Introductory Soils Lab 10 Soil-based N Management NRES 201 Page 1 of 9 LABORATORY 10. SOIL-BASED NITROGEN MANAGEMENT 10.1 SOIL TESTING FOR PHOSPHORUS AND POTASSIUM. Soil testing refers to rapid chemical analyses that assess the available nutrient status of a soil, and is commonly employed as a basis for liming, and also to predict crop requirements for phosphorus (P) and potassium (K) fertilization. As discussed in laboratory 9, lime requirements are estimated from soil pH measurements with or without the use of a buffer solution. Soil tests currently used in Illinois for P and K have their origin in work by Dr. Roger Bray and his colleagues in the 1940s and 1950s. In both cases, the soil sample is treated with a chemical extracting reagent that removes some but not all of the soil’s P or K, and the extract is then analyzed colorimetrically (P) or by flame photometry (K). The method of analysis is less critical than the choice of extractant, which ultimately determines the degree of selectivity in estimating only the nutrient form(s) that affect plant growth. The classic Bray-Kurtz P1 test uses 0.025 N HCl + 0.03 N NH4F as the extractant, and mainly estimates Ca- and Al-bound P. In contrast, the usual reagent for K testing is 1 N NH4OAc, which recovers the exchangeable fraction but not clay-fixed or mineral forms. Numerous other extractants have been developed for soil P and K testing, including some designed for simultaneous extraction of multiple nutrients. The latter option has become increasingly popular with soil testing labs as a means of expediting the testing process while expanding the number of nutrient tests that can be offered. The most common multi-element extractant is the Mehlich-3 reagent for P, K, Ca, Mg, Mn, Zn, and Cu, which contains NH4F, HOAc, NH4NO3, HNO3, and a chelating agent (EDTA). Soil tests are of little value unless calibrated to crop response. Calibration is a major undertaking that requires replicated field trials, in which yield measurements are made for plots fertilized to range in the input of a single nutrient, such that the same plot area is sampled for soil testing and yield measurement. Numerous factors affect soil test calibrations, such as the crop(s) to be grown, soil type and sampling depth, crop rotation, planting rate and pattern, and fertilizer and tillage practices. Not all of these factors can be standardized for routine soil testing, but three in particular deserve special attention, namely sampling depth, the type of crop, and plant population. The standard sampling depth for pH, P, and K is 7 inches. This practice was adopted to represent the normal depth of moldboard plowing, within which sampling and nutrient management are simplified considerably, but is far from ideal, considering that the zone of active rooting typically extends to 6 feet with corn, and to 5 feet with soybean. Automated systems are now available with the capability for routine sampling to 24 inches, and would be a vast improvement but for the lack of corresponding soil test calibrations. Recalibration is further warranted because current P and K tests were developed at a time when corn was typically planted at a rate of 8,000 to 12,000 plants/acre, whereas modern planting rates often exceed 30,000 plants/acre. The effect in the latter case will be an upward shift in calibration, as more plants need more food. A further shift may be necessary, depending on the crop to be grown. For example, wheat requires less K than corn, but is more responsive to P.Introductory Soils Lab 10 Soil-based N Management NRES 201 Page 2 of 9 10.2 YIELD-BASED MANAGEMENT FOR NITROGEN. Unlike P and K, soil testing has not been utilized for N fertilization of corn in Illinois. Instead, a yield-based system known as the proven yield method has been in use since the 1970s. With this system, an expected yield goal is multiplied by 1.2, with adjustments to account for N credits from previous cropping or a recent application of manure. Although not generally recognized, several assumptions are implicit to the proven yield method, as listed below: 1. Two-thirds of crop N uptake is from fertilizer, with one-third from the soil. 2. All soils should be fertilized with N. 3. Fertilizer N efficiency is constant, regardless of soil type, the time or method of N application, and management or cropping practices. 4. Higher yields need more fertilizer N. 5. Whole-field N management is adequate. These assumptions are at odds with existing information from scientific research. For example, numerous studies using isotopically labeled fertilizer N (15N) have demonstrated for the growing season as a whole, greater uptake of unlabeled soil N, relative to labeled fertilizer N. The following figure is from one such study conducted in western Illinois, and indicates that, even with 240 lb N/acre, the soil still supplied the majority of N for crop uptake. Figure 10-1 Comparison of soil and fertilizer N uptake by corn.1 Soil N occurs largely in organic forms, whereas plants require inorganic N for uptake, in the form of ammonium (NH4+) or nitrate (NO3–). Soil microorganisms are responsible for 1Stevens, W. B., R. G. Hoeft, and R. L. Mulvaney. 2005. Fate of nitrogen-15 in a long-term nitrogen rate study: II. Nitrogen uptake efficiency. Agron. J. 97:1046-1053. 0501001502000 60 120 180 240N Applied (lb/acre)N Uptake (lb/acre)Total N Soil N Fertilizer NIntroductory Soils Lab 10 Soil-based N Management NRES 201 Page 3 of 9 converting organic N to NH4+-N, a key process known as mineralization. Due in part to the long-term impact of management and cropping practices, there are inherent differences among soils in their capacity to supply plant-available N, which decreases crop response to N fertilization. Figure 10-2 documents these differences for 69 N-response trials conducted at on-farm sites in Illinois between 1990 and 2003. In particular, note the spread of points along the x-axis (optimum N rate) when optimum yield (y-axis) was relatively similar. This is exactly what would be expected if mineralization varied, such that a high N rate was required at some sites (points to the right) but not others (points to the left). Such variation would explain why there was no significant relationship between optimum yield and N rate, invalidating assumption #4 in the proven yield method. A different approach must be taken to improve fertilizer N recommendations.