Soils, Fertilizers and Plant Nutrition 1

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1 Soils, Fertilizers and Plant Nutrition 1 E. A. Hanlon 2 PLANT ESSENTIAL ELEMENTS Soil is the natural medium for the growth of land plants and is the source of most of the nutrients necessary for plant growth and development. The sixteen essential nutrient elements required by plants and the forms in which they are absorbed are shown in Table 1. The nutrients are required in varying amounts by plants, as may be noted from the table. For convenience, the nutrients used in largest quantities are referred to as macronutrients and those used in smallest quantities as micronutrients. Although the bulk (93-99%) of plant tissue is comprised of carbon, hydrogen, and oxygen, those elements seldom if ever limit plant growth. It is the nutrient elements obtained from the soil that usually limit crop development. When the soil does not provide sufficient quantities of nutrients for the plants, fertilizer must be applied to the soil to satisfy the crop nutrient requirement. MACRONUTRIENTS FROM SOILS AND FERTILIZERS Important points concerning the macronutrients obtained from soils and frequently supplemented by fertilizers are discussed in this section. Table 1. Nutrient elements essential for plant growth. Essential Element Chemical Symbol Chemical Form(s) Used by Plants Typical Percent of Plants Fresh Weight Carbon C CO , HCO 3 45 Hydrogen H H + (water) 8 Oxygen O H 2 O, other oxides 41 MACRONUTRIENTS Nitrogen N NH + - 4,NO Phosphorus P HPO ,H 2 PO Potassium K K Calcium Ca Ca Magnesium Mg Mg Sulfur S SO ,SO This document was published December 1992 as RF-AA003, Florida Cooperative Extension Service. For more information, contact your county Cooperative Extension Service office. 2. Extension Soil Scientist, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville. The Institute of Food and Agricultural Sciences is an Equal Opportunity/Affirmative Action Employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, or national origin. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean

2 Soils, Fertilizers and Plant Nutrition Page 2 Essential Element Chemical Symbol Chemical Form(s) Used by Plants Typical Percent of Plants Fresh Weight MICRONUTRIENTS Iron Fe Fe ++,Fe Manganese Mn Mn ++,Mn Copper Cu Cu +,Cu Zinc Zn Zn Molybdenum Mo -- MoO Boron B --- BO Chlorine Cl Cl - < Table 2. Amounts of nitrogen fixed by well-nodulated Nitrogen legumes Biological N A group of bacteria called Rhizobium which live in association with the roots of leguminous plants can fix atmospheric nitrogen for the host plant. The wide renge of relative amounts of nitrogen fixed by various legumes are shown in Table 2. If poor growth of the legumes is obtained, the amounts of nitrogen may be much less than those reported here. The age old question of whether or not to add nitrogen to a leguminous crop can be answered by a simple basic consideration. Legumes that produce tiny seeds, such as true clovers, usually respond to some N fertilizer when they are very young. The nitrogen supply in the seeds is quite small and young seedlings have a limited root system with which to absorb nitrogen. Larger seeded legumes, such as soybeans, have not consistently shown a response to added nitrogen. The nitrogen supply is greater in the larger seeds, and nodulation generally takes place before the seed supply of nitrogen is depleted. Fertilizer Nitrogen The main source of available soil nitrogen is from commercial fertilizers. Nitrogen is reported on the Florida fertilizer tag in four different forms: Crop Nitrogen Fixed Per Crop Year lbs/acre Alfalfa Clover Crimson Clover Vetch Soybeans Velvet Beans Beans Cowpeas Lupines Overall Average: 120 Ammoniacal Nitrogen (NH 4+ ): When ammoniacal N is added to the soil, it may be immediately absorbed by the clay and organic colloids. As long as it remains as ammonium, it is held against leaching unless the soil has extremely low cation exchange capacity. However, bacteria immediately begin to transform ammonium to nitrate. Complete transformation generally requires from one day to two weeks, depending on the environmental conditions which govern nitrification. The optimum conditions for nitrification are: Nitrate Nitrogen (NO 3- ): This form of nitrogen is immediately available to plants. Nitrate is quite soluble, is not held by soil colloids, moves with the soil water, and is readily leached. The principal form of nitrogen taken up by plants is nitrate. - Optimum temperature range is 80 to 90 degrees F. Essentially stops above 125 degrees F and below 40 degrees F. - Proper moisture - a soil too wet or too dry will not support nitrification. The number of

