The profitability of a crop is based on the amount and quality of the yield and the inputs incurred. One of the main inputs is fertilisation and it is not worth doing it “guesstimating”.
The soil’s macro- and micronutrient content, organic matter and pH level determine soil productivity. The availability of the 17 most important elements for plant growth is required. Depending on their quantitative requirements, they are divided into macronutrients: C, H, O, N, P, S, K, Ca and Mg and the micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni. A deficiency of macronutrients can result in stunted plant growth, while an excess is generally not harmful to plants – with the exception of nitrogen. Micronutrients are essential to plants as catalysts for physiological processes and can determine the size and quality of the yield achieved. Both deficiency and excess of micronutrients can be harmful to plants.
Organic matter has a sorptive and structure-forming function in the soil and is also a source of minerals after mineralisation. Humus (humus) and mineral substances are jointly responsible for the capacity of the soil sorption complex.
Soil pH (pH) is a basic parameter of soil fertility, determining the transformation of applied organic and inorganic fertilisers, i.e. also the effectiveness of fertilisation. Most crops thrive best on acidic / slightly acidic to slightly alkaline soils. Acidification of the soil, especially on post-glacial soils, is a natural, continuous process, so their pH must be monitored systematically and corrected by liming. Liming is generally carried out every four years, but can also be done more frequently.
The appropriate frequency of soil analyses depends on the physico-chemical properties of the elements to be determined and their movement in the soil. Testing for pH and abundance of phosphorus, potassium, magnesium and micronutrients is carried out as standard on a 3-4 year cycle. In integrated production, test results for the basic macronutrients are valid for 4 years and mineral nitrogen tests must be performed annually. The use of instruments offered for crop management by precision agriculture allows ongoing monitoring of soil abundance.
In the classical method, soil samples are taken with a special “Egner’s stick” or other tool. The method of soil sampling is described in the PN-R-04031:1997 standard. Based on the above-mentioned standard, soil sampling recommendations have been developed, taking into account the specificities of the different types of crops (agricultural, orchard, hops and nursery, under covers) and the special requirements for mineral and nitrate nitrogen. These instructions can be found on the website of the National Chemical and Agricultural Station (SCHR). They also describe the requirements for packaging and documenting the soil samples collected.
The general principles of soil sampling require that a sketch of the fields of the farm is drawn up and the extent of the area of the crops grown is plotted on it. For each crop, up to 20 – but up to 40 in orchards, nurseries and hop yards – primary samples are taken according to a predetermined pattern, preferably in a direction perpendicular to the direction of the agronomic treatment (cultivation, fertilisation). From these samples, an averaged sample of up to 0.5 kg is formed, which should represent an area of up to 4 ha of agricultural land with similar natural (soil type, type and species, topography) and agrotechnical (forecrop, cultivation, fertilisation) conditions. It is best to take soil samples in spring or autumn before sowing fertilisers. Sampling must be documented by a sampling protocol and a label attached to the sample. The samples must be durably packaged in such a way that they cannot be reopened without breaking the packaging. The protocol, in addition to other required information, should specify the sampling method (standard, regulation, SCHR manual, customer procedure) and the details of the sampler.
During soil analysis, up to 75-80% of errors that arise are due to incorrect sampling, and only 20-25% of inaccuracies arise in laboratories. Samples should not be taken at a distance of less than 5 m from the edge of a field, in places left behind by stacks, mounds, lime or manure piles, in ditches, furrows, molehills and gravel pits, in depressions and on steep slopes of the land – additional samples should be taken from these places if necessary. Sampling immediately after mineral fertiliser application, after organic fertiliser application and from excessively dry or excessively wet sites should be avoided. In fields exposed to surface run-off and soil erosion, possible nutrient movement as a result of these processes should be taken into account. In agricultural crops, samples should be taken from the arable layer to a depth of 0-20 cm, from locations previously marked on the diagram. In orchards, samples are taken from the arable layer (0-20 cm) and sub-arable layer (20-40 cm), separately for herbicide fallow and turf. In nurseries and hops, from the 0-25 cm arable layer and 25-40 cm sub-arable layer. Sampling locations in orchards, nurseries and hop gardens are set diagonally across the plot or along the rows in a zigzag pattern, alternating between the herbicide strip and the inter-row.
