Soils - Part 6: Phosphorus and Potassium in the Soil

Overview

This lesson explains the importance of phosphorus fixation and describes methods for applying phosphorus and the advantages (and disadvantages) of each. It also describes the three forms of potassium and how form determines availability of potassium to plants.

[This lesson, as well as the other nine lessons in the Soils series, is taken from the "Soils Home Study Course," published in 1999 by the University of Nebraska Cooperative Extension.]

 

Phosphorus

Next to nitrogen, phosphorus (P) is by far the most limiting nutrient in Nebraska for crop production. However, while nitrogen is limiting on most all soils for grain crops, native levels of phosphorus are adequate in many of Nebraska’s soils even for irrigated corn. This makes it important to know when to use phosphorus fertilizers and how to manage them for maximum effectiveness. If not, the return of the fertilizer investment is, for all practical purposes, lost.

Importance of Phosphorus to Plants

Phosphorus is a component of the complex nucleic acid structure of plants, which regulates protein synthesis. Phosphorus is, therefore, important in cell division and development of new tissue. Phosphorus is also associated with complex energy transformations in the plant.

Adding phosphorus to soil low in available phosphorus promotes root growth and winter hardiness, stimulates tillering, and often hastens maturity.

Plants deficient in phosphorus are stunted in growth and often have an abnormal dark-green color. Sugars can accumulate and cause anthocyanin pigments to develop, producing a reddish-purple color. This can sometimes be seen in early spring on low phosphorus sites. These symptoms usually only persist on extremely low phosphorus soils. It should be noted that these are severe phosphorus deficiency symptoms and crops may respond well to phosphorus fertilization without showing characteristic deficiencies. In addition, the reddish-purple color does not always indicate phosphorus deficiency but may be a normal plant characteristic. Red coloring may be induced by other factors such as insect damage which causes interruption of sugar transport to the grain. Phosphorus deficiencies may even look somewhat similar to nitrogen deficiency when plants are small. Yellow, unthrifty plants may be phosphorus deficient due to cold temperatures which affect root extension and soil phosphorus uptake. When the soil warms, deficiencies may disappear. In wheat, a very typical deficiency symptom is delayed maturity, which is often observed on eroded hillsides where soil phosphorus is low.

Phosphorus is often recommended as a row-applied starter fertilizer for increasing early growth. University of Nebraska starter fertilizer studies conducted in the 1980s showed early growth response to phosphorus in less than 40 percent of the test fields (Penas, 1989). Starter applications may increase early growth even if phosphorus does not increase grain yield. Producers need to carefully evaluate cosmetic effects of fertilizer application versus increased profits from yield increases.

The Soil Phosphorus Problem

Phosphorus is adsorbed by plants in the ionic forms H2PO4and HPO4=. General knowledge of ion exchange in soils would predict that these anions are not retained by the negative charged soil colloids, but move in the soil similar to nitrogen. However, phosphorus does not leach. In fact, it moves very little, even with large amounts of precipitation or irrigation. The reason for this apparent anomaly is that the soil solution contains only a very small amount of available phosphorus in these ionic forms at any one time. In fact, most soils contain less than 0.00005 grams phosphorus per liter or 0.0000068 ounces phosphorus per gallon of soil. It has been estimated that the phosphorus in the soil solution must be replenished on an average of about twice every day for normal crop growth. This is the basic phosphorus problem — to adequately re-supply the soil solution as the crop roots remove available phosphorus from the soil solution. It is the soil’s ability to re-supply the soil solution that dictates whether the crop will need additions of fertilizer phosphorus and whether those additions will be effective in the forms applied.

The ability of the soil to re-supply the soil solution with phosphorus is dependent on the complex chemistry of the soil system. However, the system can be viewed very simply with the following diagram:

Slowly Soluble or Insoluble P Form

 Soluble or Plant Available P Forms

Relatively Unavailable ————>
P minerals and         <———— Soil Solution P
Compounds of Ca, Fe,
and Al
Organic P

This is an equilibrium reaction. As soil solution phosphorus is removed by crop roots, more phosphorus becomes available from the slowly soluble sources. However, if soluble fertilizer phosphorus is placed in the soil, it reverts into slowly soluble or insoluble forms, removing soluble phosphorus from the soil solution. This phenomenon is often called “fixation.” Fixation is the primary reason why placement of phosphorus fertilizer is important. Placement of phosphorus is an attempt to limit fixation. This is done by banding the phosphorus fertilizer near the seed or by dual placement with anhydrous ammonia bands. The goal is to limit soil-fertilizer contact, while placing available sources of phosphorus from the fertilizer in a position of a high probability root contact.

