Poor drainage is often read as a lack of fertiliser
Raising the dose on waterlogged ground is money gone. Check the water first.
It nearly always goes in the same order. The fronds yellow, the fertiliser rate goes up, three months later nothing has changed, so the rate goes up again. A year passes, the fertiliser bill has doubled, and the plantation looks exactly as it did. The fault is not the fertiliser and not the amount. The fault is that the roots are in no condition to take up anything at all.
Why the symptoms look so alike
Oil palm roots need air in the soil, not just water. Ground that stays saturated too long loses its air spaces, and roots short of oxygen stop working, then rot if it carries on. The nutrients around them go untaken, so the palm shows the symptoms of a shortage even though the nutrients are right there in the soil.
Which is why the fronds yellow in exactly the same way. From a photograph of one frond nobody can tell the two apart, ourselves included. What separates them is not the colour of the leaf, it is where those palms are standing.
Mapping the yellowing palms
This is half a day's work that can save a year's spending, and it needs nothing but paper or a phone. Walk the plantation, and mark every yellowing palm on a simple plan of rows and numbers.
Even across the whole block, regardless of high or low ground, points to a nutrient shortage.
Clustered in the low ground, next to a blocked drain, or along a track that is always soft, points to water.
Scattered single points spreading slowly from one palm to its neighbour point to a root and trunk-base problem.
Running along one narrow line usually means an old track, or ground compacted under machinery.
The best time to walk it is two days after heavy rain. Whatever water is still standing then is the most honest picture of your drainage you will get, and on a dry day it is invisible.
If the yellowing palms cluster in the low ground, the problem is almost certainly water and not fertiliser. Raising the dose on waterlogged ground is money gone, and on saturated soil the urea that goes untaken is also lost faster.
The kinds of drain, and the usual sizes
The boundary drain runs round the edge of the plantation and does two jobs: holding water out and letting water off. It is the one most often present and least often cleared, and a boundary drain choked with grass and silt is the same as no drain at all.
The field drains sit inside the block, usually in the dead interrow, and carry water from the low spots into a larger drain. Their depth and spacing depend heavily on the soil and the slope, and what decides them is not a standard figure but where the water finally gets out.
One thing matters more than any dimension: a drain has to go somewhere. A deep drain with a dead end only moves the standing water, and often makes a new spot worse than the one before.
Peat and mineral soil are not the same
On mineral soil it is simple: get the excess water off. On peat it is not. A water table pulled too deep dries the peat out, makes it burn easily, and lets the surface subside, so the roots lose their footing instead. What you want on peat is not the driest ground possible, it is a water table held at a set depth, generally around 50 to 70 centimetres, with a water gate to hold it there through the dry season.
If your plantation is on peat, digging the drains deeper as an answer to standing water is a move that can cost you in the long run. This is one to talk through before the machinery comes in.
When fertiliser can go back on
Once the water is off and the soil has stopped being saturated for a few weeks. Roots need time to recover and put out new growth, and fertiliser spread before then still goes untaken. A few weeks of patience here saves a whole fertiliser round.
Once it has recovered, the real nutrient shortage can be judged, and there usually is one, because a plantation that has sat under water for months took up none of the fertiliser already given. The rates by age are in the piece on fertiliser rates, and the symptoms to compare against are in the three causes of yellowing fronds.
What you can send us
A photo of your simple plan of the yellowing palms, photos of the plantation two days after heavy rain, a photo of the boundary drain exactly as it is, and a photo of the lower fronds. Say what the soil is, how old the palms are, and what the last year of fertilising looked like.
The map is what decides it. With the spread of yellow palms in hand, most cases narrow to water or nutrients before anybody comes out to the plantation, and that is a quick question, which is free of charge.