. Avoid practices that result in soil compaction. Soil compaction reduces root growth, which is critical for efficient uptake of N and for general crop health. Minimize traffic on very moist or wet soils, limit axle loads, ensure enough tire contact area on soil to support loads, and consider using controlled traffic.

ii. Choose good hybrids or varieties.

iii. Manage soil, P, K and pH to ensure optimum levels for crop production. Soil sample each field at least every four years to guide P, K and lime applications.

iv. Carefully set planters and drills and check their performance (seed depth, spacing, soil contact and slot closure).

v. Prevent weeds from getting too large. Large weeds can drain substantial amounts of N away from the crop, even if they are successfully controlled later.

Type # 2. Phosphatic Fertilizers:

Phosphatic fertilizers can be classified into three groups on the basis of forms in which orthophosphoric acid is combined with calcium.

(i) Water soluble mono-calcium phosphate, e.g. Single Super Phosphate, 16% P₂O₅ (SSP), Double Super Phosphate, 32% P₂O₅ (DSP) and Tripple Superphosphate, 46% P₂O₅ (TSP).

(ii) Citric acid soluble, di-calcium phosphate, 34-39% P₂O₅, suitable for acid soils.

(iii) Phosphate fertilizers not soluble in water or citric acid, tri-calcium phosphate and hence not readily available to plants, rock phosphate, raw bone meal etc.

Most widely used phosphatic fertilizers are SSP, Single super phosphate. Besides these, some important complex fertilizers, of which nitro-phosphate, polyphosphate based fertilizers are also used for crop production.

Sources:

The most common phosphate fertilizers are single super phosphate (SSP), rock phosphate in acid soils, triple superphosphate (0-46-0), mono-ammonium phosphate (11-52-0), di-ammonium phosphate (18-46-0), and ammonium polyphosphate (10-34-0) liquid. All of these materials are highly water soluble.

The ammonium phosphates also are excellent nitrogen sources. Mono-ammonium phosphate and ammonium polyphosphate, either alone or with some added potassium, make excellent starter fertilizers because of their high P-to- N ratios, high water solubility, and low free ammonia.

Di-ammonium phosphate (DAP) is not recommended as a starter material because it produces free ammonia, which can harm the seed. Many starter fertilizers, however, contain DAP; thus it is critical, when that is the case, that the starter is accurately placed a safe distance (about 2 inches) from the seed, and that high rates are avoided.

Because P is relatively immobile, placement where plant roots will have easy access to the fertilizer is especially important for P fertilizers.

Characteristics:

Plants absorb most of their phosphorus from the soil solution as orthophosphate (H₂PO₄^–), regardless of the original source of phosphorus. Although orthophosphate’s negative charge prevents it from being attracted by the soil’s cation exchange capacity (CEC), it does react strongly in the soil, primarily with the large amount of iron and aluminum naturally in the soil, to form products that are very insoluble and thus unavailable to plants.

A major factor controlling these reactions is the soil pH. At low or high pH, the solubility of phosphorus (and thus its availability) is very low. The maximum availability occurs in the 6.0 to 7.0 pH range. This is another important reason to lime regularly.

The solubility of phosphorus in fertilizer varies. The legal definition of available phosphorus in fertilizer is the sum of the phosphorus that is soluble in water plus that which is soluble in a citrate solution. Regardless of the actual chemical form of the phosphorus, the analyses of phosphorus fertilizers are given as phosphate (P₂O₅).

The water solubility of this phosphorus can vary from 0 to 100 per cent. Generally, the higher the water solubility, the more effective the phosphorus source. This is especially important for short season, fast- growing crops, for crops with restricted root systems, for starter fertilizers, and for areas where less-than-optimum rates of phosphorus are applied to soils testing low in phosphorus.