The above relationship is sometimes shown in terms of labile and non-labile phosphorus forms according to the following relationship:

Non-labile P  <—>  Labile P  <—>  Soil solution P

In this relationship, non-labile phosphorus refers to slowly available forms, while labile phosphorus is an intermediate form that is rather weakly adsorbed or bound to various compounds and clay in the soil (solid phase). This is the primary phosphorus source supplying the soil solution.

The equilibrium relationship shown above between non-labile or insoluble phosphorus forms and labile phosphorus is affected by many factors, such as size of the slowly available pool, soil temperatures, kind of compounds in the pool, kind and amount of clay in soil, and the pH of the soil solution. Figure 6.1 shows the general relationship between soil pH and phosphorus availability, which is based on the kinds of phosphorus compounds associated with the various pHs. At high soil pH, most phosphorus is in the form of calcium compounds. At low or acid pH, phosphorus is combined with iron and aluminum compounds. Maximum phosphorus availability occurs at a soil pH between 6.5 to 7.0. This is why one of the most important benefits of liming acid soils is improving phosphorus availability. Reducing the pH of calcareous soils would also increase the availability of phosphorus in the soil solution by changing some of the solid phase compounds into compounds of higher solubility. Sulfur will reduce the soil pH; however, the cost is prohibitive for field crops because of the high sulfur rates required.

 
Figure 6.1.
Soil phosphorus compound in relation to soil pH.

Figure 6.2 characterizes phosphorus additions and removals from the soil system in addition to the inorganic minerals. Organic phosphorus in the form of residues, manures, or from the soil organic matter can contribute greatly to the phosphorus in the soil solution for crop growth. In some soils organic phosphorus can contribute 50 percent of the available phosphorus. Since availability of organic phosphorus is dependent on decomposition of the organic matter, soil temperature and moisture are important factors regulating how fast organic phosphorus is made available.

 
Figure 6.2. 
Relation of additions and losses of phosphorus in a soil system.

As previously indicated, available or soil solution phosphorus can revert to slowly soluble mineral forms. This fixation may also occur when available phosphorus is used by microorganisms in the decomposition of residues. This type of fixation is called immobilization and can be either long- or short-term.

The Plant Problem

While the soil system limits the amount of phosphorus in the soil solution at any one time and limits its re-supply, the plant root also has its problems. The concentration of roots in the soil volume is relatively small. It has been calculated that roots contact only about one percent of the soil volume. Phosphorus enters the root primarily by diffusion (90-98 percent), which can occur only if the phosphorus is very close to the root. Very little phosphorus enters the root by mass flow in the water (one percent). Root growth is essential for adequate phosphorus uptake or the soil solution needs to be replenished frequently. Actually since roots contact such a small amount of the soil, the soil solution in the areas of root contact must be replenished more often than twice a day or phosphorus deficiencies will occur. This makes the labile forms (those weakly bound to compounds or clay) very important in soil phosphorus supply.

Research in recent years has developed valuable models which predict phosphorus plant uptake and the factors that influence it. One of the most commonly known models has been developed by Dr. Barber at Purdue University. His model indicates phosphorus uptake is largely a function of size and nature of the root system, rate of water absorption, amount of phosphorus in the soil, and ability of the soil to supply phosphorus to the soil solution. His model includes seven plant factors and three soil factors. While phosphorus models are helpful from a research view for understanding phosphorus uptake by plants, relatively little on-farm applied use has been made of models.

