Rapid Nitrate Analysis of Soil Cores Using ISFETs

An intact core extraction procedure was tested that might be used in the field for real–time prediction of soil nitrates. An extraction solution was pushed through a soil core held between two filters, and an ion–selective field–effect transistor/flow injection analysis (ISFET/FIA) system was used to sense soil nitrates in real time. Laboratory tests were conducted using four soil types and two levels of nitrate concentration, soil moisture, core density, core length, core diameter, and extraction solution flow rate. The extraction solution flow was sampled at the exit face of the core and routed to the ISFET/FIA system. The ISFET output voltage was sampled at 100 Hz. Results of the test indicate that nitrate extraction of the soil cores was successful, and that data descriptors based on response curve peak and slope of the ISFET nitrate response curve might be used in tandem in a real–time prediction system

Evaluation of Phosphate Ion-Selective Membranes for Real-time Soil Nutrient Sensing

A real-time soil nutrient sensor would allow the efficient collection of data with a fine spatial resolution, to accurately characterize within-field variability for site-specific nutrient application. Our goal was to evaluate the applicability of a phosphate membrane to the measurement of phosphate levels in soil extractants and to determine how previously developed nitrate and potassium membranes would be affected by the presence of phosphate. A type of PVC-based phosphate membrane containing an organotin compound, bis(p-chlorobenzyl)tin dichloride, was evaluated, along with the nitrate and potassium membranes, in pH 7 Tris buffer solution and Kelowna soil extractant for sensitivity and long-term stability. The phosphate membranes in the Tris buffer solution of pH 7 exhibited a response over a range of 10-5 to 10-1 mol/L phosphate concentrations with an average slope of -28.2 +1.5 mV per activity decade of dibasic phosphate. The response speed of tested electrodes containing phosphate, nitrate and potassium membranes was rapid, reaching an equilibrium response in less than 15 s. However, the phosphate membrane in the Kelowna solution of pH 8.5 was almost insensitive to different phosphate levels from 10-6 to 10-2 mol/L due to the presence of a high concentration of fluoride in the solution. In addition, the tin compound-based phosphate membranes had limited lifetimes of less than 14 days. It is not expected that the tested phosphate membranes could be used for phosphate detection in other soil extractants, such as Bray P1 and Mehlich III solutions, because they also contain high concentrations of fluoride

Evaluation of Phosphate Ion-Selective Membranes and Cobalt-Based Electrodes for Soil Nutrient Sensing

A real-time soil nutrient sensor would allow efficient collection of data with a fine spatial resolution to accurately characterize within-field variability for site-specific nutrient application. Ion-selective electrodes are promising candidates because they have rapid response, directly measure the analyte, and are small and portable. Our goal was to investigate the ability of three different phosphate ion-selective electrodes (two fabricated with organotin compound-based PVC membranes, and one fabricated from a cobalt rod) used in conjunction with Kelowna soil extractant to determine phosphorus over the typical range of soil concentrations. Electrodes using organotin compound-based PVC membranes containing bis(p-chlorobenzyl)tin dichloride as an ionophore exhibited sensitive responses to HPO42- over a range of 10-4 to 10-1 mol/L in Tris buffer at pH 7. They were nearly insensitive to phosphate when using Kelowna soil extractant as the base solution, perhaps because of the high concentration of fluoride (0.015 mol/L) in the Kelowna solution. In addition, the life of the membranes was less than 14 days. Electrodes using another tin-compound-based PVC membrane containing tributyltin chloride as an ionophore also provided unsatisfactory results, showing much less sensitivity to H2PO4- than previously reported. The cobalt rod-based electrodes exhibited sensitive responses to H2PO4- over a range from 10-5 to 10-1 mol/L total phosphate concentration with a detection limit of 10-5 mol/L in the Kelowna solution. This detection range would encompass the typical range of soil phosphorus concentrations measured in agricultural fields. The selectivity of the cobalt electrodes was satisfactory for measuring phosphates in the presence of each of six interfering ions, i.e., HCO3-, Cl -, Br -, NO3-, Ac -, and F -, with the electrodes being 47 to 1072 times more responsive to phosphate than to the tested interfering ions

Statistical and Neural Methods for Site-Specific Yield Prediction

Understanding the relationships between yield and soil properties and topographic characteristics is of critical importance in precision agriculture. A necessary first step is to identify techniques to reliably quantify the relationships between soil and topographic characteristics and crop yield. Stepwise multiple linear regression (SMLR), projection pursuit regression (PPR), and several types of supervised feed–forward neural networks were investigated in an attempt to identify methods able to relate soil properties and grain yields on a point–by–point basis within ten individual site–years. To avoid overfitting, evaluations were based on predictive ability using a 5–fold cross–validation technique. The neural techniques consistently outperformed both SMLR and PPR and provided minimal prediction errors in every site–year. However, in site–years with relatively fewer observations and in site–years where a single, overriding factor was not apparent, the improvements achieved by neural networks over both SMLR and PPR were small. A second phase of the experiment involved estimation of crop yield across multiple site–years by including climatological data. The ten site–years of data were appended with climatological variables, and prediction errors were computed. The results showed that significant overfitting had occurred and indicated that a much larger number of climatologically unique site–years would be required in this type of analysis

Corn Stover Harvest, Tillage, and Cover Crop Effects on Soil Health Indicators

Monitoring soil health indicators (SHI) will help ensure that corn (Zea mays L.) stover harvest is sustainable. This study examines SHI changes after 5 yr of growing continuous corn with either chisel plow or no-tillage practices and harvesting 0, ∼35, or ∼60% of the stover. Two no-tillage treatments with a cereal rye (Secale cereale L.) cover crop and stover harvest rates of ∼35 or ∼60% were evaluated. All eight treatments were replicated four times in a randomized complete block design at an 11-ha site in Boone County, IA. Soil samples were collected following grain and stover harvest from 0- to 5- and 5- to 15-cm depth increments. Particulate organic matter C (POM-C) decreased when stover was removed or the soil was chisel plowed. No-till with 0% stover removal had 10 mg g–1 POM-C in the 0- to 5-cm soil layer, which was 1.9-fold higher than in other treatments. Potentially mineralizable N (PMN) was greater under cover crop treatments. Average PMN values were 56.9 and 45.5 µg g–1 PMN for no-till with cereal rye at 0- to 5- and 5- to 15-cm depths, respectively, compared with 17.5 and -3.7 µg g–1 PMN for the same no-till treatments without cereal rye. Other soil properties did not respond to increasing levels of stover removal. At this location and at the studied removal rates, 5 yr of harvesting corn stover did not decrease soil health, but POM-C data suggest that changes may be occurring. Long-term monitoring should continue to assess corn stover harvest sustainability