The Role of pH in Plant Nutrition
The pH of the soil is one of the most important factors in determining the ability of the soil to be used as a media for growing plants. The soil pH affects the uptake of essential nutrients by plants, soil microbial activity as well as the health of plants in general. The soil pH is something that must be continually monitored if optimal crop yields are to be obtained.
H2O –> H+ + OH–
The above equation represents the ionization of a water molecule into hydrogen (H) ions and hydroxyl (OH) ions. In pure water the concentration of OH– ions is always equal to the concentration of H+ ions. Experiments have shown that in pure water, the concentration of both of these ions is 10-7 moles per liter. When the concentrations of the two ions are multiplied together the total is 10-14. Research has shown, that while the actual concentration of both hydrogen ions and hydroxyl ions may vary in aqueous solutions, the product of their concentrations will always be equal to 10-14. This is a very important principle. The fact that the product of the hydrogen ion concentration and the hydroxyl ion concentration is always equal to a constant, shows that their concentrations are inversely proportional in an aqueous solution. If the concentration of one of them is very high, then the concentration of the other must be very low, so that when multiplied, their combined concentration will equal 10-14. An example is given below:
H2O –> H+ + OH–
(10-7)(10-7) = 10-14 or (10-2)(10-12) = 10-14
So, what exactly is pH? PH is a term that is used as measurement of the hydrogen ion concentration in an aqueous solution. More specifically, pH is the negative logarithm of the hydrogen ion concentration in solution. The pH scale has no units. As was discussed earlier, the actual concentration of hydrogen ions in an aqueous solution can range from 1 to 10-14 moles of hydrogen ions per liter. When the concentration of hydrogen ions in solution is 1, the concentration of hydroxyl ions is 10-14, and vice-versa.
When the negative logarithm of 1 is taken, it turns out to be 0, and when the negative logarithm of 10-14 is taken it turns out to be 14. Thus the pH scale ranges from 0 to 14, with a pH of 0 being the most acidic state and a pH of 14 being the least acidic. A pH of 7 is considered neutral. At a pH of 7, the hydrogen ions and hydroxyl ions are present in equal concentrations (10-7). At a pH above seven there are more hydroxyl ions than hydrogen ions present. When there are more hydroxyl ions in solution than hydrogen ions, the solution is considered basic. Thus, anything with a pH above 7 is considered basic, and anything with a pH below 7 is considered acidic.
Since pH is actually a logarithmic measurement, each increment of one on the pH scale actually corresponds to a ten-fold increase or decrease in hydrogen ion concentration in solution. For example, a solution with a pH of 3 will have ten times more hydrogen ions than a solution with a pH of 4. A solution with a pH of 2 will have 100 times more hydrogen ions than a solution of pH 4. In the Southeast, there are two areas of concern when it comes to pH. These two areas are: acidity of the soil and acidity of the rhizosphere region of plant roots. Both of them are extremely important when it comes to determining the type of pH management one would use when trying to grow a crop in a particular area. The pH of the rhizosphere is something that is very often overlooked in growing; this can lead to very poor productivity for a grower, particularly in the high intensity growing situations that occur in horticulture. Research has shown that the pH in the rhizosphere can be two, and sometimes more, units higher or lower than the actual soil pH. This means that if the soil is determined to have a pH of 6.0, then the pH in the rhizosphere could range anywhere from around 4.0 to 8.0. While a pH of 6.0 may not seem to be too large a problem, a pH of 4.0 will certainly result in substantially reduced yields for the grower. The pH of the rhizosphere will be addressed later on, as soil pH and acidity will be discussed below.
The pH of the soil is a dynamic quality that can have a tremendous effect on the ability of a plant to grow and thrive in it. While both acidity and alkalinity of soil can be problems in all areas of the world, soil acidity is a major problem for much of the southeast United States and will be discussed more completely than soil alkalinity problems, which occur more in the western part of the United States.