Soil Testing for Available Phosphorus

The standard soil tests for available phosphorus are generally weak acid extractants which dissolve portions of the calcium, iron and aluminum phosphates in the soil. In the sodium bicarbonate test, also known as Olsen P, for high pH soils, the bicarbonate ion removes calcium from the system, solubilizing the calcium phosphates, which are measured as an “index” of available phosphorus. All presently used soil tests for phosphorus are of no value unless correlated and calibrated with crop response to applied phosphorus. Correlation tests relate the chemical testing to plant response, and calibration studies measure field response to phosphorus applications based on soil test levels. All phosphorus soil tests are an “index” of availability. The fact that the results are reported in terms of parts-per-million or pounds-per-acre has no interpretative value by itself. The phosphorus “index” is not a “quantitative” value, i.e., it does not indicate a quantity of phosphorus in the soil. Its value is based on the correlated and calibrated predictability of economic crop response.

It is not known precisely what phosphorus compounds are dissolved or measured by the soil test extractants. The extractants probably measure all of the labile phosphorus and, perhaps, some of the non-labile forms. The tests used at the University of Nebraska Soil Test Lab (Bray and Kurtz No. 1 and the sodium bicarbonate extractants) relate well to crop response to applied phosphorus, but are not infallible. The probability of phosphorus response decreases rapidly as the soil test increases from nearly 100 percent when the test is very low, to less than 5 percent when the test is high. However, the long term economics make following UNL recommendations the most profitable decision.

Accurately predicting the precise amount of phosphorus that a given soil can supply to a given plant is essentially impossible. Phosphorus reactions in the soil are complex. The present extracting agents do not measure the actual rate of phosphorus release from the relatively unavailable form to the soil solution where it is available to the plants. The present research data base uses the soil test “index” value from various extracting agents to provide “relative level” for the soil’s ability to supply phosphorus, i.e., very low, low, medium, high, and very high. On a soil testing very low, the amount of phosphorus to apply for corn would be different than with wheat because all crops either don’t have the same ability to extract phosphorus from the soil and/or they take up phosphorus in larger amounts. How well the phosphorus recommendation meets the need of the crop from a particular field depends on several factors, e.g., how variable the field is and how well the soil sample sent to the laboratory represents the field. All chemical tests have some degree of error associated with them; but, in the case of soil phosphorus determinations, the error is very small. Of more concern is the lack of field research on the influence of subsoil phosphorus on crop yield. Considerable field research has been done to arrive at the present ability to make phosphorus recommendations, but considerably more field and laboratory research are needed to improve soil test procedures and to access the role of subsoil phosphorus. At the present time, there is inadequate support for this type of research.

Phosphorus Fertilizer Efficiency

Crop uptake efficiency of fertilizer phosphorus is very low because of the many ways that fertilizer phosphorus (which is 100 percent water soluble) can change to less available forms. Fertilizer efficiencies are affected by several factors.

1. Amount of phosphorus applied
2. Soil characteristics (pH, organic matter, texture, etc.)
3. Type of crop and root characteristics
4. Degree of soil phosphorus deficiency
5. Method of phosphorus application
6. Soil temperature and moisture

In the year of application, corn, sorghum, or soybeans will generally use less than 10 percent of the applied phosphorus. Wheat, however, has shown efficiencies of up to 30 percent. This efficiency was attained on a very low available phosphorus soil and was achieved with seed-applied phosphorus. Utilization of broadcast phosphorus application was less than 10 percent.  For more detail, please refer to the article, 'Phosphorus,' in Nutrient Management for Agronomic Crops in Nebraska.

Application Methods

There is little producers can do to change the basic soil and climatic characteristics that affect crop response to applied fertilizer. However, one can control phosphorus availability by managing the soil pH (acid soils), increasing organic matter, and by proper placement of phosphorus fertilizer. Research has shown that band application of phosphorus is much more efficient than broadcasting. Wheat studies in Nebraska have shown that profits from application with the seed are double those of broadcasting. This is because each pound of applied phosphorus with the seed increased yield much more than a pound broadcast. There is little economic justification for broadcasting phosphorus on wheat in Nebraska.

