Fertilizers (or fertilisers) are substances that supply plant nutrients or amend soil fertility. They are the most effective means of increasing crop production and of improving the quality of food and fodder. Fertilizers are used in order to supplement the natural nutrient supply in the soil, especially to correct the (yield-limiting) minimum factor.
Fertilizers are soil amendments applied to promote plant growth; the main nutrients present in fertilizer are nitrogen, phosphorus, and potassium (the 'macronutrients') and other nutrients ('micronutrients') are added in smaller amounts. Fertilizers are usually directly applied to soil, and also sprayed on leaves ('foliar feeding').
Organic fertilizers or and some mined inorganic fertilizers have been used for many centuries, whereas chemically synthesized inorganic fertilizers were only widely developed during the industrial revolution. Increased understanding and use of fertilizers were important parts of the pre-industrial British Agricultural Revolution and the industrial green revolution of the 20th century. Inorganic fertilizer use has also significantly supported global population growth — it has been estimated that almost half the people on the Earth are currently fed as a result of artificial nitrogen fertilizer use.[1]
Fertilizers typically provide, in varying proportions:
Critical values are still quite useful and are frequently referred to when interpreting a plant analysis result. A brief discussion of the known critical values for the elements included in a plant analysis is given below:
Nitrogen (N)
The critical level of N in many plants is around 3 percent. For several crops, when the N level in leaves drops below 2.75 percent, N deficiency symptoms appear and yield and quality decline. The primary exceptions are for the very young plants when the critical level may be 4 percent or more, and for leguminous plants, such as soybeans, peanuts, alfalfa, etc., where the critical N percentage is 3 to 4.25 percent. For some tree fruits and ornamentals, N levels may be as low as 2 percent before deficiency occurs. Deficiencies as well as excesses can be a problem. Nitrogen leaf levels in some varieties of pecans exceeding 3.50 percent may result in early defoliation. Nitrogen leaf levels greater than 4.50 to 5 percent retard fruit set in greenhouse tomato. High N levels (>3.50 percent) in forage crops such as fescue is thought to be related to the incidence of grass tetany.
Small changes in N content for some crops can result in large effects on yield, plant growth, and the quality of forage and fruit. Therefore, it is important that the N level be maintained within the prescribed limits of the sufficiency range by the proper use of N fertilizer.
Phosphorus (P)
The P requirement of plants varies considerably. Tree crops have relatively low P requirements with the critical values ranging from 0.12 to 0.15 percent. Grasses have higher P requirements with critical values ranging from 0.20 to 0.25 percent. Legumes and some vegetable crops have relatively higher P requirements with critical values being 0.25 to 0.30 percent or slightly higher. Most plants grow to the extent to maintain a near constant P level within the plant. When a P deficiency occurs, it is usually due to a severe inadequacy of P in the soil solution, or in some cases it may be due to a restricted root system as a result of cool-moist growing conditions. Phosphorus deficiencies normally occur early in the growth cycle of the plant when the P requirement is high. The P content of plants is initially high and declines with age. Since P is a fairly mobile element in plants, deficiencies generally occur on older tissue.
The excess range of P is not clearly known. The P level in young plants can be very high such as 0.50 to 1.00 percent, but these high levels may reflect actual need. In some instances, high P plant levels may cause imbalances and deficiencies of other elements, such as Zn, Cu, Fe, etc. Plant P can be maintained within the sufficiency range by proper P fertilization and the maintenance of the soil P level within the medium to high soil test range.
Potassium (K)
The K requirement of plants varies widely depending on plant species. The tree crops such as pecans, peaches, apples, etc., have relatively low K requirements. The critical value for K in tree leaves ranges from 0.75 to 1.25 percent. For grasses, the K requirement is higher with the critical value in leaves ranging from 1.20 to 2.00 percent. For legumes, the critical value for K generally ranges from 1.75 to 2.00 percent. The K level in a plant can change quickly as K is quite mobile and moves readily within the plant. Potassium can be easily leached from growing plants by rain to be reabsorbed through the roots. Because of K mobility, both in the plant and soil, deficiency symptoms can develop quickly. Deficiencies frequently occur during both the early and latter stages of growth, particularly during fruiting. Young plants may contain 3.00 to 5.00 percent K, although the actual requirement may not be that high. Because it is mobile in the plant, K deficiency symptoms appear in the older plant tissue first. The K concentration in the plant decreases with age. Potassium balance in plants is important. The K/(Ca+Mg) and K/N balances must be maintained at a proper level to avoid deficiencies of Mg in the first instance and K in the second. High K can induce Mg deficiency in most plant and tree crops. Plants which are Mg deficient may have high K and Ca contents as the plant tends to maintain a constant cation concentration. As a result of these balance phenomena, heavy applications of K or N fertilizer, respectively, can induce a Mg or K deficiency. Under Georgia soil conditions, K deficiency is difficult to induce unless the K soil test level is low and the soil is heavily limed or fertilized with large quantities of N. The K to N balance is becoming increasingly important in pecans. As the N level in tree leaves increase, the K level must also be increased to maintain the proper balance and prevent K deficiency from occurring. Plant K can be maintained within the sufficiency range by proper K fertilization and the maintenance of the soil K level within the medium to high soil test range.
