Features and methodology of conducting growing season experiments with soil crops

Soil-based cultures are one of the most common modifications of growing season experiments and are closest in growing conditions to field trials. The advantages of growing season experiments with soil cultures compared to field trials lie in the possibility of: 1) achieving complete soil uniformity in all pots through mixing, which allows avoiding significant discrepancies between replicates that are often encountered in field trials due to the heterogeneity of soil fertility; 2) eliminating the influence of adverse weather conditions on experimental results; 3) maintaining identical soil moisture in all variants and replicates of the experiment.

Conducting growing season experiments with soil culture allows for an in-depth study of natural and artificial soil fertility, providing a comparative assessment of the fertility of different types, subtypes, and varieties of soil, and studying the effectiveness of types and forms of fertilizers depending on soil conditions. At the same time, experiments with soil cultures, unlike field trials, provide only a qualitative assessment of the studied patterns and do not allow for their quantitative evaluation. Furthermore, a number of issues regarding the effect of fertilizers cannot be resolved using the growing season method of research with soil culture. These include the placement of fertilizers in crop rotation, the study of combining fertilizer with tillage">the tillage system soil, plant care, and other agrotechnical practices that must be studied only in field trials.

There are three fundamental differences between a growing season experiment with soil culture and a field trial in the use of nutrients. Firstly, in a growing season experiment, as a rule, nutrients from only one genetic horizon of the soil profile are used. Secondly, growing plants in glass houses, greenhouses, climate chambers, and other controlled environments facilitates a more intensive mobilization of nutrients from the soil than in field trials. Thirdly, the very process of nutrient mobilization in a growing season experiment with soil culture differs from that under field conditions. A complete resolution of issues concerning plant nutrition and fertilization requires a combination of field and growing season methods.

The setup of growing season experiments with soil culture is carried out in the following sequence: 1) preparation of growing season facilities and pots for the setup of experiments; 2) taring of pots; 3) seed preparation, soil, and fertilizers; 4) preparation of drainage, filter paper, gauze, and glass tubes for irrigation; 5) filling of pots; 6) sowing; 7) irrigation, plant care, phenological observations, and biometric measurements during the growing season; 8) harvesting and yield accounting; 9) collection of soil and plant samples; 10) processing of experimental results and report writing.

Before setting up the experiment, the growing season facilities intended for it are thoroughly cleaned of last year's plant residues and soil, and pots are removed if they were left there for winter storage. All shelves of the growing season house are washed and dried. Afterward, they are painted with light-colored paint and numbered.

For growing season experiments, metal, glass, earthenware enameled, and plastic pots with a capacity of at least 4 kg of air-dry soil are used (Fig. 92; Onishchenko L.M., 2005). Pots suitable for conducting the experiment are washed inside and out with tap water and set out to dry. Before filling, metal pots are coated with white enamel or oil paint, and on the inside with bitumen varnish, or polyethylene bags are inserted into them. A batch of identical pots must be selected for each experiment.

Fig. 92. Growing season experiment with rice in plastic pots.

The pots are thoroughly dried, tared, and numbered with varnish or black paint. The taring of pots consists of matching them by height and adjusting them to an identical mass. The pots are balanced using broken glass, pebbles, and careful selection of drainage. In terms of mass, the pots in a single experiment should not differ by more than 100 g, and by more than 0.5 cm in height and diameter.

The next operation is the preparation of drainage. Pebbles and broken glass are most often used as drainage. They are thoroughly cleaned of impurities by keeping them for several days in glass vessels with one of the acids—sulfuric, hydrochloric, nitric, or potassium dichromate. Then they are rinsed with tap water until the acid is completely removed. They are rinsed with distilled water and dried in the sun or in a drying oven.

When setting up growing season experiments with soil culture, it is important to pay attention to the soil and its preparation. The location for soil sampling must be carefully chosen in accordance with the research objectives. The upper arable horizon is used for filling the growing season pots. The soil is taken in the spring before setting up the growing season experiments with such moisture content that it does not smear, and its clods are easily broken when rubbed. The amount of soil required for setting up the experiments is determined taking into account the number of pots and their capacity. Usually, 30% more soil than the calculated amount is prepared. The delivered soil is mixed and sifted through a screen with 1 cm holes to remove stones, roots, and crop residues.

The technique for filling vegetation pots with soil requires specific skills. It is essential that each series of pots for a single experimental design be filled by one person, as this ensures uniform soil compaction in the pot. The first step in the filling process is determining the mass of soil per pot, which is done by performing a trial filling and weighing the soil. Subsequently, a weighed portion of soil is taken, placed in an enamel basin, the necessary fertilizers are added, mixed, and then poured into the pre-prepared pots with periodic, uniform compaction by hand.

How to properly design a vegetation experiment

The accuracy of the results of a vegetation experiment directly depends on the structure of its design. To obtain reliable data that can be transferred to field practice, it is important to observe the principle of factorial design — testing all possible combinations of the studied elements. For simple single-factor experiments, the design is structured so that, ultimately, a clear dependence of yield on the fertilizer dose is obtained.

