The following points highlight the nine main nutrients required for irrigated groundnut production and also considered as the major plant nutrients for enhancing the crop productivity. The nutrients are: 1. Nitrogen 2. Phosphorus 3. Potassium 4. Balanced Fertiliser Use 5. Calcium and Sulphur 6. 6. K: Ca: Mg Ratio 7. Zinc 8. Boron 9. Iron.
Nutrient # 1. Nitrogen:
The necessity for fertiliser nitrogen to groundnut is reduced because of atmospheric nitrogen fixation root nodules. The contribution of biologically fixed nitrogen has not been adequately quantified. However, there are indications that around 200 kg N ha-1 can be fixed under ideal conditions, reducing the need for fertiliser nitrogen application.
Groundnut, as a member of the cowpea cross-inoculation group, is nodulated by a large number of diverse Rhizobial strains. However, not all Rhizobial strains are efficient in fixing nitrogen through symbiosis.
Rainfed groundnut is often subjected to soil moisture stress, which also adversely affects the efficiency of nitrogen fixation. This is one of the reasons why response of groundnut to applied nitrogen is not consistent. Efficient Rhizobial strains used in groundnut are NC 92, Tal 1000 and THA 205.
Rhizobium Inoculation Procedure:
Rhizobium or Bradyrhizobium inoculation is a cheap and effective way of providing nitrogen to legumes. Cultures of Bradyrhizobium like NC 92, IGR 6, IGR 40, TAL 1000 and TNAU 14 are recommended for groundnut.
Several factors influence the success of Rhizobium inoculation and efficiency of nitrogen fixation. Intercropping with cereals leads to reduction in nitrogen fixation by groundnut; deep sowing leading to elongated hypocotyl, poor rooting, poor nodulation and poor nitrogen fixation.
Bradyrhizobium population of 102 to 104 cells g-1 soil are usually detected in many soils. A minimum of 106 cells per seed are needed for successful field inoculation. Use of higher rates (106 to 108 cells per seed) for initial inoculation may help early establishment of inoculant strain.
Common inoculation procedures are:
Direct Seed Inoculation:
Seed inoculation with inoculum pasts:
Seeds are inoculated by direct coating with Rhizobium (106 cells per seed). Seed inoculation is planned 48 hrs prior to sowing. Before coating with Rhizobium, seeds may be moistened or paste of inoculum may be used. Carrier in the paste-like inoculum helps the Rhizobial cells to stick to the seeds. Jaggery or sugar solution (10-15%) may be used as inoculum carrier.
Mix the inoculum and seed thoroughly. It is better to add half the required inoculum to the seed, mix and then add the remaining half and mix again. This will ensure black specks on each seed, indicating the presence of inoculum.
The amount of inoculum to be used is usually indicated on the container of the commercial product. Nine ml of slurry containing 4.4 g of peat inoculum per kg seed has been suggested for seed of the size of soybean.
Pelleting:
In this method, appropriate Rhizobial culture is added to the seed at the factory or main distribution point. Seed is enclosed in a clump of lime or phosphate rock along with inoculum. This implies protection of the bacteria and seed, plus amelioration of adverse conditions in the soil adjacent to the seed.
Material used in this procedure is lime stone or rock phosphate as a base with stickers such as gum-arabic (15% water solution) and methyl-ethyl-cellulose (1% solution). The procedure involves inoculation of seed using gum-arabic (40%) or methyl-ethyl-cellulose (50%); while the seed is still moist tumble it in an appropriate container with the chosen coating material like rock-phosphate or fine limestone powder.
Field Inoculation:
Groundnut seed is often treated with fungicides, which may be toxic to Rhizobum, prior to sowing to control seedling diseases. Application of inoculum in the seed furrow in peat-based slurry form appears effective. Mixing of peat in water (0.7 g per liter) and pouring the mixture below the seed (4-5 ml per seed) into the furrow, to give a population of more than 106 rhizobial cells per seed appears effective.
Results of 200 trials in different agroecological zones in the country revealed that application above 22 kg N ha-1 is not beneficial. Results of 436 trials on cultivator’s fields in groundnut growing states revealed that rainfed groundnut responds to 30 kg N ha-1 if phosphorus and potassium are also applied along with nitrogen. Rainfed groundnut on black soils responded to 20 kg N ha-1 at different locations.
On shallow red soils of Rayalaseema, rainfed groundnut responded to 10 kg N ha-1 during drought years. However, under conditions of adequate rainfall, it responded to 30 kg N ha-1. Instances of groundnut responding to nitrogen up to 60 kg N ha-1 on light soils and up to 30-40 kg ha-1 on medium to deep soils are not uncommon.
From the results of experiments, it is clear that response of groundnut to nitrogen largely depends on soil type and distribution of rainfall during crop season. In general, 20 kg N ha-1 for rainfed groundnut and 30 kg N ha-1 for irrigated groundnut can be recommended for economic yields under most crop growing situations.