Magnesium (Mg)
Magnesium deficiency occurs in many plants when the leaf level is less than 0.10 to 0.15 percent. Small grains may exhibit deficiency symptoms when the Mg level is less than 0.10 percent. When corn is less than 12 inches in height, magnesium deficiency may occur when the Mg level is below 0.15 percent. However, as corn matures, deficiencies may not be evident until the Mg level is less than 0.13 percent. For legumes such as peanuts and soybeans, the critical level is 0.25 to 0.30 percent. The critical level for cotton and pecans is 0.30 percent. Several vegetable crops such as tomato, turnips, and collards have a high Mg requirement with the critical level near 0.40 percent Mg. Magnesium is a fairly mobile element in the plant, therefore, deficiency symptoms occur in the older plant tissue. The Mg concentration in the plant tends to increase with age.
Magnesium deficiencies can be induced by excessive K and NH4-N fertilization. When the soil pH is less than 5.4, Mg availability and uptake by plants is greatly reduced. The usual cause for Mg deficiency in Georgia is generally low soil pH and/or low soil Mg. Depending on the soil conditions, the effect of K and NH4-N fertilization can vary depending on the soil pH and level of soil Mg. Continued liming with only calcitic lime will result in a Mg deficiency. Adequate soil Mg can generally be maintained by liming with dolomitic limestone to keep the soil pH between 6.0 and 6.5. Supplemental applications of fertilizer Mg may be needed in some cases to supply some of the Mg crop requirement.
Sulfur (S)
It has been generally thought that the S requirement of plants was comparable to that of P. This has not proven to be so. The S requirement for grasses is quite low, the critical value being around 0.10 percent. Sulfur deficiencies in corn do not generally occur until the S level is less than 0.13 percent in the leaves. Under Georgia conditions, legumes, cotton, tobacco, and tomatoes have a critical S level of about 0.20 to 0.25 percent. The S critical level for crops such as cabbage, spinach, turnips, and collards is around 0.30 percent. However, additional research in this area should aid in pinpointing the critical level for these crops. There is a critical N to S percentage ratio which should be maintained. As suggested by Reneau (1983) the N:S ratio may be a better indicator of the S status of corn than the S concentration. For crops such as corn, this ratio should not exceed 18:1 if S deficiency is to be avoided. Stewart and Porter (1969) suggested that a N:S ratio above 16:1 indicates a lack of S may be limiting protein formation. A ratio of 20:1 or greater indicates that S is severely deficient. For optimum corn grain yields, the N:S ratio should be maintained between 10:1 to 15:1 (M. E. Sumner, personal communication). The optimum N:S ratio for Coastal bermudagrass ranges from approximately 9:1 to 12:1 (Martin and Matocho, 1973). Maintaining the N:S ratio within the range for optimum production of Coastal also provides the N:S ratio that is about optimum (10:1 to 15:1) for ruminant nutrition (Allaway and Thompson, 1966). Sulfur deficiencies occur primarily on the very sandy soils of South Georgia and when low S containing fertilizers are used over several years. Sulfur deficiencies tend to occur early in the plant growth cycle. The proper S level can be maintained in the plant by providing a S source near the germinating seed or by adding S with sidedress and topdress N applications particularly in sandy soils. Most Georgia subsoils contain sizeable quantities of S. Provided the pH is not too low when roots enter the subsoil, sufficient S will generally be available to satisfy the crop requirement.
Since S is not a mobile element in the plant, deficiency symptoms tend to first appear in the upper or newly emerging leaf tissue.
Calcium (Ca)
The Ca requirement for plants varies widely with grasses having the lowest requirement, legumes intermediate, and fruit crops and cotton the highest. Calcium levels from 0.20 to 0.25 percent are quite adequate for pasture grasses and corn. Soybean has a critical Ca concentration in the mature leaves of 0.50 percent, while the level for peanuts is 1.25 percent. Apple leaves should contain about 1 percent Ca and peach leaves 1.25 percent. Greenhouse tomato has a critical concentration for leaves of about 1 percent. Of the crops grown in Georgia, cotton probably has the highest critical Ca concentration at 2 percent for leaves.
