Farmers and agronomists are very aware that phosphorus (P) is an essential element needed for optimum crop production in Western Canada. Most soils used for annual crops in Western Canada are very low, low or medium in plant-available soil P and are responsive to added P fertilizer. As a result, phosphate fertilizer use is second only to nitrogen (N) fertilizer with respect to use in Western Canada.
Prairie farmers use about one million metric tonnes of phosphate fertilizer annually, which has a value of over $1 billion. It is important that farmers and agronomists have a very good understanding of soil phosphorus and phosphate fertilizer management to help ensure fertilizer is carefully used and money is wisely spent.
In this article, I will focus on understanding soil phosphorus and, in my next two articles, will discuss soil P testing methods and developing wise phosphate fertilizer recommendations.
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The soil P cycle
To understand soil P, it’s important to understand how P cycles in soil. The major processes of the P cycle include:
- adsorption of P onto surfaces of inorganic constituents;
- uptake of P by plants;
- cycling through plant residues; and
- microbial influences through immobilization and mineralization.
Both inorganic phosphorus (Pi) and organic phosphorus (Po) occur in soil. Both are important sources for crop nutrition. A great deal of research has been conducted in Western Canada in the past 40 years to understand the soil P cycle. Figure 1 shows a simple illustration of the P cycle.
The left side of Figure 1 shows the various forms of Pi which come from the parent materials on which soils have formed. During soil development, primary P minerals very slowly dissolve to provide P ions to the soil solution. The solution P can be taken up by plant roots; adsorbed to mineral surfaces; precipitate with various cations to form secondary P minerals; or incorporated into the biomass and soil organic matter. The fate of soluble Pi depends on the physical and chemical conditions in the soil environment.
Depletion of solution P by plant roots can cause a rapid replenishment by exchangeable and labile P forms. “Labile P” refers to a pool of soil P that is less available to plants but can undergo rapid chemical or biological changes to recharge or replenish the soil solution P. As the labile P forms become depleted, nonlabile secondary P minerals slowly solubilize to maintain the labile Pi pool. Concurrent to the dynamic interchange between Pi fractions, the cycling of Po also contributes to the maintenance of P in the soil solution.
Microbial P is the active hub of the Po cycle. Organic P can be taken up by soil organisms or can be mineralized to enter the soil solution as Pi. Po can also be stabilized as part of the soil organic matter or interact with soil minerals.
Carbon (C) inputs, derived from litter and plant residues, provide the energy to drive the system by stimulating microbial activity. When C inputs are lacking, the turnover of labile Po slows down, and maintenance of solution P is limited to the quantity of labile Pi. Conversely, larger C inputs may result in the immobilization of solution P in labile and stable Po forms. Figure 1 shows a simplification of the P cycle but tries to portray the dynamic nature of soil P.
Crop rotation effects on soil P
In the 1980s, the University of Saskatchewan developed a sequential extraction technique to characterize the various Pi and Po forms and amounts (Figure 1). We used the technique to look at soil P changes in long-term cropping studies at Agriculture and Agri-Food Canada’s Lethbridge Research Centre on Dark Brown soil (Rotation A-B-C) and the University of Alberta’s Breton Plots on a Gray soil. We found that crop rotations and fertilizer management had dramatic effects on most Pi and Po pools. Among the observations:
- Not adding P fertilizer resulted in a continuous drain on almost all inorganic and organic P pools.
- Leaving the soil in fallow every second year accentuated the drain on Po pools.
- The addition of fertilizer inputs at both Lethbridge and Breton resulted in more dynamic P cycling. Phosphate fertilizer addition increased the size of all Pi and Po pools at both sites.
- The continuously cropped treatments that received both N and P fertilizer inputs had the highest total soil P levels of all cropping treatments.
- Continuous cropping and additional P fertilizer inputs had the most positive effects on soil P cycling at both sites.
- Continuous cropping, and the addition of both N and P fertilizers, resulted in benefits to the health and quality of soil P cycling at both long-term research sites.