For a rainfed crop, entire dose of nitrogen should be applied by placement at seeding. A fertiseed drill could be effective for placement of fertiliser along with groundnut seed. For an irrigated crop, nitrogen may be applied in two equal splits at seeding and 30 days after seeding, especially for the crop on light soils with high infiltration rates to minimise leaching losses of applied fertiliser.
Nutrient # 2. Phosphorus:
The total amount of phosphorus uptake by groundnut plant is relatively small compared to nitrogen and potassium. On sandy loan soils of Tirupati (AP), 20 kg P2O5 ha-1 of available phosphorus in the soil was adequate for pod yield of 1.0 t ha-1 under rainfed conditions. Though, the phosphorus requirement is small, large quantity of fertiliser has to be applied as the efficiency of uptake from fertiliser is low.
From the results of 200 trials under different situations in the country, it is evident that rainfed groundnut responded to around 30 kg P2O5 ha-1. The responses, in general, are highest on red soils than on black soils. Simple fertiliser trails in the country revealed necessity for 40 kg P2O5 ha-1 for rainfed groundnut. There are also results indicating the need for 60 kg P2O5 ha-1 for irrigated groundnut.
From the results of experiments it is evident that there is necessity for addition of phosphorus from 20 to 60 kg P2O5 ha-1 for economic yields under different situation. In general, rainfed groundnut should be given 40 kg P2O5 ha-1, whereas irrigated crop needs 50 kg P2O5 ha-1, especially on red soils.
These recommended rates holds good provided recommended dose of nitrogen is applied. If only phosphorus is applied to groundnut, optimum doses are 30 and 40 kg P2O5 ha-1 for rainfed and irrigated crops respectively.
Most common source of phosphorus is single superphosphate (SSP), which also supplies calcium to groundnut. Response to applied phosphorus is usually evaluated based on phosphorus content of SSP, ignoring the effect of calcium and sulphur content of SSP.
On equal P2O5 basis, response to SSP is higher than that due to diammunium phosphate (DAP) due to calcium, sulphur and trace elements in SSP. For irrigated groundnut, DAP is the most common source of phosphorus, as it almost meets the needs of both nitrogen and phosphorus.
Entire recommended dose of phosphorus should be applied at sowing along with nitrogen by placement, preferably using fertiseed drill. Several studies indicate beneficial effect of foliar application of phosphorus on pad yield of groundnut. However, its implementation at field level is limited due to practical problems.
Nutrient # 3. Potassium:
Groundnut plant takes up potassium throughout the growth period. Though, its uptake is high, groundnut in general will not show any significant response to added potassium as most of the Indian soils are rich in potassium.
Response to its application can be expected only when the available potassium content is less than 150 kg K2O ha-1. There is a mutual antagonistic effect on the uptake of K, Ca and Mg. The ratio of K: Ca: Mg is more important than total amount of any of them. High concentration of potassium in fruiting zone is harmful, especially at low levels of Ca.
Results of the experiments in groundnut growing areas of the country indicated no response to applied potassium at several situations. Even if there is response, it is limited to 20-40 kg K2O ha-1 both under irrigated and rail fed conditions.
From the available information, there is no necessity for potassium application to rainfed groundnut yielding around 1.0 t ha-1. For an irrigated crop, a dose of 40-60 kg K2O ha-1 is necessary, provided nitrogen is applied at recommended rate.
On equal K2O basis, all potassium fertilisers are equally effective. Entire dose of potassium should be applied along with phosphorus at sowing, preferably using a fertiseed drill. Marginal increase in yield due to foliar application of potassium has been reported. However, it may not be a viable practice, especially under rainfed conditions.
Nutrient # 4. Balanced Fertiliser Use:
Application of all the three major nutrients in a balanced proportion increase the pod yield of groundnut both under rainfed and irrigated conditions. Combined application of 10 N, 20 P2O5 and 20 K2O kg ha-1 resulted in 150 per cent increase in pod yield of groundnut over control and significantly higher pod yield due to N, P and K applied singly or when only two of them were applied together on light and medium textured soils.
Experiments in farmers’ fields indicated that application of NPK in 1:2:2 or 1:2:3 ratio will be economical. Results of 436 trials conducted all over the country revealed that combined application of 30 N and 40 P2O5 double the yield of groundnut compared with 30 kg N ha-1 alone and addition of 20 kg K2O ha-1 to 30 N and 40 P2O5 improved the response considerably. Similarly, application of 20 N, 40 P2O5 and 40 K2O kg ha-1 was optimum for rainfed groundnut on sandy loam soils of Tirupati (AP).
Review of work done in India on response of groundnut to fertilisers for about two decades conclusively proved the need for balanced fertiliser use for realising economic groundnut yields. Table 9.11 givens recommended fertiliser schedules (ICAR) for different states.
