The influence of crop residues and root residues on soil fertility

Post-harvest and root residues are a constant and free source of organic matter for a field. Their actual quantity depends on the biological characteristics of the grown crops and the overall productivity of the crop rotation. Reasonable management of these residues helps the agronomist maintain soil fertility without additional costs for purchased organic matter.

Mineral and organic fertilizers significantly increase the accumulation of root and post-harvest mass. This effect is observed both on individual crops and throughout the entire crop rotation cycle. At the same time, the volume of above-ground residues strongly depends on the cutting height and the chosen harvesting method.

• Maximum root residues of clover with fertilizers — 39.1 centners/ha
• Minimum post-harvest residues of potato without fertilizers — 1.1 centners/ha
• Nitrogen accumulation by clover with fertilizers — 97.78 kg/ha
• Nitrogen accumulation by potato without fertilizers — 15.68 kg/ha

By chemical composition, crop residues are far from equivalent. The highest concentration of nitrogen and phosphorus is contained in the residues of perennial legume grasses, as well as in potato roots. Fertilizers not only increase total biomass but also raise the percentage nitrogen content, phosphorus, and potassium in plant tissues.

Green manure: how cover cropping works in practice

Cover cropping (sideration) is a classic technique for enriching soil with organic matter by growing and subsequently ploughing in fresh green mass. Unlike straw or humus, cover cropping involves incorporating succulent, non-decayed plant biomass into the soil. It contains readily available sugars, starch, proteins, nitrogen, as well as an actively functioning root system.

In global agriculture, cover cropping has been used for about 3,000 years, starting from ancient China and India, where today more than 10 million hectares are allocated to it. The high value of this technology was noted in sources dating back to 79 AD, pointing out the efficiency of lupine incorporation. In Europe, this method began to spread from the 16th century in France and Spain, and from the end of the 18th century in Germany. In the first domestic experiments with lupine as a green manure, conducted from 1881 to 1905, as well as in 1888, the positive impact of cover crops on the yield of subsequent crops was proven.

1. Growing a cover crop until vegetative mass accumulates.
2. Maceration and incorporation of succulent green biomass into the soil strictly before pod formation.
3. Ploughing in the root system of plants that is still functioning at the time of soil tillage.

Nutritional value of cover crops and mineralization management

Legume cover crops work in the field as an effective and accessible nitrogen source. Nodule bacteria on their roots fix nitrogen from the air, transferring it into the soil in organic form. The green mass of lupine or sweet clover is not inferior in nutritional value to traditional manure, allowing for the compensation of organic matter deficits without costs for the purchase and transport of external fertilizers.

• Green mass yield of legumes — up to 50 t/ha
• Nitrogen content in biomass — up to 200 kg/ha
• Nitrogen equivalent — 1 t of green manure = 1 t of manure

Nitrogen in the ploughed-in green mass is initially present in the form of proteins. Soil microflora gradually mineralizes them: first, the process of ammonification occurs, followed by nitrification, which converts nitrogen into mineral forms available to plants. The speed of this process can be managed, adapting it to the needs of a specific crop in the crop rotation.

Impact on soil properties and humus balance

The root system of leguminous green manures penetrates deep into the soil, extracting potassium, phosphorus, and calcium from lower horizons and poorly soluble compounds. Upon ploughing, these elements are concentrated in the plough layer in an easily accessible form. Additionally, green manure reduces soil acidity, decreases the mobility of aluminum, which is toxic to plants, and improves buffering capacity and cation exchange capacity.

Incorporation of green manure mass fundamentally changes the physical properties of the soil. It loosens the plough layer, reduces its bulk density, which helps to eliminate the consequences of soil compaction by heavy machinery. As a result, water permeability and water-holding capacity increase, surface runoff of precipitation is reduced, and soil air is enriched with carbon dioxide, which accelerates the dissolution of soil phosphates.

Cultivation of row crops and cereal crops inevitably leads to losses of soil organic matter. A positive humus balance is ensured only by annual and perennial leguminous grasses. To maintain a non-deficit balance, it is necessary to combine green manuring with the application of straw, manure, or composts.

To accurately calculate humus accumulation, humification coefficients of various organic types are used. Straw has the greatest accumulation potential, while green manure works mainly for the rapid activation of biological processes and plant nutrition of the current season. This allows for planning a long-term nutrient balance when designing crop rotation.

Knowing the volume of organic matter depletion by a crop, the field's fertilizer requirements can be accurately calculated. For example, maize consumes about 2.2 t/ha of humus per season. These losses can be compensated for by applying litter or liquid manure, as well as by incorporating green manure mass.

• Humus consumption by maize — 2.2 t/ha
• Litter manure application rate (2.2:0.20 ratio) — at least 11 t/ha
• Liquid manure application rate (up to 4% dry matter) — 110 t/ha
• Green manure mass consumption (2.2:0.06 or 2.2:0.04 ratio) — 37–55 t/ha
• Organic matter accumulation by perennial lupine — up to 2.2 t/ha

Primarily, leguminous crops are cultivated as green manure: lupine, sweet clover, vetch, grass pea, sainfoin, serradella, and forage peas. In some cases, non-leguminous plants are used—mustard, buckwheat, amaranth, or mixtures of legumes with cereals. The effect of green manures on humus accumulation depends on the technology: whether the entire above-ground mass is ploughed in on site, only stubble and root residues are incorporated, or if green manure is applied together with straw. Thus, perennial grasses (in particular, perennial lupine), even under cereal cover and during the stubble period, accumulate up to 2.2 t/ha of organic matter.

Incorporation timing and methods for integrating green manures into crop rotation

The timing of ploughing in green manures directly affects nitrogen and humus accumulation in the soil. In soils light in texture, summer incorporation before winter crops leads to excessively rapid biomass decomposition. Spring ploughing is preferable under such conditions, as it creates better conditions for preserving nitrogen and organic matter.

Non-legume green manures (rapeseed, mustard, winter cress) effectively protect the soil from nitrate leaching during the autumn period. However, mineral nitrogen must be applied additionally for them, as well as for the subsequent crop. The choice of a specific cultivation method for green manure crops depends on soil-climatic and management conditions.

Green manure sowings are divided into independent and intermediate:

• Independent sowings. The field is occupied solely by a green manure crop, the green mass of which is ploughed in completely. This practice leads to a loss of the commercial harvest in the current season, therefore it is used mainly on low-fertility soils.
• Intermediate sowings. Green manures are grown during the time free from main crops — before their sowing or after harvesting. Depending on the sowing dates, intermediate crops are divided into undersown, post-harvest, and stubble crops.

Undersown green manures are sown under the cover of the preceding main food crop. The plant develops in the lower tier without interfering with the main one, which reduces the growing season of the green manure in that area. After harvesting the cover crop, the green manure rapidly accumulates biomass and is then incorporated into the soil. This method is most effective in regions where the period between harvesting the predecessor and sowing the subsequent crop is too short.