Tolerance of Selected Maize (ZEA MAYS L.) and Soyabean (Glycine Max L. Merr.) Cultivars to Soil Acidity
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Soil acidity is the most important growth-limiting factor especially in the tropics and sub-tropics. Increased acidity in Zimbabwe’s smallholder agricultural soils has been attributed to use of acidifying fertilizers without liming. Low input farming in some areas also contribute to increased soil acidification. Aluminium and Mn toxicity together with low P, Ca, Mg, K, N and Zn affect plant growth on acid soils. However, tolerant cultivars may allow for economic crop production under these conditions. The aim of this study was to develop a protocol for screening maize and soyabean cultivars for tolerance to Al and soil acidity and to assess response to lime by these cultivars on light and heavy textured soils. Soil samples (100) and 90 soil amendment samples (80 cattle manure, 5 ash and compost and 5 termitaria) were sampled from Chendambuya smallholder farming area (SFA) to determine the fertility status and the extent of soil acidity. Five maize (SC 403, SC 517, CZH 00013, CZH 00017 and DK 8031) and 3 soyabean (Magoye, Safari and Solitaire) cultivars were screened for tolerance to Al in modified 1/5 Steinberg solutions with 0, 4, 8 and 16 mg L-1 Al. Maize (SC 403, SC 513, SC 517, PAN 413 and PHB 30G97), and soyabean (Magoye, Safari, Solitaire and Storm) cultivars were grown on limed and unlimed acid sandy and red clay soils in Chendambuya SFA and at Domboshawa Training Centre for two seasons. Soils from Chendambuya had low pH (averaging 4.43, sem 0.08) and only 19% were heavier soils comprising mainly red clay soils (21 – 40% clay) with over 4 times higher Ca and Mg compared to lighter textured sandy soils (0 –5% clay). About 89% of the interviewed households applied cattle manure annually at an average rate of 5.3 t ha-1, while ash and compost were applied once in 3 – 4 years in low quantities, and termitaria was only spread on the ground in the first year of ploughing to level the field. The manure was of poor quality (84% containing < 1.5% N), low liming value averaging 37%, and high soil content (averaging 56%). Aluminium reduced shoot dry matter yield (SDMY) for all maize cultivars (65%, 84%, 82%, 74% and 77%reduction for SC 403, SC 517, CZH 00013, CZH 00017 and DK 8031, respectively, at 16 mgL-1 Al). Root responses varied across cultivars and were thus more appropriate in screening maize for Al tolerance. The Al tolerance index (ATI) showed that CZH 00017, CZH 00013, SC 403 and DK 8031 were more tolerant to Al (ATI of 3.1, 3.4, 4.7 and 5.0 respectively) compared to SC 517 (ATI of 1.0). Tolerant cultivars responded positively to low Al concentrations (4 mgL-1) and were more efficient in nutrient uptake and utilization. At 4 mgL-1, Al resulted in an increase in soyabean root and shoot dry matter yield. Magoye and Safari (ATI of 5 and 2.51 respectively) were relatively more tolerant to Al compared to Solitaire (ATI of 1). Nutrient (P, Ca and Mg) translocation was reduced in all soyabean cultivars. Differential cultivar tolerance to Al was observed with both maize and soyabean. Tolerance to Mn and determination of tolerance mechanisms are still areas to be researched on. Lime reduced maize stover (up to 39%) and grain yield (up to 56%) on sandy sites except at Mudzengerere site with much lower Ca and K contents (0.70 and 0.09cmolckg-1 respectively) where yields were increased (up to 43% and 92% for stover and grain respectively). Up to 69% grain yield increases were realised on red soils. Cultivar PHB 30G97 was recommended for acid soils as it yielded higher grain (6.18 tha-1 and 6.67 tha-1) than the Al tolerant SC 403 (5.29 tha-1 and 5.83 tha-1) on acid red and on Domboshawa sandy soils, respectively. Nodule numbers and weights were increased by lime especially in the second season. Lime increased grain yield for Magoye and Safari in the two seasons and for Solitaire in the second season only. Soil acidity in Chendambuya SFA was high on both heavy and light textured soils. Cattle manure availability and quality was too low to have a fertilising and liming effect on soils. Aluminium tolerant cultivars can be selected for acid soil conditions using the ATI. Although liming is profitable on heavy textured soils, liming on light textured soils may result in reduced maize yields due to possible micronutrient deficiencies and nutrient imbalances in poorly buffered soil.