The fertiliser gap Africa cannot close with rate alone

In May 2024, African heads of state signed the Nairobi Declaration at the close of the Africa Fertilizer and Soil Health Summit. Most of the coverage led on the headline number, a pledge to triple domestic production and distribution of certified quality fertilisers by 2034, alongside a commitment to double intra-African fertiliser trade over the same period.¹ It is a supply-side promise, and it follows almost twenty years of supply-side promises.

The commitment that matters more sits a few lines further down. By 2034, the signatories also undertook to ensure that at least 70 percent of smallholder farmers receive tailored agronomic recommendations to optimise fertiliser use.¹ That word, tailored, is doing an enormous amount of work, and it points at the part of the problem that another decade of tonnage will not solve.

A target measured in the wrong unit

The reference point for all of this is the 2006 Abuja Declaration, in which African Union member states committed to raising average fertiliser use to 50 kilograms of nutrients per hectare by 2015.² The target was missed, and the shortfall has been the organising fact of African soil fertility policy ever since. The International Fertilizer Development Center's tracking puts average use across Africa at 8.0 kilograms per hectare in 2006, rising to a peak of 42.5 kilograms in 2019 before falling back to 34.5 kilograms in 2022 as global fertiliser prices spiked.³

Those numbers describe a real constraint. Nitrogen removed at harvest has to be replaced from somewhere, and across much of sub-Saharan Africa it is not being replaced at anything like the rate it is removed. But a national average expressed in kilograms per hectare is a statement about volume, and volume is only one of four decisions that determine whether applied nitrogen ends up inside a maize plant.

What a rate leaves out

Nutrient stewardship is conventionally described through four decisions, the right source, the right rate, the right time and the right place. A recommendation that specifies only rate is a quarter of an answer, and it is the quarter that a farmer can follow perfectly while still losing most of the nutrient.

Consider what actually happens on a rain-fed smallholder farm. Urea broadcast onto a warm, moist soil surface begins to hydrolyse within hours, and the ammonia produced can volatilise straight into the air before rain moves it into the root zone. Nitrogen applied all at once at planting sits in the soil for weeks while a seedling with almost no root system takes up very little of it, exposed to leaching if heavy rain arrives and to denitrification if the soil saturates. The same quantity of the same product, applied in two or three splits timed to the crop's uptake curve and placed close to the row rather than scattered across the field, meets a plant that is ready to use it.

A recommendation that specifies only rate is a quarter of an answer, and it is the quarter a farmer can follow perfectly while still losing most of the nutrient.

Where the other two thirds go

The scale of the loss is not a subtlety. Raun and Johnson's often-cited estimate put worldwide nitrogen use efficiency for cereal production at roughly 33 percent, meaning that around two thirds of the nitrogen applied to the world's cereal crops was not recovered by those crops in the year of application.⁴ That figure describes global cereal production, including systems with far better infrastructure and far more precise application than most African smallholders have access to.

Read against the Abuja target, the arithmetic is uncomfortable. Reaching 50 kilograms per hectare at a recovery efficiency of one third delivers the crop rather less than a well-timed, well-placed 30 kilograms would. The policy conversation has been optimising the number on the left of that equation while leaving the multiplier on the right largely alone.

Timing is not a marginal refinement of this picture. In a preprint on nitrogen recommendations for smallholder maize systems, I set out evidence that the alignment between intra-seasonal rainfall and crop growth stage can account for 78 to 83 percent of yield variability in the rain-fed systems that cover more than 95 percent of the region's cropland, at times outweighing the rate applied.⁵ The short version is that the recommendations reaching farmers through extension services are routinely specified as a rate and a crop, with timing and placement either absent or reduced to a phrase such as "top dress at knee height" that carries no guidance on what to do when the rains arrive three weeks late.

Why the advice stayed simple

It would be easy to treat this as negligence. It is not. Simple recommendations persist because they are the only ones that survive the delivery system.

An extension officer covering several thousand households cannot tailor a nitrogen schedule to each of them. A blanket rate for a district can be printed, broadcast on radio, taught in an afternoon and remembered. A conditional rule that depends on soil texture, on the previous crop, on when the rains broke and on whether the farmer has the cash for a second purchase in February is harder to deliver and easier to get wrong. There is a genuine trade-off between the precision of advice and the probability that it is followed at all, and anyone writing about this who pretends otherwise has not spent time with an extension budget.

I spent a year as an extension officer with Kenya's Ministry of Agriculture and Livestock Development, and later ran on-farm trials with an out-grower network of roughly 1,700 farmers. The pattern I saw repeatedly was not farmers rejecting good advice. It was farmers applying their entire fertiliser purchase at planting because that was the day the labour was hired, the transport was paid for and the cash existed. Splitting an application assumes a second trip, a second labour day and money still in hand in the middle of the season. For a household managing under two hectares, those assumptions are not trivial.

What a complete recommendation looks like

The Nairobi commitment to tailored recommendations is therefore the right target, and a much harder one than tripling production. Tailoring means at minimum four things that current blanket guidance usually omits.

1. A splitting rule tied to something observable. Not a calendar date, which fails in a variable rainy season, but a crop stage a farmer can see, with explicit guidance for what to do when the season runs late.
2. A placement instruction. Banding or spot placement near the row rather than broadcasting, which is a change in technique rather than in spend and is among the cheapest efficiency gains available.
3. A source adjustment. Whether the product on sale locally is urea, CAN or a blend materially changes volatilisation risk and therefore whether surface application is defensible at all.
4. A cash-flow-aware version. If a second purchase is unrealistic, the advice should say what the best single-application strategy is rather than describing an ideal the household cannot finance.

None of this is new agronomy. All of it is well established in the literature and none of it is controversial among agronomists. The gap is not knowledge. It is that the knowledge has not been packaged into the form that actually reaches a farm, and that the policy targets everyone is measured against are denominated in kilograms rather than in kilograms taken up.

There is a version of the next decade in which African fertiliser production triples, use climbs past 50 kilograms per hectare, and recovery efficiency stays where it is. That would count as success against every headline target and would waste roughly two thirds of the additional investment. The 70 percent commitment is the one worth holding governments to, because it is the only one in the declaration that is about whether the nutrient arrives.