Application of Phosphorus Fertilizers

Last updated: June 21, 2025

Methods of Applying Phosphorus Fertilizers

Phosphorus fertilizers, as a rule, are applied in two ways: pre-sowing (pre-planting) and main. Given the low mobility of phosphates in soil and weak root development during the initial growth period, the role of pre-sowing application of phosphorus fertilizers is of great importance in the formation of high yields.

Even on soils with a high content of mobile phosphorus, the concentration of phosphate ions in the soil solution is insufficient to adequately supply plants with phosphorus in the early growth stages. Row (starter) application of phosphorus fertilizers is carried out at rates of 7–20 kg/ha P₂O₅. Only water-soluble, readily available forms are used — granulated superphosphates. Powdered superphosphates under spring conditions quickly absorb moisture, cake, and clog fertilizer sowing apparatus.

1 t of granulated superphosphate with row application gives a grain increase of 5–6 t; with main application — 1–2 t (CINAA).

A good effect is provided by water-soluble phosphorus fertilizers, most often granulated superphosphate, when applied at sowing of the crop into hills or nests. The fertilizer is applied with combined seeders. For sugar beet, potato, and other crops, superphosphate is applied with combined seeders simultaneously with nitrogen or nitrogen-potassium fertilizers. According to experimental data, 0.5 centner of granulated superphosphate or 10 kg P₂O₅ per 1 ha allows an additional 2.5–3 centners/ha of grain to be obtained. When phosphorus fertilizers are in deficit, application of superphosphate for grain crops at sowing shows good efficiency.

Top dressing with superphosphate can be effective:

when insufficient doses of phosphorus fertilizers are applied in the main application, under autumn plowing;
in areas of sufficient moisture or under irrigation;
on soils with strong chemical absorption during prolonged contact of superphosphate with soil, especially on acidic soils with a high content of sesquioxides.

In other cases, top dressings are less effective than applying similar doses before sowing or in rows.

The great diversity of soil types in Russia allows all types of phosphorus fertilizers to be successfully used for main fertilizer.

Timing of Phosphorus Fertilizer Application

Timing is of great importance for sparingly soluble phosphates. They are applied in advance, in autumn, so that part of the calcium phosphates has time to transform into more available forms by the growing season.

Depth of Phosphorus Fertilizer Incorporation

Due to the low mobility of phosphates in soil, the depth of incorporation of the main phosphorus fertilizer is of great importance. Therefore, efforts are made to create reserves of available phosphorus in the zone of the active root system. This is especially important in arid conditions, where the upper part of the arable layer dries out in summer. Thus, in an experiment with ³²P, surface application of superphosphate on pasture at a dose of 450 kg/ha P₂O₅ did not lead to phosphorus penetration deeper than 2.5 cm.

The depth and location of fertilizers depend on the method of incorporation.

Table. Placement of fertilizers in the arable soil layer depending on the method of their incorporation, %.

Depth of arable layer, cm	Method of incorporation
Depth of arable layer, cm	light harrow	heavy harrow	heavy cultivator	plow	plow with skimmer
0-3	98	75	55	11	3
3-6	2	22	21	12	4
6-9		3	23	16	12
9-12			1	16	14
12-15				23	20
15-20				22	47

Incorporation with harrows or cultivators concentrates the main part of the fertilizers in the upper (0–9 cm) layer. More uniform incorporation is achieved with a plow without a skimmer; deeper incorporation is achieved with a plow with a skimmer, but in this case little fertilizer remains in the upper layer. In the latter case, there is a need for row pre-sowing fertilizer. Applied phosphorus fertilizers do not migrate through the soil profile and remain at the places of incorporation. Only subsequent tillage changes their location in the arable layer.

Therefore, the depth of plowing for a specific crop determines the depth of incorporation of the main phosphorus fertilizer.

Determination of Phosphorus Fertilizer Rates

Based on correlation analysis of experimental data, a quantitative relationship was established between the content of nutrients in the soil and crop productivity, optimal parameters were determined for different crops, and a method for achieving them was proposed.

Table. Optimal levels of nutrient content in soil for various crops (V. G. Chernenok, 1993, 2009).

Crop	Content, mg/kg
Crop	N-NO₃	P₂O₅	K₂O
Wheat	12-15	35	400
Barley	12-15	35	400
Oats	10-12	28-30	400
Corn	10-12	40	500
Millet	10-12	40	360
Buckwheat	10-12	30-32	400
Chickpea, pea	12-15	28-30	440
Rapeseed	15-18	30-32	—

This allowed a new, more precise method to be proposed for determining phosphorus deficiency in soil and the fertilizer rate for crops using the optimization formula:

D_{P}\ (kg\ a.i./ha) = (P_{opt} - P_{fact}) \cdot K,

where the difference (P_opt — P_fact) shows the phosphorus deficiency in a given field in mg/kg soil, coefficient K = 10 on zonal soils, D_P — the amount of phosphorus fertilizers that must be applied to eliminate the deficiency and create optimal phosphorus nutrition conditions for the crop on the given field.

