Sampling soil correctly for analysis and reading the results

The sample sent to the laboratory weighs roughly one ten-millionth of the topsoil of the field it represents. That is why the FAO plant nutrition guide stresses that however accurate the analysis, the result of a sample that does not represent the field is worthless (Roy et al., 2006). The sampling guide of the Soil, Fertilizer and Water Resources Central Research Institute under TAGEM, Türkiye's agricultural research directorate, opens with the same warning (TAGEM TGSKMAE).

When to sample

• According to the Institute's guide, sample one and a half to two months before sowing or fertiliser application; do not sample on frosty or muddy days, and the soil should be dry enough not to stick to your boots. Results come out one week to ten days after the sample reaches the laboratory, longer in the busy autumn period.
• FAO notes that testing is done separately for each field and each nutrient and can be repeated every 3-4 years (Roy et al., 2006).
• For citrus in Florida, the recommendation is to sample once a year at the end of the rainy season and before autumn fertilisation (Obreza et al., 2008). When adapting this to your own calendar the principle is the same: results are comparable if the sample is taken before the fertiliser plan and at the same time each year.

Where and how many points

First divide the field into uniform parts by colour, slope, texture, soil depth, previous crop and fertilisation history; take a separate composite sample from each part. According to the guide, a composite sample is taken from 8-10 points along a zigzag line, and one composite sample should not represent more than 40 decares (4 hectares). At each point, dig a V-shaped hole 20 cm deep and cut a 3-4 cm slice from the straight side; mix the slices in a bucket, remove stones and roots, put about 1 kg of soil in a bag and write the label in pencil. If micronutrients are to be analysed, use plastic, wooden or stainless steel tools (TAGEM TGSKMAE). FAO suggests about 20 subsamples for a 1 hectare field and a final sample of about 0.5 kg for analysis (Roy et al., 2006).

Do not sample at old manure heaps and dung spots, threshing and animal resting places, spots where straw or weeds were burnt, mounds and waterlogged hollows, under trees in the field, on top of rows fertilised at sowing, or along roads, canals, streams and fences. A small sandy patch in the field is sampled separately (TAGEM TGSKMAE).

How deep

For fertiliser recommendations the depth is usually the tilled layer, 15-20 cm; one additional sample from 20-40 cm at a typical spot in the field is also recommended. In vineyards and orchards, separate composite samples are taken from 0-20, 20-40 and 40-60 cm, and the depth can be adjusted to the species and age of the tree (TAGEM TGSKMAE). FAO recommends 20-30 cm for arable soils (Roy et al., 2006). For drip-irrigated citrus, UF/IFAS recommends one core about 20 cm deep at the dripline of each of 15-20 trees, within the area wetted by irrigation, and uses soil analysis together with leaf analysis: about 100 leaves from 4-6 month old spring flush on non-fruiting twigs. Written for the sandy soils of Florida, this guide states that soil testing has no practical value for easily leached nutrients such as nitrogen and potassium and is most useful for pH, phosphorus, calcium, magnesium and copper (Obreza et al., 2008).

Reading the results

pH: The range allowing the widest nutrient availability is 5.5-7.5 (FAO, 2020). In Florida the recommended range for citrus is 6.0-6.5; outside it, deficiencies or toxicities become more likely (Obreza et al., 2008).

Lime: FAO describes soils with often more than 15% calcium carbonate as calcareous. These soils are generally low in organic matter and nitrogen, and phosphorus, zinc and iron deficiencies are common (FAO Soils Portal).

EC (salinity): Check the method: a saturated paste extract (ECe) and a 1:1 soil:water mixture give different numbers (FAO, 2020). Crop salt tolerance is expressed as ECe. According to FAO Irrigation and Drainage Paper 29, the threshold at which yield starts to decline is 7.7 dS/m for cotton, 6.0 for wheat, 3.2 for groundnut and 1.7 dS/m for maize and orange; at an ECe of 2.3 dS/m orange yield potential falls to 90% (Ayers and Westcot, 1985).

