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Managing the Nutrient Requirements of Your Strawberry Crop Gordon S. Miner |
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The efficient use of fertilizer depends on getting your strawberry crop off to a good start. There are several management decisions the grower has to make that can insure his crop has the potential to produce a good crop. These decisions are important because yield and, in some cases, fruit quality can be diminished greatly if proper management practices are not followed. Fumigation Strawberries benefit greatly from soil fumigation. This is due to the control of weeds and soil born diseases that prevail on nonfumigated soil. Methyl bromide has been the fumigant of choice for many years, but it is being phased out due to its effect on the environment and will become less available and more expensive for use on strawberries in coming years. We have evaluated chemical alternatives to methyl bromide for two years at Central Crops Research Station at Clayton, NC. All materials evaluated have been effective at controlling weeds and in improving strawberry yield over a nonfumigated control. The results of these tests are summarized in table 1. In 1998, Basamid and Vapam were as effective as methyl bromide in maintaining yield, but Telon C35 did not appear to be much of an improvement over the control. However, in 1999, all three materials were similar to methyl bromide in maintaining strawberry yield. There were no significant differences among fumigants for any other horticultural trait measured either year. Based on these studies, we fumigated our study site for the coming year using Vapam to obtain further experience with this alternative to methyl bromide. Planting Date Selecting an appropriate planting date is extremely important to realizing the yield potential of strawberry. Planting at 10 day intervals at Clayton, NC in 1995 resulted in the following market yields: September 28 - 17,320 lb/acre with 4.7 crowns/plant; October 8 - 7,864 lb/acre with 3.0 crowns/plant; and October 18 - 4,077 lb/acre with 2.7 crowns/plant. These large yield differences are unusual, but illustrate the effect planting date can have on yield performance of strawberry. The effect of planting date on strawberry yield is directly related to the severity of fall weather after planting. Cold temperatures, such as occurred in 1995, reduce plant development (specifically crown number and flower initiation) which severely limits the spring yield potential. Your county extension agent should be able to provide you with the appropriate planting window for your area. Keep in mind that it may be quite narrow and it should be considered when making arrangements for custom fumigation and setting of plants. Lime Requirements Lime neutralizes soil acidity and provides a soil environment where strawberries can thrive. It can result in a deeper root system and increase the ability of plants to take up nutrients. Dolomitic lime (which should be used in NC because it contains magnesium) decreases the soil content of exchangeable aluminum (which is toxic to plants), increases the soil content of both calcium and magnesium, and increases pH. Strawberry response to lime can be measured by both increased yield and improved fruit quality. The goal of liming is to raise soil pH to 6. However, do not apply lime indiscriminately or you can induce other problems such as manganese deficiencies. Be sure and take a soil test and apply the lime recommended by the laboratory. This should be done about three months before transplanting to insure the applied lime will react with the soil before strawberry plants are planted. Fall Fertilization A complete fertilizer should be broadcast with shallow incorporation prior to making beds. There is a wide range of fertilizer materials and rates of application that can produce a strawberry crop. However, nutrient pollution is a major concern in watersheds crossing the coastal plain and it is in everyone's interest to apply only those fertilizer nutrients required by the crop and to apply them at needed rates. Excess nitrogen and phosphorus entering rivers and sounds contribute significantly to the degradation of our surface water resources and fisheries. Many growers continue to use sulfur-coated urea in addition to a complete fertilizer in the fall. This fertilizer is expensive, usually applied to supply about 140 lb N/acre, and it contributes to overfertilization with N when added to the amount of N applied in the complete fertilizer. With additional N applied through the drip system in the spring, the total N applied in many instances ranges from 200 to 300 lb/acre. This is way in excess of that required to grow a successful crop of strawberries. The total N taken up by a 23,000 lb/acre strawberry crop is around 90 lb/acre. Only 20% of the total N is taken up before spring growth commences. Thus, there is no need to apply high rates of N in the fall as described above. Maximum yields have been attained at Clayton on very sandy soils for a number of years with only 120 lb total N/acre. In 1999, market yields were in the 35,000 lb/acre range over a number of experiments with that rate of N. Successful strawberry production can be attained with 60 lb N/acre in the fall and with phosphorous, potassium, and other nutrients applied according to soil test. In any case, a minimum of 30 lb P2O5 should be applied because we have routinely seen responses to P fertilizer even on a high P soil. In the absence of soil test, a 10-10-20 fertilizer applied at about 600 lb/acre will supply sufficient N, P, and K to grow your crop. We have not been able to measure any difference in strawberry response to various sources of N fertilizer. We have evaluated ammonium nitrate, calcium nitrate, ammonium sulfate, and urea. Urea may be the least desirable because there are soil conditions under which significant volatilization and loss of N can occur with this source. Another fertilizer we have not evaluated, but one which I feel would be just as good as the others, is potassium nitrate. Spring Fertilization In most situations, there will be no reason to inject P or K through the drip system if proper amounts were applied in the fall. The main nutrient of concern would be N (and any other indicated by tissue testing to be low). From 30 to 60 lb N/acre need to be injected with the highest rate on deep sandy soils and the lower rate on heave textured soils. Medium-textured soils would need an intermediate amount (45 lb/acre). We apply the 60 lb N/acre at Clayton in three applications with 3 weeks between applications. We apply the first one about the middle of march when visible spring growth commences. The last application is usually about the last of April, shortly after harvest is in full swing. This approach has been very successful and given yields and fruit quality equivalent to or superior to other approaches we have evaluated. The easiest and cheapest fertilizer N source to dissolve in water for injection is ammonium nitrate if tissue testing indicates all other nutrients are sufficient. Potassium nitrate is also very soluble if additional K is needed. Calcium nitrate is an excellent N source, but it is much less soluble in water. In any case, use a greenhouse grade of fertilizer that does not have a coating (usually clay) to protect it from absorbing water from the atmosphere. This coating can plug screens in the delivery line and can plug emitters in the drip tube if it bypasses the screens. Tissue Testing Monitor the crop about every two weeks in the spring by collecting 20 to 25 recently-matured leaves and petioles and having them analyzed. Your extension office should have packets and instructions for collecting and submitting samples. This tool is an excellent way of keeping track of your fertilization program and allows you to make adjustments when called for. Boron is often deficient on deep sandy soils. However, you will not notice it because visible symptoms are difficult to detect in young plants. Tissue testing is the main method of detecting deficiencies of this important element and tissue levels of boron begin to decrease markedly in the spring as growth commences. If tissue levels fall below 20 ppm, a recommendation will be made to apply 1/8 lb B/acre (5/8 to 3/4 lb Solubor/acre). You can weigh out the Solubor, dissolve it in water, and apply it one time, unless later tissue tests call for more. Care must be taken to apply boron accurately because it is extremely toxic if applied in excess. The difference between enough boron and too much is small. |
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a = Means with the same letters are not significantly different at
the 0.05 level of probability. |
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