When calculating fertilizer rates, the lower value of the optimal level should be included.

If the phosphorus deficiency is very high and it is not possible to bring its content to the optimal level in one application, it can be done in several applications, bringing it to the average level first — 25 mg/kg soil.

Creating an optimal level of phosphorus content in the soil allows the crop to realize its potential for forming a maximum yield under any moisture conditions.

An optimal phosphorus nutrition regime promotes efficient moisture use. For example, at a low nutrition level, on average over 20 years, the water consumption coefficient was 20 mm; at medium — 12 mm; at optimal — 8 mm.

With a large phosphorus deficiency in the soil, when it is not possible to bring the content to the optimal level in one application, the fertilizer rate can be determined for a certain yield increase.

Example.

To increase the yield by 5 centners, it is necessary to increase the phosphorus content in the soil by 6 mg (5 centners ⋅ 1.2 mg P₂O₅); for this, 60 kg of a.i. of fertilizer must be applied (6 ⋅ 10).

Table. Relationship between spring wheat yield and P₂O₅ content in soil (KATU, Chernenok, 1970-1990 гг.).

P₂O₅ content, mg/kg soil	Yield, centners/ha in years
P₂O₅ content, mg/kg soil	Very dry	Average	Wet
10	3-4	7-8	10-12
15	5-6	10-12	17-20
20	6-7	14-16	22-25
25	8-9	17-20	28-30
30	10	21-25	33-37
35	11-12	25-30	38-40
P₂O₅ consumption, mg per 1 centner of yield	3	1.2-1.4	0.8-1.2

Optimization of Phosphorus Fertilizer Rates

Soils with sufficient phosphorus reserves due to systematic fertilizer application are capable of providing crops with optimal phosphorus nutrition for a long time. Phosphorus mitigates the effect of extreme weather conditions on plants; a high yield can form even under drought, low, or high temperatures.

In world agricultural practice, especially in European countries, increasing the phosphorus content in soil in crop rotation is achieved by periodic application of high rates of phosphorus fertilizers. Due to the preservation of phosphorus in a plant-available form, weak migration through the soil profile, and absence of losses, as well as data on optimal levels of mobile phosphorus for crops, it is possible to calculate the rates of phosphorus fertilizers needed to achieve optimal phosphate nutrition. The main way to maintain optimal phosphorus nutrition is the application of mineral and organic fertilizers.

Agrochemistry has accumulated enough knowledge about this biogenic element, but a number of unresolved problems remain:

Low coefficient of phosphorus utilization from fertilizers by individual crops and, in general, in the agrocenosis.
Systematic application of high rates of phosphorus fertilizers and soil phosphorus accumulation leads to an imbalance of other biogenic elements, which worsens the nutrient regime.
Various substances contained in phosphorus fertilizers as impurities, including heavy metals, have a negative polluting impact on the environment and, entering plants, on agricultural products.
Immobilization (retrogradation) of phosphorus in soil as a result of chemical absorption. These processes are especially intensive on carbonate chernozems, red soils, and acidic sod-podzolic soils with a high content of sesquioxides of aluminum and iron.
Mobilization of soil phosphates. This is especially important for those agricultural regions and soils where, as a result of systematic application of large doses of phosphorus fertilizers, reserves exceeding the optimal phosphate level have been created.
Optimization of phosphorus nutrition of crops depends on the specification of crop rotations in specific soil-climatic conditions. The complexity of optimizing phosphorus plant nutrition is related to the binding of a number of biogenic elements, such as zinc and copper, and to disturbance of the nutrient balance in the soil.

The development and application of optimal phosphorus rates are connected with a complex of agrotechnical, chemical, and biological methods for mobilizing phosphorus accumulated as a result of systematic application of phosphorus fertilizers. Thus, the application of physiologically acidic nitrogen and potassium fertilizers in combination with trace elements mobilizes phosphorus on phosphorus-rich chernozems, serozems, and chestnut soils. In this case, it is possible to obtain high crop yields for a long time without applying phosphorus fertilizers. Liming of acidic sod-podzolic soils also promotes the mobilization of soil phosphates bound by sesquioxides of aluminum and iron.

When solving problems of optimizing phosphorus fertilizer with account for the phosphate regime of the soil, it is necessary to consider:

An objective assessment of the effectiveness of phosphorus fertilizers is carried out not only by the productivity of individual crops, but also of the crop rotation.
Methods for assessing the phosphate level and optimizing phosphorus fertilizer depend on the methods for determining mobile phosphorus in soil.
For an objective assessment, one should take into account both the content of mobile phosphorus by the method accepted for the given soil type and its mobility in weak salt extracts.

On sod-podzolic light loamy soil, the optimal content of mobile phosphorus in the arable layer is considered to be 10–15 mg/100 g soil. On such soils, under good agrotechnics and provision of plants with nitrogen and potassium, the average annual productivity of a field crop rotation is 45–50 centners of grain units of main products. A higher content of mobile phosphorus leads to a lower payback of phosphorus fertilizers.

