Potassium Requirements for Processing Tomatoes

Tim Hartz, Extension Specialist, UC Davis

Paste to Peel: The California processing tomato industry has traditionally been focused on the production of bulk paste, for which the primary fruit quality factors were blended color and soluble solids content. However, changes in the marketplace over the last decade (increasing sales of salsa and 'chunky' sauces, for example) have dramatically increased the need for whole, peeled fruit.

Yellow Eye and White Streak: A major quality issue for peeled fruit is color uniformity. A fruit color disorder called 'yellow eye', in which a ring of tissue around the stem scar remains yellow upon ripening, is a widespread problem. Yellow eye can range in severity from just a narrow halo of yellow around the stem scar, to the entire shoulder area of a fruit remaining yellow. When the peel is removed, the extent of yellow tissue is usually larger than was evident externally.

An associated problem is the presence of veins or streaks of white tissue that can occur throughout the fruit. Together these color disorders can be severe enough to prevent whole production fields from being used for peeled, diced products.

Survey of 140 Fields: A project was conducted to determine the cause of these color disorders, and to develop management practices to minimize their occurrence and severity. County Farm Advisors Kent Brittan, Mike Cahn, Gene Miyao, Bob Mullen, Mike Murray, and Jesus Valencia, together with Veg Crop Specialist Jeff Mitchell and I conducted a statewide survey of 140 commercial tomato fields to determine if soil fertility characteristics and fruit quality were correlated.

K, Mg and Yellow Eye: The survey showed a clear relationship between exchangeable soil potassium (K), and the ratio of exchangeable soil K to magnesium (Mg), and these color disorders. Unlike the Sacramento Valley, where high soil Mg is a common problem, soil Mg levels in Fresno, Kings, Tulare, and Kern Counties are generally moderate to low. In these counties the occurrence and severity of yellow eye and internal white tissue can be predicted mainly by the level of exchangeable soil K. Soils in which K makes up at least 3% of cation exchange are likely to produce good fruit color uniformity. Table 1 lists the minimum exchangeable soil K level to meet this 3% rule for various soil textures.

K and Yield: In addition to fruit color issues, soil K level can also affect yield, and perhaps soluble solids. From 1994 to 1998 a total of 16 field trials were conducted to evaluate the effect of K application rate and application technique on processing tomato yield, soluble solids, and blended color. The results were very consistent. In 2 of 3 trials in which the soil had < 130 PPM exchangeable K, yield was increased with K fertilization; not one of the 13 trials in which soil K > 130 PPM showed a significant yield increase. In no field did K fertilization increase soluble solids or blended color.

In six of these field trials the effect of K fertilization on the fruit color disorders was also determined. The occurrence or severity of yellow eye and internal white tissue was reduced by K fertilization in 4 of these fields, but the amount of improvement was modest, and fertilizer application in excess of 400 lb K2O per acre was required to obtain a significant response.

Summary: Given this background, my advice to processing tomato growers in the southern San Joaquin Valley is:

1) K fertilizer application in fields with soil exchangeable K < 130 PPM may be necessary to ensure maximum yield; fields with more available K are unlikely to show a yield increase with K fertilization. K fertilization rates of 50-150 lb K2O per acre are appropriate in fields with exchangeable K between 100-130 PPM, while rates as high as 250-300 lb K2O per acre may be justified in fields with very low K. Regardless of initial soil K level, do not expect K fertilization to result in increased soluble solids content or blended color score; these quality characteristics are controlled mainly by factors other than soil fertility.

2) If you are growing tomatoes destined for peeling, plant them in fields with high soil K, ideally at least as high as the levels suggested in Table 1. This is no guarantee of freedom from the color disorders, but it is the most reliable way to maximize peeling quality. Although heavy K fertilization can reduce yellow eye and internal white tissue, it is not likely to be cost effective unless you are paid a substantial premium for high peeling quality.

Table 1. Predicted minimum soil exchangeable K required for optimum tomato fruit color uniformity.

*top 12 inches of soil

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