P-application

The major objective of P management is to prevent P deficiency rather than treat P-deficiency symptoms. If low soil P supply is the reason the targeted yields are not achieved, management must focus on the buildup and maintenance of adequate soil-available P levels to ensure that P supply does not limit crop growth and N-use efficiency.

 

P is not easily lost from the system, but inputs from sources such as irrigation water and straw are generally small. P fertilizer application has residual effects that can last several years, and maintenance of soil P supply requires long-term strategies tailored to site-specific conditions that consider P inputs from all sources.

 

Sustainable P management may require the application of fertilizer P, even if a direct yield response to P application is not expected. In some soils, the indigenous P supply would decline after only a few seasons if fertilizer P was not applied. A suitable maintenance strategy would then have to balance the nutrient removal with grain and straw to replenish the soil P reserves.

 

Applying P Based on Omission Plot Yields and Yield Target

 

General Rule: Where the soil P supply is small, apply 20 kg fertilizer P2O5 per ha for each ton of target grain yield increase (difference between yield target and yield in 0 P plot).

 

The maintenance fertilizer P rates given in the table below are designed to replenish the P removed with grain and straw, assuming a low to moderate return of crop residues. Look up the fertilizer P2O5 rate based on:

the yield target (Step 1 of implementing SSNM) and

an estimate of soil P supply measured as yield in a 0 P omission plot (Step 2 of implementing SSNM).

 

Table: Maintenance fertilizer P2O5 rates according to yield targets

and P- limited yield in 0 P plots

 

Theoretically, fertilizer P application would not be required if a yield response were not expected for the selected yield target (i.e., if yield target = yield in nutrient omission plot). This “zero-P fertilizer” strategy results in mining the soil of P reserves and may affect yields in the medium to long term, especially if other nutrient sources such as straw or manure are not applied.

 

NOTES:

  • In order to avoid excessive P fertilizer use, arising from overly optimistic yield targets, the maximum yield increase over yield in the O P plot is 3 tons/ha (Table). A reduction in the yield target is suggested for cases where a yield increase of more than 3 t/ha over the yield in the 0 P plot is required.

 

  • To prevent mining of soil P reserves, the following rules of thumb can also be applied:

    • If most of the straw is retained in the field (e.g., after combine harvest or harvest of panicles only) and the nutrient input from manure is small, apply at least 4 kg P2O5 per ha for each t grain harvested (e.g., 20 kg P2O5 for a yield of 5 t/ha) to replenish P removed with grain.

    • If most of the straw is removed from the field and nutrient input from other sources (manure, water, sediment) is small, apply at least 6 kg P2O5 per ha for each t grain harvested (e.g., 30 kg P2O5 for a yield of 5 t/ha) to replenish P removed with grain and straw.

 

  • Maintenance fertilizer P rates can be reduced if

    • soils receive organic amendments such as farmyard manure. Organic material can contribute substantially to the buildup and maintenance of soil P reserves depending on nutrient concentration and amount applied. Apply organic amendments in nutrient omission plots to assess the combined nutrient-supplying capacity of soil and applied organic materials.

    • soils are periodically flooded with substantial nutrient inputs from sedimentation (e.g., Mekong Delta in Vietnam).

 

  • P applied to either rice or wheat has a residual effect on the succeeding crop, but direct application to each crop is more efficient. Phosphorus fertilizers should be incorporated in the soil before seeding or transplanting.

 

  • Fertilizer P application is not recommended if yield in a 0 P plot with favorable conditions is greater than the yield target.

 

  • It may be necessary to reassess the soil P supply after 8–10 cropping cycles.