The optimal phosphate regime on gray forest soils is close to that of sod-podzolic soils when using the Kirsanov method. The same value of optimal mobile phosphorus content was established for chernozems when determined by the Chirikov method. On carbonate chernozems, serozems, and chestnut soils, the optimal level is 3–4.5 mg/100 g soil by the Machigin method.

To optimize phosphorus fertilizer, in addition to optimal content, the following are used:

1. Balance coefficient of utilization, or balance coefficient, removal coefficient. It shows the share of nutrient removal from that applied with fertilizers and is calculated by the formula:

K_b = \frac {R}{D} \cdot 100,

where

K_b: — balance coefficient;
R: — phosphorus removal with harvest;
D: — dose of applied phosphorus.

2. Compensation coefficient, or balance intensity (K_c), the reciprocal of K_b, equal to:

K_c = \frac {D}{R} \cdot 100.

The balance coefficient is a measure of fertilizer effectiveness at the corresponding nutrient content for the given soil conditions.

The increase or decrease in the fertilizer rate (K_opt, %) in accordance with nutrient removal is calculated by the formula:

V_{opt} = \lparen \frac{100}{K_{b\ opt}} - 1 \rparen \cdot 100.

The optimal fertilizer rate is calculated by the formula:

D_{opt} = R_{opt} \cdot V\%.

Then, the degree of soil phosphorus supply depending on the content of its mobile form (K) is equal to:

K = D_{opt} - R_{opt}.

At a low soil supply of mobile phosphorus, K is 48–55 kg P₂O₅/ha; at medium — 17–20 kg P₂O₅/ha; at high — 3–6 kg P₂O₅/ha.

3. Rates of phosphorus and potassium fertilizers are calculated by the formula:

D_{P(K)} = R_P - S_O + C_{P(K)},

where

D_P(K): — dose of phosphorus or potassium fertilizers, kg a.i./ha;
R_P: — removal of phosphorus or potassium with the planned harvest, kg/ha;
S_O: — content of phosphorus or potassium in organic fertilizers, kg/ha;
C_P(K): — amount of phosphorus or potassium that increases the content of these elements by 10 mg/kg in soils with low content and by 5 mg/kg in soils with medium nutrient content, kg/ha.

The optimal level is taken as the content of mobile phosphorus in the soil at which not less than 90–95% of the maximum yield is achieved, and the missing 5–10% is compensated by fertilizer phosphorus to compensate for removal with the planned harvest.

Generalization of the results of long-term experiments allowed general principles for differentiating fertilizer rates to be developed, taking into account specific conditions.

Table. Differentiation of phosphorus fertilizer rates and phosphorus removal by plants depending on soil supply of mobile phosphorus.

Content of mobile P₂O₅ in soil, mg/100 g	Rates of P₂O₅, kg/ha	Coefficient of rate differentiation^*	Possible removal of P₂O₅, kg/ha	Coefficient of removal change	Residual phosphorus in soil, kg/ha
< 5	120	2.0	30 — 35	0.75	85 — 80
5.1 — 10.0	90	1.5	35 — 40	0.85	55 — 50
10.1 — 15.0	60	1.0	40 — 50	1.00	15 — 5
15.1 — 25.0	30	0.5	45 — 50 and more	1.15	- (25 — 30)
> 25	10^**	0.2	45 — 50 and more	1.15	- (45 — 50)

Notes.

^* The unit rate (differentiation coefficient 1) is taken as P₆₀.

^** In row at sowing.

Rates of P₂O₅ for pre-sowing application are determined by the crop. Some of them, for example corn and sunflower, can be inhibited by direct contact of seeds with superphosphate. Therefore, a soil layer is created between the seeds and the fertilizer; the P₂O₅ rates in this case are 7–10 kg/ha.

Cereal and vegetable crops, flax, and hemp are less sensitive and respond positively to granulated superphosphate at a dose of about 10 kg/ha; mixing it with seeds before sowing with an ordinary row seeder is permissible. In this case, the seeds and fertilizers must be dry, and the granules must have good mechanical strength, not grind in the sowing apparatus, and not clog it. Superphosphate must have a neutral or slightly acidic reaction. Acidic superphosphate, even with short contact with seeds (up to 2 h), reduces the germination of winter rye, barley, spring wheat, flax, and table beet seeds. At acidity less than 1%, it can be mixed with seeds of rye and beet not earlier than 2 h before sowing, and with the other listed crops — 4–8 h before. Neutralized superphosphate can be mixed with seeds of these crops one day before sowing.

On average, rates of phosphorus fertilizers vary from 30–45 kg/ha to 90–120 kg/ha P₂O₅ and depend on soil fertility, soil-climatic conditions, the planned yield, the predecessor, and accompanying fertilizers.

References

1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. B. A. Yagodin. Moscow: Kolos.

2. Evtefeev Yu. V., Kazantsev G. M. (2013). Fundamentals of Agronomy: textbook. Moscow: FORUM.

3. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.