Management of Phosphorous:

i. Sample the tillage layer of soil in each field on a regular basis and have soil analyzed to determine available soil P levels prior to applying P fertilizer.

ii. Credit all available P from manures and other organic residues to the P requirement for the crop.

iii. Fertilize soils with ‘low’ to ‘medium’ P soil test value using environmentally and economically sound agronomic guidelines. In general, soils testing ‘high’ will not respond to additional P and should not receive fertilizer unless a banded starter is needed to compensate for low soil temperatures. Phosphorus fertilizer should not be applied to soils testing ‘very high’ for soil P.

iv. Divide large, non-uniform fields into smaller fertility management units based upon yield potential or soil type and fertilize according to P levels determined through soil analysis.

v. Apply P fertilizers where they can be most efficiently taken up by the crop. Band application of P in the root zone reduces surface loss potential and enhances nutrient availability, especially in cold or P deficient soils.

vi. Incorporate surface applied P into the soil where any potential for surface runoff or erosion exists.

vii. Minimize soil erosion and corresponding P losses by establishing permanent vegetative cover, conservation tillage and residue management, contour farming, strip cropping, and other management practices as feasible. When erosion potential is serve, install structures such as diversions terraces, grass waterways, filter fences, and sediment basins.

viii. Maintain a buffer strip (where fertilizer and manure is not applied) a safe distance from surface water and drainage channels.

ix. Maintain grass filter strips on the downhill perimeter of erosive crop fields to catch and filter P in surface runoff.

x. Manage irrigation water to minimize runoff and erosion by meeting the irrigation BMPs or the NRCS approved Irrigation Water Management practice standard and specification.

Type # 3. Potassic Fertilizers:

The term muriate is derived from muriatic acid, a common name for hydrochloric acid; Fertilizer grade muriate contains 50-52% K (60-63% K₂O) and varies in colour from pink or red to white depending on the mining and recovery process used. Muriate of potash (KCI) is generally used as a potassic fertilizer. Besides, sulphate of potash (K₂SO₄) is also used as a potassic fertilizer.

Potassium occurs in the soil in three forms : as exchangeable (available) potassium (K⁺) adsorbed onto the soil CEC; fixed by certain minerals from which it is released very slowly to available form; and in unavailable mineral forms (most of the potassium in soils). Plants take up potassium as the K⁺ ion.

Potassium chloride is the most common K source. Red, pink, and white forms are available. These materials are equivalent as sources of K. The color in 0-0-60 is due to iron impurities that have no effect on the availability of K for crop growth.

Potassium-magnesium sulfate is a good source of K when there is also a need for magnesium in a fertilizer program. This product should be considered for fertilizing com, alfalfa, and small grains grown on sandy soils. The cost of potassium nitrate and potassium sulfate is usually higher.

Manure is also a source of K. The K content of manure varies with animal type, feed ration, storage, and handling practices. Manure should be analyzed to determine the amount of K that was applied.

Management Practices for Potassium:

Suggested management practices for K vary with crops. There is a higher probability of successful establishment of perennial crops such as rice, wheat, maize, potato, oilseeds, legumes, alfalfa and grasses if the soil test for K is in the medium range or higher.

For these crops, the best strategy would be to apply potassic fertilizers before seedling recently followed by annual top dressing. The annual applications should be based on the results of routine soil tests for K.

Any potash needed for corn and small grain production can be applied in a band near the seed at planting or broadcast and incorporated to the soil before planting. When applied in a band, the recommended broadcast rate of potash can be reduced by one-half of the recommended without causing a reduction in yield.

For best results, potash fertilizer needed for soybean production should be broadcasted and incorporated before planting. This crop will respond to potash applied in a band. In contrast to corn and small grains, however, highest yield increment is associated with broadcast applications.

Type # 4. Blended Fertilizers:

Blended fertilizers are made by physically mixing fertilizer materials to give a desired grade. The individual particles remain separate in the mixture, and segregation may occur. This problem can be reduced by using materials with the same particle size. The quality of the blended fertilizer, particularly the segregation of the individual fertilizer materials, should be an important factor in choosing a fertilizer.

Poor quality blends may contain the guaranteed nutrients but if there is segregation these nutrients will not be supplied uniformly to the crop. If properly made so as to reduce segregation during transportation and application, blends generally are equal in agronomic effectiveness to granulated complete fertilizers.

Blends have the added advantage of allowing a very wide range of fertilizer grades, thus making it possible to match a fertilizer exactly to a soil test recommendation. When using a blend as a starter fertilizer, try to avoid urea and di-ammonium phosphate as ingredients. Both materials produce free ammonia, which can hurt seed germination and seedling growth.

Type # 5. Liquid Fertilizers:

Liquid fertilizers are becoming more and more common. Multi-nutrient liquid fertilizers also are becoming more popular. The liquid fertilizers may be categorized into two groups: clear solutions and suspensions. The major advantage is in handling. The disadvantages are the generally higher price and lower possible analysis compared to dry fertilizers, especially when the material contains potassium.

