Soil amelioration and boron nutrition effects on the growth of sunflowers (Helianthus annuus L.) on an Avalon medium sandy loam.

Abstract

Studies with sunflowers (Helianthus annuus) on an Avalon medium sandy loam, both in pots and in the field, indicated that the poor growth of this crop on this soil was largely due to soil acidity. Marked improvements in plant growth were brought about by liming as shown by measurements of emergence, seedling mass, population, plant heights, leaf areas and yield. Based largely on soil and plant analyses, it was concluded that improved growth with soil amelioration resulted from reduced aluminium toxicity and, since lime was more efficient than gypsum in neutralizing toxic Al, lime was markedly superior to the latter in improving plant growth. An annual application of 2 400 kg agricultural lime/ha increased seed yields >5 fold in the first and >10 fold in the second season in which the field experiment was carried out. In the pot experiment, no benefit of liming above the level required to neutralize toxic Al (-pH (N KCl) 4.5) was recorded and, on the contrary, yields tended to be depressed above this level. Highly significant linear relationships between yield and exch. Al (meq100g) were recorded in the pot and field experiements and, averaged over two seasons, seed yields in the field were increased 12% for each 0,1 meq/100g reduction in exch. Al. Another factor which decreased sunflower seed yields on this soil was boron deficiency and symptoms of B deficiency in the field were identified and described. At the levels of boron in the unfertilized soil, this deficiency affected the reproductive, rather than the vegetative stage of growth and correction of B deficiency by applying 10 kg borax/ha per annum increased seed yields by 38% and 18% in the two seasons, respectively. Soil amelioration had only a slight effect on the boron nutrition of sunflowers in this study. The chemical composition of the plant tissue was found to be a suitable means of quantifying B deficiency in sunflowers, the B concentration being a slightly superior method in most cases to the Ca:B ratio in plant tissue. Using field data, critical B concentrations in (i) month-old seedlings, (ii) the topmost, fully-mature leaf at flowering and (iii) in the seed were determined. The relationships were established between the amount of borax applied to the soil and the B concentration in plant tissues and these relationships could be used as a basis for recommending corrective B fertilization.