Straw

Scientific basis for using straw as organic fertilizer

Scientific prerequisites for using straw as fertilizer:

1. Straw is a source of nutrients. The chemical composition of straw varies depending on soil and weather conditions. On average, at 16% moisture it contains: 0.5% nitrogen, 0.25% phosphorus (P₂O₅), 0.8–1.0% potassium (K₂O), 35–40% carbon, as well as sulfur, calcium, magnesium, boron, copper, manganese, molybdenum, zinc, and cobalt.

At average cereal yields of 2.0–3.0 t/ha, 10–15 kg of nitrogen, 5–8 kg of phosphorus (P₂O₅), and 18–24 kg of potassium (K₂O) are returned to the soil with straw.

2. Straw serves as an energy material for humus formation and for increasing the microbiological activity of the soil. The chemical composition of straw from cereal crops includes a large amount of nitrogen-free substances (cellulose, hemicellulose, lignin) with a small content of nitrogen and mineral elements. The C:N ratio (70–80:1) in straw affects its decomposition in the soil. Straw provides soil microflora with available carbon, but cellulose-decomposing microorganisms have a high demand for nitrogen; therefore, given its small amount in straw, microorganisms consume mineral soil nitrogen, that is, nitrogen immobilization occurs. With a nitrogen deficiency, straw decomposition processes are inhibited. For normal straw decomposition, the C:N ratio should be 20–30:1.

The effectiveness of straw fertilization increases with additional nitrogen application. A comparative assessment of fertilization with straw plus additional nitrogen compensation and with manure shows their equal effectiveness. It is important that with the applied straw and nitrogen the C:N ratio reaches 20:1. For this, when plowing in straw, an additional 0.5–1.5% nitrogen by its mass is applied, or 5–15 kg N per 1 t of straw from mineral or organic fertilizers.

3. When straw is composted under aerobic conditions, the humus yield is 7.9%; with the addition of mineral nitrogen, it is 8.5% of the straw mass. The most intensive humus formation occurs in the first 4 months of composting, during the decomposition of cellulose and hemicellulose. Moreover, humus accumulates in maximum quantities during the period of the highest microbial population.

In combination with mineral fertilizer, liquid manure, or leguminous crops used as green manures, straw is not inferior in its effect on humus content to an equivalent amount of manure.

4. Straw for fertilization helps improve the physicochemical properties of the soil, reduces nitrogen losses, increases phosphate availability and the biological activity of the soil, and improves conditions of plant nutrition. The positive effect of straw is possible when favorable conditions for decomposition are created. For example, the rate of microbial decomposition of straw depends on the availability of nutrient sources, their number, species composition and activity, soil type, cultivation level, temperature, moisture, and aeration. Thus, straw decomposition increases with the application of nitrogen, phosphorus, manganese, molybdenum, boron, and copper.

The intensity of fiber decomposition increases from sod-podzolic soils to gray forest soils and chernozems. The optimal temperature for fiber decomposition is 28–30 °C at soil moisture of 60–70% of full water-holding capacity. Decomposition intensity in the upper soil layer is higher due to good aeration, large numbers, and diversity of microbial species.

Straw enhances the nitrogen-fixing capacity and enzymatic activity of the soil.

5. Often in the first year after straw application, the yield of cereal crops decreases due to toxic compounds contained in and formed during decomposition, as well as due to worsened nitrogen nutrition of plants.

Straw fertilization is of special importance for leguminous crops. The effectiveness of straw increases when legume seeds are treated with nitragin; therefore, on areas fertilized with straw, they try first to place legumes or row crops. Straw applied in advance stimulates the nitrogen-fixing capacity of legumes and increases their yield. Nitrogen nutrition of row crops is provided as a result of mobilization of soil nitrogen during inter-row cultivation.

6. Nitrogen from mineral fertilizers reduces the depressive effect of straw on cereal crops. Nitrogen from mineral fertilizers immobilized in the presence of straw is more mobile, less resistant to acid hydrolysis, and mineralizes faster than nitrogen immobilized without straw, especially humus nitrogen. In aftereffect, straw intensifies nitrogen mobilization processes and increases plant use of both immobilized and soil nitrogen, which determines a positive effect on the yield of subsequent crops.

Methods of straw application

1. Chopped and spread straw over the field is plowed in in autumn during fall plowing or in spring in areas with sufficient moisture. This technique can be combined with green manure, which makes it possible to exclude the application of mineral nitrogen fertilizer, and also creates favorable conditions for humus formation after plowing in.

2. On soils of heavy texture and in wet climatic conditions, spread straw is not plowed in but incorporated superficially with stubble cultivators, disk harrows, or rotary tillers. This method of incorporation gives a better effect compared with plowing in with a plow. Where possible, after surface incorporation, an intermediate catch crop, preferably a leguminous crop, is sown.