When one considers soil acidity they must understand that soil acidity is actually made up of two parts: active acidity and reserve acidity. Active acidity is the concentration of hydrogen ions that are present in the soil solution. This is the acidity that one would measure with a pH meter. Reserve acidity consists primarily of aluminum and hydrogen ions that are bound to negatively charged soil colloids. These colloids are referred to as cation exchange sites, after their ability to bind positively charged ions (cations). In most cases the reserve acidity of a soil will be much greater than the active acidity. Reserve acidity will also largely determine how much lime or other amendments one will have to add to the soil to raise the pH. The active acidity of the soil can usually be remedied with a relatively small amount of lime, whereas to neutralize the reserve acidity of a soil much more will be needed. The amount of lime needed to neutralize reserve acidity will depend on the soil type, amount of cation exchange sites, age of soil, amount of aluminum present as well as several other factors. Reserve acidity is not a separate entity from active acidity though. Both of them are interlinked, and what goes on in one will directly affect the other. Their chemistry will be discussed below.
Soil consists of particles of clay, sand and organic matter (humus). These particles are mixed together in varying amounts in the soil. The relative quantity of one to another gives a particular soil its properties. In the piedmont region of Georgia, the soils have a high percentage of clay particles in them and thus we have a red clayey soil. In addition, most soil particles are charged. Usually this is a negative charge, but in some situations organic matter and iron oxides can have a positive charge. Clay particles and organic matter particles have a high amount of negative charge, whereas the negative charge of sand particles is extremely low, if there is any. When a soil particle such as clay has a negative charge, positively charged ions that are present in the soil solution want to bind to them, and they do. Many cations that are present in the soil will bind to these negatively charged soil colloids. These cations include calcium, potassium, magnesium, aluminum and hydrogen to name a few. When considering soil acidity, two ions in particular, aluminum and hydrogen, are important.
There are a number of factors that affect the overall pH of the soil. A general overview of the factors that can determine whether a soil is acidic or basic, are listed below. The specific factors that can lead to a decrease (acidification) in soil pH will be discussed later on.
Some of the factors affecting pH: · Type of parent material
· Age of the soil
· Amount of precipitation
· What crops are grown and for how long have they been grown at that particular location.
· Fertilizer program When a clay particle breaks down for example, aluminum is released into the soil solution. Often this aluminum ion will bind to another negatively charged clay particle. When the conditions in the soil solution are favorable, the bound aluminum ion will react with water in the soil solution and form a number of aluminum hydroxides. When this occurs, hydrogen is released into the soil solution- thus increasing the acidity.
Al 3+ + 3H2O –> Al(OH)3 + 3H+
Al 3+ + 2H2O –> Al(OH)2+ + 2H+
Al 3+ + H2O –> Al(OH)++ + H+
When there is a high number of hydrogen ions in solution, this reaction is not favored and aluminum will not form a hydroxide, but instead will simply stay in solution as an Al3+ ion. This in itself is not a good situation, because aluminum is toxic to plants and when it is present in high amounts in the soil solution it will be taken up by plants and can weaken or even kill them. In addition, aluminum can bind to plant nutrients, thus making them unavailable to plants.
A similar situation exists when considering the hydrogen ions that are bound to soil particles. When a situation exists where it is favorable for the hydrogen ions to move into solution they will. This occurs when the acidity (hydrogen ion concentration) of the soil solution drops. These hydrogen ions that have moved into solution and the hydrogen ions liberated in the hydroxylation reaction of aluminum are what make up the active acidity in a soil. These reactions, involving the movement of hydrogen ions into solution will continuously occur, unless something is present that takes the place of the aluminum or hydrogen ions on the soil colloids and then neutralize the hydrogen that has been released in solution. It just so happens that lime performs this function. Lime and its actions will be discussed later.
As was stated earlier, any number of cations, not just aluminum or hydrogen can be bound to a soil particle at a given time. These non-aluminum/hydrogen cations, are often referred to as the basic cations. The most common basic cations are calcium, magnesium and potassium. These basic cations, when they move into the soil solution, do not cause an increase in the acidity of the soil solution. The percentage of these basic cations in a soil is often referred to as percentage of base saturation. Having a high percent base saturation is desired, for it means that there is usually a high number of nutrients in the soil (Mg, Ca, K etc.) and that soil acidity is not likely to be a problem. In the Southeast, percentage base saturation of the soil is usually between 35 and 50 percent. In some fertile areas of the Midwest, it may be as high as 90%.
