Soil Testing Before You Plant in Ghana

Fertiliser by habit, not by evidence

Fertiliser is one of the biggest cash items on a maize plot, and it is often the least measured. Many farmers can tell you the bag rate their father used. Far fewer can tell you what the soil on that plot actually needs. That is not a criticism of him. Testing soil in Ghana has been out of reach for many smallholders, so the habit of applying the same bag rate every season has lasted.

The trouble with habit is that the soil changes. A plot cleared from forest, a plot that has grown cassava for three years, and a plot that has had poultry litter for a decade will all need different things. Put the same fertiliser on all three and you may over-feed one, starve another and leave the acid soil in the third still refusing to take up the nutrients you paid for.

We are a day-old chicks hatchery and we do not sell fertiliser or testing services. We write about soil testing because many farmers who buy chicks from us also grow maize, vegetables or groundnut, and the money they put into the land is money that could go into the birds. A test is a small outlay that tells you where the rest should go.

What a soil test tells you, and what it does not

A standard report from a soil laboratory usually covers the acidity of the soil, the main nutrients a crop takes from it, the amount of organic matter and the texture. Some reports add the calcium and magnesium available, the cation exchange capacity and the levels of micronutrients. Each figure answers a different question about the plot.

• pH shows how acid or alkaline the soil is. Acid soil locks up phosphorus and can make some nutrients harmful to the roots, which is a common problem on Ghanaian ground.
• Organic carbon or organic matter shows how much material is available to feed the soil life and hold water and nutrients.
• Nitrogen, phosphorus and potassium show what the crop can take up now, and roughly how much is likely to be available over the season.
• Texture shows whether the soil is sand, loam or clay, which affects how water moves and how often you need to feed the crop.

A test does not tell you about rainfall, pests, disease, the drainage on a flat plot or the weed seed bank. It is one piece of information, not the whole diagnosis. Read it alongside what you see in the field, and ask the laboratory or the extension officer to explain any figure you do not understand.

Where to get the soil tested in Ghana

Several routes exist. The CSIR Soil Research Institute at Kwadaso, in Kumasi, is the national reference for soil analysis and advice, and it is worth asking whether it accepts samples from individual farmers in your region. MoFA's regional and district offices can also advise on the nearest route, and some agricultural faculties at universities run soil laboratories open to outside samples. Private laboratories exist, and the quality of their work varies, so ask what methods they use before you send a sample.

Before you send anything, ring ahead. Ask for the current fee, the list of analyses offered, the sample size required, the packaging they want and how long the results take. Turnaround times and charges change, so check them with the laboratory and do not assume the figures you heard last year still hold.

If a laboratory is far away, your district extension office may be able to collect samples in batches from several farmers and send them together. That often cuts the travel and the packaging cost for each farmer and makes the trip worth the effort.

How to take a sample properly

Most bad soil tests come from bad sampling, not from bad laboratories. A sample is meant to represent the plot, and a single spadeful from the edge of a heap tells you nothing about the field. Take the time to do it right, because the result will guide a full season's inputs.

Divide the farm into areas that look and behave alike. Ground that has been under the same crop, that drains the same way and that sits on the same slope can share a sample. Separate the high ground from the low, the old cassava field from the maize field, and any patch that looks pale or grows badly. Do not average a good patch with a bad one, because the mix hides the problem.

Walk the area in a zig-zag and take 15 to 20 small cores or slices from the root layer. Use a clean spade or auger, and put the soil from each core into a clean bucket. Avoid galvanised containers, which can add zinc to the sample. Break up the lumps, remove stones, roots and leaves, mix the whole lot thoroughly and fill a clean plastic bag with about half a kilogram. Let it air-dry in the shade before you pack it for the laboratory, and label it with the plot, the date and the crop history.

Sampling depth and the areas to sample

The depth you sample depends on what the crop is. Annual crops such as maize, cowpea and vegetables root mainly in the top layer, so a sample from the first 15 to 20 cm is the usual choice. Tree crops such as mango, cashew and citrus root deeper and wider, so their samples should come from a greater depth, taken under the canopy edge.

Sample the same areas at the same time each year where you can. That gives you a comparable history, which is worth more than a single good result. If you have a plot that is clearly different, such as a new clearing or a patch next to a termite mound, give it a separate sample so its problems do not hide in the average.

Sampling mistakes that spoil the result

A handful of errors come up again and again, and each one can send a test result in the wrong direction. Avoid them and the report becomes useful.

• Sampling just after fertiliser. A recent application makes the nutrient numbers look high. Wait a few weeks, or sample before you apply the next dose.
• Sampling beside a heap. The edges of a compost pile, a manure heap or a burnt patch are not the field. Stay well clear of them.
• Sampling wet soil. Muddy samples are hard to mix and dry unevenly. Let the ground drain for a day or two before you dig.
• Using a dirty or galvanised tool. Rust, zinc and old fertiliser residue can change the numbers. Clean the spade and the bucket before each sample.
• Mixing fields together. A single bag from two different plots tells you the average of two problems, which is usually not what you need.
• Taking too few cores. Three or four scoops cannot represent a whole field. Aim for the number suggested by the laboratory, and at least 15 for an ordinary plot.

Keep a record of each sample in a notebook. Note the GPS point if you have a phone that will take one, or describe the position clearly, so the next sample can be taken from the same place.

Reading the pH and the need for lime

The pH is often the first figure to look at, because it sets how well the nutrients are available. Many of the soils in the forest and transition zones are acid, and some of the coastal soils are too. Most field crops do best in a band that is slightly acid to near neutral, and maize, cowpea and groundnut each have their own preferences. Ask the laboratory for the target range for each crop, rather than relying on a single number.

