In short
Soil testing on a farm means taking composite samples from each field at a consistent depth, sending them to a laboratory for pH, organic matter and nutrient content, and using the results to set fertiliser and lime doses.
The University of Arizona advises 15 to 20 cores per composite, mixed well, and one composite per 15 to 20 acres when you sample a whole field. Kazakhstan runs a national scheme with elementary plots, 20 to 25 point samples per composite and a seven-year cycle on rainfed land. Planva holds the field register and the plans that these decisions feed, and it does not store the test results.
Why test the soil
A fertiliser dose is a guess until the soil is measured. FAO states that the nutrients a crop needs can be derived from soil testing and from the nutrient removal by grain and straw. The University of Arizona encourages growers to test regularly, if not every season, before planting each crop, and to follow a consistent protocol so that the data represents the land.
Three ways to sample a field
| Pattern | How it works | When it fits |
|---|---|---|
| Whole field | Cores taken in a zigzag across the field, one composite per 15 to 20 acres, about 6 to 8 ha | One uniform input over the whole field. Cheapest, but it hides problem spots |
| Grid | The field is divided into uniform cells, with a composite per cell or samples at the grid points | Seeing how soil varies and tracking change over time |
| Zone | Separate composites for areas already known to differ, such as low ground, salt-affected patches or high and low yield | Site-specific management |
- Composite, not single. Collect 15 to 20 cores or scoops per site and mix them thoroughly.
- Same depth, same tool. Every core in a composite comes from one depth, taken the same way. Tilled soils are sampled to tillage depth.
- Label by field. The result is only useful if it maps back to a field and a sampling date.
What the laboratory measures
The University of Arizona lists the basics as pH, electrical conductivity (soluble salts), cation exchange capacity and the available or extractable nutrients, covering nitrogen, phosphorus and potassium and micronutrients such as iron, zinc and manganese. Methods and units differ between laboratories, so keep the same laboratory from year to year when you want to compare.
A national scheme: Kazakhstan
Kazakhstan's Rules for agrochemical survey of soils, issued by Order No. 4-1/147 on 27 February 2015, show what a state programme looks like.
| Item | Rule |
|---|---|
| Who | A state agrochemical service institution designated by the Government. Landowners and land users can request a survey on a paid basis |
| Land | Arable land, each district within one field season |
| Elementary plot | Up to 75 ha on rainfed land and up to 10 ha on irrigated land, with one composite sample per plot |
| Sample | 20 to 25 point samples from 0 to 20 cm, composite of at least 500 g |
| Measured | Humus, mobile phosphorus, exchangeable potassium, forms of nitrogen, soil pH, micronutrients and the water extract |
| Cycle | Seven years on rainfed land, with shorter periods for irrigated land, land treated with chemical ameliorants and variety trial plots |
| Output | A summary analytical statement, a passport statement for each field, a colour cartogram of nutrient levels and an agrochemical essay for the land user |
From results to a fertiliser plan
Results become useful when they meet a yield target. The nutrient level tells you how much the soil can supply, the target tells you what the crop will remove, and the gap sets the dose. The sums are in how to calculate fertiliser application rate. Keep the laboratory report, the sampling map and the date together with the field record.
If no national scheme covers you
Arizona says one composite per 15 to 20 acres and Kazakhstan allows plots up to 75 ha on rainfed land. That spread tells you plot size is your decision, so make it deliberately.
- Plot size. Smaller where soil, slope or past manuring change from one part of the field to another, larger on even ground.
- Depth. Pick one, usually tillage depth, and never change it between years.
- Interval. Retest when your inputs or yields have changed enough to move the answer. A regular interval plus a test after any big liming or phosphate programme works for most farms.
- Laboratory. Stay with one laboratory and one method so year-on-year changes mean something.
- Records. Write down the field, the sampling date, the map of cores and the laboratory report.
How this works in Planva
- Field register. Boundary, area, region and cadastral numbers for each field, with boundary import and export in Shapefile and KML.
- Crop rotation. Seasons with crop, variety, sowing and harvest dates and yield.
- Agroplans. Fertiliser by rate or quantity, planned and actual.
Soil test results are not a Planva function. Keep the laboratory reports alongside your field record. The fields and plans they inform are described in field history records.
Sources
- University of Arizona Cooperative Extension, A Guide to Effective Soil Sampling for Sustainable Crop Production, AZ2147, July 2025
- Kazakhstan, Rules for agrochemical survey of soils, Order No. 4-1/147 of 27 February 2015
- FAO, Plant nutrition for food security, chapter 8, nutrient management guidelines for some major field crops
Common questions
Collect 15 to 20 cores per site at a consistent depth with the same tool, mix them into one composite, and send it to a laboratory. Tilled soils are sampled to tillage depth.
The real Planva interface, shown with test data.
