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An optical sensor-based fertilizer nitrogen (N) management strategy that relies on visible and near-infrared spectral response from plant canopies was evaluated for irrigated wheat in the northwestern India. GreenSeeker™ optical sensor-guided fertilizer N dose, computed from an estimate of potential yield and response index, takes into account both the temporal and field-to-field variabilities and is applied only once after measuring in-season spectral response from the crop canopy. Seven field experiments were conducted in four wheat seasons to decide whether to apply the optical sensor-guided fertilizer N dose at 2nd or 3rd irrigation stage and to work out the appropriate N management before applying it. Robust relationships between in-season sensor-based estimates of yield and actual wheat yields were observed both at 2nd (R2= 0.64) and 3rd (R2= 0.86) irrigation stages of the crop. GreenSeeker-based fertilizer N management resulted in high yield levels and high N-use efficiency. Application of 30 kg N ha−1at planting and 45 kg N ha−1at 1st irrigation was found to be the appropriate N management before applying the GreenSeeker-guided dose at 2nd irrigation stage. Grain yields obtained by following sensor-guided N management were at par with those observed with the blanket recommendation of 120 kg N ha−1, but with greater recovery efficiency (by 6.7–16.2%) and agronomic efficiency [by 4.7–9.4 kg grain (kg N applied)−1]. The major outcome of this study was that applying a moderate amount of fertilizer N at planting and enough fertilizer N to meet the high N demand during the period between crown root initiation stage and maximum tillering stage before applying a sensor-guided fertilizer N dose at 2nd irrigation stage results not only in high yields but also in high fertilizer N-use efficiency in irrigated wheat.

Application of fertilizer N doses to rice (Oryza sativaL.) whenever leaf greenness as measured by chlorophyll meter (SPAD) and LCC falls below a fixed threshold is being extensively used for site-specific management of fertilizer N. As fixed threshold greenness varies with regions, rice cultivars and seasons, a dynamic threshold greenness concept in the form of SPAD-based sufficiency index has been introduced. For rice, the dynamic threshold greenness is defined as 90 % of the SPAD meter reading of leaves in an Nrichor over fertilized strip. In the present investigation, four field experiments with six cultivars were conducted at two locations to test fertilizer N management following two variants of the LCC-based dynamic threshold greenness concept—the leaf greenness one or 0.5 unit less than the LCC reading of the Nrichstrip (LCC-Nrich), vis-à-vis SPAD meter-based sufficiency index approach and blanket recommendation of 120 kg N ha−1applied in three equal split doses. The fertilizer N management following threshold greenness of one unit less than the LCC-Nrichresulted in total N application of only 60 kg ha−1and thus sub-optimum rice grain yields. The grain yield of rice, N uptake and fertilizer N use efficiency measured with N management following threshold leaf greenness of 0.5 units less than LCC-Nrichwere at par with the SPAD meter-based 90 % sufficiency index approach and were better than the blanket fertilizer recommendation in the region. The LCC-based dynamic threshold greenness strategy holds promise to efficiently manage fertilizer N rice as it can effectively take care of variations in the rice cultivars, locations and seasons.

In the Indo-Gangetic plain in South Asia, fertilizer nitrogen (N) application to irrigated wheat in two split doses at planting and at crown root initiation (CRI) stages of the crop is the general recommendation. Farmers have a tendency to apply another dose of N at maximum tillering (MT) stage to avoid the risk of N deficiency. However, appropriate criteria to decide MT stage application of N are lacking. Field experiments were conducted for three seasons with four wheat cultivars to develop criteria for using a chlorophyll (SPAD) meter and an optical sensor (GreenSeekerTM) for guiding MT stage N application. Application of 0, 60, 80, 100 and 120 kg N ha−1in two equal split doses at planting and at CRI stage constituted the main plots. The two subplots consisted of applying 0 and 30 kg N ha−1at MT stage immediately after taking SPAD meter and GreenSeeker readings. Greenness of wheat leaves at MT stage was found to be a function of N applied at planting and CRI stages. Wheat grain yield at maturity was determined by the level of greenness of leaves at MT stage—whether measured by SPAD meter or GreenSeeker (expressed as in-season estimate of yield (INSEY) computed by dividing normalized difference vegetation index by number of days between planting and sensing). Grain yield response to N application at MT regressed against SPAD readings at MT revealed that an application of 30 kg N ha−1will increase wheat yield by 1.0 or 0.5 t ha−1when color of the wheat leaves was equivalent to or less than SPAD value of 32.5 or 42.5, respectively. Robust regression between grain yield response and INSEY values revealed that application of 30 kg N ha−1at MT will lead to a grain yield increase by 1.0 or 0.5 t ha−1if INSEY values were found to be 0.005 or 0.011, respectively. It should serve as an easy and useful criterion for applying a supplementary dose of fertilizer N at MT stage of wheat.