Another banding method (dual placement) applies liquid phosphorus (10-34-0) at the same time as anhydrous ammonia with a separate tube delivery for each fertilizer. Dual placement has been found to be equal to seed application on wheat and equal to or better than row application for corn and soybeans. While band applications of phosphorus require special application equipment and require extra time at planting, these methods are generally economically superior to broadcast phosphorus. The primary exception being broadcast phosphorus applied to growing alfalfa, grass, or in no-till farming systems. When residues remain on the soil surface, research studies indicate broadcasting phosphorus can be nearly as effective as dual placement. This is attributed to increased root activity in the residue-soil interface where soil moisture and mineralizing nutrients from the residues stimulates root development. This is believed to give a broadcast application the advantages of a band application. This is sometimes referred to as a “horizontal band.” The horizontal band, which is unincorporated, has limited soil-fertilizer contact and is in a position of increased root activity.

Seed placement is another method of banding that can be very effective. The problem with seed application is that starter fertilizer contains salts from the nitrogen and potassium sources; and, when applied in excessive amounts, reduces seed germination. Phosphorus fertilizer without nitrogen has little effect on germination, but mixed fertilizers containing potassium, sulfur, and nitrogen are very damaging, unless water moves the fertilizer from the seed. A major factor affecting salt concentration in the seed row is row spacing. Since wheat is planted in 7- to 12-inch rows, the concentration of 18-46-0 fertilizer is only one-third of the concentration in a 30- or 36-inch corn row. Phosphorus fertilizers, even with nitrogen, can be safely used on wheat at normal phosphorus application rates. For row crops, such as corn, sorghum and soybeans, rates must be limited, because germination will be decreased about one percent for each pound of salt applied (pounds of nitrogen + potassium + sulfur) for corn. Soybeans are more susceptible to germination damage, and so any fertilizer should be kept from contacting the soybean seed.

Row application to the side and below the seed is favored over seed application for row crops, even though this method requires more expensive application equipment than seed applications. This method is also referred to as a “starter” method for row crops and is more effective than broadcast incorporation methods on soils low in available phosphorus. It is, however, important to remember that increased early growth from starter fertilizer application does not always indicate increased yields at harvest.

Summary

Soil phosphorus is relatively stable in soil. It moves very little when compared to nitrogen. In fact, this lack of mobility is due to the rather limited solubility of soil phosphorus compounds. Because of the limited solubility of these compounds, fertilizer phosphorus will become much less available as it reverts back to soil phosphorus compounds. Fertilizer phosphorus that reverts back to soil phosphorus compounds is not lost completely, but becomes slowly available to crops over several years. The rate depends greatly on soil type. For most Nebraska soils, applying more fertilizer phosphorus than needed for optimum yields is probably not economically justified.

Phosphorus availability is controlled by three factors: soil pH, amount of organic matter, and proper placement of fertilizer phosphorus. Acid soils should be limed to bring soil pH up to nearly 6.5. The pH of alkaline soils (over 7.0) probably cannot be practically lowered for better phosphorus availability.

Organic matter maintenance is an important factor in controlling phosphorus availability. Mineralization of organic matter provides a steady supply of available phosphorus. Organic soil phosphorus may represent 30-40 percent of the phosphorus available to Nebraska crops, and may be a major factor affecting phosphorus availability during wet, cold springs.

Placement of phosphorus is the best practical control of phosphorus availability. Placing phosphorus with seed wheat has given much better results than broadcast applications. Banding phosphorus two inches to the side and two inches below the seed of row crops provides a ready source of phosphorus for the young seedling; however, soil phosphorus must be deficient before yields can be expected to be increased.

Potassium

Potassium (K) is an essential plant nutrient. Because it is required in large amounts by plants, potassium is referred to as a macronutrient (Black, 1957). The terms primary and secondary elements also refer to macronutrients.

The major portion of potassium is contained in minerals such as feldspar and mica, and clays such as montmorillonite, vermiculite and illite. The total amount of potassium is important. However, of immediate concern to crop production is the portion of this nutrient that is in an exchangeable (available) form for plant use. Potassium is an exchangeable cation. The potassium ion has a positive charge and binds with the negatively charged soil particles.

