Gary J. Gascho

Chapter 12 Silicon sources for agriculture

Characteristics of an acceptable silicon (Si) source are: a high content of soluble-Si, physical properties conducive to mechanized application, ready availability, and low cost. Since Si is the second most abundant element in the earths crust, finding sources of Si is easy. But, Si is always combined with other elements and most sources are insoluble. Responses of crops to soluble-Si applications in sands (largely SiO 2 ) provide an example of the insolubility of one source. Slags, by-products from the processing of iron and alloy industries, have been utilized quite extensively. Their concentrations and solubility of Si and their contents of other elements vary widely. For a given source, solubility is indirectly related to particle size. A few sources are soluble, but too costly for general use. Potassium silicate is used in nutriculture for disease control in some high value crops. Sodium silicate and silica gel have also been used to supply Si in research and high value crops. Calcium silicates have emerged as the most important sources for soil applications. Of those, calcium meta -silicate (wollasonite, CaSiO 3 ) has been the most effective source in many locations with low concentrations of soluble-Si in soils. Such a material, supplied as a slag by-product from the high temperature electric furnace production of elemental P, is applied extensively to Everglades mucks and associated sands planted to sugarcane and rice. Thermo-phosphate, a commercial fertilizer used in Brazil to supply P, Ca, and Mg, also supplies soluble-Si due to high temperature manufacturing process effects on its magnesium silicate ingredient.

Influence of tractor wheel tracks and crusts/seals on runoff: Observations and simulations with the RZWQM

In simulations on the fate of agricultural chemicals applied to crops, accurate partitioning of rainfall between infiltration and runoff is fundamental to chemical runoff predictions. We evaluated the Root Zone Water Quality Model (RZWQM version 3.1) against measured runoff from two field plots (15×45 m with 3% slope) on a Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult). Six simulated rainfall events, each 25 mm h−1 for 2 h, were applied to maize (Zea mays, L.) each year. In the uncalibrated mode, RZWQM under-predicted runoff by 40% on average, with the closest fit for events that occurred after full canopy. Saturated hydraulic conductivity (Ks) accounted for the majority of the uncertainty in predicted runoff. When Ks of the surface crust was back calibrated from the measured runoff, RZWQM predicted runoff closely for the remaining plots and events. Alternatively, using different Ks values for wheel track and crop beds, running the model for each and, then, proportionally assigning runoff also led to predictions that agreed with measured runoff. When spatial and temporal changes in Ks were calibrated to specific conditions at the site, RZWQM effectively predicted runoff.

A foliar boron nutrition and insecticide program for soybean

In an attempt to increase soybean (Glycine max) yield, studies initiated in 1986 and continued until 1996 have resulted in the recommendation that soybean growers in Georgia foliar-apply 0.28 kg boron ha-1 with Dimilin at 0.07 kg active ingredient ha-1 in reproductive stages. Such applications, at the time an insecticide is needed, have resulted in economically important yield responses of soybeans grown using other best management practices. The yield response appears due to both nutrition and protection from foliage-eating insects. Initial experiments were with applications of nitrogen (N) applied by fertigation using center-pivot sprinkler irrigation systems. Such applications increased yield and protein contents of soybeans in many cases. However, fertigation is not an option for most soybeans, therefore research on foliar effects of N applications were undertaken. Foliar nitrogen applications frequently result in leaf burns, but we found that addition of boron (B) afforded some protection from those burns and provided additional increases in yield. Additional experiments indicated that B application was resulting in yield increases similar to those attained with fertigated N at far less cost. Yield increases of up to 400 kg ha-1 for a single application of 0.28 kg B ha-1 have been attained. The major gross physiological effect of B appears to be an increase in bean size. Recent studies have been directed toward applying the boron with insecticides at the time the insecticide is needed. This method of application reduces the cost of B application to the actual cost of the B source (approximately

Nitrate-nitrogen, soluble, and bioavailable phosphorus runoff from simulated rainfall after fertilizer application

3 US ha-1). Soluble disodium octaborate tetrahydrate is compatible with insecticides, including Dimilin (diflubenzuron), an insecticide which is very effective against the velvetbean caterpillar (Anticarsia gemmatalis Hubner).