Phosphorusr Biofertilizers


Phosphorus is a macronutrient that plays a number of important roles in plants. It is a component of nucleic acids, so it plays a vital role in plant reproduction, of which grain production is an important result.

It is also critical in biological energy transfer processes that are vital for life and growth. Adequate phosphorus results in higher grain production, improved crop quality, greater stalk strength, increased root growth, and earlier crop maturity. For over one hundred years, phosphorus has been applied to crops as fertilizer—first as ground bone and now as some chemical reaction product of ground rock. Yet, for all that experience, its management cannot be taken for granted.

Phosphorus is not lost into the atmosphere—rarely does it leach beyond the reach of roots—and its availability to crops can be accurately estimated by soil testing. The challenge is that phosphorus is a macronutrient in plants but behaves somewhat like a micronutrient in soils. The concentration of soluble phosphate in the soil solution is very low, and phosphorus is relatively immobile in the soil. That is important because crops take up phosphorus only from the soil solution. The crop depends on replenishment of the soil solution with phosphate from the other forms existing in the soil. The rate of replenishment, which determines the availability of phosphorus, is related to soil pH, phosphorus levels in soil, its fixation by the soil, and placement of added phosphorus. The crop manager must deal with each of these factors to avoid crop phosphorus deficiency. Phosphorus deficiency symptoms include reduced growth and yield, delayed maturity, and generally purple coloring along the edge of the lower plant leaves, especially on younger plants.

In addition, the manager needs to consider possible “side effects” of crop production; specifically, nutrient pollution of streams or other surface water near crop fields. Water can be polluted with phosphorus primarily as a result of erosion and runoff of phosphorus in the soil or phosphorus applied either from fertilizer or manure. The amount of phosphorus lost due to runoff of manure, fertilizer, or soil may be relatively small as far as fertilizer costs are concerned. However, these small losses may have serious effects on the quality of water. The main problem with phosphorus pollution is eutrophication resulting in excessive growth of plants and algae in the water. This can seriously limit the use of the water for drinking, industry, fishing, or recreation. Pollution reduction may not be simply a direct economic problem for the farmer, but a responsibility that extends beyond the farm fence. For more information see the Penn State publication “Agricultural Phosphorus and the Environment.”

Availability of phosphorus to crops

In general, crop use of any nutrient depends on a two-step process: soil supply of that nutrient in an available form, and uptake of that available nutrient by the crop. There are certain constants involved that the crop manager cannot change. Selecting among the options presented by nature constitutes management.

Soil supply

Figure 1 shows an overview of the behavior of phosphorus in the soil. The soil solution is the key to plant nutrition because all phosphorus that is taken up by plants comes from phosphorus dissolved in the soil solution. Because the amount of soluble phosphorus in the soil solution is very low, it must be replenished by as many as 500 times during a growing season to meet the nutritional needs of a typical crop. Although very little phosphorus is in the soil solution at any time, there is a large amount of phosphorus in most soils. The bulk of the soil phosphorus is either in the soil organic matter or in the soil minerals. A large proportion of the phosphorus in both of these fractions is in very stable, unavailable forms, while a much smaller proportion is in available forms that can dissolve in the soil solution and be taken up by plants. The dynamic and available phosphorus phosphorus in these fractions, such as phosphorus added in fertilizer or manure, can be quickly fixed into stable, unavailable forms in the soil. This is why, even with optimum management, the efficiency of plant uptake of phosphorus is very low—usually less than 20 percent. At the same time as the soil solution phosphorus is depleted by crop uptake, unavailable phosphorus can slowly be released to more available forms to replenish the soil solution. This slow release can sustain plant growth in many natural systems, but is usually not rapid enough to maintain adequate phosphorus availability in intensively managed cropping systems without some supplemental phosphorus in the form of fertilizer, manure, or crop residues.

Crop Uptake

Crop response to phosphorus depends on the availability of phosphorus in the soil solution and the ability of the crop to take up phosphorus. The availability of phosphorus in the soil solution has already been discussed. The ability of a plant to take up phosphorus is largely due to its root distribution relative to phosphorus location in soil. Because phosphorus is very immobile in the soil, it does not move very far in the soil to get to the roots. Diffusion to the root is only about 1/8 of an inch per year, and relatively little phosphorus in soil is within that distance of a root. Thus, the roots must grow through the soil and basically go get the phosphorus the plant needs. Therefore root growth is very important to phosphorus nutrition. Any factor that affects root growth will affect the ability of plant to explore more soil and get adequate phosphorus. Soil compaction, herbicide root injury, and insects feeding on roots can all dramatically reduce the ability of the plant to get adequate phosphorus. Young seedlings can suffer from phosphorus deficiency even in soils with high available phosphorus levels because they have very limited root systems that are growing very slowly in cold, wet, early early-season soil conditions. This is why some crops respond to phosphorus applied at planting in starter fertilizers even in relatively high phosphorus soils.

