Chapter 11. Phosphorus

Summary

The primary function of phosphorus is the transfer of energy from plant leaves to its storage in sugars and starches. Its observable effect is to enhance root development, seed size and flower development.

Once in the soil, it is so active that it is almost completely immobile. A plant needs a good root structure to find it; on the other hand phosphorus is not subject to leaching losses,

Both the pH and biological activity affect the availability of phosphorus to plants.

The value of fertilizer depends on how it is spread and the availability of water.

Table 20. Comparison Of Phosphorus Fertilizers is a comparison of fertilizers.

Phosphorus In The Plant

Phosphorus is the Power Broker. It controls and distributes the energy trapped by photosynthesis preparatory to storing that energy in sugars and starches.

It is also an essential element in every metabolic process. It is a constituent of DNA and RNA and necessary in protein synthesis. Root nodules associated with the fixation of nitrogen require an ample supply of phosphorus.

But the role phosphorus plays in energy transfer is its most important activity and the one which is most affected by a deficiency.

Seeds contain a large amount of phosphorus. A phosphorus deficiency reduces the number and size of seeds. Larger seeds can germinate from deeper into the soil, and the sprouting plants have more resistance to drought.

Phosphorus is a stimulus to root development. Roots branch out and root hairs form profusely in the vicinity of a source of phosphorus. Owing to its effect on roots, phosphorus is a major factor in determining the early growth of a plant and its vigor throughout the season.

Nitrogen and phosphorus have complementary tendencies. Nitrogen enables the plant to trap energy from sunlight, and phosphorus facilitates the actual use of the energy. Nitrogen is a necessary component of proteins, but phosphorus manages the synthesis of proteins.

In field crops, nitrogen encourages grasses, while phosphorus encourages legumes.

However, nitrogen in the nitrate form (slightly acid to alkaline soils) competes with phosphorus for takeup by the plant roots. But it is much more mobile, and phosphorus can be overwhelmed by an excess of nitrogen even if it is adequate otherwise.

A deficiency of phosphorus also, like nitrogen, produces stunted growth. On some plants the underside of leaves may be purplish, owing to the accumulation of underutilized sugars. A phosphorus deficiency delays the growth of new shoots and the development of flowers.

Phosphorus In The Soil

Limitations On Phosphorus Mobility

Owing to its high reactivity with almost anything which it contacts, phosphorus has a lower mobility than any other nutrient. It can be bound up by soil organisms, by mineral elements (particularly aluminum, calcium and iron), and by clay minerals containing aluminum or iron. Consequently phosphorus does not remain in a free state for long, and any amount taken up by plants usually comes from an area within a fraction of an inch around the roots.

One of the few agricultural benefits of a temporarily anaerobic condition is that it causes iron phosphate to change from ferric phosphate to ferrous phosphate, which is more soluble.

Otherwise phosphorus is only slowly available to plants. Furthermore, in cool weather, particularly in the spring, biological activity is low, and phosphorus availability may be low even if a soil test indicates an adequate amount.

An advantage of this immobility is that it limits leaching losses to such low levels as to be measurable only over periods of 50-100 years1. Water pollution from phosphates is caused not by leaching of phosphorus through the soil but by runoff of phosphorus-containing fertilizers from the surface.

Factors Affecting Phosphorus Mobility

Soil pH

The pH affects the limitation on phosphorus availability in several ways:

The net effect is to create a window - in most cases in the pH range from 6.5 to 6.8 - where these tendencies to immobilize phosphorus drop off.

On most acid soils the pH can be adjusted with lime, but on alkaline soils pH control is not easy. Where it is high because of arid conditions, constant irrigation to leach the salts may help, although excess irrigation washes away some of the important nutrient salts in the process. Gypsum may help by dissolving insoluble sodium carbonates. Where the soil is on top of a limestone bed, or in dry conditions, peat moss and finely ground mined sulfur are common natural materials for increasing acidity; but they may be expensive. Aluminum sulfate and sulfuric acid are synthetic alternatives.

Fortunately, biological activity reduces the damage from alkaline soils. The production of organic acids as a metabolic byproduct creates a separate environment with a reduced pH around plant roots, where activity is strongest.

