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Agrochemistry

Fertilizer system and agrotechnics of peas and grain legumes for high yields

For students

6 min read

AGROCHEMISTRY A

Unlocking the potential of peas: focusing on nitrogen fixation

Peas produce maximum yields on humus-rich chernozems, loams, and sandy loams with a neutral soil reaction. Heavy, excessively dense, acidic, and waterlogged soils are unsuitable for this crop. The main task of an agronomist when growing peas is to create optimal conditions for the activity of nodule bacteria. Through symbiosis, the plant is capable of meeting up to 75% of its nitrogen demand, but if conditions deteriorate, this figure drops below 45%.

Symbiotic nitrogen fixation begins at the two-to-three-leaf stage, peaks during the budding and early flowering period, and practically ceases by the grain filling stage. For the process to be as active as possible, the bacteria require good aeration, sufficient moisture, nutrients, active photosynthesis, and the presence of boron and molybdenum.

When planning a nutrition program, aim for the target yield and nutrient removal by pea plants:

Nutrient Consumption per 1 centner of grain and the corresponding amount of straw, kg
Nitrogen4.5–6.0
Phosphorus1.7–2.0
Potassium3.5–4.0
Calcium2.5–3.0
Magnesium0.8–1.3
  • Optimal salt pH for nitrogen fixation — 5.6–6.0
  • Base fertilizer application rate — N20–40Р40–60К0–60
  • Starter dose at sowing — Р20
  • Soil molybdenum deficiency threshold — less than 0.3 mg/kg

For an effective crop start and to ensure its nitrogen supply, follow the technological sequence of treatments:

  1. Before sowing, treat the seed with rhizotorfin and molybdenum.
  2. At sowing, apply molybdenum-enriched granulated superphosphate in the rows at a rate of Р20.
  3. Two weeks after the beginning of flowering, perform nitrogen top dressing to increase the protein content in the grain.

If molybdenum-enriched superphosphate is unavailable, be sure to treat pea seed before sowing with a 0.5% aqueous solution of ammonium molybdate using a semi-dry method. Molybdenum deficiency sharply reduces the activity of nodule bacteria.

Soybean nutrition: critical periods and deficiency diagnostics

Soybean is demanding regarding soil structure and drainage. It performs best on humus-rich chernozems with a slightly acidic reaction, as well as on yellow earths and brown forest soils. This crop is able to develop the bulk of its roots in the arable layer when groundwater is near, but waterlogged and saline areas without land reclamation should not be allocated for it.

Soybean consumes significantly more nutrients to form a harvest than other crops. The demand for elements is distributed unevenly throughout the growing season phases:

Nutrient Application rate per 1 centner of seed, kg
Nitrogen 7.7–10.0
Phosphorus 1.7–4.0
Potassium 3.2–4.0

From emergence to flowering, soybean assimilates only a small fraction of the total required nutrients:

  • Nitrogen — 6–16%
  • Phosphorus — 8.4–12.3%
  • Potassium — 9.0–23.8%
  • Calcium — 10.0–11.0%
  • Magnesium — 6.0–8.0%

The bulk of nutrients is assimilated from the start of formation to grain filling. Soybean absorbs the maximum amount of nitrogen and phosphorus during flowering and pod formation, potassium — 87–95 days after emergence, calcium — on the 70–80th day, magnesium — on the 73–80th day, and sulfur — during the pod formation phase.

The critical period for nitrogen nutrition in soybean is 2–3 weeks before the start of flowering and 2 weeks after it. Nitrogen deficiency at this time irreversibly reduces yield, and subsequent nitrogen top dressing will not be able to correct the situation.

Nitrogen starvation in soybean can be identified visually at the trifoliate leaf emergence phase. With nitrogen deficiency, leaves remain small: the first leaf takes on a uniform light green color, and the second — an uneven yellow-green. All subsequent new leaves will also grow yellow-green and mottled.

Soybean is capable of self-supplying nitrogen by 50–70% through symbiosis with nodule bacteria. However, an excess of mineral nitrogen in the soil suppresses nitrogen fixation, so nitrogen should always be less than phosphorus in fertilizer compositions. The crop itself effectively absorbs phosphorus even from poorly soluble compounds, and potash fertilizers only require application on potassium-poor soils.

When planning soybean yields, focus on the field's moisture availability. Under irrigation or with sufficient precipitation, the return on applied nutrition increases 2–4 times. A severe lack of moisture during the growing season minimizes the effectiveness of fertilizers.

Monitor seeding density. Excessive growth of vegetative mass at the beginning of the season creates strong shading and triggers moisture deficiency. As a result, the plants drop flowers and fruit set, which leads to a sharp decrease in yield.

Fertilizer application schemes and pre-sowing treatment

To start the growing season, soybean requires small doses of nitrogen. Applying N20–30 at sowing completely meets the needs of the seedlings during the initial growth phases. Moreover, such a starter dose does not harm the formation of bacterial nodules on the roots.

  • Nitrogen requirement met through atmospheric fixation — 50–70%
  • Starter nitrogen dose at sowing — N20–30
  • Yield increase on chernozem from the full scheme — 4.7 c/ha
  • Additional protein harvest — 380 kg/ha
  • Molybdenum consumption per hectare seed rate — 25–50 g
Field conditions Fertilizer scheme and doses Agronomic effect
Ordinary chernozem Application of P60 before autumn ploughing, N20 before pre-sowing cultivation, and seed inoculation with rhizotorphin or nitragin Average yield increase over 3 years was 4.7 c/ha, with an additional protein harvest of 380 kg/ha achieved
Medium and low content of mobile phosphorus and mineral nitrogen Application of N30P40–60 during primary tillage or localized sowing application of N30P30 with top dressing of N30 Top dressing is carried out during the last inter-row cultivation based on leaf diagnostics or nodule development
Fertile soils No mineral fertilizers applied Plants develop through natural soil nutrients
  1. Before sowing, treat the seed with rhizotorphin and perform molybdenum application — moisten them with an ammonium molybdate solution at a rate of 25–50 g of molybdenum per hectare sowing rate.
  2. Apply a starter nitrogen dose of N20–30 (or N30P30 locally) at sowing to support seedlings in early phases without hindering nodule development.
  3. Determine the need for top dressing using diagnostics with an OP-2 device or by assessing nodule development on the roots.
  4. Perform nitrogen top dressing at a dose of N30 during the last inter-row cultivation if tests confirm a nutrient deficiency.

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