3 Soils, Fertilizers and Plant Nutrition Page 3 organisms that carry on nitrification fluctuates with moisture content. - ph - practical range is between 5.5 and 7.0. At this ph, nitrification is not most rapid but is still quite good. Water Soluble Organic Nitrogen: Urea and calcium cyanamide are the only two materials considered as water soluble organics. When urea is added to the soil, the enzyme urease changes it to ammonium. This reaction is very rapid, since urease is always present in soils containing microorganisms. Thereafter, the ammonium is nitrified to nitrate. Water Insoluble Organic Nitrogen: Natural organics are the water insoluble organic forms of nitrogen. They must undergo a complete change before becoming available to plants so, in that sense, they release N slowly over a long period of time. Slow release materials such as sulfer-coated urea and IBDU are noted with an asterisk on the fertilizer label since they do not fit neatly into the old categories set into law years ago. The claim that natural organics are less subject to leaching needs clarification. As long as nitrogen remains in the water unsoluble organic form, or as ammonium, it is resistant to leaching. But, once this nitrogen changes to nitrites or nitrates, it is subject to leaching the same as any nitrate-n source. It is doubtful that natural organic sources of N are worth the premium price often paid for them. The pros and cons of various N sources are listed in Table 3. Phosphorus Many Florida soils are high in native phosphorus. Only the finer textured soils of north and west Florida have a tendency to fix large amounts of phosphorus. Phosphorus accumulates in most Florida soils. When a certain level of soil-test extractable phosphorus is reached, no additional fertilizer phosphorus is needed, except infrequent light applications. An excess of phosphorus nutrient has a depressing effect on the uptake of iron and zinc and has the potential to pollute groundwater when runoff or soil erosion occurs. In acid soils, iron and aluminum fix phosphorus in forms relatively unavailable to plants. This is one of the adverse effects of low soil ph. Most of the phosphorus present in the soil came from phosphorus-bearing minerals or from commercial fertilizers. When recommended liming practices are followed, little or no phosphorus is lost by leaching. Phosphorus will leach in strongly acid sands (ph < 5.0) that are low in iron and aluminum. The liming program is closely associated with phosphorus availability. Table 3. Comparison of Organic and Inorganic Sources of Nitrogen ADVANTAGES Organic Nitrogen A portion is slowly available Little danger of over-fertilizing Micronutrients are present in most materials Good fertilizer conditioners Inorganic Nitrogen & Urea Less expensive than natural organics Very soluble Readily available Can be used in liquid fertilizers Greater efficiency DISADVANTAGES Expensive Low nitrogen content May not nitrify when N needed most Low efficiency Improper use may damage plants More leaching potential May be acid-forming More acid-forming per pound of material (not so per pound of nitrogen contained) Poor fertilizer conditioner