Comparing experimental treatments with a control reveals the actual responsiveness of plants. When studying mineral nutrition, standard schemes for element combinations are used. For example, a full scheme includes eight treatments:

• 1) Control without fertilizers (0);
• 2) N;
• 3) P;
• 4) K;
• 5) NP;
• 6) NK;
• 7) PK;
• 8) NPK.

In reduced experiments, a five-treatment scheme is applied: 1) 0; 2) NP; 3) NK; 4) PK; 5) NPK. If specific forms of nitrogen fertilizers need to be compared, they are tested against a constant phosphorus-potassium background (PK). In this case, the scheme looks as follows: 1) 0; 2) PK (background); 3) background + Nm; 4) background + Naa; 5) background + Na; 6) background + Nc.

When studying the liming of acidic soils, lime application rates are calculated individually. For most soils, the CaCO3 dose is determined by hydrolytic acidity using a 1 N CH3COONa solution with pH 8.2. In humid subtropics, the calculation is based on exchange acidity in extracts of 1 N KCl solution with pH 5.5–6.0.

1. Determine the soil acidity value in mmol-eq per 1 kg of soil.
2. Divide the obtained value by a coefficient of 20 to convert the rate into grams of CaCO3 per 1 kg of soil.
3. Develop an experimental design with contrasting lime rates on a natural background and on a background of mineral fertilizers.

A standard lime experiment scheme includes 20 treatments:

• Treatment 1: Control without CaCO3;
• Treatments 2–4 (without fertilizers): 2) CaCO3 at 0.25 of acidity, 3) CaCO3 at 0.50 of acidity, 4) CaCO3 at 1.0 of acidity;
• Treatment 5: NPK without lime;
• Treatments 6–8 (on an NPK background): 6) NPK + 0.25 CaCO3, 7) NPK + 0.50 CaCO3, 8) NPK + 1.0 CaCO3;
• Treatments 9–12: repetition of treatments 5–8 with a PK background;
• Treatments 13–16: repetition of treatments 5–8 with an NK background;
• Treatments 17–20: repetition of treatments 5–8 with an NP background.

The study of plant responsiveness to microelements is conducted using schemes with different concentrations of substances. In all cases, a control treatment where no micro-fertilizers are used must be included. Recommended dosages for various application methods are provided below.

Fertilizer selection and dosage calculation

Chemically pure salts are used to create a nutrient background or to determine the natural soil nutrient availability. Unlike industrial fertilizers, they do not contain ballast elements that could distort the results. Choose salts containing only ions assimilable by plants: NH4NO3, KNO3, KH2PO4, K2HPO4, NH4H2PO4, (NH4)2HPO4.

The choice of salt depends on the soil type and the goals of the experiment. On acidic podzolic soils and red soils (krasnozems), NH4NO3, CO(NH2)2, or a mixture of 2/3 NH4NO3 and 1/3 Ca(NO3)2 are used as a nitrogen source. To create a dual nitrogen-potassium background, KNO3 is taken, compensating for the lack of nitrogen with ammonium nitrate. A phosphorus-potassium background is formed using KH2PO4 and K2HPO4, while a nitrogen-phosphorus background on chernozems is created by applying NH4NO3 and (NH4)2HPO4.

When calculating nutrition per kilogram of soil, the rates are adjusted for the specific crop. On chernozems, which are naturally rich in potassium, the rate of potash fertilizers is reduced by 2–3 times. For grain crops, the calculation is performed using standard values that help obtain comparable results.

• Nitrogen application rate for cereals — 0.15 g N per 1 kg of soil
• Phosphorus application rate for cereals — 0.1 g P2O5 per 1 kg of soil
• Potassium application rate for cereals — 0.1 g K2O per 1 kg of soil

 Cereals 0.15 0.10 0.10 Legumes 0.04–0.10 0.10–0.15 0.10–0.15 Potato 0.12 0.20 0.28 Sugar beet 0.15 0.22 0.22 Cotton 0.24 0.36 0.06–0.09 Tobacco 0.20–0.30 0.10–0.20 0.20–0.30 Vegetables 0.10–0.20 0.10–0.25 0.15–0.30

The fertilizer rates presented in Table 227 can be considered average, so both higher and lower rates should be introduced into the scheme. If industrial fertilizers are used as sources of N, P, and K, their application rate is calculated based on their nutrient content. Water-soluble fertilizers can be applied in the form of solutions.

Samples of dry fertilizers should be weighed and placed into parchment paper bags no earlier than 1–2 days before filling the vessels; the scheme variant, type and form of fertilizer, and mass in grams should be indicated on them. The number of weighed bags of fertilizer must correspond to the number of vessels with the fertilized soil. If fertilizers are applied in a solution, there is no need to weigh samples into bags. In the laboratory, one general sample is weighed, equal in mass to the sum of the single variants, and dissolved in a small volume of water. It is convenient to have solutions containing 1 g of a given nutrient per 100 ml of solution. At optimal soil moisture, 30–50 ml of fertilizer solution is usually sufficient per vessel for heavy loamy and clay soils, and 15–20 ml for sandy and sandy-loam soils. Samples with nitrogen and potassium fertilizers can be dissolved in the same volume of water and applied in one go. Distilled water equal in volume to the fertilizer solution applied in the studied variants is added to the soil without fertilizers. To calculate the nutrient rate, it is convenient to use Table 228.

