Strategies for maintaining a non-deficit humus balance in arable soil

Humus determines soil fertility and directly affects crop yield. Under natural conditions, its content is stable, but with ploughing and intensive land use, this balance is inevitably disrupted. To preserve the potential of a field and prevent land depletion, an agronomist must create conditions for a non-deficit balance of organic matter.

Why arable land loses humus and how to compensate for the losses

Humus losses in cultivated soils occur continuously. Intensive tillage accelerates oxygen access to deep layers, triggering the rapid decomposition of organic matter. Application of mineral fertilizers">Application of mineral fertilizers also activates soil microflora, which begins to intensely decompose humus to ensure its own vital activity.

The main factors for the reduction of humus stocks in the soil are:

• a decrease in the amount of incoming plant residues when replacing a natural biocenosis with an agrocenosis;
• acceleration of the mineralization of organic matter due to frequent mechanical tillage and increased aeration;
• decomposition and biodegradation of humus under the influence of physiologically acidic fertilizers;
• increased mineralization during drainage or irrigation measures;
• development of water and wind soil erosion.

To replenish losses of organic matter, a complex of agricultural practices is used in practice. Their choice depends on the specialization of the farm and the availability of resources. The main task is to ensure a constant influx of fresh organic matter into the soil.

• application of organic fertilizers in pure form or in combination with mineral fertilizers;
• ploughing in green manures (siderates), as well as the conservation of post-harvest and root residues;
• inclusion of grain legumes and legume-grass mixtures with a predominance of the legume component in crop rotation;
• incorporation of chopped straw in combination with nitrogen fertilizers;
• use of various organic wastes of agricultural origin.

The rate of destruction (mineralization) of humus is not uniform. It depends directly on the particle size distribution of the soil, as well as on the crops grown and their cultivation technology. On light sandy soils, organic matter decomposes faster, and in bare fallow, losses reach their maximum.

• Mineralization of fresh organic matter over 2 years — 70–80%
• Humification of fresh organic matter — 20–30%
• Annual loss of humus stocks — 1.5–2.0%
• Nitrogen content in humus — about 5%
• Humification coefficient of manure (at 25–50% dry matter) — 20–25%

How to calculate the humus balance and application rates of organic matter

Humus consumption is closely linked to the nitrogen nutrition of plants. If the nitrogen balance in the field is negative, crops begin to compensate for the deficit at the expense of soil organic matter. The total amount of loss depends on the type of plants and their productivity.

For accurate planning of the organic application rate, it is necessary to calculate the volume of humus destroyed per season. This can be done using a special formula for nitrogen removal:

Hm = (Ym · KN + Ym · KR · KNP) · 0.6 · 20

where:

• Hm — quantity of mineralized humus, centners/ha;
• Ym — yield of main product, centners/ha;
• KN — nitrogen removal per 1 centner of main product and the corresponding amount of by-product, kg;
• KR — coefficient of plant residue yield in relation to the main product;
• KNP — nitrogen removal per 1 centner of plant residues, kg;
• 0.6 — coefficient of soil nitrogen removal in relation to the total nitrogen removal by plants;
• 20 — coefficient of conversion of nitrogen into humus.

All necessary reference data for calculations are contained in regional agrochemical guidelines. They take into account the specifics of local soils and climate.

In parallel with decomposition, the process of humification occurs in the soil — the transformation of fresh organic residues into humus substances. Every year, thanks to root and post-harvest residues, humus stocks are partially replenished. Cereals return 0.4–0.6 t/ha of humus to the soil, row crops — 0.2–0.3 t/ha, and perennial grasses — 0.5–1.0 t/ha. The humification coefficient of cereal and perennial grass residues is equal to the humification coefficient of bedded manure, while for row crops it is half that amount.

For long-term planning, it is important to understand the dynamics of fertility for several years ahead. Expected humus stocks over a full crop rotation can be projected using the organic matter accumulation formula:

St = (S0 + Kг · A · t) · (1 – Км)

where:

• St — humus reserves after t years, t/ha;
• S0 — initial humus reserves, t/ha;
• Кг — humification coefficient of fresh organic matter in fractions of a unit (A is taken as a unit);
• А — quantity of fresh organic matter entering the soil, t/ha;
• t — forecast period, years;
• Км — humus mineralization coefficient in fractions of a unit.

Humus accumulation increases the share of its labile fraction in the soil. This active humus is capable of intensive mineralization. It is precisely this that is consumed for the current plant nutrition during the growing season.

To maintain a stable non-deficit balance, regular compensatory doses of manure are required. Its volumes vary significantly depending on the soil-climatic zone. Estimated application rates are presented below.