Organic matter: Read it together with texture; for the new EU-level criterion, see the guide “Soil health indicators and monitoring” in this section.

Nitrogen, phosphorus, potassium: The most suitable measurement for nitrogen is water-soluble nitrate. For phosphorus, Bray-1 suits acid soils and Olsen (sodium bicarbonate at pH 8.5) suits neutral and alkaline soils; among the major nutrients, soil tests for phosphorus are the most reliable. Potassium is measured as exchangeable K extracted with ammonium acetate. The general classes FAO gives for soils with medium cation exchange capacity (10-20 cmol/kg) are shown below; FAO also stresses that only locally developed limits should be used for specific soils and crops (Roy et al., 2006).

From results to a fertiliser plan

1. Determine the class of each nutrient: the lower the value, the larger the external input needed; at very high values no additional fertiliser is usually required (Roy et al., 2006).
2. Subtract the nutrients expected from manure and legumes; FAO asks for recommendations to be adjusted for these contributions.
3. If the report gives elements (P, K) and the fertiliser label gives oxides (P2O5, K2O), convert: 1 unit of P = 2.29 units of P2O5, 1 unit of K = 1.2 units of K2O (Roy et al., 2006).
4. On calcareous soils, apply phosphate fertiliser in a water-soluble form and in bands; side-dress nitrogen instead of applying it at sowing, and do not leave ammonium fertilisers on the surface; use zinc sulphate against zinc deficiency. FAO also recommends generous animal manure to prevent iron and zinc deficiencies (FAO Soils Portal).
5. Plan nitrogen more tightly in nitrate vulnerable zones. The plain of Thessaly is a Nitrate Vulnerable Zone under the EU Nitrates Directive; the action plan for the Pinios basin recommended cutting nitrogen fertilisation by 50% for cotton, maize and sugar beet, legume cover crops in autumn and winter, and crop rotation, measures predicted to reduce the nitrogen surplus by 30-35% (JRC, 2020).

Where to get soil tested

In Türkiye, samples can be taken to the nearest soil testing laboratory by the farmer or sent through the provincial and district agriculture directorates (TAGEM TGSKMAE). In Larissa, the Department of Soil and Water Resources of the ELGO-DIMITRA Institute of Industrial and Forage Crops carries out certified analyses of soil, water and plant tissue samples (ELGO-DIMITRA).

Sources

1. T.C. Tarım ve Orman Bakanlığı TAGEM, Toprak Gübre ve Su Kaynakları Merkez Araştırma Enstitüsü. Toprak Numune Alınması (accessed 29 September 2026)
2. FAO, 2006. Roy, R. N., Finck, A., Blair, G. J., Tandon, H. L. S.: Plant nutrition for food security: A guide for integrated nutrient management, FAO Fertilizer and Plant Nutrition Bulletin 16 (accessed 29 September 2026)
3. FAO, 2020. Soil testing methods: Global Soil Doctors Programme, a farmer-to-farmer training programme (Global Soil Partnership) (accessed 29 September 2026)
4. FAO, 1985. Ayers, R. S., Westcot, D. W.: Water quality for agriculture, FAO Irrigation and Drainage Paper 29 Rev. 1, Chapter 2 (Table 4, crop salt tolerance) (accessed 29 September 2026)
5. FAO Soils Portal. Management of some problem soils: Calcareous soils (accessed 29 September 2026)
6. UF/IFAS, 2008. Obreza, T. A., Zekri, M., Hanlon, E. A., Morgan, K., Schumann, A., Rouse, R.: Soil and Leaf Tissue Testing for Commercial Citrus Production (SL253.04/SS531) (accessed 29 September 2026)
7. ΕΛΓΟ-ΔΗΜΗΤΡΑ, Ινστιτούτο Βιομηχανικών και Κτηνοτροφικών Φυτών (Λάρισα). Τμήμα Εδαφοϋδατικών Πόρων (accessed 29 September 2026)
8. European Commission Joint Research Centre (JRC), 2020. Fact sheet: Pinios River Basin (accessed 29 September 2026)