A number of optical sensing tools are now available which can potentially be used for refining need-based fertilizer N topdressing decisions. Algorithms for estimating field-specific fertilizer N needs are based on prediction of yield when crop is growing in the field. The present study was conducted to establish and validate yield prediction models using spectral indices measured with proximal sensing using GreenSeeker canopy reflectance sensor, chlorophyll meter (SPAD), and two different types of leaf colour charts (LCCs) in five basmati rice genotypes across different growth stages. Regression analysis was performed using normalized difference vegetation index (NDVI) recorded with GreenSeeker sensor and leaf greenness indices measured with SPAD meter and LCCs developed by Punjab Agricultural University, Ludhiana (India) (PAU-LCC) and International Rice Research Institute, Philippines (IRRI-LCC). The exponential relationship between NDVI and grain yield exhibited the highest coefficient of determination (R2) and minimum normalized root mean square error (NRMSE) at panicle initiation (35 days after transplanting) stage and explained 38.3% to 76.4% variation in yield using genotype specific models. Spectral indices pooled for different genotypes were closely related to grain yield at all the growth stages and explained 53.4% to 57.2% variation in grain yield. Normalizing different spectral indices with CGDD (cumulative growing degree days) decreased the accuracy of yield prediction. Normalization with days after transplanting (DAT) however, did not reduce or improve the predictability of yield. The ability to predict grain yield with different models was validated with an independent set of data collected from two experiments conducted at different sites with a range of fertilizer N doses. The NDVI based genotype specific models exhibited robust linear correlation (R2= 0.77, NRMSE = 7.37%, n= 180) between observed and predicted grain yield only at 35 DAT, while the SPAD, PAU-LCC and IRRI-LCC consistently provided reliable predictions (with respective R2of 0.63, 0.60, 0.53 and NRMSE of 10%, 10%, 13.6%) even with genotype invariant models with 900 data points obtained at different growth stages. The study revealed that normal values of spectral indices—NDVI, SPAD, PAU-LCC and IRRI-LCC can be satisfactorily used for in-season estimation of grain yield in basmati rice. As LCCs are very economical as compared to chlorophyll meter and canopy reflectance sensor, these can be preferably used by small holder farmers in developing countries.

Transformations of applied phosphorus (P) to unavailable residual soil P is the major cause of limited P supply in most of the P-deficient soils. The effect of the incorporation of crop residues (rice straw [RS] and wheat straw [WS]) and organic manures (farmyard manure [FYM] and green manure [GM]) on P release in soil and its bio-availability to various summer and winter crops was investigated in laboratory and screen house experiments. Surface (0–0.15 m) soil samples collected after 32 years of differential fertilization to maize–wheat–cowpea fodder crop rotation, were examined for adsorption/desorption behavior of P, after incubating with organics of varying C:P ratios. Incorporation of crop residues increased P adsorption maxima as well as resistance to P release in soils. Increased buffering capacities in crop residue-incorporated treatments decreased P desorption in soil, whereas the incorporation of organic manures decreased P sorption, maximum buffering capacity (MBC), bonding energy, and increased P concentration in soil solution. Although the incorporation of crop residues decreased P release in soil its bio-availability in the soil–plant system was crop-specific and varied with the time of incorporation of organics. Raya showed increased P uptake with incorporation of both RS and WS, whereas in the case of berseem increased P uptake occurred only with wheat straw. Phosphorus uptake in rice, maize, and soybean decreased with the incorporation of both RS and WS. Incorporation of crop residues 1 day before the sowing of summer crops decreased P uptake, whereas incorporation 3 weeks prior to the sowing of winter crops improved P bio-availability. Incorporation of organic manures with a narrow C:P ratio, however, improved P uptake in all the crops under investigation, in both the seasons. The results thus emphasized that adsorption parameters calculated from the examination of soil samples should not be used independently for making fertilizer P recommendations. Crop effects (root exudates) and their interaction with P reaction products in soil and synchronization in P release from organics and crop uptake need to be considered to understand the virtual behavior of P bio-availability in the soil–plant system.

