The most widespread mistake on a maize plot is to order the fertiliser formula first and look at the soil afterwards. On a strongly acid soil the phosphorus applied becomes fixed and the plant never sees it: the nutrient is paid for, it is sitting in the soil, and it stays unavailable. So the answer to the question above is yes, for any plot you intend to fertilise seriously, and one test every two to three cycles is enough if you keep the same rotation. What remains is doing it properly, because a rushed sampling produces a report that describes no real plot.
What the laboratory report actually decides
A soil test report is not there to tell you whether your soil is good. It is there to settle four decisions in a set order.
- pH determines whether the nutrients you apply will be available. It is corrected before anything else, and the correction takes several months to act, which makes it a decision for the previous season.
- Available phosphorus decides the basal dressing, the one placed at sowing that cannot be made up mid cycle.
- Exchangeable potassium decides the basal complement and whether a top dressing is worthwhile.
- Organic matter and texture decide how nitrogen is split, because a sandy soil low in organic matter loses nitrogen faster than a clay soil.
The report therefore does not replace a dose table, it makes one applicable. The reference doses by crop remain those of the agricultural services, which we set out in our article on NPK 15 15 15 dosage.
Sampling: the method that makes the result usable
The FAO guide to establishing a plant nutrient analysis laboratory, published by Motsara and Roy in 2008, sets the reference method. The figures below are taken from it.
- Divide the plot into uniform units. Each uniform portion is sampled separately, and one unit should not exceed 1 to 2 hectares. A damp lower slope, a termite mound zone and a former livestock pen are three different units, even on the same plot.
- Multiply the sampling spots. Depending on the size of the field, 10 to 20 spots are needed to make up a single composite sample. One hoe stroke at the plot entrance describes nothing.
- Move at a regular interval. The guide recommends cutting a slice of the plough layer every 15 to 20 steps, following a zigzag route.
- Respect the depth. Sampling covers the worked layer, from the surface to plough depth, roughly 0 to 22 cm. Fact sheet SL454 from the University of Florida, written for citrus groves, uses 8 inches of depth, which comes to about the same thickness.
- Mix, then reduce. The cores are gathered and homogenised in a non metal container, then reduced by quartering down to about 0.5 kg of soil to send to the laboratory. Both the University of Florida and the Alabama extension service start from 15 to 20 cores mixed together, and settle on a submitted quantity of 200 to 400 mL in one case, about one pint in the other.
- Air dry before shipping. Moist soil shipped in a sealed bag ferments, and the nitrogen result becomes unusable.
The habits that distort the result
Four habits are enough to make a report misleading, and none of them shows on the final document. Sampling just after a fertiliser application, which artificially inflates phosphorus and potassium. Sampling in wheel tracks or on a former manure storage area, two places that look nothing like the rest of the plot. Using a rusty auger or bucket, which contaminates the micronutrient figures. And mixing different soil units in the same bag, which produces an average matching no zone you will actually fertilise.
One calendar precaution matters as much: always sample at the same point in the cycle from one test to the next. Comparing a report from the end of the rainy season with one from the middle of the dry season amounts to comparing two different plots. The University of Florida fact sheet recommends annual sampling at the end of the rainy season, before the next fertilisation, which transfers to our calendars: sample 4 to 6 weeks before land preparation, so the result comes back before you order inputs.
Reading the report line by line
Two grids make it possible to place a result. For pH, the classes used by the FAO guide are as follows.
| Measured pH | FAO class | What it means in the field |
|---|---|---|
| Below 4.6 | Extremely acidic | Correction essential before any phosphate dressing |
| 4.6 to 5.5 | Strongly acidic | Phosphorus largely fixed, liming or amendment to plan |
| 5.6 to 6.5 | Moderately acidic | The common working range of soils in the region, standard dressing applicable |
| 6.6 to 6.9 | Slightly acidic | Nutrient availability close to the optimum |
| 7.0 | Neutral | No pH correction to plan |
| 7.1 to 8.5 | Moderately alkaline | Watch iron, zinc and manganese |
| Above 8.5 | Strongly alkaline | Micronutrient lock up likely, further diagnosis useful |
For the nutrients, the classes depend on the extraction method the laboratory uses, and the laboratory’s own grid always takes precedence over an outside one. As a reading benchmark, interpretation guide EC 1478 published by the Oregon State University extension service, by Marx, Hart and Stevens, rates phosphorus extracted with Bray P1 as low below 20 ppm, medium from 20 to 40 ppm, high from 40 to 100 ppm and excessive above 100 ppm. Potassium is rated low below 150 ppm, medium from 150 to 250 ppm and high from 250 to 800 ppm. Calcium is low below 1,000 ppm and high above 2,000 ppm, magnesium low below 60 ppm and high above 180 ppm. Those thresholds were established for the soils of that state, which makes them an order of magnitude for reading rather than a local reference: compare them with the grid attached to your own report.
