IRRI agronomy challenge: how much fertilizer

Author // Dr. Achim Dobermann Categories // Achim Dobermann's blog

January 17: I’m not impressed by how the crop looks so far - there are some pretty bare looking patches - but we have to keep going and I hope that it’ll take off soon. The next thing we need to do is figure out how much fertilizer to apply.

Achim Dobermann discusses adding fertilizer to the crop.
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I think farmers know very well that they need to apply fertilizer, but they also want to keep things simple. Their decisions are much influenced by fertilizer availability and price, own experience, and convenience in applying the nutrients needed. Scientists, on the other hand, like to talk a lot about soil testing, soil nutrient supply, crop nutrient requirements, nutrient cycling, balanced nutrition, nutrient use efficiency, nutrient losses, or even site-specific nutrient management. They also tend to aim for a precision in calculating a fertilizer rate that may not have much practical meaning.

Let’s start with some common sense

Our field has a fertile, deep clay soil with high organic matter content (about 4%); hence we really don’t need to apply extra organic materials to further improve soil quality or yield. We also don’t have cheap manure or other organic fertilizer at hand (a problem that most farmers face too); if we had it, we would apply it to our vegetable garden, not the paddy field.

We want to use a NPK compound fertilizer to supply basic quantities of macronutrients needed in the form of one product – mainly because it’s more convenient as compared to having to apply different single-nutrient fertilizers. We haven’t seen any serious micronutrient deficiencies in this field before, so we’ll not apply those.

We know that we are going to grow a wet-seeded rice crop. Experience by farmers and researchers shows that for wet direct-seeded rice no fertilizer needs to be applied before or at sowing; the first dose will only be needed about two weeks after sowing.

Fortunately, we can use a nice software tool for coming up with a concrete recommendation for our field – Nutrient Manager for Rice. In the video I’m fiddling with a tablet computer on which an Android version of Nutrient Manager is running, live in the field. All it requires is answering a series of questions about the field (e.g., size, where it is located), past and intended management practices that determine the nutrient demand by the crop or the nutrient supply from soil, crop residues or irrigation water. It’s based on more than 15 years of on-farm research on site-specific nutrient management in rice, looking into all these relationships in great detail. You can play with it yourself or with the web version to see how it works.

I entered a previous dry season yield of about 6.3 t/ha to obtain a recommendation for an attainable yield of about 7 t/ha (it actually says that it may be a yield in the range of 6.8 – 7.3 t/ha), resulting in this recommendation:

  • Early, 12-16 days after sowing: 35 kg N/ha, 32 kg P2O5/ha, 28 kg K2O/ha
  • Active tillering, 28-32 days after sowing: 47 kg N/ha
  • Panicle initiation, 43-47 days after sowing: 47 kg N/ha, 28 kg K2O/ha

This looks quite reasonable, but...

I doubt that we really need to apply that much potassium in this field because as a soil scientist I know that the soil is very rich in potassium (K) and that there is also a lot of K in the deep-well irrigation water used in the IRRI farm. The software can’t know about this very unique situation. Under typical conditions Nutrient Manager also recommends top-dressing of K at panicle initiation when yield matches or exceeds 7 t/ha, which may not be necessary in our field because we haven’t noticed a clear yield response to that in the past.

What to do? I decide that we should drop the K at panicle initiation, but keep the small amount of K included with the early application because it’s included in the NPK compound fertilizer (“14-14-14”) we have easily available. It’s also widely used by Philippine rice farmers. So, as a pragmatic compromise, our schedule will roughly be:

  • about 2 weeks after sowing - 5 bags 14-14-14 per ha
  • about 4 weeks after sowing - 2 bags urea per ha
  • about 6 weeks after sowing - 2 bags urea per ha

If the weather turns out to be really nice (much sunshine) and we do have a good crop stand, we may need some more nitrogen to capture the extra yield potential. We’ll decide that later in the season.

January 19: Applying nitrogen (N) and phosphorus (P) at this early stage is particularly important to promote early growth and tillering of the young seedlings. I applied the first fertilizer dose on the moist soil surface. Right after that we irrigated for the first time, which should move the nutrients nicely into the root zone. Just a shallow water layer is enough for now, keeping things nicely wet.

Lessons learned

Achim Dobermann applies fertilizer to the plot

Achim Dobermann applies fertilizer to the plot.

I needed to apply 1 ¼ bags of 14-14-14 for the 0.25 ha field, i.e. five strips of about 5 m width x 100 m length each, using the tracks created during seeding. I found out that a common plastic bucket was a reasonable measurement unit to use. One bucket was about ¼ of a 50-kg bag of fertilizer, or roughly 12.5 kg, which is just what I needed for one pass across the field. It’s also something one can easily carry while having to spread the fertilizer by hand. It took me over one hour to apply the fertilizer, more than twice as long as the seeding. I now also understand better why farmers like compound fertilizers such as the 14-14-14 I used. It was nicely granular (not sticky, good feel) and easy to apply. Moreover, since the nutrient content in it is not so high, it also doesn’t make that much difference if you’re not perfect in spreading it evenly, which is something I worried about a lot. Clearly, with the “technology” I had available one cannot be very precise in the field. I wish I had a simple implement (drop spreader?) to be more precise in dropping the fertilizer. We’ll see in a week or two whether I’ve created a patchy mess.


About the Author

Dr. Achim Dobermann

Dr. Achim Dobermann

Achim is a soil scientist and agronomist with 25 years experience working in Asia, North America and Europe. He is recognized internationally as an authority on science and technology for food security and sustainable management of the world's major cereal cropping systems. He has authored or co-authored over 250 scientific papers and two books on nutrients in rice and has received numerous awards from various academic, government and industry organizations. He is a Fellow of the American Society of Agronomy and a Fellow of the Soil Science Society of America.

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