Feeding the Soil, Not Just the Crop: What KALRO's Research Says About Using DAP Fertilizer Wisely in Kenya

Introduction

Di-ammonium phosphate (DAP, 18% N : 46% P₂O₅) is the default planting fertilizer for the overwhelming majority of Kenyan farmers, used on over 90% of surveyed potato plots and standard practice for maize, tomato, and most other row crops.¹ Its popularity is well earned — it delivers a concentrated, immediately available dose of phosphorus exactly when a young root system needs it most. But KALRO's own field research, including long-term monitoring at its national potato research centre, has documented a real cost to using DAP carelessly: declining soil pH, phosphorus fixation, and yields that fall even as fertilizer bags pile up in the store. This short article sets out what KALRO's work says a Kenyan grower should actually watch for.

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1. Apply the Recommended Rate — Not a Guess

KALRO's published recommendation for potato is 500 kg/ha of DAP, yet field surveys found farmers in Nyandarua County — Kenya's main potato-growing county — applying only around 300 kg/ha on average, and farmers in Bungoma applying as little as 137 kg/ha.² This under-application is not a safe compromise: the same research found that farmers who applied rates closer to KALRO's recommendation harvested, on average, 3 tonnes per hectare more than those applying below it.² The lesson generalises beyond potato — under-dosing DAP relative to the crop- and zone-specific recommendation is a direct, measurable yield cost, usually driven by cost concerns or simply not knowing the correct rate rather than any deliberate agronomic choice.

2. DAP Is Not a One-Size-Fits-All Product

KALRO researchers have been explicit that Kenya's fertilizer culture has relied too heavily on blanket, national-level recommendations rather than rates matched to local soil type, crop, and agro-ecological zone — and that this is now a priority research gap.³ A new KALRO–CGIAR collaboration is actively building machine-learning models, combining decades of agronomic trial data with soil, climate, and seasonal-forecast data, specifically to move Kenya away from blanket DAP rates toward site-specific fertilizer advice that adapts to the individual field rather than asking the field to adapt to a generic bag rate.³ Practically, this means a grower should treat the standard "one bag of DAP per acre" heuristic as a starting assumption to be checked against a soil test, not a universal rule.

3. Watch Soil pH: DAP Can Quietly Acidify Your Field Over Time

This is the single most important, and least appreciated, finding in KALRO's own research. Long-term soil monitoring at KALRO's Tigoni potato research station showed soil pH declining year after year under continuous DAP use, even though most Kenyan potato-growing soils were already acidic to begin with, owing to acidic parent rock.⁴ Across eight monitored sites in Nyandarua County, soil pH ranged from 4.0 to 5.72, with phosphorus levels already low at 10–30 ppm — precisely the conditions in which continuous DAP application accelerates acidification rather than correcting the underlying deficiency.⁴ Soil-science research from Western Kenya reinforces why this matters agronomically: acid soils don't just tolerate less fertilizer well, they actively fix phosphorus and release aluminium in forms that are toxic to roots, compounding the very deficiency DAP is meant to fix.⁵

Practical implication: DAP should not be applied year after year on the same acidic plot without a soil pH check. Where pH has fallen into the 4.0–5.0 range, correcting acidity with agricultural lime before or alongside fertilizer application is necessary — DAP alone cannot substitute for lime on soils this acidic, and continuing to apply it without liming can lock further phosphorus out of reach of the crop.⁴⁵

4. Placement Matters as Much as Rate

Phosphorus, DAP's main nutrient contribution, does not move readily through soil once applied, which is why Kenyan extension guidance consistently emphasises placing DAP directly in the planting furrow, close to where the seed or seedling root will establish, and covering it with a thin layer of soil before the seed goes in — never letting fertilizer granules touch the seed directly, which risks scorch.¹ Broadcasting DAP over the surface, rather than placing it in-furrow, wastes much of its phosphorus content to fixation before the young root system ever reaches it — an especially costly mistake on the acidic, phosphorus-fixing soils described above.

