K-application

The general strategy for K management follows the same principles given for P, but the K uptake requirement of rice is much greater than for P. Furthermore, >80% of K taken up by rice remains in the straw after harvest, making straw an important input source to consider when calculating fertilizer K requirements.

 

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

 

Applying K

General rule: Where the soil K supply is small, apply 30 kg fertilizer K2O per ha for each ton of target grain yield increase (difference between yield target and yield in 0 K plot).

 

The maintenance fertilizer K rates given in Table B are designed to replenish the K removed with grain and straw by considering the amount of straw returned to the field from the previous crop.

 

Look up the required fertilizer K2O rate in Table B based on

  • the yield target (Step 1 of implementing SSNM),

  • the estimate of soil K supply measured as yield in a 0 K omission plot (Step 2 of implementing SSNM), and

  • the amount of K recycled with straw yield and the straw management level in the previous season (Table A).

 

Substantial mining of soil K reserves may affect yields in the

medium to long term, especially if most straw is removed.

 

Table A. Input of K with recycled straw according to yield and straw management practices in the previous season.

 

Table B: Maintenance fertilizer K2O rates according to yield target and K-limited yield in 0 K plots.

 

NOTES:

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

 

  • In the short term, fertilizer K application would not theoretically be required if a yield response is not expected for the selected yield target (i.e., if yield target = yield in 0 K plot). This strategy results in mining of soil K reserves and may affect yields in the medium to long term, especially if other nutrient sources such as straw or manure are not applied.

 

  • To prevent mining of soil K reserves, the following general rules 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 3.5 kg K2O per ha for each t grain harvested (e.g., 17.5 kg K2O for a yield of 5 t/ha) to replenish K 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 12 kg K2O per ha for each t grain harvested (e.g., 60 kg K2O for a yield of 5 t/ha) to replenish K removed with grain and straw.

 

  • The maintenance fertilizer K rates given in Table B can be reduced if:

    • soils receive organic amendments such as farmyard manure. Organic material can contribute substantially to the buildup and maintenance of soil K 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; or

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

 

  • When the total amount of K fertilizer to be applied is small, it can all be applied at seeding or transplanting. For larger applications (40–120 kg K2O per ha), apply in two splits (50% as basal application before crop establishment or within the first 2 wk after crop establishment and 50% at panicle initiation, PI). With large applications (>120 kg K2O per ha), apply in three splits (1/3 basal, 1/3 at PI, and 1/3 at heading to first flowering).