Phosphorus And Water

The major mechanism for plant roots to absorb phosphorus - as well as other anion nutrients (nitrogen, sulfur, boron, molybdenum, and silicon) - is by solution in soil water. Although the solubility of phosphorus in water is low, it is adequate for plant growth if water flow is steady throughout the growing season.

The reverse is also true. Phosphorus is important in good root development, and good root development is necessary to enable the plant to find water. Consequently, an adequate supply of phosphorus is essential at the beginning of the season.

Moreover, the placement of phosphorus fertilizers affects its availability. Phosphorus fertilizer topdressed or banded results in high growth within a small volume of the soil. In dry weather, the lack of well-spaced roots limits the plant's ability to take up water. Dry weather will also cause root development downward in search of moisture, away from the fertilized zone. More care than usual is necessary in order to assure a satisfactory supply of water.

Consequently, if an irrigation system is in place to assure a sufficient supply of water throughout the season, topdressing or banding is probably the most efficient way to utilize fertilizer. Otherwise a better procedure is to broadcast the fertilizer and thoroughly till it under.

Saturation of the Phosphorus Reservoir

One way to overcome the tendency of the soil to absorb phosphorus is to load it down with fertilizer to such an extent that all the mechanisms which can tie up phosphorus are overpowered. This is one rationale for banding phosphorus fertilizer.

The strategy often works, but it can be dangerous. Phosphorus may be present at such excessive levels as to have a harmful effect on crop growth. The major hazard of a phosphorus overload is the reduction of trace element availability, particularly of iron, manganese and zinc. Phosphorus must be unusually high to be so detrimental, but occasionally it is.

The role of organic matter And biological activity

Organic matter and the activity of soil organisms have a strong influence on the availability of phosphorus. Any which is released by decaying residues is readily available.

Phosphorus picked up by fungi is distributed throughout the innumerable extensions of their microscopic threads (mycelia). Upon death of the fungi, the released phosphorus is apportioned more evenly throughout the soil. A consequence is that phosphorus broadcast onto a pasture is soon well distributed. This reduces the need for irrigation, stated earlier, in soils topdressed with phosphorus fertilizer.

Organic matter can break up the aluminum-phosphate bond in an acid soil, because aluminum has a stronger affinity for organic matter than it does for phosphorus.

Soil organisms cause the production of organic acids as waste products of their metabolism. These acids are effective in dissolving inorganic phosphorus. The particularly high activity surrounding plant roots produces a high concentration of acids, which is especially favorable to phosphorus availability.

Some fungi invade the roots of plants for the purpose of extracting carbohydrates. The value of these fungi - mycorrhizae - is that they accumulate minerals, including phosphorus, which they pass on to the roots. Fungi assisting the plant in obtaining phosphorus is analagous to nodule-forming bacteria which provide nitrogen to legumes.

As is true with nitrogen fixation, however, this exchange and cooperation between plant and microorganism is an agent of last resort. If the plant can obtain phosphorus (or other minerals) by an easier route with less expenditure of carbohydrates, it will do so in order to divert its energy elsewhere. Mycorrhizae are useful only when available phosphorus is low. They are probably responsible for the success of trees in soils poor in phosphorus and may be most useful to perennials.

Organic matter and biological activity are often the predominant sources of phosphorus, especially in alkaline soils. Plowed sods produce a good crop the first year because of the phosphorus released by the decaying residues.

Root activity

By a straightforward but technical chemical process, the roots of plants facilitate the breakdown of insoluble calcium phosphates, releasing the phosphorus. This process may occur with any plant having a high calcium requirement. It has been demonstrated with squash and undoubtedly is a factor in the ability of many calcium-loving legumes to make direct use of rock phosphate.

Phosphorus Fertilizers

Fertilizers

Table 20. Comparison Of Phosphorus Fertilizers lists only those organic materials which offer a generous supply of phosphorus. Others, such as cow manure, hay and seed meals are good for maintaining phosphorus, and possibly they might supply enough to growing plants even though soil phosphorus is low. But their value is questionable for building up soil phosphorus where the phosphorus/nitrogen balance is low.