High levels of soil acidity can be quite detrimental to plants. A number of reasons for this are listed below: · Low pH levels can adversely affect the uptake of nutrients by plants.
· At low pH levels, some elements, such as aluminum or manganese can become readily available in quantities that are toxic to plants.
· Many pesticides that are used are less effective at low pH levels.
· Beneficial bacteria, such as the bacteria that convert ammonium to nitrate, can be harmed. It is true that most crops will do best when soil pH is between 6.0 and 7.0, but many crops grow best at lower pH levels. These acid loving plants do best in a pH range from 5.0 to 6.0, too low for most plants. Included in this low pH group are: blueberries, sweet potatoes, watermelons and azaleas to name a few. Overall though, acidifying the soil is something that people want to avoid.
In order to avoid acidifying the soil, it is valuable to know what causes soil acidification. One of the primary causal factors of soil acidity is leaching. Water is continually moving through the soil profile, and over time, this water movement causes nutrient elements, that were bound to soil colloids move into solution and are leached out of the soil, being replaced by aluminum and hydrogen ions. Leaching is more prevalent in areas of high rainfall and old soils. Leaching is a major cause of soil acidity in tropical areas. In addition, if soil nutrients are not replaced after a crop is harvested then acidity levels in the soil are likely to rise because nutrient elements are leaving the soil in the harvested crop and are being replace with aluminum and hydrogen ions, not nutrient ions. Lastly, one of the leading causes of soil acidification, especially in high intensity horticultural crops, is the use of acidifying fertilizers, in particular the overuse of ammonium containing fertilizers. The process by which ammonium containing fertilizers can cause acidification will be discussed next.
Ammonium has been used as a source of nitrogen in fertilizers for many years. It is cheap and does not leach out of the soil as readily as nitrate. For these reasons, farmers rely upon it heavily as a source of nitrogen. Using high amounts of ammonium fertilizer will cause acidification of the soil and the rhizosphere of a plant. Acidification occurs when, ammonium is broken down in the soil to form nitrate. In this process, called nitrification, hydrogen ions are released into solution. Nitrification is carried out by two types of soil inhabiting bacteria: nitrosomonas and nitrobacter. The reaction occurs over two stages: in the first stage ammonium is converted to nitrite, in the second reaction the nitrite is converted to nitrate. Nitrite is very toxic to plants, but in is unstable in the soil and is quickly converted to nitrate. The reaction is shown below:
The above reaction illustrates how the acidity of the soil solution can be increased when using ammonium fertilizers. In agronomic crops this may not be as big a problem as it is in horticultural crops. This is because crops are usually grown much more intensely than agronomic crops, often fertilizer is applied every day, and thus acidification from ammonium fertilizers is a big problem. Acidification of the rhizosphere is also a major problem that can result from using ammonium fertilizers. The acidification process is the same as the one that occurs in soils, but it is much more concentrated in the rhizosphere. The nitrifying bacteria are much more dense in the rhizosphere than they are in the soil and thus convert more ammonium to nitrate here per unit area in the rhizosphere. Thus, the hydrogen concentration becomes more intense here than in the soil and this is why the pH of the rhizosphere can be two or more units lower in the rhizosphere than in the soil.
In addition to the acidification effects of using large amounts of ammonium fertilizers there are other detrimental processes that can occur. In horticulture crops that are very intensively grown, often the bacteria cannot keep up with the ammonium being applied and there is excess ammonium “sitting” around in the soil solution. This ammonium will be taken up by plants searching for much needed nitrogen. Ammonium by itself is toxic to plant cells and once inside a plant it must be combined with carbon compounds to be detoxified. This process “steals” carbon that could be used for growth, but is instead being used for detoxification purposes. In addition, very high levels of ammonium will burn the young undifferentiated portions of plant roots. It is here that much of the nutrients for a plant are absorbed. When this area is physically damaged it can no longer absorb enough nutrients for proper growth and the plant will suffer. This “overloading” of ammonium was never thought to be a problem in the past. This is because in the past, most research was done on agronomic crops. In these crops, fertilizer is not applied as frequently as in horticultural crops and the nitrifying bacteria generally can “keep up” in converting ammonium to nitrate. However, since most horticulture crops are grown much more intensively, the overloading effect of ammonium has become and issue.