4 Soils, Fertilizers and Plant Nutrition Page 4 Potassium Potassium differs from other plant nutrients in that it doesn t enter the structure of the plant. The potassium remains in the plant sap and is easily leached from dead plant tissue. No decomposition is necessary to release potassium. Plants have a tendency to absorb more potassium than they need and, under such conditions, a nutrient antagonism might develop. The major portion of plant available potassium in Florida soils comes from commercial fertilizers. The amount of potassium returned to the soil from plant and animal residues depends on the quantity of the residue and on its potassium content. The major portion of the loss of soil potassium is through crop removal and leaching. Crop removal accounts for most of the loss except where large amounts of potassium are applied at one time and leaching becomes a major factor. Calcium Calcium, discussed under liming, is generally applied to the soil as limestone and as impurities in fertilizers. With few exceptions, calcium will not limit production on soils with ph > 5.5. Magnesium This element is frequently deficient in Florida soils and addition of fertilizer magnesium is often needed. Mg can be economically supplied in dolomite when lime is needed, but another source must be used if the soil ph does not merit liming. Sulfur Two fairly recent developments have reduced the sulfur added to agricultural lands: (1) use of high analysis fertilizers which contain little or no sulfur contaminants, and (2) reduction of air pollutants which contained significant amounts of S. As a result, fertilizer S is now recommended for certain crops where deficiencies have been noted. FERTILIZERS Definitions of several common fertilizer terms may be appropriate since frequent misuse is observed. Some of the terms are burdened with concepts which should have ceased use long ago but which are kept around because of their presence in fertilizer laws. Grade The guaranteed percentages of nitrogen, phosphorus, and potassium in a fertilizer. The elements are expressed in terms of N, P 2 O 5, and K 2 O, in that order. Though phosphorus and potassium are not present in the oxide form, their contents remain expressed that way because custom, law, and widespread usage have resisted change to the elemental mode of expression. Examples of grades are: , , , Complete Fertilizer An unfortunate term with which we are burdened. Avoid using it. Instead use "N-P-K fertilizer". This is any fertilizer which contains appreciable quantities of nitrogen, phosphorus, and potassium. Bulk Blended Fertilizer A physical mixture of two or more fertilizer materials, usually prepared locally to the specifications of the client and applied by spreader truck. Over 70% of the fertilizer sold in Florida is bulk blended. Chemical Compositions The chemical compositions of some fertilizer materials are shown in Tables 4 and 5. It must be noted that these are average values and some deviation in nutrient content is normal. All fertilizers sold are covered by the Florida Fertilizer Law. A fertilizer tag which shows the guaranteed analysis must accompany the fertilizer. The "approximate calcium carbonate equivalent" is a measure of the acid-forming potential of the fertilizer. A minus sign indicates the number of pounds of calcium carbonate needed to neutralize acid formed when one ton of the material is added to the soil.

5 Soils, Fertilizers and Plant Nutrition Page 5 Table 4. Plant nutrient content of some fertilizer materials Percentage of: Approx. Calcium Carbonate Salt Material N P 2 O 5 K 2 O Other Nutrients Equivalent (CCE) Index SOURCES OF ONE MACRONUTRIENT Ammonia, anhydrous Ammonia, Aqua to Ammonium nitrate Zn Nitrogen solutions to Phosphoric acid to Potassium chloride Mg, 0.03 B, 47 Cl (muriate of potash) Sodium nitrate S, 0.07 Cu, B Sulfur S to Urea Urea-form SOURCES OF TWO OR MORE MACRONUTRIENTS Ammonium Ca, 4.4 Mg, 4 S 0 61 nitrate-lime Ammonium sulfate S,0.1 Zn, 0.3 Cu Ammonium sulfate S nitrate Ammonium phosphate nitrate Ammonium phosphate sulfate Ammonium phosphate, mono Ammonium phosphate, di Basic slag, open hearth Mg, 15 S, 0.2 Mn, 0.02 Zn, 0.02 Cu, 0.03 B to Ca, 0.3 Mg, S, 0.03 Mn, 0.03 An, 0.02 Cu, 0.02 B to Ca, 3.4 Mg, 0.3 S, 2.2. Mn Calcium cyanamide Ca, 0.06 Mg, S, 0.04 Mn, 0.02 Cu Calcium nitrate Ca, 1.5 Mg, S Colloidal phosphate Dolomite Ca, 11 Mg, 0.3 S, Mn Gypsum Ca, 0.4 Mg, 17 S 0 8 Kieserite (emjeo) Ca, 18.2 Mg 0 ---

