Besides that which is essential for photosynthesis, namely carbon dioxide, water, and sunlight, grapevines also require a range of nutrients to grow, survive and prosper. These nutrients are split into two groups depending on the scale of requirement, macro and micronutrient, the former being those required in larger amounts. Phosphorus is essential for plant growth. It is a component of cell membranes and DNA and plays a vital role in photosynthesis, the movement of sugars, and carbohydrate storage within the vine. Deficiency of phosphorus in vines can result in reduced vine vigour and yellowing of the interveinal area of basal leaves. In extreme cases, this may be followed by early defoliation of these leaves. Poor bud initiation and fruit set may also be observed. In this article, I will explore phosphorus in viticulture from soil to bottle.

Phosphorus in the soil

Phosphorus is a widely distributed element, the 12th most abundant in the Earths crust, to which it contributes about 0.10% of its total weight. Phosphorus is one of the atoms which makes up the molecule ATP, lipids, DNA, and RNA nucleic acids, and in nature always occurs as the phosphate ion. Phosphate is an anion, derived from phosphoric acid, most commonly chemists are referring to orthophosphate when discussing plant nutrition.

Despite phosphorus cycling through the environment, there is no phosphorus in the atmosphere itself, this is in contrast to the nitrogen cycle where there is nitrogen in the atmosphere.

The total phosphorus content of most surface soils is low, averaging only 0.6%. This compares to an average soil content of 0.14% nitrogen and 0.83% potassium. The phosphorus content of soils is quite variable, ranging from less than 0.04% in the sandy soils of the Atlantic and Gulf coastal plains to more than 0.3% in soils of the northwestern United States.

Soil phosphorus can be broadly categorised into two groups, organic and inorganic. Approximately 30-65% of total soil phosphorus is in organic forms, which is not plant-available, while the remaining 35-70% is in inorganic forms. Organic forms of phosphorus include dead plant/animal residues and soil micro-organisms. Soil micro-organisms play a key role in processing and transforming these organic forms of phosphorus into plant-available forms. Inorganic phosphorus moves to the root surface through diffusion

The principal inorganic source of phosphorus is apatite, the original source of all phosphorus. Other important phosphorus-bearing minerals are wavellite and vivianite. The solubility of these phosphorus compounds, as well as organic phosphorus, is extremely low, and only very small amounts of soil phosphorus are in solution at any one time.

Since the main source of phosphorus is found in rocks, the first step of the phosphorus cycle involves the extraction of phosphorus from the rocks by weathering. Weather events, such as rain and other sources of erosion, resulting in phosphorus being washed into the soil. There are two forms of soluble P are H₂PO₄^– and HPO₄^2-, both negatively charged anions. Plants take up H₂PO₄^– and HPO₄^2- at varying rates, often dependent on soil pH, When the soil pH is less than 7.0, H₂PO₄^– is the predominate form in the soil.

Phosphorus enters the soil, where it is eventually available to plants, through chemical fertilizers, manure, biosolids, or dead plant or animal debris, it cycles between several soil pools (plant-available, sorbed and mineral pool) via processes such as mineralization, immobilization, adsorption, precipitation, desorption, weathering, and dissolution. The Alabama A&M & Auburn Universities have explored these processes in more detail.

When plants and animals die, decomposition results in the return of phosphorus back to the environment via the water or soil. Plants and animals in these environments can then use this phosphorus, and step 2 of the cycle is repeated. A number of human activities have had a significant impact on the phosphorus cycle including but not limited to the use of fertilizer, distribution of food products, and artificial eutrophication.

Phosphorus is removed from soil by crop/plant uptake, runoff and erosion, and leaching. Surface runoff is the major pathway for phosphorus loss from soils. Runoff water carries away both soluble phosphorus and particulate phosphorus from the soil surface. Leaching is the loss of soluble phosphorus from sub-surface soil as water percolates vertically down the soil profile.

Phosphorus availability

Availability of phosphorus is not static across sites, studies and experience show that there is a range of factors which influence the availability of phosphorus to plants. These factors are considered as part of the wider ecosystem of the site and a holistic approach to agriculture fostering biodiversity certainly bolsters the overall health and performance of the soil and vine. Whilst considering the site as a single diverse organic system, a number of independent variables can be considered in relation to phosphorus availability.

Soil pH – P-sorption occurs when the orthophosphates, H₂PO₄^– and HPO₄^2-, bind tightly to soil particles making them unavailable. At low pH, soils have greater amounts of aluminium in the soil solution, which forms very strong bonds with phosphate. In fact, a soil binds twice the amount of phosphorus under acidic conditions, and these bonds are five times stronger.

Organic matter – Soils high in organic matter contain considerable amounts of organic phosphorus that are mineralized. In addition to supplying phosphorus, organic matter also acts as a chelating agent and combines with iron, thereby preventing the formation of insoluble iron phosphates. Heavy applications of organic materials such as manure, plant residues or green manure crops to soils with high pH values not only supply phosphorus but upon decomposition, provide acidic compounds, which increase the availability of mineral forms of phosphorus in the soil.

