Variable Rate Fertilizers for Grape Nutrient Management

Transcription

1 Variable Rate Fertilizers for Grape Nutrient Management Joan R. Davenport, Jaimi M. Marden, and Lynn Mills Washington State University Irrigated Agriculture Research and Extension Center N. Bunn Road Prosser, WA Introduction There are many possible causes of yield and quality variability across a vineyard; plant nutrient availability is one of these (Gladstones, 1994). Typically in a vineyard setting, nutrients are applied across an entire field or large sections of a field at a uniform rate. In a uniformly fertilized field, nutrient availabilty reflects the existing plant nutrient variability in the field as well as soil chemical or physical properties that influence nutrient availability within the field. Many factors can influence the variability of nutrients within a field (Brady and Weil, 1999). Because of this variability, soil scientists have long sought ways to manage nutrients differently across fields. In the late 1980 s, variable rate application (VRA) fertilizer equipment was developed for large scale, row crop farming. In the late 1990s, a VRA fertilizer spreader was developed commercially for narrow row (ca. 9 ft) fruit crop systems. Yield monitoring was employed in a study of the effects of different pruning treatments on Concord grape (Vitis labrusca L.) yield and quality (Wample et al, 1999). In the initial year of the study, there was a great deal of yield variability that was not related to pruning treatment. This, coupled with advances in technologies to apply fertilizer at variable rates throughout the vineyard, resulted in a companion study to evaluate the ability to reduce yield variability through variable rate fertilizer applications. The objective of this study was to evaluate changes in soil chemical properties after 4 years of variable rate fertilizer applications. Materials and Methods In the spring of 1998, a 10 A. vineyard was soil sampled intensively at 80 grid points (Fig. 1). Soil samples were collected for two depths a surface sample from 0 12 and a subsurface sample from making a composite sample at each grid point and depth with three cores obtained with a 2 i.d. bucket auger.

2 Figure 1: Vineyard (10 A) outline, elevation (m) and soil sampling points for 1998 (initial sampling points) and from (2001 sampling points). Samples were returned to the lab and divided in half. One-half of each sample was sent to a commercial test lab for analysis of ph, P, K, organic matter, and cation exchange capacity. The other half was air-dried. A subsample was extracted with 2 N KCl and analyzed for NO 3 -N colorimetrically (Mulvaney, 1996). An additional subsample was used to determine soil texture (data not reported). Table 1: Levels of statistical significance (P) for main and interactive effects. ph NO 3 -N P K Year Soil Depth Year*Soil Depth

3 The data from the soil analysis was used to develop a variable rate fertilizer application across the field. Fertilizer was applied using an AgChem orchard spreader with N, P, and K variably applied across the field (Table 1, 2). In 1999, 2000, and 2001 soils were collected at a subset of original sample points with some additional intermediatary points in areas of interest (Fig. 1). Each year, one-half of each sample was sent to a commercial test lab for analysis of ph, P, and K, and the other half was analyzed for NO 3 -N as described above. The same procedure was used to apply variable rate fertilzer application across the field (Table 2). Samples were collected in the spring of 2002 to determine the net effect of four years of variable rate fertilizer application on plant nutrient levels in the soil. Table 2. Average soil test parameters for 5 years and coefficients of variability (CV) ph NO 3 -N (ppm) P (ppm) K (ppm) Surface soil (0 12 ) Average CV Average CV Average CV Average CV Average CV Subsurface soil (12 30 ) Average CV Average CV Average CV Average CV Average CV Soil N, P and K data were classified into groups that correspond to the ranges of soil test values for suggested rates of fertilizer application in grape (Dow et al., 1976). For example, Table 3 shows the soil test ranges for how much K fertilizer should be applied when the soil test K value is in a range. Each of these ranges is the classification used for this analysis. Thus, a soil test K of 100 ppm falls in the range of ppm, and since this is the second range, it would be classifed as a 2.

4 Table 3: Classifications for potassium based on soil test values. Soil Test K Value Apply this amount (lbs/a) Classification (ppm) K (elemental K) K 2 O (fertilizer K) > Data, both raw and classified, were analyzed by analysis of variance using SAS PROC GLM (SAS, 1998) and means, standard deviations, and coefficient of variability were determined for each element in both surface and subsurface depths in each year using EXCEL. GIS maps showing the spatial variability across the fields were made with ArcView 3.2 (ESRI, Redmond, WA), using inverse distance weighting and classifying with the soil N, P and K classes described above. To determine how variable a nutrient was in a field, and how that changed from year to year, we calculated an index known as the Coefficient of Variability (CV, Steel and Torie, 1960). CV is a measurement of variability across a set of numbers; in the case of this study, across a field. Thus, for each different factor we looked at (e.g., soil P) numbers that are higher have more varibility in this factor than numbers that are lower. Results and Discussion All of the parameters in this study were significantly different by year and soil depth and, with the exception of P, the interaction between year and soil depth (Table 1). Soil ph had very low variability compared to the plant mineral nutrients. With the exception of 1999, there was less variability in ph in surface soil compared to the subsurface (Table 2, Fig. 2). Both surface and subsurface soil ph decreased in 1999 and 2000 then increased again in 2001 and A uniform rate application of 550 kg/ha of granular S was made in the springs of 1998 and 1999 but not in sucessive years which likely accounts for initial decrease and subsequent increase in soil ph. Actual NO 3 -N, P and K had higher variability than the classified values (Table 2).