Horticulture crops not only include many fruit and nut crops, which are grown on trees, but woody landscape plants as well. The problems that can occur with heavy use of ammonium on tree crops are substantial. This is because these tree crops may stay in the soil in one place for 30 years or more, unlike an annual vegetable crop, which is planted every year. Having a crop planted for a very long period of time can magnify the acidification process because it would be nearly impossible for one to till the soil around the plant while that plant remains in the ground. Doing so would likely destroy the root system of the tree. The fact that one cannot till the soil, will prevent them from being able to remedy the acidifying effects of ammonium with lime. Though it will be discussed in more detail later on, lime will work to neutralize acidic soils. In order for it to work well though, it must be tilled into the soil. Studies have shown that if lime is strictly applied to the surface of the soil, it will take years for the lime to move just a few inches downward in the soil profile. Thus it could take ten years for the lime, that one applied, to reach the roots of a tree, where acidification is a problem. After that long, the tree will likely be in very poor condition due to the fact that the roots will have been damaged and nutrient uptake was impaired, therefore opening the door for diseases to invade the tree. It has been proposed that this acidification effect of ammonium could be partially to blame for peach tree short life in Georgia.
Although there are a number of factors which contribute to the short life of peach trees here in Georgia, the acidify effects of ammonium could be one of them. Most peach growers use only ammonium as a nitrogen source. Thus, after a period of time, the surrounding soils and rhizosphere of peach trees becomes very acidic. This will lead to nutrient uptake problems as well as root burn, which in-turn will lead to a plant that is much more susceptible to infectious agents such as nematodes. Eventually the tree is so disease ridden that it becomes unproductive and must be destroyed. Peach tree short life is a major problem in Georgia; peach trees here only live to be about 6 or 8 years on average. In other parts of the country, such as California, peach trees are productive for up to 20 years. Undoubtedly, the acidifying effect of ammonium is not the only reason for peach tree decline in Georgia, but it is a major contributor. A diagram of the root rhizosphere is shown below.
The problem with using ammonium fertilizer can easily be remedied by using nitrate-based fertilizers. First, it should be noted that one does not have to do away with using ammonium fertilizers and should not do away with using ammonium as a source of nitrogen. Instead, use a combination of ammonium and nitrate containing fertilizers. It has been proposed that a 60/40 mix of nitrate and ammonium fertilizers be used. That way one will get the benefits that both have to offer. By using some ammonium, a grower will save some money. Also, ammonium is not leached as readily as nitrate, thus by using some ammonium one gets a sustained source of nitrogen for their crop. By only using 40% ammonium though, one may not run into the acidification problems that occur with using only ammonium. Also it has been noted that, a good fertilization with ammonium in the spring may lead to a quick burst of available phosphorous, which is also need by the plant. Ammonium fertilizers do have their place in a nutrition program it’s just that too much ammonium can lead to problems.
Nitrate fertilizers are not without their problems either. To begin with, nitrates are very expensive. For some growers, it may not be economically feasible to spend the extra money on nitrates. For example, a grower who grows annual transplants in a green house, who uses new potting media every year, may not benefit much from using nitrate fertilizers, especially if they only grow the crop for a few weeks and don’t make much money off the crop. Nitrate is also very easily leached from the soil. If one only fertilizes with nitrates and if soil levels of nutrients are not monitored closely, then a grower could see a nitrogen deficiency in some plants after a long period of rainfall or heavy irrigation. If a grower is accustomed to fertilizing with ammonium, and they decide to switch to a nitrate based fertilizer, then they will likely have to fertilize more often. If they do not change their fertilization schedule, then they could run into problems associated with nitrogen deficiencies. Nitrate based fertilizers, such as potassium nitrate, will work to increase the alkalinity of soils as well. Though, as a whole, this is not a problem with as far-reaching as the acidification problems with ammonium, it does happen.