Sampling can be contracted and carried out by a company providing such services, which will use its expertise to select the sampling sites. However, it is worth participating in such an activity. In integrated production, soil samples for mineral nitrogen testing must be taken by a representative of an external body (e.g. SCHR).
Samples for nitrogen assessment are taken slightly differently. For the purposes of fertiliser advice, soil samples are taken from levels: 0-30 and 30-60 cm, in the pre-spring, from fields of winter crops before the application of nitrogen fertilisers. From fields intended for spring crops – in the pre-spring / spring, before sowing of these crops. To assess the effects of fertilisation, samples are taken after harvesting the crops, from levels: 0-30, 31-60, 61-90 cm. Testing soil nitrogen at this time also enables the correct determination of the amount of fertilisation needed for the next crop.
Sampling is done with sticks of increasing length and decreasing diameter, in the same holes made in the soil, from layers: 0-30 cm, then 31-60 cm, and when assessing post-harvest nitrogen content, also from layer 61-90 cm.
The minimum number of sub-samples making up one bulk sample for testing is 10 for each layer, and up to 15 when organic fertilisation is used.
Aggregate samples for testing shall be made by combining the subsamples from each layer separately, after thorough mixing and removal of stones, plant debris, etc., and reducing to approximately 0.25 kg. From large fields with low variability, one aggregate sample for testing should come from an area of no more than 4 ha, in the case of smaller fields with higher soil variability, the sample should come from an area of no more than 1 ha. Samples may be stored for 2 to 3 days at a temperature of not more than +5° C. Longer storage requires soil samples to be frozen at a temperature no higher than -18° C.
The disadvantage of the traditional method of soil chemical analysis is its rather long waiting time for laboratory results. However, there is often a need to carry out analyses very quickly, for example when the time between harvesting a pre-crop and sowing the next crop is short. Precision farming tools allow both small and large farms to optimise fertilisation very quickly and accurately using modern technology.
On smaller farms, including, for example, orchards, the method of scanning soil samples directly on site can be used. The scanning is based on the use of X-ray fluorescence methods and near-infrared spectroscopy, the so-called NIR. The results of the measurement, together with fertilisation recommendations, can be obtained in a very short time as information in an app on the phone, with the possibility of printing out a table.
In the case of fields with a large area and high variability, soil scanning methods in conjunction with GPS location can be used to optimise the number of samples taken, allowing for the delineation of homogeneous soil zones.
The method of soil electromagnetic scanning, based on the principle of analysing the electro-conductivity of the soil (continuous measurement of the EC coefficient) at one or two depths, uses special sensors. The soil map created from the soil scan data allows us to get an idea of the differences in soil structure in the area of the field under study. No soil contact is required during the measurement, allowing us to carry out the survey regardless of the current state of the crop.
Another method is the electromagnetic method, where the scanner induces a primary electromagnetic field that is modified by the properties of the soil (conductivity, changes in soil structure, moisture content) creating a secondary electromagnetic field. The device records the magnitude of the secondary field and the software records and interprets the results, displaying them on the operator’s terminal.
By taking soil samples from the homogeneous zones thus defined, we can be sure that the results obtained are the most accurate in terms of soil nutrient abundance, as we are not mixing soils with different parameters in a single aggregate sample. In addition, good knowledge of the homogeneous zones can be used to calculate the sowing rate and depth (so-called variable seeding rate) according to soil conditions, optimising yields within a field.
Soil variability data can also be used as valuable information on the value of agricultural land, its fertility, particularly useful when buying/selling or leasing.