5 Figure 2: Surface (top) and subsurface (bottom) soil ph from spring soil sampling over a 5 year period where 1998 was sampled prior to variable rate fertilizer applications and 2002 after 4 seasons of variable rate fertilizer management in in. With the exception of 2002, surface NO 3 -N was higher than subsurface NO 3 -N (Fig 3). Variability in surface NO 3 -N decreased with time until the last year sampling. The same trend was found in subsurface NO 3 -N with the exception of 1999 where nitrate in general was very low, likely due to higher December March precipitation in that year compared to other years of this study. The increase in subsurface NO 3 -N over surface NO 3 -N in 2002 and the increase in variability was likely due to a change in management practices. Until 2002, this vineyard was irrigated using rill irrigation. In 2002, the irrigation system was changed to an overhead sprinkler system that resulted in higher overall soil moisture at both surface and subsurface depths (Fig. 4).

6 Figure 3: Surface (top) and subsurface (bottom) soil nitrate nitrogen from spring soil sampling over a 5 year period where 1998 was sampled prior to variable rate fertilizer applications and 2002 after 4 seasons of variable rate fertilizer management in in CV - Surface Subsurface

7 Figure 4: Seasonal changes in soil moisture content in 0 3 (top) and 3 5 (bottom) soil depths in 2000 with rill irrigation and 2001 with over canopy sprinklers. In general, P was higher in the surface than the subsurface soil while variability was higher in th subsurface depth (Fig. 5). In all years, average surface soil P was at or above the 20 ppm (Table 2), the point beyond which no P fertilizer is needed (Dow et al., 1976). Average subsurface P was always slightly below this level. As a result, little P fertilizer was applied. Although there were differences in soil P concentration by year, there was neither a net increase or decrease in P over the duration of the study and, if anything, variability increased with time. Thus, variable rate P management, in this high available P situtation, did not seem effective.

8 Figure 5: Surface (top) and subsurface (bottom) soil phosphorus from spring soil sampling over a 5 year period where 1998 was sampled prior to variable rate fertilizer applications and 2002 after 4 seasons of variable rate fertilizer management in in. CV - Surface Subsurface

9 The upper limit of soil test K for adding K fertilizer is 240 ppm (Dow et al., 1976). The surface soil had a small portion of the field with K at this level before the variable rate fertilizer applications were made. The surface soil K plateaued in 2000 and by 2002 was lower than at the start of the experiment while (Table 2). The subsurface soil K was always below the 240 ppm level and continually decreased with time (Fig. 6). Of the three plant nutrient elements, K had a consistently low variability (14-26%, Fig. 6). The low variability and the decrease with time imply that variable rate management for this nutrient may be beneficial. Figure 6: Surface (top) and subsurface (bottom) soil potassium from spring soil sampling over a 5 year period where 1998 was sampled prior to variable rate fertilizer applications and 2002 after 4 seasons of variable rate fertilizer management in in. CV - Surface Subsurface

10 Conclusions Based on the soil test results following four years of variable rate fertilizer application of N, P and K to grape, this practice appears to have benefit for N and K management as these nutrients showed either no change or a decrease in variability with time. On the other hand, soil test P variability in the surface soil, above the root zone, remained relatively constant but root zone P increased in variability with time, indicating that variable rate management for this nutrient was not effective. Literature Cited: Brady, N. C., and R. R. Weil The nature and property of soils, twelfth edition. Prentice Hall, Upper Saddle River, N.J. Dow, A. I., W. J. Clore, A. R. Halvorson, and R. B. Tukey Fertilizer Guide for Irrigated Vineyards. Washington State University Cooperative Extension Service FG-13, Pullman, WA. Gladstones, J Viticulture and environment. Winetitles, Underdale, South Australia. Mulvaney, R.L Nitrogen - Inorganic forms. p In J.M. Bigham (ed.) Methods of Soil Analysis Part 3: Chemical Methods. No. 5 in SSSA Book Series. ASA-CSSA-SSSA, Madison, WI. SAS Institute SAS/STAT user s guide for personal computers, Ver. 8. SAS Inst., Cary, NC. Steel, R. G. D, and J. H. Torie Principles and procedures of statistics. McGraw-Hill, London. Wample, R. L., L. Mills, and J. R. Davenport Use of precision farming practices in grape production. p In P. Robert, R. H. Rust, and W. E.Larson (eds). Proc. 4th Internat. Conf.on Precision Ag., Minneapolis-St Paul July 19-22, ASA/CSSA/SSSA Press, Madison, WI.