Calcium deficiencies are not unusual, although the crops where Ca is particularly important are the fruit crops, such as apples, peaches, and tomato. Calcium deficiency will significantly affect fruit quality. Brown rots, easy bruising of fruit, and blossom-end rot of tomato are frequently associated with inadequate Ca. Pod-rot in peanuts is also a Ca deficiency. These deficiencies are not easily "uncovered" by leaf analysis. When Ca deficiency is severe, newly emerging tissue is affected. The margins of the leaves tend to stick together, giving a ragged edge to new leaves. Older leaves will show a browning of the margins. Since Ca is not a mobile element, deficiencies occur in the newer tissues. The Ca level in plants tends to increase with the age of the plant.
There is increasing evidence that Ca is more like a micronutrient, as the critical concentration may be in the parts per million range. Several plant physiologists have grown plants successfully at low Ca levels in artificial growth media. In these experiments, the balance of Ca with the other essential elements such as Mg, Cu, Fe, B, and Mn was critical. Calcium was found to be sufficient with plant and leaf concentrations between 600 ppm to 1000 ppm. It is known that relatively little Ca is in a soluble form in many plants. Crystals of calcium oxalate have been observed in the leaves of most fruit trees as well as some field crops which are thought to have high Ca requirements. Therefore, the sufficiency of Ca in such plants may be related to the soluble fraction in the leaves rather than the total. Unfortunately at this time, all of the current literature related to Ca and its sufficiency concentration are based on total Ca contents of sampled plant parts. No doubt there is need to change the method of analysis for Ca to determine the soluble Ca content and relate this to sufficiency range standards.
Manganese (Mn)
Manganese deficiency normally occurs when the leaf tissue concentration is less than about 15 ppm. Depending upon the crop, ample but not excessive concentrations of Mn may range from 15 to over 1,000 ppm. Although there is limited data to delineate when toxicity occurs, leaf levels in excess of several hundred ppm are probably toxic to many plants. Plants which are sensitive to Mn deficiency are equally sensitive to excessive Mn. Growth of soybeans, which are particularly sensitive to Mn deficiency, is reduced when leaf Mn levels approach 200 ppm (Ohki, 1976). Several plant species have higher Mn critical levels. For example, the critical Mn level for alfalfa is about 25 ppm.
Some plants can tolerate extremely high Mn levels without detrimental effects. Pecan leaves may contain up to 1000 ppm Mn with seemingly no adverse effect. Similarly, cotton and peanuts will accumulate Mn up to 500 ppm without apparent toxicity. However, a high Mn level in plants is a sign of low soil pH, and is frequently associated with Mg deficiency. When the Mn concentration in peach leaves exceeds 150 ppm, this is generally a good indication that the soil pH is low according to George Cummings.
The Mn level in plants is usually quite high at the initial period of growth. It decreases rather rapidly and then levels off to remain fairly constant during most of the season. Since Mn is not a mobile element, deficiency symptoms will occur in the newer leaves or upper portion of the plant.
Iron (Fe)
Iron analyses are probably invalid unless the leaf tissue has been washed in dilute acid or detergent solutions. Therefore, for unwashed leaves, iron analyses are of no real value. When soil contamination is suspected, usually Al is also high.
The Fe content in a plant can vary considerably. In general, when the Fe concentration in leaves is 50 ppm or less, deficiency is likely to occur. The grasses and corn have a lower Fe requirement, the critical level being 20 ppm. Iron toxicity has not been reported for any field crops growing under natural conditions in Georgia. The only Fe sensitive field crops would be pecans and soybeans, with possible deficiency occurring only on soils with pH's at 7.0 or above. Iron deficiency is common in Centipede grass and azaleas, particularly when grown in soils with pH's above 6.0
Iron deficiency is very difficult to correct in some crops. The application of some forms of Fe to the soil is not practical. Foliar applications of Fe have been found to be effective in correcting Fe deficiencies in plants such as turf grasses. However, on crops such as pecans, foliar applications for correction of low Fe levels have been erratic.
Since Fe is an immobile element in plants, Fe deficiencies appear in the new tissue or upper portion of the plant. Iron deficiency symptoms may appear early in the growth of the plant only to disappear in several days or weeks. The Fe level in the plant usually remains fairly constant during the growing season.
Boron (B)
Boron requirements vary considerably among crops. The optimum range in leaf tissue of most crops is from 20 to 100 ppm. Some crops are particularly sensitive to B and can be injured when the leaf B level is too high. For example, B levels in excess of 50 ppm have been associated with B toxicity in peaches. The B critical level for corn is about 4 ppm, while alfalfa, cotton, peanut,and soybeans have critical levels of 20 ppm. Corn, having a fairly low B requirement, is also sensitive to excess B. Toxicities may occur when the B level in young corn leaf tissue exceeds 25 ppm. Members of the Papilionaceae and Cruciferae have fairly high B requirements with critical levels being about 25 to 30 ppm B in the leaf tissue. Those plants which have fairly high B requirements are also ones with fairly good tolerance to excessive B. Boron is not a very mobile element and deficiency symptoms occur in the newly emerging tissue. The B concentration in leaves remains constant during the growth cycle. Boron deficiencies result in various physiological diseases in plants, such as "hollow heart" in peanuts, a fairly common disorder occurring in Georgia peanut fields.
