Posts Tagged ‘vegetable irrigation’

High Evapotranspirational Demand in Vegetable Crops

Friday, July 6th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Vine crop growers have commented that they are having a hard time keeping enough moisture in their beds. Certainly, irrigation system concerns need to be accounted for (emitter volume, emitter spacing, length of run, etc.). However, high evapotranspirational demand (water taken up by the plant and evaporated from the soil) may make providing enough irrigation water a challenge.

This is particularly the case when there are high temperatures and clear skies during June and early July when day lengths are long and when plant water uptake is peaking. In 2012 during this period we have had consistently higher solar radiation and higher evapotranspirational demand compared to 2011.

Stress in Vegetables

Friday, July 15th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

In troubleshooting vegetables in the summer months, we see fields where the major symptom is an overall lack of vigor and this poor vigor is due to one or more stress factors. Hot weather makes this stress more pronounced. Pests such as root and crown rot fungi, bacterial and fungal wilt organisms, and insects such as squash bugs can damage plant roots, stems, and vascular systems, limiting water uptake, and causing excess stress. However, there are many stresses that don’t involve diseases or insects. The following are some other causes of excess stress in vegetables this time of year.

Soil Compaction
Plants will have limited rooting in compacted areas and therefore cannot take up adequate water or mineral nutrients. In addition, compacted soils have reduced air exchange. Plants will often be stunted and will wilt early in the day in high temperatures. Cultivation can alleviate surface compaction but will be ineffective on deeper compaction.

High Soil Temperatures
Soils that have limited water holding capacity can have excessively high soil temperatures during long hot days in late spring and early summer. Late planted crops on black plastic mulch are very likely to be exposed to high soil temperatures and surface roots will often be damaged. Overhead irrigation over the black plastic mulch is very beneficial to reduce heat loads until plants have sufficient canopies to shade over the mulch.

Drip Tape and Drip Irrigation “Diseases”
Issues with drip irrigation can often be the cause of plant stress due to inadequate water. This includes plugged emitters; leaks due to insect or animal chewing that limit water flow further down the tape; leaky connections reducing flow; tape twisting and binding, again limiting flow past the point of the bind,; improper tape selection or improper irrigation timing leading to under application of water; limited well capacity also leading to under application of water; too wide of emitter spacing for the crop or soil; too wide of bed for a single tape (with double rows) and others drip irrigation problems. Over application of water in drip irrigation also can be an issue, especially in lower field areas and where soil types change in the field. This can lead to saturated beds limiting oxygen for roots. The keys to avoiding drip irrigation associated problems is to monitor fields closely, note any areas that look stressed, and investigate whether or not the drip irrigation is functioning properly. Soil moisture monitoring devices can aid greatly in detecting problems.

Inadequate Overhead Irrigation
Under-watering can lead to additional plant stress. Plugged nozzles are a major problem that often goes uncorrected. Excessive runoff due to compacted soils can lead to reduced water intake.

Excessive Fertilization
Salt induced stress conditions can occur when excess fertilizer, manure, or high salt compost is applied or when high salt index fertilizer is applied too close to vegetable plants.

Fertigating Drip Irrigated Vegetables

Thursday, June 17th, 2010

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Fertigation is the term used when soluble fertilizer sources are delivered through the irrigation system to crops. Drip irrigation is an ideal means to fertigate and to deliver mineral nutrients to vegetables during the growing season. Nutrients are carried with the irrigation water right to the root zone where they can be efficiently taken up by vegetable plants.

There are several strategies for fertigating vegetable plants. One strategy is to split fertigation so that crop nutrient needs, after preplant fertilizers are accounted for, are delivered in 4-5 applications just prior to critical growth stages. For example, for fruiting vegetables, the first fertilizer application through the drip system would be done after planting when plants have become established, the next prior to rapid vegetative growth, the next at flowering or early fruit formation, and the last during fruit expansion. For crops that have long fruiting and harvest periods, an additional application would be made after first harvest to encourage continued production.

Other strategies use weekly applications or applications of fertilizers through the drip system every time the crop is irrigated. In these systems, smaller amounts of fertilizers are applied each time and rates are increased as plants get larger. This requires a somewhat higher level of management.

