Phosphorus Inputs and Location

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Field crops Field crops Phosphorus P-K-Mg Fertilization Localization

Example of the impact of localization (here nitrogen)

Phosphorus is an essential element for crops but its very low mobility in the soil often penalizes its assimilation and impacts yields, especially when the soil is poorly supplied. This article explores how localizing phosphate fertilizer as close as possible to the sowing improves its efficiency, promotes root development, and allows yield gains.

Localizing phosphate fertilization for better results?

Phosphorus is a very immobile element in the soil (only 2 mm per year) and it is also a major constituent of plants (notably present in ATP). In addition, its availability to plants is often low which leads to direct yield losses when the Olsen P threshold (available P) falls below 20 mg/kg of soil.

	Phosphorus (P2O5)	Potash (K2O)
Highly demanding Yield strongly penalized in poor soil	Sugar beet, Rapeseed, Potato	Sugar beet, Potato
Moderately demanding Yield slightly impacted in poor soil	Wheat after wheat, Durum wheat, Silage maize, Barley, Pea and Faba bean	Rapeseed, Maize, Pea and Faba bean, Sunflower
Low demanding Yield not impacted in poor soil	Oats, Soft wheat “in rotation”, Grain maize, Rye, Sunflower	Oats, Soft wheat, Durum wheat, Barley, Rye

This threshold varies depending on the phosphorus needs of the crops. Soil type must also be taken into account to adjust threshold contents to plan or not the inputs. Here are the Arvalis charts on this subject here.

The last parameter to reason mineral element inputs is the calculation of crop exports notably thanks to the content of each species. The COMIFER method allows a simple and effective calculation, see the document here.

Role of pH in availability

Soil pH plays a determining role in the dissolution/precipitation balance of a calcium phosphate. Indeed, in Basque soils, P precipitates with calcium which makes it poorly available. Acidic pH favors the dissolution of calcium phosphates but promotes the complexation of P with Iron (Fe) and Aluminum (Al).

Because of these phenomena, phosphorus applied in spring on an autumn crop will not be fully optimal and application on wide row spacing and short cycle crops (maize) will also be less effective.

To maximize the benefit of a P application, localization at sowing or band applications before sowing and/or incorporated (if not equipped) can improve P use.

Effect of phosphorus localization and soil content

Localized application of 40 units of super 45 improves wheat yield by 2.3 q/ha on average. However, a threshold (20 mg/kg Olsen phosphorus) is observed beyond which phosphorus fertilization has practically no effect compared to the unfertilized control. However, a long-term neglect sees soil content decrease by about 2 mg/kg per year.

The low measured impact of phosphorus localization on cereals is also related to the good performance of their root system (partly explaining their low to medium phosphorus demand) and their narrow row sowing method. Thus, crops responding best to phosphorus localization are generally wide row spacing crops such as rapeseed in autumn or maize in spring.

Yield is really improved by phosphate fertilizer only when the soil is poorly supplied with this element. Localization allows a small increase but it remains low.

Compilation of 10 P fertilization trials / soft and durum Wheat 40 u of Super 45 (Arvalis, 1995-2017)

Apply phosphorus early so it is available and effective on the crop

This trial conducted at Montans (81) by Arvalis in 1998 on winter soft wheat on a poorly supplied soil (Olsen P content of 20 ppm) highlights an effect of application timing. In the case of phosphorus-poor soils, a pre-sowing application is better utilized than an application at 2 tillers. Indeed, it is at the young stage that plants are most sensitive to phosphorus deficiency. It is also noted that this effect disappears when the doses applied are high (60 kg/ha/year).

Phosphorus fertilization of cereals - Chambers of agriculture

Phosphorus localization and early sowing

In this 2009 trial at Calmont (31), the starter fertilizer, here 14-48 localized at sowing (5-5 cm) allowed a gain of 15 q/ha for sowing in cold conditions, on March 20. However, there is no visible difference for sowing during April with or without starter.

Cold temperatures limit maize root growth and thus penalize its exploration. Moreover, according to Arvalis (LD trial at Presly (18) - 2001 40 and 80 kg P2O5/ha), phosphorus localization promotes denser rooting in the 0-30 cm horizon, increasing the explored soil volume.

Here, the combined effects of cold and less root development without starter really penalized the yield of early sowing without P localization.

Calmont (31) 2009 trial

Beware of toxicity in case of contact with seeds

Almost all fertilizers are salts. When dissolved in the soil, they increase the salt concentration of the solution. An increase in salt concentration raises the osmotic potential of the solution. The higher the osmotic potential of a solution, the more difficult it is for seeds or plants to extract the water they need for growth.

The renewed interest in fertilizer spreading in or near the sowing row highlights the importance of remembering that an increase in salt concentration in the fertilizer band can damage seeds and seedlings. Placing fertilizer at least five centimeters from the seed generally avoids injury. However, excessive fertilizer application in a starter band can cause damage, especially in dry weather.

The salinity index of sodium nitrate is defined as 100. Fertilizers with a salinity index above 100 produce a higher osmotic potential than an equivalent weight of sodium nitrate. Fertilizers with a salinity index below 100 produce a lower osmotic potential than an equivalent weight of sodium nitrate.

Ammoniacal toxicity is also significant in case of localized nitrogen application.

Emergence losses of the crop when 75 units of nitrogen are applied in localized form range from 5% (urea) to 20% (ammonium nitrate).

To avoid any risk of burning, it is recommended to limit fertilizer doses (N, P, K and S) applied to 50 units in the row for sowing at 25 cm spacing. However, be careful to halve the dose in case of sowing at 50 cm to avoid concentrating the input compared to a 25 cm spacing.

Generally, in maize, doses applied are 40 l of 14-48 at seed level in the row. When inputs are higher, it is recommended to apply the surplus offset (5-5 cm).

Factors that increase the risk of damage are:

• A low CEC
• The higher the salinity index of a fertilizer, the more aggressive the product will be
• The higher the NH4 content, the more aggressive the product will be
• The higher the fertilizer dose, the more aggressive the product will be.

Conclusion

Unlike nitrogen, phosphorus fertilization responds very little in terms of yield. That is to say, the doses applied are not at all correlated with yield. Except when the soil is poorly supplied with phosphorus (less than 20 mg/kg Olsen P) the response to fertilization is visible and immediate on yield.

There are actionable levers to make the applied P effective on the current year's crop. One can notably localize the fertilizer in or near the sowing furrow. This promotes root system development which offers better resilience and slightly improves yield.

Then, the closer the application is positioned to the sowing, the better its effect on yield.

In general, when P application is positioned close to the seed in space (localized) and time (close to sowing), P is better used by the target crop of the year. Otherwise, P will fertilize crops of subsequent years with still the risk of losses by Erosion which represents a source of pollution and an economic loss for the farmer.

Source

This article was written by Jasmin Razongles, agronomy engineering student in work-study at the National Center for Agroecology.

This page has been written for the NBSOIL project, with the financial help of the European Union, with the help of the Centre National d'Agroécologie, of Ver de Terre Production and of Neayi