Plant uptake of soil P
Many physical, chemical and biological factors affect plant uptake of soil P. In the top 20 cm (eight inches) of surface soil, plant roots occupy and contact less than one per cent of the soil volume. As a result, only an exceedingly small amount of P at the root surface is intercepted and absorbed by roots.
Most of the P taken uptake by roots is by mass flow and diffusion. As roots absorb water from the soil solution, a convective flow of water moves toward roots carrying P by mass flow. However, mass flow toward plant roots is often not sufficient to supply plant P requirements. As P is taken up by root hairs, the P concentration in solution near the root surface is reduced. This creates a P concentration gradient radiating from around the root. This causes P to diffuse toward the root along this gradient from an area of higher concentration toward the root surface where the P concentration is lower.
The proportion of P supplied by interception, mass flow and diffusion mechanisms depends on root characteristics, rate of water absorption and the levels of solution Pi and adsorbed P on the soil surface.
The soil-plant root interface is referred to as the rhizosphere. The rhizosphere zone is very dynamic in which living roots release exudates into the soil. These organic compounds stimulate microbial activity. As a result, the population of microorganisms in the rhizosphere zone can be up to 10 times higher than in the bulk soil. In P-deficient soils, microbes in the rhizosphere can intercept P before it can be taken up by roots. The release of organic C by roots into the rhizosphere affects the solubility and uptake of P and other nutrients.
The conditions in the rhizosphere differ from bulk soil, in that the preferential uptake of ions and water results in depletion or accumulation of ions in the rhizosphere. Prominent pH change in the rhizosphere is caused by differences in the cation/anion uptake, especially with nitrate-N (NO3- -N) and ammonium-N (NH4+-N) supply. Nitrate is negatively charged, and ammonium is positively charged. Preferential uptake of cations such as NH4+ result in higher net excretion of rates of hydrogen (H+) causing a pH decline in the rhizosphere. Preferential nitrate-N uptake results in higher net excretion of rates of hydroxyl ions (OH-) causing an increase of rhizosphere pH. The portion of P in soil solution can be influenced by changes in the rhizosphere pH. The ability of plants to preferentially change the pH at the soil-root surface can be a significant factor in influencing soil P availability and uptake.
Fine root hairs, which are tubular extensions of root cells, are a result of lateral cell growth and can increase the surface area of the outer roots by two to 10 times. Root hair length varies from 0.1 to 1.5 mm. Root hair density varieties from 50 to 500 million per square metre of root surface. Most plants have root hairs; however, some crops, such as canola, only have an extremely small amount of root hairs or none at all. Root hairs can aid in ion uptake by plants and are most important in the uptake of immobile nutrients such as P.
Vesicular arbuscular mycorrhizae (VAM), which are soil fungi, can form a symbiotic relationship with plant roots of some crops. The mycorrhizae use plant carbohydrates for their growth and in return supply nutrients for plant growth. The primary benefit of VAM is to form hyphae that extend from the plant roots to increase P uptake, water and other nutrients such as copper and zinc. VAM is particularly beneficial in soils with low plant available P. However, not all crops can be infected or benefit from VAM, such as canola.
Labile organic P compounds can be mineralized rapidly in soils by an enzyme produced and released by roots called acid phosphatase. Some research has shown that plant-produced phosphatase can hydrolyze Po for improved plant nutrition. More research is needed to better understand which plants can produce and release acid phosphatase and the benefit to P crop nutrition.
The interaction between soil and plant roots is highly complex and is influenced by soil types, crop species, root characteristics, forms of root exudates, pH changes in the rhizosphere, microbial activity, types of enzymes produced, and influence of VA mycorrhizae.
Summary
This is a brief explanation of the soil P cycle, effects of cropping and fertilizing on soil P, and soil-plant root factors that affect plant uptake of soil P. Research here in Western Canada has clearly shown that continuous-cropped soils, with the addition N, P and other needed fertilizers, have resulted in the greatest benefits to the health and quality of soils and to good P cycling in soils.
In my next two articles, I’ll discuss testing methods for plant available soil P and how to develop wise fertilizer P recommendations.