Recommended fertiliser schedule for rainfed groundnut in Andhra Pradesh, at present is 20 N, 40 P2O5 and 50 K2O kg ha-1. For an irrigated, crop additional 10 kg N ha-1 is recommended as top dressing at 30 DAS.
Recommended fertiliser schedules are based on several experiments conducted under different agroecological conditions. It is important to note that earlier to the introduction of fertilisers as the source of nutrients supply to the crops, farmers depended on farm yard manure (FYM) as a source of nutrient supply. Farmers used to apply around 15 t ha-1 of FYM for groundnut crop once in 2 to 3 years.
The FYM, under rainfed conditions, besides supplying nutrients aids in improving water receptivity of soils, which is more important than supply of nutrients. Application of FYM alone used to give pod yields around 1.0 t ha-1, which is not much different from that due to fertilisers as a source of nutrient supply to groundnut.
It is not uncommon to observe farmers depending on FYM alone for groundnut cultivation, especially when the area is relatively small. As such, it is desirable to apply at least around 5 t ha-1 of FYM for increasing groundnut productivity besides improving soil health.
The traditional practice is to apply FYM only to groundnut crop in cropping systems such as groundnut – sorghum/pearlmillet/cotton/maize (2 or 3 years rotation) and grow the subsequent crops on the residual effect of farm yard manure applied to groundnut.
Nutrient # 5. Calcium and Sulphur:
These two nutrients are absorbed by pegs and developing pods and the common source of supply is gypsum. Adequate calcium is essential in root and pod zones for yield and quality of kernels. Calcium deficiency leads to unfilled pods (pops). Sulphur is directly involved in the biosynthesis of oil.
About 1.0 meq 100 g-1 of soil in the root zone depth and 3.0 meq 100 g-1 of soil in the pod zone are considered threshold values of calcium sufficiency. About 100 ppm of heat soluble sulphur is the critical limit of available sulphur for groundnut.
Supply of calcium and sulphur to groundnut crop through CaO or gypsum has been observed to increase the yield by more than twice depending on its availability in the soil. Application of gypsum near pegging zone at flowering (30 DAS) appears to be ideal.
On sandy loam soils, application of gypsum at 250 kg ha-1 for irrigated crop and 500 kg ha-1 for rainfed crop resulted in highest yields by meeting the calcium and sulphur needs of groundnut. Response of groundnut to gypsum ranges from 250 to 1000 kg ha-1 in different situations.
Gypsum is recommended as top dressing at flowering (30 DAS) if soil test shows less than 400 kg Ca ha-1 and/or the ratio of calcium to potassium is less than 3:1 for Virginia groundnut. For Spanish types, gypsum is necessary for soils testing less than 225 kg ha-1 or the ratio of calcium to potassium is less than 3:1. If heavy rains occur within two weeks after application, a second lighter application of gypsum may be necessary around three weeks after first application.
Nutrient # 6. K: Ca: Mg Ratio:
Groundnut is rather sensitive to unbalanced nutrient supply. The ratio of K: Ca: Mg is more important than the total amount of any of them. Increase in concentration of Mg in nutrient solution decreased the uptake of K by groundnut.
Similarly, uptake of Ca deceased with increasing concentration of Mg. There is a mutual antagonistic effect on the uptake of K, Ca and Mg. The ideal ratio of K: Ca: Mg for groundnut is 4:4:2. Under field conditions, a ratio of 4:2:0 will be optimum both for rainfed and irrigated crops.
Nutrient # 7. Zinc:
Zinc deficiency is common on sandy and sandy loam soils. Zinc deficiency affects flower production and reduces the number of pegs penetrating into the soil. Results of experiments all over the country indicated increase in pod yield ranging from 12 to 30 per cent due to application of zinc sulphate at 10 to 50 kg ha-1.
Application of zinc sulphate to soil at 25 kg ha-1 once in two years corrects the deficiency. If zinc deficiency is noticed in the standing crop, foliar application of 0.2 per cent zinc sulphate along with 0.2 per cent lime can correct the deficiency.
Nutrient # 8. Boron:
Boron deficiency lads to hollow heart. Boron deficiency has been reported in Punjab and Tamil Nadu. The threshold level for boron is 0.25 ppm. Deficiency can be corrected by soil application of 5-10 kg ha-1 of boron depending on the level of deficiency. Excess born inhibits iron uptake resulting in typical burning of leaf margin. Its deficiency in standing crop can be corrected by 0.1 per cent borax spray.
Nutrient # 9. Iron:
Iron chlorosis is largely due to its reduced availability in the soil. Immobilisation of iron in the soil may be due to high levels of lime, high pH (more than 7.6) or high level of bicarbonates in soil or irrigation water. Spraying 1.0 per cent ferrous sulphate mixed with 1.0 per cent ammonium citrate around 50 DAS corrects the iron deficiency.