Zai Pits, Tied Ridges and Half Moons: Holding the Rain Where It Falls

In much of eastern, central and northern Kenya, the short rains are the main growing season, and they are not generous. A few heavy storms, a dry week, a few more storms. The crop does not fail because the season brought no rain. It fails because much of the rain ran off the field in the first hour, carrying topsoil and fertiliser with it, and the crop then went thirsty through the dry spell that followed.

In-field water harvesting tackles that directly. Instead of collecting water in a tank or a pan, you shape the field itself so that rain soaks in where it falls, right at the roots. The best known methods are zai pits, tied ridges and half moons. They need no pump, no pipe and no fuel, only a jembe, a shovel, some manure and labour. Extension services and research organisations have promoted them across the drylands of Africa for decades, and they are among the cheapest ways a smallholder can make a short season go further.

This guide explains how each method works, which crops and soils suit it, how to lay it out, and how much work to expect. It complements our guides to land preparation before the rains and planting in the short rains. Dimensions below are the ranges commonly given in extension material; your county agricultural officer can adapt them to your soil and rainfall.

Why Rain Runs Off Instead of Soaking In

Rain soaks into soil at a limited rate. When a storm delivers water faster than the soil can take it, the surplus collects on the surface and flows downhill. On bare, crusted or compacted soil, which is common at the start of the short rains after a long dry season, that rate is low, so a large share of a heavy storm can run straight off the field.

All in-field methods do the same two things: they hold the water on the surface for longer, so it has time to soak in, and they concentrate it near the plant, so the root zone gets a deeper wetting than the open ground between rows. Adding manure or compost in the pit or furrow improves the soil's ability to hold the water once it is in.

Zai Pits: Small Holes, Big Difference

Zai pits come from the Sahel, where farmers in Burkina Faso and neighbouring countries used them to bring degraded, crusted land back into production. The idea is simple: a grid of small planting pits, each with a handful of manure or compost, dug in the dry season before the rains.

Size and spacing. Extension guidance commonly gives pits of roughly 20 to 40 centimetres across and 10 to 20 centimetres deep, spaced somewhere between 60 centimetres and one metre apart, with the rows staggered so that water missed by one pit flows into the next. Larger pits, sometimes called "big zai", are used for crops like bananas, pawpaws or fruit trees. Pile the dug soil on the downhill side of each pit as a small lip.

Filling. Put a generous handful or two of well rotted manure or compost into each pit and mix it lightly with soil. Fresh manure can burn seedlings, so use well decomposed material. Because the manure is concentrated where the crop grows, a modest quantity goes further than when it is broadcast across the field.

Planting. Sow sorghum, millet, maize, beans or cowpeas in the pits once the rains have wetted them, following the seed rate on the pack. Thin to the recommended number of plants per pit.

Labour. Zai pits are hard work to dig the first time, especially on crusted ground, and that is their main drawback. The good news is that pits can be reopened and reused in following seasons, often with less effort, and the work can be spread across the dry months. Many farmers start with a small block close to the homestead, see the result, and expand from there.

Tied Ridges: Turning Furrows Into Basins

Tied ridges are ridges and furrows made along the contour, with small cross ties of soil built across the furrows every few metres. The ties turn each furrow into a row of small basins. When it rains, water collects in the basins and soaks in instead of running along the furrow and off the field. Ridges suit maize, sorghum, beans, potatoes and sweet potatoes, and they can be made with a jembe, an ox drawn ridger or a tractor implement; our guide to tractor implements covers ridgers.

The two things that matter most are following the contour and making the ties lower than the ridges. If the ridges run up and down the slope, water races along the furrows, breaks the ties and causes erosion. If the ties are as high as the ridges, a big storm overtops both together at the lowest point and cuts through. With the ties a little lower, any excess spills gently from one basin to the next along the furrow rather than breaking out over the ridge.

Peg the contour first with a water level pipe or an A-frame, then make the ridges parallel to the pegged line. In practice the ties are spaced a few metres apart, closer on steeper ground; ask your extension officer for the spacing that suits your slope and soil.

Tied ridges can be formed before planting or added at the first weeding, when soil is moved toward the crop anyway. Our guide to weeding maize and beans in the short rains covers timing, and earthing up at that stage is a natural moment to build the ties.

Half Moons for Trees, Pasture and Steeper Ground

Half moons, also called semi-circular bunds, are crescent shaped earth banks with the open side facing uphill. Runoff from the ground above collects inside the crescent. They are widely used for fruit trees, for rehabilitating bare grazing land with grass seed, and on slopes where full terraces are too much work. Lay them out in staggered rows along the contour so that water spilling round the tips of one half moon is caught by the one below. Plant a tree or sow grass inside each crescent, and mulch the surface to keep it from crusting.

Which Method Suits Your Farm?

On steep land none of these methods replaces proper terracing. They work best on flat to moderately sloping fields, and they work even better combined with mulch, which shades the soil, slows evaporation and stops the surface crusting between storms. Crop residues from the last harvest, cut grass or prunings all make good mulch where termites and livestock allow.

Timing: Is It Too Late This Season?

The ideal time to dig pits and form ridges is in the dry weeks before the rains, when the work can be spread out and the structures are ready to catch the very first storms. If the short rains have already started on your farm, all is not lost. Tied ridges can still be formed on a field that is already planted, at the first weeding, and they will catch every storm that falls for the rest of the season. Zai pits can be dug on a small block for a late planting of a short season bean or cowpea variety, if the seed pack and your extension officer agree there is time for it to mature.

Most importantly, mark this season's lessons on the ground while you can still see them. Note where water ran off and where gullies started during this season's storms, peg those areas, and plan to dig there in the dry weeks of January and February, ready for the long rains. A field that is shaped in the dry season gives the crop the full benefit of every storm from the first day.

Making Manure Go Further

Manure is the other half of the zai system, and on many farms it is in short supply. Concentrating it in pits rather than spreading it over the whole field is one way to stretch it. Keeping it under a roof or a cover so rain does not wash the nutrients out is another. Collect it from the cattle boma, the goat house and the poultry house, mix it with crop residues and let it rot down properly before use. If you plan to add fertiliser as well, place it in the pit or furrow near the seed as your agronomist or the bag label advises, rather than broadcasting it where runoff can carry it away.

A Note on Waterlogging

These methods are built for dry areas and short, erratic rains. In high rainfall areas, or in a very wet season, holding water on the field can drown crop roots, and clay soils that drain slowly are most at risk. If the pits or basins stay full for more than a day or two after rain, open the ties at the low end or switch to ordinary ridges with a drain. Our guide to farm drainage explains how to lead surplus water away safely.

Step by Step: Preparing a Block This Season

Hand Tools for the Job, With Prices

These prices are from the Fuga price list at the time of writing; check the site or call a branch for current prices and stock.

A heavier jembe bites into crusted ground better for pit digging, while a lighter one is kinder for long days of ridging; many farmers keep one of each. A bucket is the easiest way to measure out the same amount of manure into every pit.

Hold the rain where it falls, feed the soil where the roots are, and follow the contour. For a jembe, shovels and a level pipe, see our farm equipment range, or WhatsApp 0716 961 018 and we will help you kit out a work team for the season.

Frequently Asked Questions