Potassium is absorbed by plants in larger amounts than either magnesium or calcium; in fact, nitrogen is the only nutrient absorbed in larger amounts than potassium. Potassium is a vital component of numerous plant functions, including nutrient absorption, respiration, transpiration, and enzyme activity. Potassium is unique because it does not become part of plant compounds, but remains in ionic form in the plant. Potassium remaining in plant residues after harvest and in manure are quickly returned to the soil by water leaching through the plant materials and manure.

Source

Nebraska soils are “mineral” soils, which implies that these soils were formed from minerals such as feldspar, mica, hornblends, etc., and secondary minerals and clays. Because different minerals contain varying amounts of potassium, and since all soils are not formed from the same minerals, it is important to note that soils differ in their ability to supply potassium to a growing crop. Evidence of this variability is shown in Table 6.1.

The most commonly used chemical extracting agent to estimate exchangeable and solution potassium is 1/10 molar ammonium acetate at pH 7. However, this test does not measure total potassium in the soil. The reported value from the chemical analysis is an index of the soil’s ability to supply potassium to different crops. Table 6.1 indicates the difference in exchangeable potassium, as determined by the potassium soil test for soils found in various parts of the Midwest. It can be seen that soils vary in their ability to provide extractable potassium to the crops being grown.

Form and Availability

The behavior of each plant nutrient in soil is unique. Each nutrient’s behavior is a combination of attributes that depends on the parent minerals involved and the solubility and mobility of the nutrient in question. Potassium, unlike nitrogen and phosphorus, is not associated to any great extent with organic matter. Total amounts of potassium in soil will vary from 0.3 to more than 2.5 percent. While total content of potassium is important, it is of little value in determining how well a given soil can supply potassium to growing plants. The general terminology used to describe potassium is shown in Figure 6.3.

1. Relatively Unavailable Forms

Depending on soil type, from 90 to 98 percent of soil potassium is in this form. Minerals containing most of the potassium are feldspar and mica. These minerals are the source of soil potassium, and they release potassium very slowly to the more available forms as they weather and break down.

 

 Figure 6.3. Three forms of soil potassium
 

 2. Slowly Available Forms

Potassium in this form is part of the internal structure of the clay minerals forming the colloidal fraction of the soil. Slowly available potassium cannot be replaced by ordinary cation exchange processes and is referred to as “non-exchangeable” potassium. As shown in Figure 6.3, this form is in equilibrium with the available forms and, consequently, acts as an important reservoir of slowly available potassium. An equilibrium exists between “non-exchangeable,” “exchangeable” and “soil solution potassium,” as shown by the arrows in Figure 6.3. Because of this equilibrium, it is possible for some of the potassium applied as fertilizer to be temporarily converted to the “non-exchangeable” form. This is an important reaction in that it helps reduce leaching of potassium from applied fertilizer, especially on sandy soils.

3. Readily Available Forms

Readily available potassium is composed of exchangeable potassium and potassium in the soil solution. Exchangeable potassium is absorbed on the soil colloid surfaces and is available to plants. However, higher plants obtain most of the potassium from the soil solution phase.

The equilibrium between these different forms of potassium is “dynamic.” That is it is always changing; thus, that portion of the total potassium in the different forms ranges from one to two percent for readily available; one to ten percent for slowly available; and 90 to 98 percent in unavailable form.

Perhaps an example using the Thurman loamy sand from Table 6.1 may help clarify the relationships between the different potassium forms shown in Figure 6.3. There are about 247 pounds of available potassium per acre in the top six inches of this soil. If two percent of the potassium were in the soil solution and exchangeable forms, then the soil would contain 247 pounds/.02 = 12,350 pounds potassium per acre six inches of soil.

 Table 6.1.  Exchangeable potassium for crop producing soils from different areas1
 
 Table 6.2.  Relative proportions of total potassium in available and unavailable forms
 

Based on the figures above, the percentage of total potassium in the top six inches of the Thurman loamy sand is 0.61 percent (12,350/2,000,000), which is in the expected range for sandy soils of Nebraska.