Balanced crop nutrition – Adequate supplies of other plant nutrients tend to increase the absorption of phosphorus from the soil. Application of ammonium forms of nitrogen with phosphorus increases phosphorus uptake from fertiliser as compared to applying the phosphorus fertiliser alone or applying them separately.

Soil composition – As the volume of clay increases in the soil, the p-sorption capacity increases as well. This is because clay particles have a large surface area for which p-sorption can take place. The mineral composition of soil also influences the phosphorus adsorption capacity. Soils with a high content of Al3+ and Fe3+ tend to have the greatest phosphorus adsorption capacity.

Application timing – Fixation of soil phosphorus increases with time of contact between soluble phosphorus and soil particles. Consequently, more efficient utilization of fertiliser phosphorus can be obtained by applying fertiliser shortly before planting, particularly in soils with high phosphorus-fixing capacities.

Soil temperature, aeration, moisture and compaction – Phosphorus absorption is decreased by low soil temperature and poor soil aeration. Excessive soil moisture or soil compaction reduces the soil oxygen supply and decreases the ability of the plant roots to absorb soil phosphorus. Compaction reduces aeration and pore space in the root zone, thus reducing uptake and subsequent plant growth. Compaction also decreases the soil volume that plant roots penetrate, limiting their total access to soil phosphorus.

Function in the vine

Plants take up phosphorus via several pathways: the arbuscular mycorrhizal pathway and the direct uptake pathway. It is required as a component of cell membranes and genetic material (DNA/RNA) and for carbon dioxide fixation, sugar metabolism, and energy storage and transfer. Phosphorus is mobile within the grapevine and can move from mature organs to areas of new growth.

Animals obtain their energy by oxidation of foods, plants do so by trapping the sunlight using chlorophyll. However, before that energy can be used, it is first transformed into a form which the organism can handle easily. This carrier of energy is the molecule adenosine triphosphate or ATP.

ATP functions by losing the endmost phosphate group when instructed to do so by an enzyme. This reaction releases energy, which the organism can then use to build proteins. The reaction product is adenosine diphosphate, and the phosphate group either ends up as orthophosphate (HPO₄) or attached to another molecule. Additional energy can be extracted by removing a second phosphate group to produce adenosine monophosphate.

Deficiency and management in viticulture

Vines growing in soils deficient in phosphorus can display poor vegetative and reproductive growth. In turn, this can influence berry composition and subsequent must and overall wine quality. Excess phosphorus in the soil can reduce nutrient uptake and adversely affect vine growth which may negatively impact on berry composition. Vines low in phosphorus may show stunted shoots and poor fruitfulness. Early in the season, this deficiency may show as a bronze/red colouration between the main veins in older leaves, due to the plant assigning available phosphorus to younger, more promising vegetative growth.

Applied phosphorus is not readily leached in medium-heavy soils; however, in sandy soils, it is readily leached. Phosphorus may also be lost from the soil when the surface soil is eroded (something winemakers are tackling in Piedmont) whilst large amounts are also removed during harvest, usually at a rate of approximately 0.6 kg per ton of grapes.

Where deficiencies exist, they must be managed, in order to effectively manage, vineyard managers must first test for plant nutrient status. Whilst phosphorus deficiency can be identified via leaf discolouration and inhibited growth, this is reactive opposed to proactive, by the time leaf discolouration has occurred the vine is deficient and the damage has to some extent already been done. Whilst soil organic matter and phosphorus can be tested for it is worth performing annual tissue sampling in order to take a proactive approach. The below table provides a broad guideline to interpreting tissue results against expected nutrient status.

Phosphorus fertilisers are available in three forms: water-soluble, citrate soluble, and citrate insoluble. Interestingly water-soluble phosphorus has been to shown in some cases to positively influence rhizospheric bacteria, a key factor in overall soil and vine health. Water-soluble phosphorus fertilisers are available to plants relatively quickly. However, citrate insoluble phosphorus fertilisers release phosphorus slowly and in some cases may take years to become available to vines.

The application of phosphorus fertiliser in a band is generally most effective, generally in a narrow strip underneath the vines where the majority of vine feeder roots are found or between vine rows (after weeds are cleared) in autumn when annual cover crops are sown. Phosphorus fertiliser is best applied in autumn or early spring to take advantage of any rain, particularly where fertigation is not an option.

Phosphorus moves slowly from point of application, except on sandy soil, the application strategy ought to aim to avoid phosphorus fixing. In soils prone to fixing, repeat fertiliser applications may be required in order to saturate fixation sites and improve availability to the roots. The application of phosphorus fertiliser prior to planting should be based on pre-plant soil analysis. Phosphorus fertiliser should be applied as a surface band along the proposed vine rows and incorporated into the soil.

In established vineyards, periodic testing of plant tissue and/or soil can be useful in proactively monitoring phosphorus reserves in both vines and soil. Adequate phosphorus is required throughout the growing season to optimise vegetative growth and production. However, because of its key role in the reproductive processes of the vine, phosphorus availability is critical early in the season to maximise yield.

Featured image credit Vidacycle.