Of the inorganic materials listed, two natural products are hard rock and colloidal rock phospage - also called soft rock phosphate. The alternative name for hard rock phosphate from Florida is pebble phosphate. The usual use for these is in the production of commercial phosphorus fertilizers, four of which are listed in table 20. Comparison Of Phosphorus Fertilizers .

Colloidal rock phosphate is the variety commonly available to organic agriculture. When rock phosphate from Florida is mined, a very finely-divided low-grade ore is removed by washing it away to a settling basin. After the water has evaporated, any sediment which has a phosphate content of 20% or more is sold as an animal feed supplement; the rest is marketed as colloidal phosphate for fertilizer use.

Of the four synthetic fertilizers listed in table 20. Comparison Of Phosphorus Fertilizers , superphosphate is the oldest. It was first manufactured in England in the middle of the nineteenth century by dissolving bone meal in sulfuric acid. The new product became so popular among farmers that bones soon became scarce. Englishmen scoured Europe looking for them and earned a reputation as “the Ghouls of Europe”. Eventually, the industry was rescued when rock phosphate was discovered in North Africa, and production of superphosphate increased steadily up to recent years.

Today, however, superphosphate is considered inefficient because of its relatively low phosphorus content and has been superseded by triple phosphate and by mono-ammonium phosphate and di-ammonium phosphate. Owing to its sulfuric acid parentage, superphosphate is a combination of calcium phosphate and calcium sulfate, or gypsum, while triple phosphate contains no sulfate. The two ammonium phosphate fertilizers are a mixture of ammonia and phosphoric acid and are now the most popular phosphorus fertilizers in the world.

Comparisons

The ideal fertilizer appears to be poultry manure; it is cheap and loaded with nitrogen and phosphorus. However, cage layer manure, the strongest, is difficult to deal with nonprofessionally and should be used carefully for several reasons:

Applications of cage layer manure on most soils should not exceed 5 tons/acre, or 250 lbs/1000 sq ft. It is not a pleasant material, but no soil which regularly receives it is acid or low in phosphorus.

Bone meal is the oldest phosphorus fertilizer. Owing to its high cost, it is popular today principally among caretakers of small gardens. Its nutrient content is usually specified by available NPK content, typically 1-11-0, rather than the total content referred to in table 20. Comparison Of Phosphorus Fertilizers .

At one time, farmers manufactured their own bone meal by roasting the bones of slaughtered livestock or by soaking bones in urine or water and allowing them to ferment. Bones have also been composted by mixing them with wood ashes or quicklime and covering them with soil for several weeks.

In terms of the cost per pound of phosphorus, hard and colloidal rock phosphates is less expensive than bone meal; but the availability of the phosphorus is much lower. Rock phosphates are the skeletal remnants of marine animals, which have a similar composition to the bones of land animals, namely a combination of calcium phosphate and lime. Over long periods of time, however, while the deposits were still under water, the carbonates in the lime were slowly replaced by fluorides, resulting in a much more stable material. Colloidal phosphate has about 2% immediately available phosphate compared to 11% for bone meal; hard rock phosphate may have 3% immediately available phosphate.

Experimental results comparing hard and colloidal rock phosphate do not seem to exist, but in any event available phosphorus is low in both products. The choice of one or the other on the basis of a miniscule availability misses the point of using rock phosphate. Rock powders are applied either because they are cheap or because of the decision to use fertilizers whose nutrients are released by the biological activity of the soil.

Colloidal rock phosphate particles are so fine that they are hazardous to lungs and should be handled with the use of a respirator.

Rock phosphate does have a high availability in acid soils. It has been used with great success in the black soils of Illinois.

Despite its high cost, bone meal is often preferred to rock phosphate, particularly on small gardens, for three reasons:

  1. it is easier to obtain
  2. its higher availability is significant
  3. it is easier to spread.

Where immediate effect on plant takeup is essential, the limited availability of phosphorus in rock phosphate is the major impediment to its widespread use. Where the soil pH is above 6, usually optimum for other reasons, phosphorus availability in either rock phosphate or colloidal phosphate is low without the help of biological activity. Bone meal offers a higher initial availability and is more suitable in a near-neutral soil, but that portion which is not initially available is slow to dissolve. Rock and colloidal phosphate, and bone meal to a lesser extent, are useful mainly for their long-term benefits.