One further point to bring out from this example is that Figure 6.3 shows 10 percent of the readily available potassium is normally in the soil solution. The potassium soil test gives readily available potassium, which is the potassium in solution plus the exchangeable potassium. Thus, from Table 6.1, there are 722 pounds of readily available potassium in the top three feet of the Thurman loamy sand. Assume this soil is capable of producing 150 bushels of corn per acre, which has a potassium requirement of approximately 200 pounds. Further, assume the corn will draw its potassium from the top three feet. Thus, at any given time, the amount of potassium available for plant use is ten percent of 722, or 72 pounds of potassium per acre. This is approximately 30 percent of the total potassium required to produce the 150-bushel corn crop. Thus, during the growing season, the soil is continually supplying the potassium required for crop production. It is for this reason that soil test results, properly interpreted using correlation and calibration data, provide an index of how well different soils can supply potassium, and not a measure of pounds of nutrient.

Factors Influencing Potassium Behavior in Soil

The exact mechanism by which some of these factors influence the reaction of potassium in soil is not clearly understood. Some factors that are known to influence potassium in soil are: (1) soil type, (2) temperature, (3) wetting and drying cycles, (4) pH and (5) aeration and moisture.

Fortunately, the majority of Nebraska’s soils abound with minerals that readily release potassium from the non-exchangeable form. Also, most of Nebraska’s soils are rich in potassium-bearing minerals throughout the subsoil, which reduces the influence of factors, such as temperature and moisture content.

Normally, as the topsoil dries, potassium becomes less available to plants. However, with abundant potassium in the subsoil, lack of availability is not a problem. Some of the sandy soils in Nebraska do not have abundant potassium in the subsoil; and, thus, the soil test for potassium is more important on these soils. Excessive moisture tends to reduce aeration and reduces the plant’s ability to absorb potassium. This is more a problem on soils with smaller amounts of available potassium in the topsoil that tend to have wet subsoils in the spring, such as good corn growing soils of Illinois, Indiana, and eastern Iowa. To add to this, wet soils stay cool, and reduced temperature slows down chemical reactions that release potassium to the available form. Cool, wet soils also reduce root proliferation, which reduces plants’ ability to absorb potassium. However, this is not a problem with most of Nebraska’s soils. The article, 'Potassium,' from Nutrient Management for Agronomic Crops in Nebraska, can answer further questions.

Losses of Soil Potassium

Potassium losses are caused by:

1. Crop removal
2. Fixation
3. Leaching

Fixation and leaching are not serious problems for Nebraska soils. As was previously discussed, fixation can result in temporary “tie-up” of potassium. Over a period of time, however, potassium eventually becomes available. Leaching is not a problem on silt and loam soils, but can be a problem on sandy soils.

By far, crop removal accounts for the largest loss of potassium from soil. In general, potassium content of grain is less than straw or stover. Corn cut for silage would remove about 195 pounds of potassium per acre. Much of this could be returned in the form of animal manure applications from the livestock operation. Corn harvested for grain would leave the stover in the field; and, since potassium is water soluble, it would be quickly returned to the soil. Table 6.3 shows rates of potassium removal for common crops grown in Nebraska.

 Table 6.3.  Average Removal of Potassium by Crop Production
 

Summary

The nature of soil potassium can be summarized by Figure 6.4. Available soil potassium is found associated with the clay complex and soil solution. It is in equilibrium with the slowly available minerals, which are constantly supplying available potassium. Potassium is added by fertilizers and crop residues. Fertilizer potassium may be fixed and become slowly available; it may be lost by crop removal, leaching and erosion.

 
Figure 6.4. 
Available potassium in relation to additions and losses.

References

Black C.A. 1957. Soil Plant Relationships. John Wiley and Sons, Inc., NY.

Buckman, H. L., Brady, N. C. 1960. The Nature and Properties of Soils. 6th Ed., The Macmillian Company, NY.

Ferguson, Richard. Krista DeGroot, Editor. Nutrient Management for Agronomic Crops in Nebraska, EC 01-155-S.  2000.  University of Nebraska-Lincoln Extension Service.

Kilmer, V.S., S. Younts, and N. Brady, Editors, The Role of Potassium in Agriculture. 1968. American Society of Agronomy, Madison, WI.

Penas, Ed, 1989 Soil Science Research Report, Dept. of Agronomy, University of Nebraska-Lincoln.