5. Know When DAP Is the Wrong Tool

DAP supplies nitrogen and phosphorus but no potassium and essentially no secondary or micronutrients. On soils already trending acidic, extension agronomists increasingly recommend blended alternatives — NPK formulations such as 23:23:0 or 17:17:17, which release nutrients more gently and often carry supplementary micronutrients such as zinc and sulphur — specifically to reduce the acidifying pressure of continuous straight-DAP use.⁶ KALRO's Tigoni findings point the same way: the underlying recommendation is not to abandon DAP, but to stop treating it as the automatic, permanent default for every field regardless of what the soil is actually telling the farmer.⁴

6. Nitrogen Top-Dressing Still Has to Follow

Because DAP's nitrogen content (18%) is modest relative to a growing crop's full-season demand, it is a starter fertilizer, not a complete nutrition programme. Standard Kenyan cereal extension practice — consistent with the fertilizer regimes used in KALRO-linked programmes such as the Kenya Cereal Enhancement Programme — pairs a single DAP application at planting with subsequent top-dressing using calcium ammonium nitrate (CAN) once the crop reaches knee-high stage, to meet the nitrogen demand DAP alone cannot cover through to maturity.⁷

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Conclusion

KALRO's research converges on one central point: DAP is an effective, well-proven fertilizer, but only when it is used at the correct, crop-specific rate, placed correctly in the planting furrow, and periodically checked against soil pH rather than applied by habit season after season. The most common and most damaging Kenyan farming mistake with DAP is not overuse — it is under-application paired with silent, uncorrected soil acidification. A soil test every few seasons, and a willingness to lime or switch to a blended NPK product where pH has fallen too far, is cheap insurance against a slow, compounding yield decline.

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Footnotes / Sources

This article synthesises and paraphrases KALRO research findings and KALRO-linked extension programme guidance on DAP fertilizer use; it does not reproduce source text verbatim. For binding, field-specific fertilizer advice, consult KALRO's county extension offices or commission a soil test before your next application.

Footnotes

1. General DAP placement and rate guidance for Kenyan maize and vegetable production, cross-checked against KALRO-linked variety and extension programmes. See "Maize Farming in Kenya 2026," Farmers Trend: https://farmerstrend.co.ke/crops/maize-farming/maize-farming-in-kenya-2026-commercial-guide/; and Agcenture, "Ultimate Guide to the Best Fertilizers for Every Crop in Kenya": https://agcenture.com/2021/04/09/kenya-recommended-fertilizer-application-rates-per-crop. ↩ ↩²

2. KALRO/CIP field data on farmer DAP application rates versus the KALRO-recommended 500 kg/ha rate for potato, Nyandarua and Bungoma counties. "Fertilization – Intervention Options Along the Potato Value Chain in Kenya," Netherlands Potato Consultative Foundation (NPCK): http://modernizationoption.npck.org/fertilization/. ↩ ↩²

3. Kenya Agricultural and Livestock Research Organization (KALRO) and CGIAR (Alliance of Bioversity International and CIAT; IITA). "Tailored fertilizer advice for Kenya's diverse soils and crops," workshop held at KALRO Katumani Research Centre, 2024–2025. Alliance of Bioversity International and CIAT: https://alliancebioversityciat.org/stories/tailored-fertilizer-advice-kenya-diverse-soils-crops. ↩ ↩²

4. Muthoni, J. et al. "Soil fertility situation in potato producing Kenyan highlands: Case of KALRO-Tigoni." International Journal of Horticulture, citing long-term soil pH monitoring data (2015) from KALRO's Tigoni national potato research centre and the eight-site Nyandarua County soil survey (Martha et al., 2014). Available at: https://hortherbpublisher.com/index.php/ijh/article/html/2840/policy and https://www.researchgate.net/publication/312278330. ↩ ↩² ↩³ ↩⁴

5. Soil acidity, phosphorus fixation, and aluminium toxicity mechanisms in Kenyan agricultural soils. "Integrated Effects of Lime and Fertilizer Applications on Soil Properties and Sorghum Performance in Acidic Soils of Western Kenya," International Journal of Research and Innovation in Social Science (IJRISS): https://rsisinternational.org/journals/ijriss/view/integrated-effects-of-lime-and-fertilizer-applications-on-soil-properties-and-sorghum-performance-in-acidic-soils-of-western-kenya. ↩ ↩²

6. Extension guidance on blended NPK alternatives to continuous straight-DAP use on acidifying soils. "2026 Fertilizer Guide: Understanding NPK Ratios For Maximum Maize Yields": https://crazykanairofarming.com/2026-fertilizer-npk-maize-kenya/. ↩

7. DAP-plus-CAN fertilizer regime as used in KALRO-linked cereal extension programming. The Kenya Cereal Enhancement Programme (KCEP) Maize Extension and Training Manual: https://www.kcepcral.go.ke/wp-content/uploads/2017/04/KCEP-Maize-Extension-and-Training-Manual.pdf. ↩