3. Straw is also used as a mulching material to combat water and wind soil erosion. Mulching creates favorable conditions for water infiltration into the soil, reduces and sometimes completely eliminates the risk of surface runoff, promotes uniform distribution of water over the soil surface, improves the structure of the arable layer, and reduces moisture evaporation.

4. Leaving stubble and straw when conventional tillage is replaced by moldboardless tillage reduces wind speed above the soil surface by 40–60%, decreasing the risk of wind erosion; therefore, in zones subject to wind erosion, soil cultivation is carried out moldboardless without incorporating straw.

5. On areas fertilized with straw, they try first to place legumes or row crops. When sowing cereal crops on these areas, nitrogen fertilizers are applied at a rate of 8–10 kg nitrogen per 1 t of straw. Nitrogen applied with straw is not taken into account in the mineral fertilizer balance, since it is included in the general soil nitrogen turnover and plays a role only with systematic application of straw as fertilizer in crop rotation.

Plowing in straw with added nitrogen is more effective in autumn, because phenolic compounds toxic to plants formed during decomposition are leached and decomposed from the root-inhabited layer during the autumn-winter-spring period.

Application of straw with added nitrogen is highly effective under row crops with a long growing season; with systematic use in crop rotations, its effectiveness increases over time: yield increases of crop rotation crops from 0.1 t/ha feed units increase to 0.2–0.3 t/ha for each tonne of straw.

According to G. E. Merzlaya’s generalization of long-term experiments at VNIIUA, straw, when nutrient doses are equalized with mineral fertilizers, is equal to manure in its effect on crop yields and soil fertility. For example, on a thick low-humus chernozem in experiments of Drabov Experimental Station, when nutrient doses were equalized for straw and manure under sugar beet, the yield was 40.8 and 40.5 t/ha, whereas with application of only straw at 4–6 t/ha it was 35.7 t/ha, with addition of 90 kg/ha nitrogen to straw it was 37.9 t/ha, and in the unfertilized control it was 33.5 t/ha.

In studies of Sumy Experimental Station on chernozem, when straw, bedding manure, and beddingless manure were applied under maize in doses equivalent in nutrients, the green mass yields were respectively 54.5 t/ha, 52.9 t/ha, and 53.2 t/ha; in the unfertilized control, 41.4 t/ha.

In experiments of Krasnodar RIA on leached chernozem, winter wheat yield with application of 5 t/ha straw was 2.66 t/ha, with 5 t/ha straw and N₅₀ it was 3.20 t/ha, and in the unfertilized control it was 2.80 t/ha.

On typical micellar-carbonate chernozem in experiments of Stavropol RIA, the yield of winter wheat in the background treatment was 2.89 t/ha, background + 10 t/ha straw was 3.00 t/ha, and background + 10 t/ha straw + N₂0 was 3.13 t/ha.

Systematic application of straw increases its effectiveness, and nitrogen deficiency manifests only in the first years. In subsequent years, more nitrogen is released than fixed, so the aftereffect of straw is observed even without additional nitrogen application.

Application techniques and methods of straw incorporation

Mineral nitrogen fertilizers can be replaced by beddingless liquid manure at a rate of 6–8 t per 1 t of straw. Such a combination has an effect similar to bedding manure.

On straw left evenly across the field after a combine harvester, one can apply semi-liquid manure, liquid manure, manure slurry, effluents, or other organic fertilizers at a rate of 15–20 kg/ha nitrogen with incorporation by a stubble cultivator or disks to a depth of 6–8 cm. In this case, its decomposition accelerates and is not accompanied by accumulation of toxic substances. Main soil tillage to the adopted depth is carried out at the usual times for the specific zone.

Effectiveness of straw application

The effectiveness of using straw as fertilizer with the addition of mineral nitrogen or in combination with beddingless manure or green manure has been confirmed in many soil and climatic conditions. Thus, in Belarus on typical sod-podzolic, strongly podzolized, light loamy, and light sandy loam soils, separate application of 3 t/ha chopped straw and 27 t/ha liquid manure had the same effect on the yield of crops of a rotation link (potato, barley, perennial grasses) as 30 t/ha bedding manure.

The use of straw as fertilizer in the world is significantly greater than in Russia. For example, the share of straw in the total volume of organic fertilizers in Germany currently is: under sugar beet — 72%, under wheat — 71%, under winter barley — 58%, whereas in the early 1970s this indicator did not exceed 15–25%.

References

1. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. Mineev V. G. Moscow: D. N. Pryanishnikov VNIIA Publishing House.

2. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. B. A. Yagodin. Moscow: Kolos.