POTASSIUM FERTILIZER APPLICATION IN DRIP AND MICRO-JET IRRIGATED ALMONDS

Potassium fertilizer recommendations for California almond [Prunus dulcis (Mill.) D.A. Webb] production were developed decades ago for flood and sprinkler irrigated full-coverage orchard conditions. High rates, up to 930 kg K ha, at considerable expense were often suggested on soils exhibiting high levels of K fixation. Today, almonds are widely produced under low volume irrigation, which greatly limits the wetted soil area and restricts almond root systems to a relatively small portion of total soil volume. A field trial was conducted comparing fall applied surface banded potassium sulfate (BKS) with in season injected potassium sources: sulfate (KS), thiosulfate (KTS) and mono-potassium phosphate (MKP) through three irrigation systems: single line drip (SLD), dual drip (DD) and micro-jet (MJ) on two almond cultivars, ’Nonpareil’ and ‘Butte’ at the Nickels Soil Laboratory. Yields for the SLD and DD were nearly the same in 1997 and 1998 but somewhat lower than the MJ. In 1999, the DD had the highest yield followed by the MJ and then the SLD. Three year average yields were 3111, 2990 and 2832 kg ha for the MJ, DD and SLD systems, respectively. The Butte cultivar had significantly higher yields in 1997 and 1998 as well as the 3 year average yield of nearly 400 kg ha higher than the NonPareil cultivar. The 1999 yields showed a strong trend for response to applied potassium with some significant differences over the control and between fertilizer sources and placement. The highest yield was recorded with 1.13 kg K tree MKP, then by 2.26 kg K tree from banded KS and injected KS, followed closely by the 1.13 kg K tree of injected KS and KTS with the control yielding 800 kg ha less than the highest yielding MKP. Leaf total K increased significantly with applied K over the three year study with MJ showing the greatest increase followed by DD and then SLD system. INTRODUCTION The irrigation of almonds is accomplished with a number of different systems that may apply the necessary water to a very limited soil volume up to flooding the entire soil surface and wetting all of the soil. High yielding almond orchards with declining leaf potassium levels on the West side of the Sacramento Valley and other areas of the state have given growers cause for concern regarding how best to apply potassium. The 1996 Almond Production Manual (Brown and Uriu, 1996) suggests 9.08 kg K2SO4 tree is desirable for a multiple year quantity to adequately supply the potassium nutrition needs of the orchard. As orchards often have planted densities of 150 to > 300 trees ha this resulted in considerable expense, or, if potassium chloride were applied at these high rates, chloride toxicities may result. Fertilizers are applied to the soil surface in a band or broadcast before irrigation or winter rains, or through the irrigation system. In this way the fertilizer is placed in the wetted soil where tree roots will come in contact with the potassium. If added through a low volume or smaller wetted area system, even potassium fertilizers, which are not easily moved in soils, have been taken up readily by trees (Uriu et al., 1980; Andreu et al., 1997). Some fertilizers, however, are not dissolved easily or water quality characteristics prohibit trouble free injection into irrigation systems. Growers choose to apply potassium fertilizers on the soil surface in a band approximately 257 Proc. V th IS on Irrigation of Hort. Crops Eds.: I. Goodwin and M.G. O’Connell Acta Hort.792, ISHS 2008 1.2 m from the tree row as an alternative. The availability of several new formulations of potassium: as sulfate, phosphate and thiosulfate make it easier to inject into irrigation systems. Potassium chloride is the most economical source of potassium but the addition of chloride represents a potential detrimental effect. Given the different types of application now being utilized, it seems prudent to evaluate the relative efficiency of potassium uptake from several sources and methods of placement under three of the more typical low-volume irrigation systems (Schwankl et al., 1999). The objectives of this study were: 1) to evaluate three irrigation systems: micro-jet (MJ), single (SLD) and dual line drip (DD) as they influence the response of potassium fertilizers on the nutrient concentrations in leaves and nut meat yields of almonds; and 2) to evaluate the effects of fall banded potassium sulfate (BKS) versus in-season injected potassium thiosulfate (KTS), mono-potassium phosphate (MKP) and several rates of potassium sulfate (KS) on the nutrient concentrations in leaves and nut meat yields of almonds. MATERIALS AND METHODS The Butte and NonPareil cultivars of almond [Prunus dulcis (Mill.) D.A. Webb] of the irrigation system comparison trial on the Marine Avenue location of Nickels Soil Laboratory, Arbuckle, California were selected for this study. The orchard was planted in the spring of 1990 to 4 cultivars: Butte (B), NonPareil (N), Carmel (C), and Monterey (M) in a B-N-C-B-N-M sequence but only the first two rows of Butte and NonPareil were utilized in this experiment. The soil at the site was an Arbuckle gravelly loam (Fineloamy, mixed, thermic Typic Haploxeralf) having the characteristics given in Tables 1 and 2. Even though the soil exchangeable-K levels are in the adequate range (> 100 mg kg), the clay content is rather low and after 5-10 years of high yield production, leaf K concentration often drop below 10 g kg indicating a potassium response can be expected. Each individual plot size was 5 trees with a 4.9 m in-row spacing and 6.7 m between rows in a “diamond” arrangement (307 trees ha). There was a total of 36 individual plots; three irrigation systems by six fertilizer treatments by two almond cultivars, each having 5 trees in this experiment. The two cultivars: NonPareil and Butte served as two replications for the irrigation and fertilizer treatments. The six treatments applied annually were: Control (no K), 0.38 kg K tree as KS, 0.75 kg K tree as KS, 0.38 kg K tree as MKP, 0.38 kg K tree as KTS, and 0.75 kg K tree as BKS. Each treatment was randomly assigned within the three irrigation systems and two cultivars. Liquid KS was injected as Great Salt Lake ESP (1-0-8), KTS was injected as liquid 00-25, and MKP was a dry granular material (0-52-34) added to water to inject as a liquid. Liquid fertilizer injection units were designed, built and installed the summer of 1996 to inject fertilizer for each 5 tree plot. The two main irrigation systems, one for the drip (both SLD and DD) and one for the MJ, were turned on approximately one hour before any fertilizer was injected to improve the uniformity of application. Dry granular potassium sulfate (0-0-50) was applied on the soil surface in a band 7-10 cm wide approximately 1.22 m from the tree on both sides of the tree row. The trial was initiated with the band applications being made the fall of 1995, which is the normal grower practice. Liquid materials were injected through cylinders for each individual plot of 5 trees which were 15 cm in diameter and various lengths (60 up to 150 cm) to accommodate the different volumes of liquid fertilizer having a range in potassium concentration. Potassium fertilizer injection treatments were applied in July and August of 1996 (both after leaf sampling); May and June of 1997 (both before leaf sampling); and June and July of 1998 and 1999 (first before and the second after leaf sampling) with the 0.75 kg K tree injected rates being split into 2 applications. Leaf samples (25 leaves from each of the 5 trees per plot) were taken three times each year and analyzed for Total N, P, K, S, Zn, Mn, B, and Cl to evaluate the nutrient status. Leaf samples were taken on April 4, July 7, and October 7, 1997; April 2, July 7, and October 8, 1998; as well as March 31, June 29, and October 13, 1999 to evaluate nutrient concentrations. Yield data were collected on August 27, (NonPareil) and