Phosphorus availability from residual P sources in soil is virtually the chemistry of metastable P compounds, i.e., their solubility and interaction with other soil and plant components. Identification of these compounds in soils under long-term application of fertilizers and farmyard manure is unstated and is prerequisite to comprehend the processes of P transformations and cycling in soil-plant system. X-ray diffraction analysis of surface and subsurface soils fertilized for 32 years under a maize–wheat–cowpea (fodder) cropping sequence revealed that P reaction products in soil depends upon the kind of fertilizer added to the soil as well as on management practices. Precipitation of applied P occurred mainly as octa-calcium phosphate, hydroxy apatite and variscite. In addition to these compounds, precipitation as $$\hbox{CaNH}_{4}\hbox{PO}_{4}\cdot\hbox{H}_{2}\hbox{O}$$was also observed in soil receiving diammonium phosphate rather than super phosphate as a fertilizer P source. Continuous application of farmyard manure (FYM) for three decades inhibited the conversion of applied P to more stable forms such as variscite and hydroxy apatite. The different P reaction products govern the availability of residual soil-P to growing plants. The reaction products formed were associated only with the sand and silt fraction of soil. Phosphorus in the clay fraction is mainly P adsorbed on exchange complexes or entrapped in lattice of silicate minerals.

An understanding of the adsorption–desorption behaviour of phosphate (P) in soils after three decades of long-term fertilization would be an invaluable supplement to our knowledge of the chemistry of P in soils and would assist in developing P application strategies for successive crops. With this objective and within the framework of a long-term experiment, we collected surface soil samples (0–0.15 m) from agricultural crop land on which a rotation of maize-wheat-cowpea (fodder) crops had been grown for 32 years. The soil samples were investigated for adsorption and desorption behaviour of P, and the buffering capacities of soil samples were computed from the adsorption data. Soils differed widely in their P supply characteristics. Phosphate adsorption increased while percentage P adsorbed decreased gradually with increasing levels of added P. Adsorption maxima and the extent of P adsorption was at its maximum in unfertilized soil, and P adsorption decreased with increased P applications. The incorporation of farmyard manure with optimal P levels decreased P adsorption even more than a super-optimal application of P fertilizers. Bonding energy and Freundlich constant ȁ8nȁ9 also decreased with increased P application and with the incorporation of farmyard manure. The adsorption and desorption of applied P were inversely related, and the soils that adsorbed P the most readily released it the least into the soil solution, and vice-versa. Computation of maximum buffering capacities (MBC) indicated that increased levels of P application decreased MBC and increased P supply in soil. The beneficial effects of an annual incorporation of farmyard manure with optimal P levels were clearly pronounced, and P availability in these soils was found to be even higher than the soil receiving the super-optimal application of P fertilizers. Various adsorption–desorption parameters were significantly related with the P uptake of different summer (rice, maize, soybean) and winter (wheat, berseem and raya) crops. Adsorption maxima, desorption maxima and maximum buffering capacity are the major parameters governing P availability in soils.

The article presents a discussion of optical sensor-based management of nitrogen for irrigated wheat in the Indo-Gangetic plains in northwest India, adapted from "Assessment of the Nitrogen Management Strategy Using an Optical Sensor for Irrigated Wheat," by Bijay-Singh et al.