6 Soils, Fertilizers and Plant Nutrition Page 6 Percentage of: Approx. Calcium Carbonate Salt Material N P 2 O 5 K 2 O Other Nutrients Equivalent (CCE) Index SOURCES OF TWO OR MORE MACRONUTRIENTS (continued) Limestone (calcite) Ca, 3 Mg, 0.1 S, Mn, 0.05 Zn Magnesium sulfate Ca, 10.5 Mg, S Nitrate of soda B potash Potassium magnesium Mg, 23 S 0 43 sulfate Potassium nitrate Mg, 0.2 S, 0.1 B Potassium sulfate Mg, 18 S 0 46 Superphosphate, ordinary Ca, 0.2 Mg, 12 S 0 8 Superphosphate, concentrated Table 5. Additional micronutrient sources Ca, 0.3 Mg, 1.4 S, 0.01 Mn, 0.01 Cu, 0.01 B Percentage of: 0 10 MATERIAL Mn Zn Cu Fe B S Borax Copper Sulfate Iron Sulfate Manganese Oxide Manganese Sulfate Solubor Zinc Oxide Zinc Sulfate Much of the information in Tables 4 and 5 was obtained from "The Fertilizer Handbook", published by The Fertilizer Institute, Washington, DC. Nutrient contents of non-manufactured materials used as fertilizers are listed in Table 6. These may be used by persons wanting to use only naturally occurring materials, as in the case of the popular but misnamed "organic" farmers or gardeners. Some handy conversion factors are presented in Table 7. ANIMAL MANURES Manures have been used as fertilizers for centuries. Although their plant nutrient content is generally low, manures contain some quantities of all of the essential elements. In many situations, the application of even a modest quantity of manure

7 Soils, Fertilizers and Plant Nutrition Page 7 provides enough of a deficient nutrient (especially a micronutrient) to dramatically increase plant growth. Table 6. Plant nutrient content of some non-manufactured materials used as fertilizer Average Percentage of: Material N P K 5 0 Ca Mg S Mn Zn Cu B Sodium nitrate, Chilean Blood, dried Bone meal, raw Bone meal, steamed Castor pomace Cocoa shell meal Cotton seed meal Dolomite Limestone (calcite) Gypsum Tankage Guano, Peruvian Potassium chloride Peat Rock phosphate Sewage sludge Seaweed (kelp) Soybean meal Tobacco stems Tung nut meal Wood ashes The nutrient content of manure is quite variable. Factors influencing the quality include the age and kind of animal, the feed it consumed, the amount and kind of litter or bedding used, and the manner in which the manure was handled. Representative values shown in Tables 8 and 9 should be used only as general guidelines and should not be used for making important planning or management decisions. Some interesting generalities about manures: - Approximately 16 tons of manure (at 75% moisture) are produced for each ton of livestock per year, regardless of species.

8 Soils, Fertilizers and Plant Nutrition Page 8 - About 500 pounds of absorbent litter are needed to absorb 600 pounds of liquid in each ton of manure. - Addition of superphosphate to manure prevents loss of ammonia by volatilization. Use 40 lbs/ton for horse and sheep manure and 25 to 30 lbs/ton for poultry, swine, or cattle manure. However, intensive livestock and poultry operations already have an excess of P in the manure and would certainly not add more even if it saves N.

9 Soils, Fertilizers and Plant Nutrition Page 9 Table 7. Nutrient Conversion Factors To Convert Column 1 to Column 2, Multiply By: Column 1 Column 2 To Convert Column 2 to Column 1, Multiply By: Ca CaO CaO CaCO Cl KCl Cl NaCl K K 2 O K 2 O KCl K 2 O KNO K 2 O K 2 SO Mg MgO MgCO 3 CaCO MgO MgCO MgO MgSO Mn MnO MnO MnSO N KNO N NaNO N NH N NH 4 NO N (NH 4 ) 2 SO P P 2 O S SO S SO Table 8. Average percentage composition of macronutrients in fresh manure Percentage of: Type of Manure N P 2 O 5 K 2 O Ca Mg S Cattle Horse Sheep Swine Poultry (layers) Poultry (broilers)