From the report to a maize fertilisation plan
The order of decisions is always the same, and it is not negotiable.
- Correct the pH if the report rates it strongly or extremely acidic. Until that is dealt with, raising the phosphorus rate simply fixes more of it.
- Place the basal dressing at sowing according to the phosphorus and potassium levels measured, keeping to the reference dose for the crop.
- Split the nitrogen more finely the lower the organic matter and the sandier the texture. The application calendar follows the crop stages, which we cover in our article on the stages of growing maize.
- Add a foliar application on an identified deficiency, and only in that case. A foliar feed corrects a visible deficiency quickly, it does not replace soil fertilisation, as we point out in our guide to NPK, foliar and organic fertilisers. On maize, the protocol for our own range is detailed in the article on AgriBiosol, foliar fertiliser for maize.
- Record what was applied, and where. A report without a fertilisation history reads badly at the next cycle, because you no longer know whether a high phosphorus figure comes from the soil or from last year’s bag.
An organic amendment deserves a place in that plan as soon as organic matter is low. It acts on the soil’s capacity to hold water and nutrients, and so on how steady the crop is, and its effects accumulate from one cycle to the next, unlike a mineral fertiliser.
When to sample, and how often
- First test: before committing to a new plot, or ahead of a major change of crop.
- After that: every two to three cycles on a stable rotation, more often on an intensively cropped irrigated plot.
- Always: 4 to 6 weeks before land preparation, at the same point in the cycle as previous times, and never in the weeks following a fertiliser application.
Where JOGOO comes in
JOGOO Agriculture organises the sampling, the dispatch to the laboratory and the translation of the report into a fertilisation plan, then supplies the basal fertilisers, the nitrogen top dressing, organic amendments and the matching AgriBiosol foliar range. For the detail of that offer, see our Fertilisation and Plant Nutrition page and, for plot level support across several cycles, our Agricultural Consulting and Engineering page. Go through the Contact page for a quote covering your area and the number of soil units to analyse.
Frequently asked questions
How many sampling spots for a one hectare plot?
Between 10 and 20 spots gathered into a single composite sample, according to the FAO guide published by Motsara and Roy, moving every 15 to 20 steps. Beyond 1 to 2 hectares, or as soon as the soil visibly changes, a second sample has to be made up.
At what depth should I sample?
In the worked layer, from the surface to plough depth, roughly 0 to 22 cm according to the FAO guide. Fact sheet SL454 from the University of Florida uses 8 inches, which comes to the same thickness of soil explored by the feeding roots.
How much soil should be sent to the laboratory?
About 0.5 kg after mixing and quartering according to FAO. The University of Florida gives 200 to 400 mL and the Alabama extension service about one pint of the mixture: all three sources describe the same well homogenised handful of earth.
Does a pH of 5.2 rule out growing maize?
No, but it penalises it. The FAO guide rates that value as strongly acidic soil, a situation in which part of the phosphorus applied becomes fixed and unavailable. Correcting the pH is then decided before any increase in the phosphorus rate.
Can the thresholds found online be trusted?
Only as an order of magnitude. The classes in guide EC 1478 from Oregon State University, for instance, were established for the soils of that state and for Bray P1 extraction. The reference grid is the one your laboratory attaches to the report, because it matches its extraction method.
Does a soil test have to be repeated every year?
On a stable rotation, every two to three cycles is enough. An irrigated plot under intensive market gardening, with several cycles a year and frequent applications, justifies annual monitoring, at the same point in the cycle each time.
Sources
- Motsara M. R. and Roy R. N., 2008, Guide to laboratory establishment for plant nutrient analysis, FAO Fertilizer and Plant Nutrition Bulletin 19
- Kadyampakeni D. M., Morgan K. T., Schumann A. and Ferrarezi R., Soil Sampling Procedures, fact sheet SL454 / SS667, University of Florida IFAS Extension
- Alabama Cooperative Extension System, Home Soil Testing: Taking a Sample
- Marx E. S., Hart J. and Stevens R. G., Soil Test Interpretation Guide, EC 1478, Oregon State University Extension Service
