Pea Fertility Requirements
Understanding the fertilizer requirements of field pea is critical to reaching optimum yields. To achieve 50 bushels of pea seed per acre, pea crops (including vines and pods) will require approximately:
- 150 lb. (N) Nitrogen
- 45 lb. (P) phosphate as P2O5
- 140 lb. (K) potash as K2O
- 13 lb. (S) sulphur
- over 100 lb. (Ca) calcium
- 15 lb. (Mg) magnesium
Field pea has the ability to fix nitrogen from the air. The process works like this:
- pea forms a symbiotic relationship with specific bacteria, which live in association with plant roots
- the bacteria infect the plant roots and form nodules
- the bacteria use nutrients from the plant and provide nitrogen to the plant in return
For this reason, most of the nitrogen required by pea can be provided from the soil and fixation. This can greatly reduce and often eliminate the need to add N fertilizer. Inoculation with the proper strain of rhizobium bacteria is essential to ensure fixation.
Soil tests are also important in deciding whether or not N fertilizer should be applied:
- generally, if soil tests are above 40 lb. N/ac. in the 0 to 24 inch depth, no additional N fertilizer is required
- in soils testing less than 20 lb. N/ac., a small amount of starter N may be beneficial
- in a cold/wet or hot/dry spring, when nodules are slow to develop, plants may be unable to obtain sufficient N from the soil, resulting in a nitrogen deficiency
With pea and nitrogen, it’s possible to have too much of a good thing. Excess N fertilizer will reduce the amount of N fixed by a pea crop, delay crop maturity, increase disease levels and reduce standability.
Mid-season N applications are normally not recommended. An exception would be under conditions of failed inoculation and obvious N deficiency.
Adequate levels of phosphorus are critical for optimum yield and early maturity. Phosphorus deficiency restricts top and root growth, resulting in spindly stems with fewer branches.
Phosphorus moves poorly in soil, so it should be placed near the seed. On the other hand, germination and emergence can be reduced if too much phosphate is placed with the seed.
- recent field pea research in Alberta has found that up to 30 lb./ac. seed-placed P2O5 has not reduced germination or emergence – in fact, under very good seedbed soil moisture conditions, even higher rates of seed-placed phosphate are safe
- this research has also shown that although phosphorus is a limiting factor in many Alberta soils, build-up of soil phosphorus tends to raise available soil phosphorus levels and phosphorus fertilizer responses are often not dramatic
Pea crops generally need more potassium than cereal crops and often almost as much potassium as they do nitrogen. Only 20 to 25 per cent of the plant potassium is in the seed, however. The rest is in the leaves and stems and is normally returned to the soil.
Many Alberta soils are medium to high in exchangeable potassium, often ranging from 400 to 1000 lb. of potassium/ac. in the 0 to 6 inch depth of soil. Potassium deficiencies are most likely to occur on sandy soils that are intensively cropped or on Grey-Black transition soils and Grey Wooded soils.
- potassium fertilizer is not required when soil tests show greater than 300 lb. potassium/ac.
- banding or seed-placing potassium are the most efficient methods of application
- because large amounts of seed-placed potassium with pea crops may reduce germination and emergence, it may be best to either band it before seeding or sideband it at the time of seeding (this is especially true if phosphorus and sulphur fertilizers are also being applied)
Pea crops have a reasonably high sulphur requirement. Much of the topsoil sulphur is contained in soil organic matter. This is slowly released as sulphate-sulphur (SO4-S), the form of sulphur that plants require. Sulphate-sulphur is similar to nitrate-nitrogen in that both are mobile in soil.
Some soils are deficient in plant-available sulphur in the topsoil but have enough sulphur in the subsoil to meet crop requirements.
- in wetter, cooler conditions, plants may suffer from a lack of sulphur before plant roots grow down into the subsoil containing sulphur
- sulphur deficiencies are frequently a problem in the Black and Grey Wooded soil areas of Alberta and occasionally a problem in the Brown and Dark Brown soil areas
- for testing purposes, soil samples should be taken from the 0–6, 6–12 and 12–24 inch depths to determine the amounts of sulphur at each depth
- if sulphur is required, apply a sulphate containing fertilizer such as ammonium sulphate (21-0-0-24)
- elemental sulphur fertilizer won’t be available to the plant in the year it is applied – elemental sulphur is best used in a longer term program to build soil sulphur levels
Producers can build up micronutrient fertility levels that are low or deficient in much the same way that the better producers build up and maintain adequate sulphur levels for canola production.
To achieve a 50-bushel yield of pea, the micronutrients needed are:
- 0.06 lb. (B) boron
- 0.09 lb. (Cu) copper
- 0.63 lb. (Fe) iron
- 0.45 lb. (Mn) manganese
- 0.09 lb. (Zn) zinc
- trace amounts of (Mo) molybdenum
- field pea can tolerate what is considered low or deficient Cu, Zn or B levels
Note that pea is not grown continuously on the same land – other rotation crops such as flax, wheat, canola and barley may respond optimally to the topped up levels of these minerals
- the three remaining micro-nutrients – iron, manganese and molybdenum – have a much more critical effect
- most Alberta soils are adequate for iron but high pH soils or alkaline soils may lock up manganese availability so that a foliar application of this micronutrient may be necessary
- on the other hand, molybdenum becomes much less available in acidic soils (below pH 6.5) especially at pH 5.5 or less
- molybdenum is absolutely essential in the nitrogen fixation process in legumes – without it, no nitrogen can be fixed (in Europe, producers may apply 200 to 300 grams of actual molybdenum to the seed crop or soil every few years or lime the soil to bring up the pH and release more molybdenum)
- based on soil test results, micro-nutrient fertilizer should be applied in test strips the first year. Sandy, low organic matter may show best response
- For more information, recommended soil micronutrient levels are tabulated in the fact sheet: Minerals for Plants, Animals and Man, Agdex 531-3 and Micronutrient Requirements of Crops, Agdex 531-1.