To serve as a useful guide to fertilizer management, a nutrient diagnostic method should identify cases in which a nutrient limits yield, as well as cases in which the nutrient does not limit yield. Erroneous diagnoses can lead either to unnecessary fertilizer application or failure to apply fertilizer when a yield response would result. We propose a process of prescient diagnostic analysis to evaluate the utility of diagnostic methods based upon comparing individual diagnoses to corresponding responses to fertilizer applications. A nutrient diagnostic method is evaluated upon the incidence and yield consequences of its true and false positive and negative diagnoses. Evaluation criteria integrate these effects to facilitate comparison of various diagnostic systems. We propose a protocol for evaluating diagnostic methods based upon this reasoning.

Little information is available on the effects of levels of soil P and K on soybean [Glycine max(L.) Merr] root growth and morphology. Soybeans were grown for 9, 16, and 21 days in Raub silt loam soil (fine‐silty, mixed, mesic Aquic Argiudoll) to measure the effect of increasing soil P and K on root growth and morphology and its relation to shoot growth, nutrient status of shoots, and net nutrient influx. Increasing soil P increased shoot weight, root surface area per plant, root surface area per g of root, P, K, Ca, and Mg influx and P, K, Ca, and Mg concentration of shoots. Adding P decreased the root surface area per g of shoot and increased root surface area per g of root. Increasing soil K increased shoot weight (with high P soil), P, K, and Ca influx, and P and K concentration of shoots. Potassium decreased Mg influx and Mg concentration of shoots. Significant positive P ✕ K interactions were found for secondary root radius, P, K, and Mg influx and shoot concentration showing that P and K had their greatest effect when the other nutrient concentration in the soil was high. The effect of P and K on nutrient concentration of shoots followed the same pattern as that for nutrient influx. Since adding P or K changes root surface area per g of shoot, influx of other nutrients changes when they are added. Where influx increases, a sufficient soil level of each nutrient is then needed so that soil supply does not limit influx.

No abstract

are enhanced. Sugarcane germination can also be promoted by increasing soil Ca (Mohandas et al., 1983). Louisiana sugarcane (Saccharum spp.) is produced mainly on For some crops, gypsum is effective in reducing the heavy-textured soils that offer less than ideal conditions for growth and function of the root system. Cultural practices that improve the incidence of soil-borne diseases (Kao and Ko, 1986). soil environment could benefit sugarcane production by increasing Moreover, mixing 11.2 and 22.4 Mg ha Ϫ1 of by-product root growth and reducing the incidence of ratoon decline. The objecgypsum into heavy-textured soils has been shown to tive of our research was to determine the effect of gypsum and comincrease sugar yields of ratoon sugarcane in Louisiana posted, municipal-biosolids application on root growth, crop yields, (Breithaupt et al., 1991), though it was not determined and leaf nutrient concentrations of sugarcane grown on a silty clay whether gypsum affected sugarcane root growth. loam soil. Gypsum mixed into the rows at 2.24, 4.48, and 8.96 Mg ha Ϫ1 Other research showed a yield increase of 15% in did not affect (P Ͼ 0.05) root growth or cane and sugar yields. Likewheat (Triticum vulgare Vill.) and sorghum (Sorghum wise, both subsoil-and within-row applied compost at a rate of 44.8 Mg vulgare Pers.) with gypsum addition (Thomas et al., 1995). ha Ϫ1 did not affect cane or sugar yields compared with the control. Gypsum increased Ca, S, Mn, and Zn leaf concentrations, but had Gypsum applied in irrigation water increased sugar yield no effect on N, P, K, Mg, Cu, and Fe concentrations. Subsoil and and juice extraction percentage of sugarcane (Kumar within-row compost increased leaf S concentration; within-row comet al., 1999). Gypsum also increased yield in corn (Zea post increased leaf K; and subsoil compost increased leaf Zn, but mays L.) and alfalfa (Medicago sativa L.) up to 50%. reduced leaf Mn compared with the control. Compost application did This yield response was partially attributed to higher not increase Mn, Cu, Fe, or Zn concentrations in sugarcane leaf tissue exchangeable Ca and S, and a complementary reduction beyond acceptable limits. Within-row applied compost reduced (P Ͻ in exchangeable Al (Toma et al., 1999). 0.05) root surface area compared with the control, and reduced sugar Compost improves soil structure by enhancing aggreyields compared with the subsoil compost treatment. This suggests gate stability (Tate, 1987), which results in improved that, at the compost rate used in our study, subsoil rather than withinrow application of compost, is the preferred practice for sugarcane water holding capacity and aeration. Similarly, the benegrown on this soil.

Due in part to the difficulty of isolating roots from soil and measuring them, little research has been conducted on root growth and morphology and its relation to shoot growth, nutrient status of shoots, and net nutrient influx. A soil experiment was conducted to measure the effect of increasing soil K and soil bulk density on the above parameters and their interaction. A 2 × 2 × 3 factorial experiment (soil bulk density × K added × date of harvest) was conducted in a greenhouse using soybeans [Glycine max (L.) Merr.] grown in Raub silt loam soil (Aquic Argiudoll). Increasing soil K increased root and shoot growth but had a variable effect on net nutrient influx and plant nutrient content. Increasing soil bulk density reduced root growth, shoot growth, and nutrient composition of shoots but increased net nutrient influx. The detrimental effect of increasing soil bulk density on shoot growth and nutrient status of soybean plants may have been caused by decreased root growth, coarser roots, and lower root surface area/g shoot. Changes in nutrient influx and root surface area/g shoot in some cases counteracted each other in determining plant nutrient status.