Table 228 – Salt samples containing 1 g of nutrient Salts N P2O5 K2O

 NH4NO3 2.86 – – (NH4)2SO4 4.72 3 6.07 3)2·4H2O 8.44 – – KNO3 7.22 – 2.15 K2SO4 – – 1.85 KCl – – 1.58 NH4H2PO4 8.21 1.62 – (NH4)2HPO4 4.72 1.87 – KH2PO4 – 1.92 2.89 K2HPO4 – 2.46 3.70 Ca(H2PO4)2·H2O – 1.78 –

As a source of magnesium in vegetative experiments, MgSO4·7H2О is used, containing 16.4% MgO; the source of sulfur is Na2SO4 or CaSO4. Microelements are most often used in the form of pure salts MnSO4·5Н2O (22.8 4·7H2O (22.8 4·5Н2O (25.5 % С 4·7H2O (21.0 % Со), (NН4)6Mo7O24·4H2O (54.3 % Мо) and H3ВО3 (17.5 % В).

Fertilizer rates in vegetative experiments with soil cultures (H, mg/kg) can be determined based on the fertilizer rates recommended for field trials. The conversion is performed using the formula:

10  D  h   where: D – rate of a mineral nutrition element applied to the soil under field conditions, kg/ha;

P – content of the active ingredient in the fertilizer, %; h – average thickness of the soil layer fertilized under field conditions, cm; ρ – soil density, g/cm3;

10 – constant value obtained during the derivation of the formula.

In vegetative experiments, plants have better lighting conditions and supply of carbon dioxide from the atmosphere, do not experience competition from weeds, i.e., they are under conditions of more intensive involvement of nutrients in metabolism. Therefore, in vegetative experiments, fertilizer rates are higher than those used under field conditions.

Filling the vessels begins with control variants, i.e., those to which no fertilizers are applied. If the variants differ in types, forms, or rates of fertilizers, then, when starting to fill a new series of vessels, the basin should be thoroughly cleaned and hands washed. Mixing the soil with fertilizer should be carried out for 3–5 minutes. The heavier the soil in terms of particle size distribution, the longer the mixing should be.

Seed for sowing can be dry, soaked, or germinated, but must be varietally pure. The sowing process includes the following operations:

• leveling the soil surface in the vessels;
• moistening the soil (if necessary);
• marking the nests with a special template;
• placing the seed in the nests;
• covering the seed with soil previously removed from the vessel;
• sprinkling the soil surface with sand at a rate of 200 g per vessel.

Cereals and grain legumes are sown with germinated seed to a depth of 1.5–2.0 cm, small-seeded crops (perennial legumes, grasses) – to a depth of 0.5 cm. After sowing, the vessels are covered with sheets of paper, and if they remain outdoors, additionally with polyethylene film to avoid soaking by rain. The sheets of paper and the film are removed after the first seedlings emerge.

Planting rates for containers with a diameter of 15–20 cm:

The amount of seed sown in a container should exceed the desired number of plants by 5–10 pieces. Two to three days after emergence, once they have strengthened, excess seedlings are removed with tweezers, leaving an equal number of plants in each container. If the objective of the experiment, and therefore the scheme, provides for fertilizer application during the growing season, they are applied in the form of liquid top dressing.

Reliable data for cereals and flax can be obtained with 3-fold, for legumes and oilseed crops – 4–5-fold, and for root and tuber crops – 5–6-fold replication of the experiment.

To prevent plants from lodging and breaking, wire frames or thin slats are inserted into the containers. The height of the slat frame for cereals, grain legumes, and perennial grasses is 40–50 cm, with four pieces per container. Strings are stretched between them to provide support for lodging plants.

Rules for maintenance and conducting observations in a vegetation experiment

Plant maintenance in a vegetation experiment consists of weeding, irrigation, and planned top dressing. The accuracy of research results directly depends on strict control of growing conditions and the exclusion of random factors. Containers with plants are arranged according to replications — it is advisable to place one treatment in a single row for ease of observation. On dry sunny days, the carts are rolled out to an open area, which must be protected with netting against poultry.

For irrigation, distilled or tap water is used. It must be left to stand in containers the day before to avoid temperature shock to the roots from excessively cold water. Soil moisture in the containers is maintained at an optimal level for most crops — 60–70% of full water capacity.

1. Before filling the containers, the initial humidity and full water capacity of the soil are determined.
2. When irrigating potatoes, tomatoes, sunflowers, and corn, an adjustment is made for the mass of the plants themselves, as these crops form a large vegetative mass.
3. To equalize lighting and heating conditions, the containers are regularly rearranged: the outer ones are moved to the center, and the central ones to the edges.

Any changes in the state of plants during the growing season are recorded in a workbook separately for each container. This allows tracking the dynamics of the development of experimental crops and noticing physiological deviations in time. Final harvesting is carried out strictly upon plant maturation.

• Dates of onset of growing season phases;
• Morphological changes in plants;
• Agrochemical indicators and biological activity of the soil.