Introduction

The south-western part of Haryana mainly consists of arid tracts of the state. The soils of this zone are sand to loamy sand in texture, low in organic carbon, low to medium in available phosphorus, low to high in available potassium and low to medium in sulphur. A major part of this zone has a rainfall about 300 to 550 mm per annum with less than 25 rainy days. Temperature variations are quite high touching 47 to 48°C during summer and occasionally as low as around 0°C during winter. Annual potential evapotranspiration is 1,600 to 1,700 mm with water deficiency in the range of 1,200 to 1,400 mm. The main crops of the region are Indian mustard (raya), wheat and gram in the Rabi (the dry season during winter) and pearl millet, cotton and clusterbean which are grown in the Kharif season (the rainy season during summer monsoon).

About 50% of the soils are low to medium in available potassium so that response to K on these soils is basically due to inherently low K fertility. Additionally since the farmers in the area do not apply potash fertilizers, soils are therefore continuously being mined for K.

In order to study the potash response in major cropping systems of south Haryana, field experiments were carried out at the Regional Research Station, at Bawal (Haryana Agricultural University) under the IPI-HAU Research Project. The results from these experiments were later used by IPI and the HAU to set up demonstration plots on farmers’ fields, which were aimed at educating the farmers about the importance of potash application for field crops.

Response of potassium in pearl millet-mustard rotation

Materials and methods
To study the potassium response in pearl millet and mustard crops in a pearl millet-mustard rotation, a field experiment was conducted during the Kharif season of 2006 at CCS HAU, Regional Research Station, Bawal (Haryana). The experiment for pearl millet was laid out in a randomized block design with three replications. There were four treatments of increasing potash supply but all with the same rate of N and P application i.e. K at (0, 20, 40 and 60 kg K₂O/ha) and N (120 kg/ha) and P₂O₅ (60 kg/ha). The soil of the experimental field was loamy sand in texture (Typic Ustochrept) and initially (Kharif 2002) the pH was 8.56, EC 0.21 ds/m, organic carbon 0.20%, available P 13.34 kg/ha and available K₂O 160 kg/ha. Fertilizer application (all P and K) was made as per treatment with half the N applied as a basal dressing, and the remaining half applied as two equally split doses as a top dressing at thinning and at the ear emergence stage. Pearl millet (Cv. HHB-117) was sown on 11.07.2006 and was harvested on 27.09.2006.

To study the residual effect of K in mustard in the pearl millet-mustard rotation, mustard (Laxmi RH-8812) was sown on 24.10.2006. A uniform dose of N₈₀P₃₀ with 25 kg ZnSO₄/ha was applied in all treatment combinations in Kharif 2006. A basal application of half the N, and all the P and ZnSO₄ was made at sowing, while the remaining half of the N application was top dressed at first irrigation. The mustard crop was harvested on 20.03.2007.

Results
Growth and yield attributes
Table 1 shows that there was significant increase in the number of tillers and in earhead length of pearl millet at 40 kg K₂O/ha and in the 1000 grain weight at 60 kg K₂O/ha. The residual effect of potash on mustard was also revealed by the increase in the number of branches, the number siliquas/plant, siliqua length and the 1000 grain weight. The effect, however, was significant only at 60 kg K₂O/ha over the control. There was no significant difference between the 40 and 60 kg K₂O/ha rates of application.

Yield
The yield data (Table 2) indicates that the pearl millet crop responded significantly, up to 40 kg K₂O/ha. The increase in grain yield was 4.80, 9.48 and 14.14%, with the corresponding increase in straw yield 5.04, 10.84 and 14.59% at 20, 40 and 60 kg K₂O/ha respectively over the control. The residual effect of K on mustard in the pearl millet-mustard rotation revealed that mustard responded significantly at 60 kg K₂O/ha. The increase in mustard seed yield was 2.94, 5.18 and 8.30%, while the corresponding increase in mustard straw yield was 2.70, 4.83 and 7.49% at 20, 40 and 60 kg K₂O/ha respectively over the control.

K-uptake
Application of potassium significantly increased the pearl millet K-uptake (Table 3). Grain K-uptakes were increased by 0.84, 1.94 and 2.78 kg/ha, with corresponding increases in straw Kuptakes of 18.45, 51.75 and 72.97 kg/ha at 20, 40 and 60 kg K₂O/ha respectively over the control. The K-uptake by the mustard crop also increased significantly due to residual effect of K. Mustard seed K uptakes were increased by 1.63, 3.12 and 4.56 kg/ha with corresponding increases in mustard straw K-uptakes of 7.85, 11.60 and 16.35 kg/ha at 20, 40 and 60 kg K₂O/ha respectively over the control. As crop straw contains most of the accumulated plant K, the way that this resource is managed has a large impact on the long-term availability of this nutrient, particularly on soils with relatively low native K concentrations. Straw management affects the nutrient balance and the fertility requirement for the following crop. When straw is incorporated into the soil following harvest, it can improve soil physical and chemical properties and serve as a source of nutrients for the following crop. Since most of the K remains in the straw following harvest, much of this can be recycled for subsequent crop growth following decomposition.

Although incorporation of the straw is the most desirable straw management, in India most of the straw is removed from the field for feeding cattle and used as a source of energy (cooking and heating). This means that there is a high likelihood of finding responses to K application on many soils.

Protein content
Application of potassium in the pearl millet-mustard rotation also increased the grain/seed protein content; this increase being significant over the control at 40 kg K₂O/ha in pearl millet grain and at 60 kg K₂O/ha in mustard seed (Table 4).

