Nutrient Management for Strawberry Production

For the past three years, our work on sandy soils has shown that about 120 lb N/acre is optimum for 'Chandler' strawberry production under plastic culture with about one-half the N applied in the fall and the remainder applied through drip irrigation in the spring. This rate of N is markedly lower than that used by many growers, but apparent benefits include reduced vegetative growth and firiner fruit without sacrificing market yield. The optimum N rate probably will vary among soils and may even be lower for heavier-textured soils.

We have not obtained any yield or fruit quality benefit from drip applying additional K in the spring beyond that applied in the fall according to soil test. The recommended rate for our site is 120 lb K.0/acre. Drip rates of K2O as high as 200 lb/acre have been evaluated with no apparent benefit.

Our research site is high in soil P and we have not investigated fertilizer P in our studies. Tissue tests have indicated P levels in plants have been sufficient during most of the growing season, but they do tend to drop to low levels during -harvest. For that reason, we are initiating a study in 1996 to determine if we would benefit from fertilizer P on this high P soil. This study consists of fall-applied and drip-applied rates of P and we should be able to report some results next year.

Proper Sampling - Key to Success in Monitoring Nutrients

Leaf and petiole analyses provide the best means of monitoring nutritional status and correcting deficiencies that may occur. They not only ensure that yield and quality are optimized but protect against applying excess nutrients in the environment and incurring unnecessary expense. The key to success in a nutrient monitoring program is to sample the correct tissue in a timely manner. Research studies indicate that the most recent mature trifoliate is the best indicator of the status of most elements essential for plant growth. These include the major elements P and K, the secondary elements Ca, Mg, and S, and the micronutrients Fe, Zn, Cu, and B. The petiole from this same trifoliate is the best indicator of N status.

Time of sampling

Both leaf and soil samples should be taken in the fall if questions arise concerning plant establishment vegetative growth and crown development. The best procedure is to take leaf samples from good and bad plants for comparison. A soil core should be taken from the root zones of plants sampled in each of these respective areas.

Spring sampling for monitoring should begin about March 1 when spring growth begins, and continue at two-week intervals throughout the growing season. Earlier sampling may be necessary in southern and eastern regions. A total of 6-8 samples should be taken in each field during the growing season.

Best Indicator Sample

The most recent mature trifoliate (Fig. 1) and the associated petiole are the best indicators of nutritional status. This is the trifoliate that is fully expanded but is not dull from age nor slick with a light green sheen associated with immaturity. Fifteen to 20 leaves and associated petioles are required for a good sample. The petiole should be detached from the plant near the crown. Petioles should then be removed from the trifoliate and placed in a small envelope within the leaf container. It is important that the entire petiole be sent to the laboratory for evaluation. Partial petioles are not a reliable indicator of N status.

Corrective Measures

Leaf and petiole samples should be used to predict the need for corrective measures and adjustments to the feeding program. Until a problem is identified, growers should continue to apply N as planned (preplant plus spring-applied). In most cases, 60 lb N/acre applied preplant followed by 60 lb N/acre in the spring should be adequate. Heavy-textured soils may require less N.

Manure Applications

Farm manure applications on strawberry fields can improve tilth, enhance water-holding capacity, and provide nutrients. They are particularly a good source, of micronutrients. They can also lead to over fertilization and soft fruit. If a decision is made to use these materials, the manure should be analyzed prior to application to determine nutrient availability. Ideally, the farm manure should be applied only at rates needed to supply N and other elements needed for fall growth. Where manures are used in strawberry production, growers should proceed with caution in the spring. Leaf and petiole samples become critical parts of management. Routine samples should be taken to identify the need for additional N in the spring.

Spring-Applied N - Management Options for Consideration

Growers have asked two questions about the procedure for drip-applying N in the spring. These are (1) how early should they commence drip-applying N and (2) what schedule should they use for drip-applying N in the spring? The second question is very appropriate because the work load and inconvenience of having to deal with fertilizer injection increases with the frequency of application. Our initial studies used a frequency of two times a week because we had no information on this issue. The next year, we found that injection once a week was equivalent to two times a week. Since then, and based on grower concerns, we initiated studies to investigate both questions.

Time of drip N Initiation

An experiment was conducted in 1994 and 1995 consisting of 4 starting dates for the initiation of spring fertigation and 5 drip-N rates in 1994 and 4 drip-N rates in 1995. The starting dates are given for each year in Table 1 and they were 1 week apart. The base fall N rate for all treatments was 50 lb/acre.

The total N applied varied with starting date and drip-N rate. For instance, commencing N applications on March 8, 1994 resulted in 11 weeks of N application. Commencing N applications on March 30 resulted in only 8 weeks of application. There was no interaction between date of N initiation and N rate so yield for each date of initiation is the mean across N rates. The mean total N rate applied for each date is indicated in parentheses. Likewise, for each drip-N rate the total N applied varied with the length of time it was applied. The ranges in total N, depending on length of time drip-N was applied, is indicated beside rates of drip-N.

There were no significant differences among starting dates of N initiation either year. Thus, there appears to be some leeway in when to initiate drip-N application and it does not have to be at the earliest possible moment in the spring. However, we know the N requirement of the crop begins to increase rapidly in the spring and it would not be advisable to wait to long after plants commence spring growth before initiating N application.

There was a significant response to drip-N rate, but there were no differences in yield among the 0.5, 1.0 and 1.5 lb/acre/d rates in 1994. The total N varied from 78 to 166 lb/acre among these rates, depending on the date of drip initiation. In 1995, there was no 0 lb N/acre/d rate but 1.0 was better than 0.5 lb N/acre/day. We have been recommending 0.75 lb N/acre/d and this represents the mean of the 0.5 and 1.0 lb N/acre/d that were suitable in 1994 and 1995, respectively.

It is important to note that the high-rate of N (2.0 lb/acre/d) diminished yield in both years compared to the optimum N rates. Thus, use of excessive N appears to be detrimental to yield as well as fruit quality (firmness) which we have reported previously.

Drip N frequency

An experiment was conducted in 1994 and 1995 consisting of four different intervals between drip N applications. The total N applied was 134 lb/acre in all treatments and consisted of 50 lb/acre fall-applied and 1 lb/acre/d drip N over a 12-week period (Table 2). The intervals between injection of N were 1, 2, 3, or 4 weeks. Thus, on the first date of drip N application, the 1-week interval treatment received the equivalent of one week of N (7 lb/acre), the 2-week interval treatment received a two weeks equivalent of N (14 lb/acre), and the remaining treatments received their allotted quantities. The number of applications were 12, 6, 4, and 3 for the 1, 2, 3 and 4-week interval treatments, respectively.

There were no significant differences in total or market yield either year across treatments (Table 2). The mean value across years were remarkably similar. Thus, spoon feeding is not required and drip N can be spring-applied in three or four applications provided growers do not over irrigate and leach N from the root zone. This reduces the work load associated with injecting N and should make the procedure more acceptable to growers while contributing to greater N use efficiency.

The Southern Region Small Fruit Consortium
Campus Box 7553
1017 Main Campus Drive / Suite 1100
NCSU Centennial Campus
Raleigh, North Carolina 27695-7553

919.515.5365