FERTILIZER GUIDE FOR CALIFORNIA VEGETABLE CROPS

K. B. Tyler and 0. A. Lorenz, Department of Vegetable Crops, University of California, 1991

The usual questions about fertilizing California vegetable crops concern nutrient requirements, nutrient sources, application methods, rates, or timing. This guide attempts to provide brief answers to some of these inquiries.

Fertilizer Requirements of California Vegetable Crops
Field experiments, crop nutrient surveys, and grower experience have shown that California vegetable crops usually respond to the application of nitrogen (N) fertilizer with increased yield, improved quality, or both.

Phosphorus (P) application is also important for vegetable crops, although approximately 50% of the state's soils, which produce vegetables, are adequately supplied with levels of available P sufficient for maximum productivity and high quality. Appropriate soil analyses usually give reliable indications whether or not P fertilizer is needed for the next vegetable crop. P is especially important for enhancing early seedling growth.

Potassium (K) is required less often that either N or P, however California does have some K deficient soils where vegetable crops are currently produced. K fertilizer is usually applied to these soils and particularly for crops such as celery and potatoes, which have high K requirements. Soil analyses can be used to indicate whether or not the next crop is likely to respond to K application.

The secondary plant nutrients, calcium, iron, and magnesium, and all micronutrients, with the exception of zinc, are seldom needed for vegetable production in California. Some California soils are very low in available zinc and certain sensitive vegetable crops such as beans, onions, and sweet corn may be stunted and produce poorly unless zinc fertilizer is applied.

Nutrient Sources for Vegetables
N fertilizers that have proven most effective for California vegetable crop production are those which contain ammonic nitrogen rather than nitrate. Urea, which rapid' y hydrolyzes in soils to produce ammonic nitrogen, is also a satisfactory source of N for vegetable crops. Nitrate fertilizers, which generally cost more than urea or ammonic fertilizers, are more subject to leaching and denitrification losses in soils. Controlled release nitrogen materials have not performed well in UC vegetable crops comparisons with urea and ammonic fertilizers.

Most water-soluble P fertilizers, such as ammonium polyphosphate, ammonium phosphate, single and treble superphosphate, and phosphoric acid, perform satisfactorily for California vegetable production. Rock phosphate and other P materials, which are predominantly citrate-soluble, but not water-soluble, have performed poorly in vegetable field experiments on alkaline and calcareous soils.

Muriate of potash or potassium chloride, the most economical and commonly-used K fertilizer in the U.S., has been a satisfactory source of K in many vegetable field experiments in California. Sulfate of potash, another good source of K, is more costly but has a lower salt index than muriate of potash and should be considered for vegetable production in soils of borderline salinity or where chloride toxicity might be a problem. Other lesser known sources such as potassium nitrate, potassium magnesium sulfate, and potassium phosphate have not had extensive testing for California vegetable crops.

Application Methods & Placement
Application methods include broadcast followed by soil incorporation, soil injection or banding, and water run in furrow, sprinkler, or drip irrigations. While the broadcast method continues to be used for preplant fertilizer application for vegetables, soil injection or banding often excels in comparisons between the two methods for fertilizer efficiency.

Soil injection is a more precise and sure method for postplant fertilizer applications, however the ease and economics of applying fertilizer, particularly N, through irrigation systems may offset any soil injection advantages of precision placement and little or no loss from volatilization. Careful selection of fertilizer materials and good control of irrigation waters can help to avoid or minimize fertilizer loss in water run applications.

A rule of thumb for fertilizer placement for soil injection is to place the fertilizer band close enough to the seed line or plant row for expanding roots to intercept, but sufficiently removed to avoid burn or injury. A commonly suggested placement of fertilizer bands for vegetables is 2 or more inches to one side of the seed or plant row and 4 to 7 inches below the soil surface.

Application Timing
As a general rule for best results, all P and K and part of the N fertilizer should be applied before or at planting time with remaining N applied in one or more applications following plant stand establishment. For a number of short season vegetable crops, including green beans and potatoes, a single, preplant or at-planting application of all fertilizer materials is sufficient for maximum yield and quality. Other, long-season vegetables, such as garlic and onions, usually benefit from split applications of nitrogen.

Starter fertilizer application, in which an N and P fertilizer is banded at planting at a low rate 1 to 3 inches below the seed, may be an important exception to the general rule above. When starter fertilization is practiced, particularly for direct-seeded tomatoes, it is recommended that most of the crop's N be sidedressed after thinning. Additional P and K, if needed by the crop, can also be sidedressed after thinning or be applied before planting the crop.

Nutrient Application Rates
A decision to apply any plant nutrient, as well as how much to apply, should be considered in light of the vegetable crop to be produced, the soil and its current level of nutrient availability, recent cropping history of the field, and grower experience. Table 1, which shows suggested ranges of fertilizer application rates for 24 California vegetable crops, may be used as a guide for new or prospective producers of these crops.

The desert valley soils of southern California require heavy applications of P fertilizer for winter vegetable production. Thus, the suggested ranges for P fertilizer in table 1 should be approximately doubled for desert valley winter vegetable crops.

Specific application rates within the ranges in Table 1 should be determined after consideration of all available information such as previous cropping records and soil analyses. Soil analyses, in particular, are useful for verifying the status of P and K availability in soils for vegetable crops and Table 2 lists appropriate soil deficiency and sufficiency ranges for P and K for vegetable crops.

NOTE: The actual rate of fertilizer for any given vegetable crop should be chosen with consideration to soil type, recent croppng history, and soil test results. Soil testing is extremely valuable for determining P and K requirements.

Table 1.

VEGETABLE FERTILIZER RATES
Vegetable Crop	N lbs/acre	P2O5 lbs/acre	K2O lbs/acre		Vegetable Crop	N lbs/acre	P2O5 lbs/acre	K2O lbs/acre
Artichoke	50-100	50 - 100	0 - 100		Lettuce	120 - 180	100 - 200	0 - 200
Asparagus	80 - 150	50 - 100	0 - 100		Muskmelon	100 - 150	60 - 120	0 - 100
Bean	60 - 120	60 - 120	0 - 100		Onion	120 - 300	60 - 150	0 - 200
Broccoli	100 - 200	80 - 150	0 - 200		Pepper, Chili	80 - 150	60 - 120	0 - 100
Brussel Sprouts	100 - 200	80 - 150	0 - 200		Pepper, Sweet	120 - 240	100 - 250	0 - 200
Cabbage	100 - 200	80 - 150	0 - 200		Potato, Irish	150 - 300	100 - 200	0 - 200
Carrot	80 - 150	80 - 150	0 - 100		Potato, Sweet	80 - 120	60 - 120	0 - 100
Cauliflower	100 - 200	80 - 150	0 - 200		Spinach	80 - 150	60 - 150	0 - 200
Celery	200 - 300	100 - 200	0 - 300		Squash, Summer	80 - 150	60 - 120	0 - 150
Corn, Sweet	120 - 240	60 - 120	0 - 150		Tomato	100 - 200	60 - 150	0 - 200
Cucumber	80 - 150	50 - 200	0 - 200		Tomato, Cherry	100 - 200	80 - 150	0 - 200
Garlic	100 - 200	60 - 150	0 - 200		Watermelon	100 - 160	60 - 120	0 - 200

NOTE: Adapated from Soil and Plant Testing in California, University of California Division of Agricultural Science Bulletin 1879 (1983).

Soil extracts: PO₄-P: 0.5M pH 8.5 sodium bicarbonate (NaHCO₃). K: 1.0M ammonium acetate (NH₄OAc).

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