10 Soils, Fertilizers and Plant Nutrition Page 10 Table 9. Percentage composition of micronutrients and moisture in fresh manure Percentage of: Type of Manure Mn Zn Cu B Fe Moisture Cattle Horse Sheep Swine Poultry Table 10. The following data are presented to illustrate the variability of fresh manures. Broiler litter %N %P 2 O 5 %K 2 O range average DETERMINING FERTILIZER REQUIREMENTS With the Aid of Soil Tests Fertilizer is needed when the soil cannot supply sufficient quantities of the essential nutrients for the crop being grown. Fertilizers supplement the nutrients supplied by the soil. Since soils vary tremendously in their nutrient supplying capabilities, the most logical means of predicting what can be expected from the soil is by testing the soil. Soil tests are useful in predicting fertilizer needs when the levels measured in the laboratory are related to crop yields through extensive field studies. This correlation of soil tests to crop yields is probably the least appreciated aspect of "soil testing", yet without correlations the test values per se are useless. Without Soil Tests In the absence of soil tests, one can still sometimes estimate the fertilizer requirements. Experience with the crop and soil is used to decide which nutrient elements should be added and the quantities of each. This approach frequently leads to over-fertilization with some elements and under-fertilization with others. In commercial agriculture where large sums are spent on fertilizer, it is a poor substitute for soil testing. DIAGNOSING NUTRIENT DEFICIENCIES IN PLANTS Foliar symptoms Foliar symptoms of nutrient deficiencies vary considerably between plant species. In addition, insect, disease, nematode, drought, and pesticide damage often produce foliar symptoms. General descriptions of symptoms produced by nutritional deficiencies should be used only in conjunction with other available information when trying to diagnose field problems. Tissue Testing Tissue testing is divided into separate phases which are quite different. Rapid tissue testing Green tissue is usually taken for this type of analysis. Various chemicals included in these "quick tests" are used to test each element using green tissue or the extract from green tissue. These tests may give useable information, but accuracy is sacrificed for speed and field-use convenience. Confirmation using dry tissue testing of suspected nutritional problems is recommended.

11 Soils, Fertilizers and Plant Nutrition Page 11 Dry tissue testing This type of testing usually involves the use of dried leaves or plants. The entire dried leaf or plant is ground up and the total content of selected nutrients is determined. Since tissue testing is standardized, results of analyses done on a split sample by different labs should be comparable (this is not generally the case with soil tests due to differences in extractants). There are several drawbacks to leaf analysis for estimating fertilizer requirements. Leaf analysis is more time consuming and more expensive than either rapid tissue testing or soil analysis. The plant part sampled and stage of development are critical to interpretations of the results. FERTILIZER PLACEMENT The characteristics of the soil, the kind of crop, and the nature of the fertilizer materials should be considered when choosing methods of fertilizer application. Points about fertilizer placement are listed below: - Provide adequate quantities of plant nutrients within the root zone. - Irregular distribution can lower fertilizer efficiency. - Early stimulation of the seedling is usually advantageous. At least part of the fertilizer should be placed within reach of young seedling roots. - The rate and distance of fertilizer movement depend upon the character of the soil. Nutrient elements may move upward during dry periods, and may be carried downward by rain or irrigation water. - Soil-supplied nutrients when in dry soils are of little or no benefit to the plant. - Excessive concentrations of fertilizer in contact with seed, roots, or legume inoculant may be injurious. Crops vary in their tolerance to soluble fertilizer salts. - Water-soluble fertilizer of relatively low plant food content has a greater salt content per unit of plant food and has a greater tendency to produce salt injury than does a fertilizer containing more concentrated materials. - Nitrogen and potassium carriers are more readily soluble than phosphatic fertilizer materials and cannot be safely concentrated in as large amounts near seeds or plant roots. - Reduction of soil moisture content increases the concentration of salts in the soil solution. Soil drying increases the possibility of injury. - Except in strongly acidic, sandy soils, phosphates move slowly from the point of placement. Therefore, phosphatic materials should be placed where they will be readily reached by plant roots. - Placement of fertilizer in bands reduces contact with the soil, thereby delaying the conversion of phosphorus to forms not available to plants. Banded fertilizer rates may be safely reduced by as much as 50% compared to broadcast rates. LIQUID AND DRY FERTILIZERS COMPARED There is no difference in the response obtained from application of liquid or dry formulations of the same fertilizer materials. Reactions which occur in the soil are the same for both types of materials. A liquid material is neither more mobile nor more available than a dry product. Irrigation or rain following application of either type of fertilizer aids in plant uptake of the applied nutrients.