Neil Miles

Quantities of phosphorus fertilizer required to raise the soil test value

To calculate nutrient requirement it is necessary to know how much of that nutrient must be applied to overcome any buffering effects and raise the test value to a desired level. A glasshouse study was conducted on a wide range of Natal soils to establish this information for phosphorus. The upper 200-mm depth at 54 sites was sampled. Soils varied greatly in texture (4–83% clay), organic carbon (0.2–9.2%) and clay mineralogy (kaolinitic-sesquioxic through to illitic and smectitic). Soil from each site was treated with four levels of P and taken through a number of wetting and drying cycles over a six-week period. Analysis for P using the Ambic, Bray no. 1 and Truog extradants showed that the quantity of applied P (kg ha−1) required to raise the soil test value by a unit amount (1 mg L−1), i.e. the P requirement factor, varied greatly for the different soils. The range in values for these three extradants was 2.5–37.9; 2.0–17.7; and 2.3–30.3, respectively. The level of P sorption was found to be strongly r...

Silicon reduces impact of plant nitrogen in promoting stalk borer (Eldana saccharina) but not sugarcane thrips (Fulmekiola serrata) infestations in sugarcane.

The stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae) is a major limiting factor in South African sugarcane production, while yield is also reduced by sugarcane thrips Fulmekiola serrata Kobus (Thysanoptera: Thripidae). Borer management options include appropriate nitrogen (N) and enhanced silicon (Si) nutrition; the effect of N on sugarcane thrips is unknown. We tested the effects of these nutrients, in combination with resistant (N33) and susceptible (N27) sugarcane cultivars, on E. saccharina and F. serrata infestation. Two pot trials with three levels of N (60, 120, and 180 kg ha-1) and two levels each of calcium silicate and dolomitic lime (5 and 10 t ha-1) were naturally infested with thrips, then artificially water stressed and infested with borer. Higher N levels increased borer survival and stalk damage, while Si reduced these compared with controls. Silicon significantly reduced stalk damage in N27 but not in N33; hence, Si provided relatively greater protection for susceptible cultivars than for resistant ones. High N treatments were associated with greater thrips numbers, while Si treatments did not significantly influence thrips infestation. The reduction in borer survival and stalk damage by Si application at all N rates indicates that under field conditions, the opportunity exists for optimizing sugarcane yields through maintaining adequate N nutrition, while reducing populations of E. saccharina using integrated pest management (IPM) tactics that include improved Si nutrition of the crop and reduced plant water stress. Improved management of N nutrition may also provide an option for thrips IPM. The contrasting effects of Si on stalk borer and thrips indicate that Si-mediated resistance to insect herbivores in sugarcane has mechanical and biochemical components that are well developed in the stalk tissues targeted by E. saccharina but poorly developed in the young leaf spindles where F. serrata occurs.

Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake

Reports of sugarcane yield responses to silicon (Si), coupled with mounting evidence that elevated crop Si levels reduce both biotic and abiotic stresses, account for the interest in the Si nutrition of this crop. In terms of managing Si supplies to sugarcane in South Africa, uncertainties exist regarding, first, the reserves of plant-available Si in soils, and second, the reliability of soil-test methods for predicting Si availability. In this study, extractable Si was measured in 112 soils collected from sugarcane-producing fields in South Africa. Soils were selected on the basis of dominant soil types and included Inceptisols, Alfisols, Mollisols, Vertisols, Oxisols, Entisols, and Ultisols, varying widely in chemical properties, texture, and extent of weathering. Extractants employed were 0.01 M calcium chloride (CaCl2) and 0.02 N sulfuric acid (H2SO4). Silicon extracted with 0.02 N H2SO4 ranged from 2 to 293 mg kg−1, whereas with 0.01 M CaCl2 the range was 5 to 123 mg kg−1. With both extractants, extractable Si decreased significantly with decreasing pH, exchangeable calcium (Ca), and total cations. In soils with potassium chloride (KCl)–extractable Al+H levels of greater than 0.5 cmolcL−1, extractable Si levels were consistently low, suggesting that soluble Al is implicated in reducing plant-available Si levels. Extractable Si levels were not related to the Bache and Williams P-sorption indices of soils. In the second part of the investigation, sugarcane leaf Si concentrations from 28 sites were related to soil extractable Si levels. The CaCl2 soil test proved markedly superior to H2SO4 as a predictive test for leaf Si levels.

Liming an acid soil treated with diverse silicon sources: Effects on silicon uptake by sugarcane (Saccharum spp. hybrids)

ABSTRACT Highly weathered soils are typically acid, with low plant-available silicon (Si), and may also be high in soluble aluminum (Al). We tested whether pre-liming an acid soil prior to silicate fertilizer application improved Si uptake by sugarcane (Saccharum spp. hybrids), feasibly through elimination of reactive Al. Three trials with potted sugarcane were grown in an acid soil. Treatments included 11 different Si sources and 1- or 3-month pre-liming periods. Soil and plant tissue Si content and yield were determined. Pre-liming for a 1- or 3-month period did not increase plant Si uptake and reduced available soil Si throughout. Alkaline Si sources produced significantly more plant-available Si and greater plant uptake than non-alkaline sources. Silicon and lime treatments did not significantly increase yield. We conclude that pre-liming an acid soil before Si application is unlikely to significantly improve Si uptake by sugarcane.

Provision of nitrogen as ammonium rather than nitrate increases silicon uptake in sugarcane

Silicons role in ameliorating a range of biotic and abiotic plant stresses is beyond doubt, yet means to maximise its uptake via the roots from applied silicon sources and thereby enhance crop yields have not been fully explored. Our study found that reduction of rhizosphere pH through provision of nitrogen fertilizer to sugarcane as ammonium rather than nitrate increased silicon uptake from a low-silicon soil amended with calcium silicate slag. We propose that ammoniacal fertilizers have potential for enhancing the solubilisation of silicate slags by acidifying the rhizosphere and increasing silicic acid solubility and availability for plant uptake.

Effect of residue and fertiliser management on soil fertility in a long-term sugarcane trial in South Africa

Understanding the impact of long-term continuous addition of sugarcane residues and fertiliser on soil fertility is important for development of best management practices. The experiment commenced in 1939. The fertility of the 0–10 cm soil layer was affected by the management options, but neither mulching nor fertiliser impacted significantly on soil organic carbon (SOC). However, the combination of mulching and fertiliser application increased the SOC content in the 0–2 cm soil layer significantly. It also increased soil acidity, as well as the concentration of iron, manganese, zinc and silicon, whereas reducing pH, the effective cation exchange capacity and the concentration of two of the basic cations, namely calcium and magnesium, in the 0–20 cm soil layer. Fertiliser affected yield significantly. In the absence of fertiliser, mulching had a significant effect on yield. Soil moisture and yield correlated linearly. In conclusion, increasing the addition of organic matter to continuously fertilised soils accelerated soil acidification leading to the loss of exchangeable basic cations. It is, therefore, important to take cognisance of the acidification potential of added residue under a greencane harvesting (mulching) system; this consideration emphasises the need for soil conditions to be rigorously managed on the basis of regular soil analysis.