Managing Soils for Phosphorus for Phosphorus

The availability of phosphorus to crops is more than just having phosphorus in the soil. It will depend on soil pH, how supplemental phosphorus is applied, crop root growth, and the other management factors that influence root growth.

Soil Test

The most important tool in phosphorus management for crops is a soil test. Soil testing reveals soil pH, the soil phosphorus level, and determines the recommended application amount of phosphorus for the crop to be grown. Consistent and representative soil sampling is very important for correct interpretation of soil test results. Take as many cores as practical. Sampling depth is extremely important for both pH and phosphorus, especially in reduce and no-tillage systems where there is little or no mixing to homogenize the soil. In Pennsylvania, the recommendation is to sample to “plow depth,” even in no-till fields where phosphorus is concentrated within several inches of the soil surface.

There is no specific “available” fraction of phosphorus in soils. The available phosphorus is what is in solution plus what can be expected to become soluble from minerals and organic matter over the growing season. Therefore, soil tests cannot extract the exact available amount from the soil, but rather an amount that reflects what might become available. Research on Pennsylvania soils is then used to interpret the amount extracted by the soil test in terms of what is required for optimum crop production. This research has shown that on our soils, if the Mehlich 3 soil test used, in Pennsylvania extracts between 30 and 50 parts per million (ppm) phosphorus it is optimum for production of agronomic crops. Below 30 ppm phosphorus, additional phosphorus must be applied to build up the soil for optimum crop production. Above 50 ppm phosphorus, there will be no benefit to adding additional phosphorus. In some cases, applying a small amount of phosphorus as a starter on soils testing above 50 ppm may be beneficial. In the optimum range range—between 30 and 50 ppm phosphorus—phosphorus is often recommended to offset crop removal (Table 1) and thus maintain the soil in the optimum range over time.

Inorganic Phosphorus Fertilizers

By Pennsylvania law, mineral phosphorus materials sold as fertilizer must be labeled with the percentage “available phosphoric acid”, which is defined as the amount of fertilizer phosphorus that dissolves in neutral ammonium citrate. This analysis must be given as a percent P2O5/A) in the material. Fertilizers do not really contain any P2O5/A) but this expression is a carryover from past analytical methods. Fertilizer recommendations are also given as pounds of P2O5/A) per acre and are based on the amount of this “available phosphoric acid” that should be available to the crop over the period of the growing season. Mineral phosphorus materials that have not been reacted with acid, such as raw rock phosphate and basic slag materials, must also be labeled with the total P equivalent and the material’s degree of fineness. The phosphate availability of phosphorus materials that have not been reacted with acid is low, as the availability then depends on reaction in acidic soil, particle size determines the speed of that reaction. Bone and other natural organic phosphate materials must be labeled only with the total P content. Don’t confuse total P with available P—the availability of phosphorus in these forms depends on the mineralization, or breakdown, of the material by bacteria in soil and cannot be guaranteed.

Immediate phosphorus availability can be defined by the percentage of the available P that is water-soluble. This is not a labeling requirement, but it is described for various materials in Table 2. A high percentage of water solubility is important for short-season, fast-growing crops, crops with a restricted root system, crops receiving a starter fertilizer application, and crops grown in a low phosphorus soil where less than optimum rates of phosphorus are applied. Where the importance of high water solubility, or quick reaction in the soil, is not so great (such as in fertilizing a permanent pasture or where soil phosphorus levels are already optimum), a more economical form of phosphorus can be applied. Most of the common phosphorus fertilizer materials are highly water soluble (Table 2).

Although the calcium orthophosphate fertilizers are manufactured by reaction with an acid, they do not acidify the soil. The ammonium phosphates and the ammoniated superphosphates, on the other hand, do ultimately have an acidic effect on soil because of the ammonium nitrogen that they contain—not because of their phosphate content.

The physical form of the applied phosphorus does not make any difference to the plant if the materials have similar chemical properties. The same reactions eventually occur in soil whether liquid or solid fertilizer is applied. Though all of the phosphorus in true solution fertilizer will be water soluble, the same materials applied in dry form are just as efficient.

Manure Phosphorus

Average manure phosphorus values for various animal types are shown in Table 3, but however good the averages are, the manure phosphorus content on individual farms may vary considerably from the average. The true value can only be known by manure analysis.