Methodology for calculating the balance of nutrients and soil humus

It is impossible to control the nutrient cycle on a farm without preparing a balance sheet. The balance shows how many nutrients have entered the soil and how many have been consumed to produce a harvest or lost as a result of leaching and erosion. Regular calculation of this indicator allows the agronomist to precisely adjust the application rates, forms, and types of fertilizers for a specific field.

• Nitrogen from precipitation — 5 kg/ha per year
• Nitrogen losses from erosion — 10–15 kg/ha per year
• Phosphorus losses from erosion — 5 kg/ha per year
• Potassium losses from erosion — 100 kg/ha per year

How to calculate the incoming part of the balance

The input of nutrients consists of several sources. First of all, mineral and organic fertilizers are taken into account. Their volumes are taken from field history logs, field passports, and application reports. The rates are calculated per 1 hectare of crop rotation area, and the content of elements in organic matter is determined by laboratory analyses of the farm or reference data.

The second source is seed and planting material. The input of nutrients here is calculated directly through the sowing rate and the average concentration of substances in them. Additionally, atmospheric precipitation is taken into account. Practically all elements enter the soil with rain and snow, but nitrogen (on average 5 kg/ha per year, mainly in ammonium form) enters in the largest amounts.

Special attention is paid to nitrogen fixed by grain legumes. Its amount in the soil is calculated using the biological nitrogen accumulation formula. The calculation is based on an assessment of the mass of crop residues, the magnitude of harvest losses, and fixation coefficients:

N_enr = (M_cr × 2.5% × N) + (M_hl × % × N) × C_f − N_rem × (1 − C_f)

Or according to the simplified version: N_enr = N_b − N_rem, where:

• N_enr (or N_enr) — soil enrichment with biological nitrogen, kg/ha;
• N_b — biological nitrogen of crop residues, kg/ha (calculated as: N_b = [(M_cr × 2.5) × %N + (M_hl × %N)] × C_f);
• N_rem — nitrogen removal from the soil by legumes, kg/ha (calculated as: N_rem = N_h × (1 − C_f));
• M_cr — mass of dry stubble and root residues, c/ha;
• M_hl — mass of harvest losses for all cuts, c/ha;
• N_h — total nitrogen in the hay harvest for all years of legume cultivation, kg/ha;
• C_f — nitrogen fixation coefficient (the ratio of fixed nitrogen to total);
• 2.5 — adjustment coefficient for the completeness of organic matter recording.

The nitrogen fixation coefficient (C_f) depends on the crop and the type of residues. For clover, lupine, and sainfoin, it is equal to 0.7; for alfalfa — 0.8; for peas and vetch — 0.6. For stubble residues of grain legumes, this coefficient is 0.3–0.4, and for residues of leguminous grasses — 0.5–0.7.

Nitrogen is also accumulated in the soil by free-living microorganisms. The volume of this non-symbiotic fixation depends on the soil-climatic zone. The calculations take into account the natural activity of soil biota on specific soil types.

When calculating the nitrogen balance in long-term trials, minimum standards of non-symbiotic fixation are used. They directly depend on the level of field saturation with fertilizers and liming materials. This allows adjusting planned indicators taking into account the actual agrochemical load.

How to account for unproductive losses and removal by harvest

The expenditure part of the balance consists of nutrient removal by plants and inevitable losses from the soil. The main expenditure item is the economic removal by the marketable part of the harvest and by-products. It is extremely variable and depends on the cultivar, weather, and nutrition level. For calculations, standard removal indicators per 1 ton of product approved for a specific region or determined experimentally on the farm are used.

The second expenditure item is the leaching of nutrients from the root-inhabiting layer with downward water currents. The intensity of this process depends on the soil texture. Light sandy soils lose nutrients significantly faster than loams.

The third expenditure item is losses from water and wind erosion. They cause irreversible destruction of the top, most fertile horizon. Every year, erosion removes a significant amount of nitrogen, phosphorus, and potassium from the topsoil, which must be compensated for when planning a fertilizer application system.

How to account for hidden losses: nitrogen volatilization, potassium leaching, and weeds

When calculating the balance of plant nutrition, one cannot be limited only to the direct removal of elements with the harvest. A portion of nitrogen and potassium is lost irreversibly due to gas volatilization, leaching by rain, and the "co-authorship" of weeds. Gaseous nitrogen losses from fertilizers account for at least 10–15% for broadcast crops and 15–20% for row crops. If you use organic matter, 10 to 15% of nitrogen will escape into the air, and in general, on arable land and pasture lands, these losses can fluctuate between 10 and 50% of the applied volume.

