Replenishment of soil solution organic and inorganic P in a sterile and nonsterile grassland soil amended with 0 and 235 kg P ha -1 for 13 consecutive years was investigated in a recirculating column system. In sterilized treatments, P liberated from soil biomass, initially increased solution organic and inorganic P concentrations to about 0.3 and 0.6 Ixg P cm-3 in the 0 and 235 kg P treatment, respectively. Sterilization effects were larger than the residual fertilizer effect. Subsequently, in sterilized treatments were microbial activity was lacking, removal of solution P over the duration of the experiment reduced organic P concentration to the detection limit (0.001 txgPcm 3). Organic P concentrations in the nonsterile treatment were maintained at about 0.015 ixg Pcm -3 which was higher than inorganic P concentration. Inorganic P concentrations were about 0.002 and 0.008 Ixg P cm -3 in the nonfertilized and the fertilized treatment, respectively. Inorganic P buffer power was greater in the nonsterile treatments, but abiotic buffering alone could not account for the measured inorganic P concentrations found during desorption. It was concluded that biomass P is a major factor controlling organic and inorganic P solution concentrations in this systems.

Information on soil±plant nutrient balance in India and elsewhere is scarce and mostly generalized. A review of earlier studies on nutrient balances was done to assess their relevance for researchers, policy makers, and farmers' understanding, to manage soil fertility for sustainable crop productivity. An on-farm nutrient balance study during 1995/1996 and 1996/1997 was designed to understand nutrient balances resulting from farmers' practices in semi-arid tropical regions of India. This diagnostic study targeted sorghum-based cropping systems and groundnut-based cropping systems in low rainfall areas of the Indian SAT. Selection of farmers for this study was done through a systematic survey and analyses of factors effecting farmers' decision making for nutrient inputs. Intensive plot-speci®c nutrient input and output measurements were carried out on 53 farmers' ®elds for sorghum-based systems and 45 farmers' ®elds for groundnut-based system in this study. Topsoil mineral nitrogen (N) content observed in 2 years at the beginning of the crop season in two locations of Andhra Pradesh, India, was surprisingly high and exchangeable potassium (K) contents also indicated suf®cient supply in most ®elds. Available phosphorus (P) in the majority of ®elds in both locations was around threshold levels, and just suf®cient for most crops. The nutrient balance in sorghum-based systems indicates a moderate to higher negative balance of potassium leading to soil mining for potassium supply in these systems. Nitrogen and P balances were generally positive. Although the groundnut-based system accumulated 53% of its N requirements through BNF, negative balances of N and K were observed mainly due to low applications of these nutrients. Application of K along with options for improving BNF of groundnut are suggested. Better nodulating groundnut cultivars and ef®cient rhizobium strains need to be introduced. High positive balances were observed in the systems whenever commercial crops like castor and cotton were sown in the rotation. This indicates farmers' preferences for applying excess quantities of FYM to commercially important crops even in dryland farming systems. The replenishment costs of mined nutrients annually in different cropping systems were calculated based on current market prices of inorganic fertilizers.

Organic phosphorus is often a major part of total phosphorus in soil solution. The role of this fraction as a P source for plants and the mechanism involved in its transfer from soil to plant is still unclear. We studied the utilization of organic phospharus in 0.01 M calcium chloride extracts by barley and its hydrolysis by isolated acid and alkaline phosphatases. Calcium chloride extracts were used as a nutrient solution in 24 hrs assays. Concentration of organic and inorganic P in equilibrium calcium chloride extracts was 7.8 and 1.8/~mol P L -1 , respectively, which was similar to the soil solution P concentration. When soil microbial biomass was destroyed by autoclaving, organic P concentration increased to 64.8 ~mol P L-1 whereas the inorganic P was hardly changed. Inoculation of the autoclaved soil with non-sterile soil and incubation for 5 days decreased the organic P concentration to 27.9 #mol P L -l but did not change inorganic E In this study barley plants utilized organic P from all extracts. The greatest reduction of organic P concentration occurred in fresh extracts of the autoclaved soil. Inorganic P was depleted to traces in all extracts. Organic P was hydrolyzed by isolated acid and alkaline phosphatases. We conclude that organic P in soil solution is a heterogeneous pool of organic P compounds originating from microbial biomass. Its initial availability to plants was high but its susceptibility to phosphatase hydrolysis was quickly reduced but not completely lost.

Information on soil±plant nutrient balance in India and elsewhere is scarce and mostly generalized. A review of earlier studies on nutrient balances was done to assess their relevance for researchers, policy makers, and farmers' understanding, to manage soil fertility for sustainable crop productivity. An on-farm nutrient balance study during 1995/1996 and 1996/1997 was designed to understand nutrient balances resulting from farmers' practices in semi-arid tropical regions of India. This diagnostic study targeted sorghum-based cropping systems and groundnut-based cropping systems in low rainfall areas of the Indian SAT. Selection of farmers for this study was done through a systematic survey and analyses of factors effecting farmers' decision making for nutrient inputs. Intensive plot-speci®c nutrient input and output measurements were carried out on 53 farmers' ®elds for sorghum-based systems and 45 farmers' ®elds for groundnut-based system in this study. Topsoil mineral nitrogen (N) content observed in 2 years at the beginning of the crop season in two locations of Andhra Pradesh, India, was surprisingly high and exchangeable potassium (K) contents also indicated suf®cient supply in most ®elds. Available phosphorus (P) in the majority of ®elds in both locations was around threshold levels, and just suf®cient for most crops. The nutrient balance in sorghum-based systems indicates a moderate to higher negative balance of potassium leading to soil mining for potassium supply in these systems. Nitrogen and P balances were generally positive. Although the groundnut-based system accumulated 53% of its N requirements through BNF, negative balances of N and K were observed mainly due to low applications of these nutrients. Application of K along with options for improving BNF of groundnut are suggested. Better nodulating groundnut cultivars and ef®cient rhizobium strains need to be introduced. High positive balances were observed in the systems whenever commercial crops like castor and cotton were sown in the rotation. This indicates farmers' preferences for applying excess quantities of FYM to commercially important crops even in dryland farming systems. The replenishment costs of mined nutrients annually in different cropping systems were calculated based on current market prices of inorganic fertilizers. #

The relationship between nutrient influx (In) and solution concentration at the root surface (Clo) has not yet been determined for roots growing in soil because of difficulties in measuring Clo. Corn was grown on two soils with 12 and 21% clay. Each soil had five K levels ranging from low to very high. Potassium influx (In) was determined from K uptake between two harvests and root length. Clo was then calculated from the average soil solution concentration and In by assuming that diffusion is the main transport mechanism for K to the root. Potassium influx plotted against Clo showed a saturation curve, typical of a Michaelis‐Menten kinetic relationship. The Michaelis‐Menten uptake parameters, maximum influx (Imax) and Michaelis constant (Km), were obtained by the “Hanes” plot. There was close agreement, without lack of fit, between calculated and observed data. The proposed procedure therefore appears to be suitable for estimating the uptake kinetics of roots growing in soil. Requirements for applications of the method are discussed.