Protein yield (protein yield = grain yield * % protein) was significantly increased due to K application, probably via the effect of potassium promoting photosynthate mobility improving the utilization of nitrogen. Protein yield of pearl millet was increased from 252 to 340 kg/ha and that of mustard from 322 to 370 kg protein per ha (Fig. 1).

Response of potassium in pearl millet-wheat crop rotation

Materials and methods
To study the potassium response in pearl millet and wheat crops in a pearl millet-wheat rotation, a field experiment was conducted during 2006-2007 at CCS HAU, Regional Research Station, Bawal (Haryana). The experiment was laid out in a randomized block design with three replications in the same plots during the fifth year of study. The plot size was 5 x 8 sq. m. There were six treatment combinations viz. N₉₀P₆₀K₀, N₉₀P₆₀K₃₀, N₉₀P₆₀K₆₀, N₁₂₀P₆₀K₀, N₁₂₀P₆₀K₃₀ and N₁₂₀P₆₀K₆₀ for pearl millet and a uniform dose of N₁₂₀P₆₀ with the N₉₀ treatment and N₁₅₀P₆₀ with the N₁₂₀ treatment combinations was superimposed for the wheat crop. The soil of the experimental field was loamy sand in texture (Typic Ustochrept) and the initial (Kharif 2002) pH was 8.56, EC 0.21 ds/m, organic carbon 0.20%, available P 13.34 kg/ha and available K₂O 160 kg/ha. Fertilizer applications (all P and K) were made as per treatment with half the N for pearl millet and wheat given as a basal dressing, while the remaining half dose of N for pearl millet and wheat was top dressed in two applications. Pearl millet (Cv. HHB-117) was sown on 11.07.2006, while wheat (PBH-343) was sown on 09.11.2006. The pearl millet crop was harvested on 27.09.2006 and wheat was harvested on 29.03.2007.

Results
Growth and yield attributes
There was a significant increase in pearl millet in the number of tillers and earhead length at both levels of N up to 30 kg K₂O/ha whereas plant height and 1,000 grain weight were not affected significantly by K application (Table 5).

The residual effect of K on wheat was also revealed by increases in the number of tillers, spike length and 1,000 grain weight but the effect was significant only at 60 kg K₂O/ha over the control at both levels of N.

Yield
The yield data (Table 6) indicated that pearl millet crop responded significantly up to 30 kg K₂O/ha at both levels of nitrogen. The increase in grain yield was 11.61 and 16.65% with 90 kg N/ha whereas this increase was 11.67 and 17.46% with 120 kg N/ha at 30 and 60 kg K₂O/ha, respectively over the control. The corresponding increase in straw yield was 9.42 and 14.08% with 90 kg N/ha and 9.81 and 14.32% with 120 kg N/ha. The results of K residual effect on wheat in the pearl milletwheat rotation also revealed that the wheat crop responded significantly at 60 kg K₂O/ha with both levels of N. The increase in grain yield was 7.27 and 7.20% while the increase in straw was 7.60 and 7.92% at 60 kg K₂O/ha with lower and higher levels of N, respectively over the control.

K-uptake
Application of potassium significantly increased K-uptake by pearl millet and wheat crops (Table 7). The increase in pearl millet grain K-uptake was 1.76 and 2.25 kg/ha with N₉₀ and 2.00 and 2.93 kg/ha with N₁₂₀ at 30 and 60 kg K₂O/ha respectively over the control while the corresponding increase in pearl millet straw K-uptake over the control was 23.88 and 42.00 kg/ha at N₉₀, and 36.25 and 57.22 kg/ha at N₁₂₀ respectively. Similarly, the increase in wheat grain K-uptake was 2.35 and 4.93 kg/ha at N₉₀, and 2.84 and 5.75 kg/ha at N₁₂₀ with 30 and 60 kg K₂O/ha respectively over the control, whereas the increase in wheat straw K-uptake was 10.37 and 20.87 kg/ha at 90 kg N, and 11.14 and 23.34 kg/ha at N₁₂₀ with 30 and 60 kg K₂O/ha respectively, over the control.

Again, the high K-uptake by the straw shows the importance of the fate of the straw for K management. When straw is removed, three to ten times more potassium is lost from the soil compared to harvesting only seed. This shows that straw management readily influences the response to K fertilization by the following crop in the rotation.

Protein content
The protein content of pearl millet and wheat grain increased due to K application (Table 8). The pearl millet grain protein content increased from 9.85 to 11.29% at 90 kg N/ha, and from 10.01 to 11.51% at 120 kg N/ha with the increase in K level from 0 to 60 kg K₂O/ha. Similarly, the wheat grain protein content increased from 10.62 to 11.74% at 120 kg N/ha, and from 10.86 to 11.87% at 150 kg N/ha due to the increase in residual K level from 0 to 60 kg K₂O/ha.

Protein yield of grains was significantly increased as a result of K application regardless of the N level. These results show how potassium improves nitrogen use efficiency by favoring protein formation.

Conclusions
This study demonstrates the importance of potassium fertilization in field crop production in Haryana. The optimal rate of K fertilization was either 30 or 40 kg K₂O/ha for pearl millet, and there was a positive residual effect of the K application for the following crop, with an optimal response of 60 kg K₂O/ha for both mustard and wheat crops. Increase in grain protein content was also observed with optimal K nutrition, indicating improvement in crop quality. This indicates more effective utilization of available nitrogen in the presence of K.

When market demands exist for higher protein content, high protein grains can be sold by the farmers at a premium, providing additional profits. These results demonstrate, and further confirm, the importance of balancing nutrient inputs in crop production to optimize yield, quality, and grower profit.

Edited by E.A. Kirkby.