Copper (Cu)
The normal range of Cu in many plants is fairly narrow, ranging from 5 to 20 ppm. When the Cu concentration in plants is less than 3 ppm in the dry matter, deficiencies are likely to occur. When Cu levels exceed 20 ppm in mature leaves, toxicities may occur. There is some variation in the critical values for various plant species; however, most critical values have been determined to be somewhere between 3 to 10 ppm for most crops. The Cu level in leaves tends to remain constant during the growing season.
Copper deficiency symptoms often depend on plant species or variety and the stage of deficiency. In the early stages of deficiency, symptoms are generally reduced growth. In the moderate to acute stages of deficiency on crops such as wheat, terminal or new leaves are pale green, lack turgor, and become rolled and yellowed; older leaves become limp and bent at the ligule. The leaves die and dry to a bleached gray (Reuther and Labanauskas, 1966).
Zinc (Zn)
The normal range of Zn in most plants is between 20 to 100 ppm. Zinc deficiencies occur in a wide variety of plants when the leaf level drops below 15 ppm. The critical Zn value for apple is about 14 ppm with the first symptom of the deficiency being small fruit size. Zinc deficiency in pecans occurs when the Zn leaf level is 30 ppm or less.
In order to avoid Zn deficiency, Zn levels in most crops should be maintained at 20 ppm or better, except for pecans when 50 ppm Zn is the desired minimum.
Zinc toxicity is an uncommon problem and does not generally occur until the Zn level exceeds 200 ppm. However, in crops such as peanuts, Zn toxicity has been reported in Georgia when tissue levels reach 220 ppm (Keisling and others, 1977). More recently (Parker and Walker, 1986) reported that Zn levels up to 287 ppm did not adversely affect peanut yields nor show any of the symptoms associated with Zn toxicity. However, the author has observed plants exhibiting Zn toxicity symptoms, described by Keisling and others (1977), with Zn concentrations of 117 ppm. Apparently, there are other plant growth factors or nutrient relationships in addition to just the Zn concentration that affect the manifestation of Zn toxicity. One such relationship appears to be the Ca:Zn ratio in the tissue. Upon evaluating unpublished data of Parker in which the Zn concentration in tissue varied from 50 to 302 ppm, and Zn concentrations could not be related to Zn toxicity, the author noted that when the Ca:Zn ratio was less than approximately 45 to 50:1 Zn toxicity symptoms were evident. However, when the ratio was greater, where the Zn concentration was 302 ppm, no toxicity symptoms were detected. Continued research in this area should elucidate the nature of this relationship. Excessive Zn also interferes with the normal function of Fe in plants giving rise to symptoms similar to Fe deficiency.
Zinc is not a very mobile element in plants, and deficiency symptoms occur in the newly emerging leaves. Stunting is a frequent symptom associated with Zn deficiency. Zn concentration in leaves remains fairly constant with a fairly rapid increase at the end of the growth cycle.
Aluminum (Al)
Aluminum is not considered a plant nutrient; therefore, it is not required by plants. However, its presence in plants can affect the normal function of some other elements. As with Fe, probably no accurate measure of the Al status of the plant can be obtained unless the tissue is free from dust and soil contamination. High Al in plants is usually an indication of very low soil pH or poor soil aeration due to compaction or flooding. Aluminum levels in excess of 400 ppm in young tissue or 200 ppm in mature plants and leaves are undesirable.
Molybdenum (Mo)
Molybdenum deficiencies occur in many plants when the plant concentration is less than 0.10 ppm. Toxicity levels in plants have not been established. Molybdenum is quite toxic to animals if the forage being consumed contains more than 15 ppm Mo. The Mo requirement of legumes is higher than that of other plants since Mo is essential for the fixation of atmospheric N by the symbiotic bacteria. For the non-legumes, Mo is probably not needed if all the N requirement is supplied by the ammonium form. Molybdenum is essential for the conversion of nitrates to ammonium in the plant. In Georgia, Mo application has been found beneficial for alfalfa, particularly when the soil pH is low. The need for Mo on soybeans has also been confirmed in Georgia, particularly on the heavier Piedmont, Mountain, and Limestone Valley soils which are low (approximately 5.5 or less) in pH. Significant responses to Mo application have not been consistent on Coastal Plain soils. [2]
Citations
[1] http://en.wikipedia.org/wiki/Fertilizer#Inorganic_fertilizer_.28synthetic_fertilizer.29
[2] http://aesl.ces.uga.edu/publications/plant/Nutrient.htm#N
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