Rock phosphate, as well as other rock powders, are reputed to become more available when spread with animal manure. Although the evidence for this belief is weak, I have sometimes recommended the combination as a desperate measure. Some studies conclude that the mixture is effective, and others that it is not. The effectiveness may depend on the state of the manure. In theory, the organic acids of manure are said to dissolve the rock phosphate. But fresh manure tends to have a high pH, which may cancel the effectiveness of the acids. Rotted manure, however, is somewhat acid and may be more efficient.

An alternative to increasing the near-term usefulness of rock powders is to spread them before turning under a planting of green manures. The decay of the vegetation stimulates a high biological activity and the production of organic acids; this will hasten the availability of the rock powders.

Despite its limited availability, rock phosphate can be efficiently utilized by some plants. There is little universal agreement on what those plants are, but on everybody's list are buckwheat, sweetclover and mustard; other recommendations are Indian corn and rape. Most legumes are better than average at picking up rock phosphate, and most grasses and small grains are worse than average.

For a philosophical comparison of rock phosphate and the synthetic, acidulated fertilizers, see chapter 1. Introduction . Where organic certification standards forbidding the use of synthetic phosphorus fertilizers are in force for philosophical reasons or marketing purposes, rock or colloidal phosphate has to be the preference. Otherwise, the synthetics are worth considering, especially if they are less costly and their use is restricted to an initial period of building up phosphorus reserves. Among the synthetics, triple phosphate or superphosphate should be the choices2.

Before any inorganic phosphorus fertilizer is used, one should determine that phosphorus is indeed deficient. Organic residues contain more phosphorus than most people realize and are often sufficient for maintaining the soil supply. Most of the phosphorus in residues is inorganic, but both the organic and inorganic forms have a high availability. One exception is starting a crop in a cold Spring; this may warrant supplemental phosphorus even if the soil reserve is adequate.

Spreading Rates

The phosphorus test differs from tests for other major nutrients in that the result does not state how much phosphorus the soil contains, but only whether or not adding fertilizer is warranted. Consequently a low test result does not by itself indicate how much fertilizer is likely to be necessary.

The amount that is necessary depends upon the fixing power of the soil, that is, the power of the soil to lock up fertilizer phosphorus. The fixing power depends upon the nature of the soil and upon the soil pH. Several states have developed tests to measure this fixing power, or they have successfully correlated the phosphorus test with the fixing power. The University of Vermont, for example, bases a fertilizer recommendation on phosphorus and aluminum tests, on the assumption that aluminum is responsible for locking up phosphorus. This method works very well for acid soils in Vermont and possibly in other states in New England but not for soils with a low aluminum content.

In a state where recommendations are based upon the fixing power of the soil, and where the soil and pH are characteristic for that state, then the fertilizer recommendation may be very good. Otherwise one has to make a choice based on other, usually average considerations.

One rule of thumb, when planning to use a soluble phosphorus fertilizer, is to determine the amount of phosphorus needed for growing the crop and then increase this by about 50%. Tables 3. Estimated Fertilizer Requirements - Field Crops and 5. Average Nutrient Requirements For Vegetables , for example, can be used to make an estimate of the amount required.

The rock phosphate choices are usually used for long-term benefits rather than to meet an immediate demand. A rate of approximately 1 ton/A is customary. This application is based upon the notion that such a quantity will supply crops for 4 years, which is about as long as one can plan in advance. But the amount is arbitrary, and more or less could be spread with the same results, unless, of course, that 2% immediate availability is essential.

Bone meal is intermediate between rock phosphate and the soluble synthetics, and so intermediate rates are appropriate.

Table 20. Comparison Of Phosphorus Fertilizers also shows how much of each of the fertilizers is needed to supply a given amount of phosphorus.


1 Under extreme conditions, leaching of phosphorus can be significant, for example in very coarse soils with little organic matter or clay, or in peat soils with little aluminum or clay. Some loss also occurs from the leaching of soluble organic substances containing phosphorus.    [return to text]

2 see chapter 10. Nitrogen - Other Considerations for an argument against the use of ammonium phosphate or any fertilizer containing ammonia or an ammonium salt    [return to text]

© 2013 Robert Parnes

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