If the pH is low, the usual remedy is agricultural lime, applied on the advice of the test. Lime does not work overnight. It needs months to react with the soil, so apply it well before the crop goes in, and mix it into the top layer rather than spreading it on the surface and hoping. Too much lime is also a problem, because it can lock up other nutrients, so follow the laboratory's advice on the quantity.

Check the source of the lime before you buy it. Quality varies, and a bag that looks like lime may have a low neutralising value. Ask the supplier for the analysis, or ask the extension officer which products are reliable in your area. The price of lime changes from one town to another, so check the local market and current rates.

Organic matter and the soil you cannot see

Organic matter is the number that many farmers ignore until the yield falls. It feeds the soil life, helps the soil hold water and nutrients, and keeps the surface from crusting after rain. Ghanaian soils are often low in it, particularly on sandy ground that has been cropped for many years with little return of residue.

The fix is steady and slow. Leave crop residues on the field where you can, use mulch as described in our guide to mulching to hold soil moisture, and add compost or manure that has been properly composted. Rotation helps as well, because a legume returns both nitrogen and organic material. Expect the change to take several seasons, and use the soil test each time to see whether it is moving in the right direction.

Do not chase a single target number. Ask the laboratory what level is normal for soils in your zone and crop type, and look for a rising trend rather than a single good year.

Nitrogen, phosphorus and potassium on the report

Most reports rate the nutrients as low, medium or high, or give a figure in parts per million. Read the rating as a guide to where the field is short. Phosphorus is often the limiting nutrient on Ghanaian soils, particularly where the soil is acid, because the phosphorus is held in forms that the crop cannot easily reach. Potassium may be low on sandy soils and on fields that have lost their crop residue for years. Nitrogen is harder to read from a single test, because it moves quickly through the soil with the rain and the crop.

The laboratory or your extension officer should give you a fertiliser recommendation for each crop, based on the test and the yield you expect. Use that recommendation rather than the bag rate on the shop shelf. If you are unsure what the figures mean, ask for a written explanation, and take the report to the extension office so the person advising you can see the whole picture.

Split the nitrogen where the recommendation allows it. A part at planting and a part at the top-dressing stage often suits maize better than one large dose, because it reduces the losses from heavy rain. Follow the stage timings from your extension office, since they depend on the variety and the rainfall.

Turning the results into a plan

Here is an example of how a report can become a plan. Suppose a plot in the forest zone tests at pH 4.9, with low organic matter, low available phosphorus and medium potassium, on sandy loam. The farmer wants maize this season.

The order of work matters. Lime first, because the acid soil is holding back the phosphorus and the nitrogen the crop needs, and the lime needs time. Add compost or the residues from the last crop to build the organic matter. Apply the phosphorus recommended by the laboratory, mixed into the root zone at planting. Split the nitrogen between planting and top-dressing. Plant a legume on the next cycle to add nitrogen back, and test the plot again in three to four seasons to see whether the pH and the organic matter are moving.

The farmer does not need to fix everything in one season. A plan that corrects the biggest limit first, and records the result, is worth more than a long list of inputs applied at random.

Testing again and keeping records

A soil test is a snapshot. The plot changes with every crop, every fertiliser application and every season of rain. For a plot in regular use, a retest every three to four seasons gives a useful trend, and a test before a major change, such as a new clearing or a shift from maize to vegetables, is worth the money.

Keep the reports in a folder with the plot map, the crop history and the inputs you applied. Note the yield, the date of planting, the rainfall you recorded and any problems you saw. In a few years you will have a record that tells you more about your land than any general guide can, and it will make the next purchase decision easier.

Ask your extension officer whether the district keeps soil records for your area. Some districts hold results from earlier programmes, which can help you see a pattern before you test your own plot.

A checklist before you buy inputs

1. Divide the farm into areas that have the same history, slope and drainage, and label each one.
2. Find out the current fee, sample size and turnaround time from the laboratory before you send anything.
3. Take 15 to 20 cores per area from the right depth, using a clean spade and a clean bucket.
4. Mix the cores, air-dry the sample in the shade and pack about half a kilogram in a labelled bag.
5. Record the GPS point or a clear description of where the samples came from.
6. Read the pH first, then organic matter, then the nutrients, and ask for a written explanation of anything unclear.
7. Take the report to the extension officer and ask for the recommendation for your crop and your yield target.
8. Check the current price and quality of any lime, fertiliser or compost before you buy.

Soil testing on a mixed farm

On a mixed farm the soil test is the starting point for the crops and for the manure. You can test litter and compost as well, because the nutrient content of poultry manure varies widely with the feed, the bedding and how long it has been stored. A litter analysis tells you how much to apply, so that you do not overload a bed with nitrogen and salts. Our notes on composting poultry litter in Ghana and using poultry manure as fertiliser explain how to treat the material before it goes on the land.

The same farm plan can also cover rotation. A soil test shows whether the next crop should be a legume, and it helps you decide where the nightshades should go. See our guide to crop rotation in Ghana for the cycle, and our piece on growing maize in Ghana for how the crop responds to the nutrients on the plot.

If you are adding birds to the holding, our day-old chicks page explains the breeds and the first steps. Ross 308 and Cobb 500 start from GH₵20 per bird, and our prices page lists the other lines. You can compare the breeds, place an order through the order form, or call and WhatsApp us on +233 24 915 1503. Our partner page sets out how farmers grow with us, and about us explains who we are.

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