Suspension fertilizers, which are much less common, are fluids in which the components’ solubility has been exceeded and in which very fine insoluble particles are kept from setting out by the inclusion of clay. Again, the major advantage of these materials is in handling.

Suspensions also can be formulated at much higher analyses than can the clear solutions. Analyses similar to those for dry materials are possible. The major disadvantage of suspensions is that they require constant agitation, even in storage. Furthermore, suspension fertilizer cannot be used as a carrier for certain other chemicals.

Manure Nutrient Management:

Nutrient management is not the same for all types of soils and crops. It is important to recognize the differences among farms and realize how they affect the choice of appropriate management strategies. Available manure to the farmers generally is not adequate to meet total crop nitrogen needs. Thus, additional nutrients in the form of purchased fertilizer or other sources are required for achieving optimum crop yields.

A well-planned nutrient management program emphasizing economic and agronomic efficiency should reduce the need for purchased inputs and thus improve farm profitability. Practices to maximize the safe use of manure and to balance nutrient inputs with removals from farm fields over time are emphasized, rather than nutrient use efficiency.

Manure Nutrient Content:

Manure is a good source of all crop nutrients, including the major and micro-nutrients, but nitrogen (N) generally is the manure nutrient with greatest value and often the highest potential for pollution. Therefore, manure is best used for crops that have large N requirements.

In many manure storage systems, there is considerable variation in the manure nutrient content, even within the storage unit. For example, in liquid manure storage, the phosphorus content may be higher in the bottom of the storage than in the top.

Also, because the ammonium nitrogen can vary with depth in the storage, the manure nitrogen availability to the crop will change as the storage is emptied, unless there is adequate agitation. In these situations, no manure in the storage actually matches the average results from the analysis of a single composite sample.

To overcome this problem, it is recommended that a detailed analysis of the manure be performed at least once after the manure storage is constructed to determine the amount and nature of the variation.

On farms with a deficiency of nutrients, using manure can greatly reduce fertilizer needs and thus their expense. On farms with an excess of manure, however, these nutrients can represent an environmental threat if they are not used properly.

However, in recommending fertilizers based on soil testing, the amount of organic manure should be taken into account. Continued excessive application of manure and/or fertilizer nutrients can create crop, environmental, and economic problems.

Environmental problems result from leached N or runoff of manure into surface water. Not only are these nutrients lost to crop use, but their value as fertilizer replacement also is lost.

In addition, excessive nutrients in the soil may contribute to excessive vegetative growth, lodging, high non-protein N levels in crops under stress, delayed maturity or poor storage quality of vegetables, and lower uptake of magnesium and/or zinc.

To conserve manure nutrients and reduce the pollution potential of manure, the following basic recommendations may be taken into considerations:

i. Conserve the liquid portion; this portion contains half the total N and nearly all of the K.

ii. The manure should be broadcasted uniformly throughout the field with exact amount. Actual rates should be based on meeting crop nutrient needs. Manures should be broadcasted far from streams, lakes, ponds, sinkholes, and wells so that runoff does not wash manure into them. Whenever possible, cover crops should be used to hold the nutrients against loss.

iii. To take better advantage of its nutrients, the application of manure may be done just before the growing season, preferably to N-requiring crops.

iv. If possible, manure should be incorporated soon after application to conserve as much N as possible.

Soil testing is extremely important in manure management. Soil tests provide information that can be used to decide where manure can do the best in supplying nutrients required by the crop. The lower the soil test level, the higher the probability of a response to adding the nutrient. Soil tests also help monitor the effect of manure application on soil phosphorus and potassium levels.

Nitrogen also may leach from the plant root zone and adversely affect the environment. In addition, the associated organic matter, which is beneficial at low rates, may become undesirable at excessive levels.

Heavy surface applications of manure with no incorporation may inhibit herbicide activity, provide a favorable area for insect pest development, or create an odour problem. As more farmers store their manure and spread it in the summer, compaction is becoming a more serious consideration in manure management.

Yield decreases due to compaction usually are related to restrictions in the plant’s root system, and these restrictions prevent the plant from getting the nutrients and water it needs. The soil test for K is the best management tool for predicting the amount of potash needed in a fertilizer program. Available K in soils is estimated by measuring the total of solution K (water soluble K) and exchangeable K.