It’s finally getting hot and dry and feeling like summer. Mosquitoes are showing up with a vengeance this week and that topic seems to come up in every phone call. Fields that don’t still have floodwaters on them are extremely dry and we need to keep up with our flood and furrow-irrigated management. The top ends of row rice fields are drying rapidly, so if you’re not using soil moisture sensors, try to stay on top of regular irrigations.
Questions have heavily turned toward fields hitting reproductive growth. With that, the discussions center around timing midseason nitrogen and possibly potassium deficiencies emerging. More on those topics are covered in this update. As a lead in, we have plenty of time to make proper midseason N applications, and we have plenty of time to correct potassium deficiencies – don’t rush to decisions that could cost you more money with less benefit.
The coming week has some chance of rainfall in the forecast for just about every day. Who knows how much anyone will get at any point, or at all, but it looks like a very unsettled week. Some of the earliest planted fields in the state could see heading by next weekend if temperatures hold up to drive the crop there.
Midseason Nitrogen (N) Timing
The majority of rice acres in Arkansas currently enrolled in the DD50 Rice Management Program are entering reproductive growth. Table 1 shows acres reaching ½” internode elongation (IE) which occurs approximately one week after the onset of reproductive growth (beginning internode elongation – BIE; green ring).
It’s important to note that our recommendations for midseason N timing have changed in recent years. Data from 2012-2018 has shown how we can improve the timing of our midseason N applications compared to previous recommendations.
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The current recommendation is to apply midseason N after beginning internode elongation AND 4 weeks after preflood N was incorporated by the flood. You must meet both conditions before applying midseason N to maximize your benefit.
Fig. 2 shows the percent of optimum yield based on timing of midseason N after the flood was established and indicates that 4-5 weeks after preflood N is incorporated is the optimum time to apply midseason N. Fig. 3 is another way of looking at the same data but based on days after BIE. In Fig. 3, ½” IE corresponds with 7 days after BIE.
Again, the absolute earliest to ever apply midseason N is BIE, but only if it’s been 4 weeks since the flood was established to incorporate the preflood N.
Identifying and Correcting Potassium Deficiency in Rice
Potassium (K), also referred to as “potash” is a plant essential element that is often limiting in Arkansas soils. Most potassium deficiencies will be observed on lighter textured sandy loam and silt loam soils as clay soils often contain potassium concentrations well above the threshold for optimal rice growth.
Most people fail to realize that K is needed in relatively equal or slightly greater amounts in the rice plant as nitrogen (N). Potassium is critical for water regulation in the plant, aids in several metabolic functions, and is a key component in the plant’s defense system against disease.
Oftentimes, some of the first indications of K deficiency are the early onset of “nibbler” diseases such as brown spot or narrow brown leaf spot (Cercospora). These diseases are only able to break through the plant’s natural defense system when the plant is stressed – most often due to hidden hunger or moderate to severe K deficiency.
Identifying Potassium Deficiency
Potassium deficiency is rarely diagnosed preflood and is almost always seen several weeks post-flood as the rice is growing rapidly following N applications. Visual K deficiency symptoms in rice are expressed following the typical “mobile plant nutrient” rules of thumb:
- The symptoms will always appear on the lower older leaves. Since K is mobile in the plant it will be moved or remobilized from the older plant growth (lower leaves) to the new growth (upper leaves). Potassium remobilization within the plant allows the leaves that are most actively photosynthesizing to maintain as high of a level of productivity as possible.
- Symptoms of K deficiency are expressed as yellowing or chlorosis of the leaf margins that will progress to necrosis or cell death if the deficiency is not corrected. Nitrogen is also mobile in the plant and can have similar symptoms but can be easily differentiated if you know the keys to look for. When you have N deficiency the whole plant will tend to be yellow and stunted and the yellowing and chlorosis on the lower leaves will move down the midrib of the leaf, not the leaf margins.
- Lower leaves will eventually die or senesce as the nutrients are moved to the upper leaves. Lower leaves can senesce when the canopy is very thick and they are no longer receiving sunlight and actively photosynthesizing, but this will often appear as whole leaf chlorosis and death rather than a pattern described for either N or K.
Tissue testing is another method of identifying or confirming K deficiency in rice. To effectively use tissue testing to identify nutrient deficiencies the correct plant part must be sampled. For rice that is near or past green ring, we suggest sampling the Y-leaf which is the leaf blade on the uppermost collared leaf.
Fifteen to twenty Y-leaves are required to have enough sample to complete the analysis and should be collected from the entire field or area of interest. Recent research on using Y-leaf tissue K concentration indicates that >1.6% tissue K is optimal from green ring through mid-boot. Our goal should be to keep the Y-leaf tissue K concentrations above 1.6% during this timeframe.
Timing of Potassium Deficiency
Potassium deficiencies are rarely diagnosed preflood due to the low demand by the plant and relatively small amount of plant biomass before N fertilizer application and irrigation.
Once rice has been fertilized with N and enters the rapid vegetative growth stage the amount of biomass increases rapidly and if there is not sufficient K in the soil to meet this growing demand we can experience either hidden hunger or visible K deficiencies.
More often than not, we start to observe visible K deficiency symptoms around ½ internode elongation (sometimes following midseason N application to pureline cultivars) which can progress as the plant continues to grow and K remains limited. Although K deficiency symptoms can occur at any time during the growing season they tend to be more pronounced and easiest to identify ~4 weeks after flooding.
Correcting Potassium Deficiency
If a K deficiency is identified 100 lb potash per acre (60 units K2O per acre) is recommended at the first onset of symptoms and yield can be salvaged until the late-boot growth stage. Timing of K application is critical with earlier identification of the deficiency and application of potassium to deficient rice leading to higher yield potentials.
Being proactive in the management of soil nutrients is always the best course of action. However, it is always good to be on the lookout for potential K deficiencies near midseason as this is when they are most likely to occur and be “obvious”. If you are concerned about K deficiency a tissue test may help identify potential hidden hunger.
Please remember that there is a wide window of opportunity for successful application of K from preplant to late-boot, but the earlier a potential K deficiency is identified the larger the return on investment.