1 Fluid Journal Fall 2000 Summary: Foliar fertilization of soybeans with macronutrients at early vegetative stages is likely to increase yields under some conditions, even in high testing soils. There were no consistent differences between products, rates, or frequencies of application except for two considerations. The high rate (6 gal/A) of 10-10-10 did not affect or reduce yield. A single application of 3 gal/A of 3-18-18 usually produced the highest yield increases. No simple set of measurements can be used to predict responses. Results suggest, however, that responses will be more likely when effective early nutrient availability is low (which does not necessarily mean a low-testing soil) and/ or when climatic factors limit plant growth in late spring or summer. Results for one year also suggest that responses are more likely in ridge-till or no-till fields. In these instances, responses as high as 10 bu/A are possible. Across all conditions, especially with predominantly high- testing soils as those used in this study, expected average response is about 1 bu/A. Little effort has been dedicated to the study of foliar fertilization of soybeans during the early vegetative stages. Fertilization at early stages could increase yields by different mechanisms compared with fertilization at reproductive stages. Field observations and research with P and K in Iowa suggest that nutrient deficiencies may occur during early growth of corn or soybeans when topsoil is dry in late spring or early summer, even for fields that have been fertilized. Because fertilizers are usually incorporated into the first 4 to 6 inches of soil with chisel/ disk tillage, or are not incorporated with no-till, deficiency symptoms may be partly explained by inhibited activity of roots when this layer is dry. This situation may occur often in soils with low P and K below the 6-inch soil layer. In these situations, foliar fertilization could result in increased growth and higher yield. There are also physiological reasons for expecting positive responses of soybeans to foliar N fertilization during early vegetative stages. Although soil N uptake and N fixation can occur Drs. Antonio P. Mallarino and Mazhar UI-Haq What About Foliar Fertilization Of Soybeans? Though a practice not widely used by growers, renewed interest by researchers has producted some positive yield responses in Iowa studies. simultaneously, the development over time of these processes is different. Measurable amounts of N fixation are usually first evident several weeks after emergence. The fixed N increases slowly until a maximum is reached during pod set and early seed filling, and then declines sharply. Soil N uptake reaches a peak at early- to mid-flowering and usually declines rapidly afterwards. Responses to soil-applied N have been ineffective in well nodulated soybeans. It has been shown that as soil nitrate increases nodule weight and size, N fixation decreases. Although high rates of foliar-applied N would cause serious leaf damage, small rates could stimulate growth without inhibiting nodulation. Thus, small amounts of N, P, and K applied at early critical periods could be effective if foliar fertilization is viewed as a complement for soil P and K FOLIAR FERTILIZATION OF SOYBEANS - 21 TRIALS IN 1994 ALL TRIALS 7 RESPONSIVE TRIALS SOYBEAN YIELD (bu/acre) 60 55 50 45 40 35 Check Figure 1. Soybean yield response to early foliar fertilization 21 trials, Mallarino, et al., Iowa State University, 1994. 2 gal 3-18-18 2+2 gal 3-18-18 3 gal 3-18-18 3 +3 gal 3-18-18 4 gal 3-18-18