Compositional nutrient diagnosis and profitability of nutrient management options for Soybean grown on a ferralsol
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The commonly used omission nutrient approach helps in identification of limiting nutrient(s) but does not indicate nutrient imbalances in tissues, resulting in inadequate management recommendations to close nutrient limited yield gaps. Reinforcing with the compositional nutrient diagnosis (CND) approach is useful in developing balanced fertilization guidelines. Field diagnostic trials were conducted on 30 farms to identify nutrients limiting soybean production and to develop CND norms to diagnose nutrient deficiencies/imbalances on Ferralitic soils in Kiboga and Kyanwanzi, Central Uganda in 2015. A randomized complete block design in a split plot treatment arrangement covering good, medium and poor field fertility classes was established with the following treatments: Control (No fertilizer inputs); Inoculation (I); I+P; I+P+K; I+P+K+Ca; I+P+K+Ca+S; I+P+K+Ca+S+Mg and I+P+K+Ca+S+Mg+Zn+Mo. Grain yields increased by 26% and 42% above the control (1280 kgha-1) due to inoculation and I+P respectively (i.e. N and P most limiting) but did not reach the potential yield of soybean of 2500 kgha-1. Adding macronutrients (K, Ca. S, Mg) and micronutrients (Zn and Mo) to I+P was not beneficial even though yields were significantly higher than the control. The preliminary CND norms for the different nutrients were: V*N=4.79±0.34, V*P=2.03±0.43, V*K=3.39±1.62, V*Ca=- 0.72±0.53, V*Mg=-2.74±0.90, V*Na=-0.20±0.81, V*Zn=-4.76±0.22, V*Fe=-3.33±0.28, V*Mn=- 4.34±0.43, V*Cu=2.36±0.22 and V*Rd=7.84±0.33. Using the critical nutrient imbalance index (CNDr2) of 6.79, nutrient imbalance ratios were 3.32 for I+P and 3.12 and 2.97 for I+P+K+Ca+S treatments on good and on the medium and poor fields respectively, representing the least nutrient imbalances. Negative interactions were found for P-K, N-Mg, K-Cu and Mg-Rd and positive interactions to Ca-Mg, N-Na, N-Rd and Na-Rd in both the high and low yielding populations. The largest profitability was obtained from I+P treatment but how much this may change with additional costs under balanced fertilization will need to be assessed as balanced fertilizer recommendations are developed.