Soil mineralized nitrogen also evaporates, and the intensity of this process depends directly on the particle size distribution of the soil and the cultivated crop. Under row crops, where the soil is loosened more often and better aerated, these losses are significantly higher than under cereals. Precise guidelines for calculating these non-productive losses are given in the table below.

Besides nitrogen, potassium is actively leached from the topsoil. In heavy loamy soils, potassium losses with filtering waters are 10–20 kg/ha, while in light sandy and sandy loam soils, they increase to 40–60 kg/ha. Partially, this consumption is compensated by atmospheric precipitation, which brings an average of 2–6 kg/ha of potassium. It is also necessary to take into account weed vegetation: weeds take up about 10% of the volume of nitrogen, phosphorus, and potassium that is removed by cultivated plants.

• NPK consumption by weeds — 10% of crop removal
• Potassium input from precipitation — 2–6 kg/ha
• Potassium losses in loams — 10–20 kg/ha
• Potassium losses in sands and sandy loams — 40–60 kg/ha

Calculation methodology: types of balance and working formulas

In agronomic practice, the study of nutrient balance is conducted in two directions. The experimental direction solves theoretical issues, while the scientific-production one helps to calculate optimal fertilizer doses for the planned yield, taking into account soil fertility and environmental requirements. Depending on the scale and tasks, the balance is divided into three types: extra-farm, farm-level, and biological.

The extra-farm balance evaluates only the input of elements with fertilizers and their alienation with marketable crop and livestock production on the scale of a zone or the entire country. The biological balance provides the most complete picture of the cycle, taking into account the return of elements with harvest residues and root residues, as well as microbiological losses and leaching. For a specific field, the farm-level balance is most often used, which compares the removal by harvest and all types of losses with the fertilizer application, seeds, and input with precipitation. This balance can be gross (without considering element assimilation coefficients) and effective (with their consideration).

Mathematically, the balance of major elements is calculated using the following formulas:

• Nitrogen: B_N = (N_f + N_s + N_b) − (N_r + N_g + N_l), where N_f is fertilizer nitrogen, N_s is seed nitrogen, N_b is biological nitrogen (soil enrichment), N_r is removal by harvest (main and side products), N_g is gaseous losses, N_l is leaching losses.
• Phosphorus: B_P = (P_f + P_s) − (P_r + P_e), where P_f is fertilizer phosphorus (determined by content in fertilizers), P_s is seed phosphorus (by seeding rate), P_r is removal by harvest, P_e is erosion losses.
• Potassium: B_K = (K_f + K_s + K_p) − (K_r + K_e + K_l), where K_f is potassium from fertilizers and chemical soil conditioners, K_s is seed potassium, K_p is potassium from precipitation, K_r is removal by harvest, K_e is erosion losses, K_l is leaching.
• Meso- and micronutrients: in the input, the intake with mineral and organic fertilizers, soil conditioners, precipitation, irrigation water, and seeds is taken into account; in the expenditure — removal by harvest, losses from filtration and erosion.

To evaluate the quality of the balance, two key indicators are used — intensity and capacity. Intensity shows, as a percentage, which part of the nutrient removal with the harvest is compensated by their input. If this indicator is less than 100% (coefficient < 1), the balance is deficient; if it equals 100% (coefficient = 1) — it is balanced; if it is greater than 100% (coefficient > 1) — it is positive. The balance capacity represents the sum of the total removal and input of elements into the soil, characterizing the overall intensity of the nutrient cycle in the field.

The efficiency of fertilizer application cannot be assessed based on the results of a single season. To understand how the content of nutrients in the soil changes under the influence of harvests, it is necessary to calculate the balance in dynamics over a long period. Only multi-year balance calculations show the real proportional contribution of each nutrient source and help to adjust the fertilizer system in time.

Without long-term monitoring, the soil becomes depleted, even if you regularly apply mineral fertilizers. For example, long-term field studies have shown that the nutrient balance in the soil can remain acutely deficient. The situation is especially critical with potassium and nitrogen, when the removal of elements with the harvest significantly exceeds their return via fertilizers.

Hidden deficiency of meso- and microelements

Intensive farming inevitably disrupts the natural balance of elements in the soil. On one hand, the increasing productivity of modern crops raises their removal from the field. On the other hand, agronomists are increasingly switching to highly concentrated ballast-free mineral fertilizers, which contain practically no trace element impurities. As a result, nutrient deficiency grows, leading to a decrease in yield and a deterioration in product quality.

Data from long-term stationary experiments show that the use of only standard mineral fertilizers is not enough to ensure a positive balance of microelements. The removal of these substances by plants consistently exceeds their input. The following elements are in the risk group:

• molybdenum;
• boron;
• copper;
• manganese;
• cobalt;
• zinc.