As was noted earlier, soil and rhizosphere pH can greatly affect the uptake and availability of inorganic nutrients for plants. Some nutrients become more available in acidic soils while others become less available. The same is true for alkaline soils. In general the most ideal pH for plant growth is between 6.0 and 7.0. In this range, all of the essential nutrients are available for uptake.
Nitrogen, one of the primary macronutrients is most available to plants between a pH of 6.0 and 7.5. At a pH lower or higher than those, nitrogen becomes less available. Phosphorous, another of the primary macronutrients, is most available between a pH of 6.5 and 7.0. As the soil becomes more acidic, phosphorous availability greatly decrease. This is because, at low pH levels there is much more aluminum and iron available. This aluminum and iron will bind to the phosphate in the soil, causing it to become insoluble and thus unavailable. At high pHs, phosphorous availability also declines rapidly due to the fact that calcium is much more available at these higher pH levels and this calcium will bind to phosphorous, again making an insoluble salt that is unavailable to be taken up by plants. Potassium, the third of the primary macronutrients, is greatly decreased in availability as the pH of the soil drops below about 6.0. In general as the soil becomes more alkaline potassium availability does not drop, instead it stays relatively constant once a pH of 6.0 is reached. One important thing to note regarding potassium, is that in general soils contain a large amount of potassium, perhaps more than any other nutrient. Unfortunately, most potassium in the soil is bound up in clay minerals and is thus unavailable. So, despite being found in soils in relatively high concentrations, only a small portion of that potassium is usually available for plants.
All of the secondary macronutrients: magnesium, calcium and sulfur become less available to plants as the pH of a soil drops below 6.0. Above this point all of the secondary macronutrients are readily available, even at highly alkaline pH levels. However, even though these elements are readily available at pH levels near 10.0, plants could not be able to utilize them in this highly alkaline region because they (plants) would not be able to tolerate and grow in such alkaline conditions.
All micronutrients, except molybdenum, become more available to a plant as the pH of a soil becomes more acidic. Included in this group are: iron, manganese, boron, copper, and zinc. Chlorine is usually present in high enough quantities and needed in such small amounts by plants that its availability is never an issue. As the soil decreases in pH, many of these elements become extremely soluble and in some cases, such as with manganese, can become so available to the plant that they can cause toxicities. As soil pH increase the micronutrients generally become less available to plants. In order to understand the complete picture though, one must make a distinction between an element being available and whether or not an element will be in the soil in sufficient quantities. Indeed all micronutrients, except molybdenum, become more available at low pHs (below 6.5), but two of these micronutrients in particular are likely to be deficient on acid soils. Both manganese and boron are very soluble at a low pH. If a grower’s soil has been acidic for any period of time, these elements have probably been leached out of the soil by rainfall or irrigation. Thus in most cases, having an acidic soil will mean that you will have a deficiency in boron and manganese, because of the fact that they are more available (soluble) in acidic soils.
In addition to affecting nutrient uptake, soil pH can alter the effectiveness of many pesticides. Generally pesticide adsorption to soil colloids increases in acidic soils, and pesticide leaching will increase in alkaline soils. This is due to the fact that in acid soils there are many free floating hydrogen ions. These ions will bind to basic pesticides to form cationic complexes, which will bind to negatively charged soil colloids. Thus, most pesticides will become strongly adsorbed in acidic soils and their effectiveness will be reduced. They become more leached in alkaline soils because of the fact that in these soils there are fewer hydrogen ions present in the soil solution. Since fewer hydrogen ions are present, fewer cationic complexes are formed and a lesser amount of pesticide is bound to negatively charged soil colloids. Thus, pesticides in general are considered to be most effective when the soil environment is at a near neutral pH.
For the most part, acidic soils are more of a problem than alkaline soils, especially in Georgia. Though high pH soils occur in California, they tend not to pose as big a problem to growers as acidic soils do in the Southeast. In addition, by using ammonium fertilizers, growers on alkaline soils can lower their pH to near neutral levels. What could be considered a detrimental practice in Georgia may be beneficial in California. Also, elemental sulfur can be spread on basic soils and easily reduce the pH. Very small quantities, just a few kilograms per acre of sulfur needs to be applied in order to reduce the pH of that soil one pH unit. Changing the pH of acidic soils is more of a challenge, especially in Georgia, and will thus be discussed in more detail.
