Spatial variability of soil boron availability and its impact on canola yield
Dubyk, Clayton Alec
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Soil boron (B) is influenced by factors including soil pH, texture and organic matter (OM). It was hypothesized that response of canola (Brassica napus) to B fertilization similarly may be related to these factors. The objectives of this study were to examine the spatial variability of available soil B (i.e., hot water soluble B (HWS-B» and identify soil chemical and physical factors controlling both available soil Band consequent canola yield response to applied fertilizer B. The spatial variability of HWS-B and canola response to B fertilization was assessed at two sites identified as having marginal (Carrot River) to deficient (Smeaton) levels ofHWS-B. At each site, a single 128-point linear sampling transect was established and soil samples, taken at 3-m intervals, were analyzed for various physical and chemical properties. Canola was seeded along the transects and three B treatments were imposed, i.e., an untreated control, 2 kg B ha' as surface applied Granubor'", and 0.5 kg B ha-I as foliar applied Solubor'". At crop maturity, seed yields were determined. Hot water soluble B was highly variable and was associated with, in decreasing order of importance: OM » pH > texture> inorganic C. Soil B availability increased with increasing OM. The effect of soil pH on B availability varied. At Smeaton, where acidic soils dominated, HWS-B increased with increasing soil pH whereas at Carrot River, where alkaline soils dominated, HWS-B levels typically decreased with increasing soil pH. Canola yield responses to B fertilizer were highly variable and the HWS-B test failed to predict responses. Although yield responses were weakly correlated with soil pH, correlations were inconsistent between sites and treatments. Apparently, response of canola to B fertilization was controlled by a suite of soil factors controlling B availability, and these factors varied both within and between sites. Wavelet analysis revealed that at spatial scales greater than 55 m, variability in HWS-B and OM were statistically significant. The spatial scale of the HWS-B variability suggests that successful evaluation of crop responses to B fertilization may be compromised if variability in soil B availability is not recognized and accounted for.