Introduction to Phosphate as a Fertilizer

Phosphate fertilizers are manufactured from raw phosphates that are either minerals such as apatite (mainly calcium phosphate) with a low cadmium content or former seabed sediments with a high cadmium content.

 

Phosphate Fertilization and Soil Deficiency  

 

organic fertilizer are manufactured from raw phosphates that are either minerals such as apatite (mainly calcium phosphate) with a low cadmium content or former seabed sediments with a high cadmium content.

 

Phosphate fertilization plays a crucial role in maintaining soil fertility, as vast agricultural areas can suffer from poor productivity due to phosphate (P) deficiency. The term available phosphate is used because phosphorus is the least mobile of the major plant nutrients. If it is not in a soluble form, plants struggle to absorb it effectively.  

 

Recognizing Phosphorus Deficiency  

Phosphorus deficiency is less visually distinct than deficiencies of many other nutrients. Affected plants are often stunted, with thin stems and a spindly appearance, yet their foliage may appear dark green or even bluish. Without healthy plants for comparison, phosphorus-deficient plants can seem normal. In severe cases, deficiency may cause leaf yellowing, premature senescence, and even purpling of leaves and stems. However, other stress factors, such as cold temperatures, can also lead to purple pigmentation. Phosphorus is particularly essential in meristematic tissues, where rapid cell division and growth occur.  

 

Phosphorus Mobility in Plants  

Because phosphorus is highly mobile within plants, when its supply is limited, older leaves transfer phosphorus to newer, rapidly growing tissues. As a result, symptoms such as purpling and premature leaf senescence are most noticeable in older leaves. Phosphorus-deficient plants often experience delayed maturity, reduced flowering, and poor seed quality, ultimately impacting overall crop productivity.

 

 

Phosphorus in Soil  

 

The challenge of maintaining phosphorus fertility in soil involves three key issues. First, the total phosphorus content in soil is naturally low—typically only one-tenth to one-fourth that of nitrogen and about one-twentieth that of potassium. In the top 15 cm of soil per hectare, phosphorus levels range from 200 to 2,000 kg, with an average of around 1,000 kg.  

 

Second, most phosphorus compounds in soil are highly insoluble, making them largely unavailable for plant uptake. Even when phosphorus is present, its accessibility to plants is often limited.  

 

Third, when soluble phosphorus sources—such as fertilizers or manure—are applied, they undergo fixation, a process in which phosphorus reacts with soil minerals to form insoluble compounds. Over time, this significantly reduces its availability to plants. Understanding these fixation reactions is essential for determining the optimal amount and method of phosphorus application in soil management.

 

Phosphorus Fixation and Soil Management  

 

Fixation reactions in soil limit the immediate availability of phosphorus from fertilizers and manure, with only 10 to 15% typically taken up by plants in the year of application. To compensate, farmers often apply two to four times the amount of phosphorus expected to be removed by the crop harvest.  

 

Over time, repeated phosphorus applications have saturated the soil’s fixation capacity, significantly increasing available phosphorus levels in many agricultural areas. In soils with high phosphorus accumulation, fertilization is only necessary to replace the phosphorus removed during harvest. In fact, many agricultural soils in industrialized countries have accumulated so much available phosphorus—due to long-term manure or fertilizer use—that additional applications are often unnecessary until phosphorus levels naturally decline.  

 

This trend is reflected in U.S. fertilizer use statistics, which show a shift toward reducing phosphorus applications where soil reserves are sufficient. While the long-term build-up of phosphorus has improved soil fertility, it has also led to certain environmental concerns, highlighting the need for balanced and sustainable phosphorus management.

 

Unlike in industrialized nations, phosphorus overuse is not a common issue in many developing countries, particularly in sub-Saharan Africa, where per capita food production has been declining in recent years. In these regions, phosphorus fertilizer application is only a fraction of the amount removed during harvest, leading to long-term soil depletion.  

 

After years of phosphorus mining, many soils in these areas now face severe phosphorus deficiencies, making it the primary limiting factor in food-crop production. This shortage also indirectly impacts nitrogen availability, as most nitrogen-fixing legumes struggle to grow under low-phosphorus conditions.  

 

Reversing the decline in food production across sub-Saharan Africa will require addressing these critical phosphorus deficiencies through improved soil management and sustainable fertilization practices.

 

Phosphorus in the Environment

 

Unlike certain nitrogen compounds produced during the nitrogen cycle (such as ammonia, nitrates, and nitrosamines; see Chapter 13), phosphorus added to aquatic systems from soil is generally non-toxic to fish, livestock, or humans. However, both excessive and insufficient phosphorus levels can have significant, widespread negative effects on environmental quality. The main environmental concerns related to soil phosphorus are land degradation from insufficient phosphorus and accelerated eutrophication from an overabundance. Both issues stem from phosphorus’s essential role as a plant nutrient.

 

In many highly weathered soils, particularly in warm, humid, and sub-humid regions, the capacity to supply phosphorus for plant growth is limited. This is primarily due to the extensive losses of phosphorus over long periods of intense weathering, as well as the low availability of phosphorus in the forms of aluminum and iron, which dominate in these soils.

 

Undisturbed natural ecosystems in these regions typically contain sufficient phosphorus in the biomass and soil organic matter to support a healthy population of trees or grasses. Most of the phosphorus absorbed by plants comes from the decomposition of plant residues. As long as the system remains undisturbed, phosphorus losses are minimal.

 

However, once the land is cleared for agriculture, whether through timber harvesting or forest fires, significant phosphorus losses can occur through eroded soil particles, runoff water, and biomass removal. Within just a few years, much of the phosphorus that previously cycled between plants and soil may be lost. The remaining inorganic phosphorus in the soil becomes largely unavailable to plants. As a result, the soil's ability to supply phosphorus diminishes rapidly, leading to sparse regrowth of natural vegetation. On agricultural land, crops quickly fail to produce viable yields.

 

Leguminous plants, which are expected to replenish soil nitrogen levels, are particularly affected by phosphorus deficiency. Low phosphorus availability inhibits proper nodulation and slows the biological nitrogen-fixation process. These spindly plants, lacking both phosphorus and nitrogen, offer minimal vegetative cover, leaving the soil vulnerable to erosion from heavy rainfall. This erosion further depletes soil fertility and reduces its water-holding capacity. As the soil continues to degrade, it can support less and less vegetation, accelerating the cycle of degradation.