The most commonly used material for increasing soil pH is lime. Lime is often defined as, a calcium containing soil amendment that works to increase soil pH. Lime is not a fertilizer. Instead, it is called a soil amendment. This is because growers do not apply lime as a source of nutrition for plants, though it does contain calcium and sometimes magnesium, but it is not used strictly for the purpose of providing plants with those nutrients. Growers use it to raise soil pH, the fact that it does contain some nutrients is just and added benefit.
Lime works in a two-step process. When lime is applied to the soil it breaks up into calcium (sometimes magnesium) and carbonate ions. The calcium ions will then move to the cation exchange sites on the soil particles and bind to them, in the process knocking hydrogen ions and aluminum ions off the charged particle. The hydrogen ions then move into the soil solution, where they bind to the carbonate ion to form carbonic acid, which is then quickly broken down to water and carbon dioxide. The aluminum that is knocked off the soil colloid moves into the soil solution and reacts with water to form aluminum hydroxide, which is inactive, and hydrogen ions. These hydrogen ions then react with the carbonate ion and become carbon dioxide and water.
Liming is extremely important in maintaining adequate soil pH. In order to be sure that one is doing an effective job of liming, a few guidelines should be noted. To begin with, lime will not move through the soil at an appreciable rate. Studies have shown that it takes months and years for lime applied to the soil surface to move just a few inches downward in the soil profile. Thus, when applying lime, one should work it into the soil while tilling. Particle size of the liming material is also very important in determining its effectiveness. As particle size decreases, the total surface area per unit of weight will increase. This increase in surface area is desirable, for it means that a greater proportion of the lime is going to react in a shorter period of time. Particle size is graded on a scale that is based on the size of the mesh screen in a sieve. The greater the mesh size number-the smaller the actual open area for a product to fit through. A sixty-mesh screen will have smaller openings than an eight-mesh screen. Studies have shown that smaller lime particles are more effective than larger ones, thus when purchasing lime, one should look for a lime with at least 70% of the particles being of a sixty-mesh size. Some studies have shown that 1.8 tons of a very finely ground lime (80%+ will pass through a 60 mesh screen) will have an effect equal to 3.9 tons of a courser lime (20-30% passing through a 60 mesh screen). Having, some larger particles mixed in will be beneficial though, because of the fact that these larger particles will indeed break down more slowly and provide a “sustained release” for the lime.
Not all liming materials are equivalent in their effectiveness. The effectiveness is based on the effect that a certain amount of a particular liming material will have on raising pH, when compared to a standard. The standard lime that is used is calcitic limestone (calcium carbonate), and the effectiveness of other limes compared to it is measured in equivalents of calcium carbonate. Pure calcium carbonate will have and equivalent of 100. Anything that is more effective in raising the pH than the pure calcium carbonate will have an equivalency of greater than 100, while anything that does not work as well will have an equivalency of less than 100. A small table demonstrating this is shown below.
As noted above, not all limes are equal. There are a number of different liming products available. Some are better than others. Several common liming products will be discussed below.
· Calcitic (CaCO3) and dolomitic (CaMg(CO3)2) limestone – These two liming materials are the most popular. Depending on purity level, their neutralizing effectiveness (CaCO3 equivalents) can range from 70% to about 100%. They are readily available to the grower, and in addition to providing a liming material they also provide a source of calcium and sometimes magnesium, both of which are required by actively growing plants.
· Calcium hydroxide (Ca(OH)2) – This is often referred to as slaked lime. Calcium hydroxide is very powerful and extremely fast acting, much smaller amounts of this material are needed than most others to achieve the same effect. It is a very caustic compound. If too much is used growers may find themselves in a situation in which their soil pH is too high to grow anything. If overused, calcium hydroxide can increase soil pH to the 12 range. Despite the disadvantages it is sometimes used when a quick response is desired.
· Calcium oxide (CaO) – This is often referred to as quicklime or burned lime. Like calcium hydroxide, calcium oxide is a caustic agent that can quickly raise the soil pH. Though not as powerful as calcium hydroxide, it can easily raise the soil pH to undesirable levels if too much is used. In addition, if spread on the surface of the soil it can rapidly cake.
· Slags– There are a number of materials that fall under the title of slag. Slag can be a byproduct from the manufacturing of steel from pig-iron. It is usually cheap and readily available to growers within a reasonable distance from a steel manufacturing operation. Before applying slag though, one should have it tested for purity as well as effectiveness. In addition, since it is a waste product from a manufacturing process one should look for any contaminants, such as heavy metals. This is important because if a heavily contaminated slag is applied to the soil, a grower could do irreversible damage to their entire growing operation.
· Marl– Marl is a soft deposit of mostly CaCO3. When dry, it is a powdery substance. Its effectiveness is based on the percentage of impurities in it. Often, impurity levels can be quite high. It is usually spread wet.
It was stated earlier that lime is a soil amendment and not a fertilizer. Likewise, lime does not need to be applied on as regular a basis as most fertilizers. Usually one will apply fertilizer several times in a growing season, perhaps even several times in a week. Lime however, usually only needs to be applied every few years. Sometimes growers can go up to ten years between applications, though this is a rare case. Because of the low solubility of lime, it takes long period of time before it is no longer effective in controlling the pH of the soil. In addition, the buffering action of the soil as well as residual effects of previous lime applications will increase the length of time that lime is effective in the soil.
The amount of lime that one would use would be determined by testing the pH of Soil. Obviously if one does a pH test of their soil and it turns out that their soil has a pH of 7.0 or greater then it is likely that they will not have to lime, unless they plan on using a great deal of ammonium fertilizer. If this is the case, then they might have to add lime in anticipation of the acidifying effect of the ammonium. Most cooperative extension agencies will test the pH of a grower’s soil and provide them with a lime recommendation either for free or for a small fee. Usually in order to test pH one will use an indicator solution or a pH meter.
For in the field use, indicators are more commonly used to test pH, though there are very small portable pH meters available that can be used in the field. Usually a series of soil samples are taken from random points in the field that are representative of the field as a whole. One would not take a soil sample from a very low spot in the field for example. Most of the time a soil solution is made with water, the soil is filtered and the pH of the remaining solution is then measured using either an indicator, such as bromothymol blue, litmus paper or a pH meter. In some places, such as the southeast, where there are fewer dissolved salts in soil solution it is difficult to get a good measurement using a pH meter. To alleviate this problem, calcium chloride salt is added to increase conductivity between the electrode on the pH meter and the solution being tested. Calcium chloride is used because it is completely soluble in water. Whether or not calcium chloride was added should be noted on the test, for a pH test of the same soil sample will vary up to 0.5 units between the two types of tests. In general, the pH results of the tests which used calcium chloride will be a little higher than those tests which used just water.
Soil and rhizosphere pH is one of the most important factors to consider when growing a plant. With the exception of water, there are few other things that can have such widespread effects on the growth of a crop. Regardless of how much fertilizer one puts out, if pH is not controlled, then that fertilizer may not be of any use because it will be unavailable for uptake by the plant. In addition, pH will affect the activity of pesticides applied to the soil. Above all else though, pH can have a tremendous impact on the overall health of a plant. If grown in an unsuitable pH, a plant will be weak and susceptible to disease problems. One should also not overlook that fact that the pH of the rhizosphere, can be two or more pH units above or below the pH of the soil. Since the rhizosphere surrounds the roots of the plant, one could say that the plant is actually growing within the rhizosphere, more so than the soil itself. Thus if the rhizosphere pH is 4.0, then the plant is growing in a pH of 4.0, regardless of whether the soil pH is near neutral. Since the rhizosphere is the actual place where absorption of nutrients takes place, the pH of this area is surely, of equal or more importance than the pH of the soil ten feet away from the plant. One of the most important things a grower could do today is regularly monitor his or her pH; it’s easy to do, and the dividends that one would gain are immense.
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