For general vegetable fertigation through the drip, a 1-1-1 N-P2O5-K2O ratio soluble fertilizer (such as 20-20-20) is recommended. Where phosphorus (P) levels are very high, lower P ratios are appropriate (such as a 21-5-20). In some vegetables, only nitrogen (N) sources will be needed if soil fertility (P and K) are high. Soluble potassium nitrate and calcium nitrate are often used in combination in crops such as tomatoes and plasticulture strawberries to provide N, K (potassium), and Ca (Calcium).

Fully soluble fertilizers must be used for fertigation. Those in dry form must be mixed with water until they fully dissolve to create a concentrated stock solution. Those already in liquid form should be checked to make sure there has been no salting out of nutrients during storage – if salting out has occurred, you will need to make sure the fertilizer re-dissolves by agitation prior to use. It is important to know how much fertilizer is contained in these liquid stock solutions to match to injection rates.

A good quality fertilizer injector matched to the flow rate of your drip system is important to deliver the fertilizer the length of each bed uniformly in the field. Run the drip system to fill the drip tubes and come to steady pressure, start injecting, and then continue injecting using an injection rate that matches the irrigation period. You may then run the irrigation for a short period after fertigation to flush the lines. It is important not to over-irrigate as nutrients may be moved out of the root zone (especially N). Fertigation rates should be base on a mulched acre – that is only the amount of ground covered by plastic mulch.

For more information on fertigation go to our Commercial Vegetable Production Recommendation guide http://ag.udel.edu/extension/vegprogram/pdf/CIrrigation.pdf starting on page C-5.

Are You Over-Watering or Under-Watering Your Drip Irrigated Vegetables with Plastic Mulch?

Thursday, July 2nd, 2009

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

One of the most common problems in drip irrigated vegetables with plastic mulch is over-irrigating or under-irrigating. The following are guidelines from the Delaware Commercial Vegetable Production Recommendations guide:

“Calculating the length of time required to apply a specific depth of water with a trickle irrigation system is more difficult than with sprinkler systems. Unlike sprinkler systems, trickle systems apply water to only a small portion of the total crop acreage. Usually, a fair assumption to make is that the mulched width approximates the extent of the plant root zone and should be used to calculate system run-times. Table C-4 has been prepared to calculate the length of time required to apply one inch of water with a trickle irrigation system, based on the trickle tube flow rate and the mulched width. The use of this table requires that the trickle system be operating at the pressure listed in the manufacturers specifications.”

Hours Required to Apply 1 Inch Water to Mulched Area (Table C-4 from the Delaware Commercial Vegetable Recommendations)

Trickle Tube Flow Rate

Mulched Width (ft)

gph/100 ft

gpm/100 ft

2.0

2.5

3.0

3.5

4.0

8

0.13

15.5

19.5

23.5

27.0

31.0

10

0.17

12.5

16.5

18.5

22.0

25.0

12

0.20

10.5

13.0

15.5

18.0

21.0

16

0.27

8.0

10.0

11.5

13.5

15.5

18

0.30

7.0

8.5

10.5

12.0

14.0

20

0.33

6.0

8.0

9.5

11.0

12.5

24

0.40

5.0

6.5

8.0

9.0

10.5

30

0.50

4.0

5.0

6.0

7.0

8.5

36

0.60

3.5

4.5

5.0

6.0

7.0

40

0.67

3.0

4.0

4.5

5.5

6.0

42

0.70

3.0

4.0

4.5

5.0

6.0

48

0.80

2.5

3.0

4.0

4.5

5.0

50

0.83

2.5

3.0

4.0

4.5

5.0

54

0.90

2.5

3.0

3.5

4.0

4.5

60

1.00

2.0

2.5

3.0

3.5

4.0

“On coarse-textured soils, applying an inch of water to the mulched width may not be effective. Because water is not held in large pore spaces, it moves below the plant root zone, carrying nutrients and pesticides beyond the reach of the plant roots. Table C-5 has been prepared to calculate the maximum recommended irrigation period for trickle irrigation systems. The irrigation periods listed are based on the assumption that 50 percent of the available water in the plant root zone is depleted. Soil texture directly influences the water-holding capacity of soils and, therefore, the depth reached by irrigation water.”

Maximum Irrigation Periods (Hours) for Trickle Irrigation Systems to Result in a Water Infiltration Depth of 12-18 Inches (Table C-5 from the Delaware Commercial Vegetable Recommendations)

Trickle Tube Flow Rate

Soil Texture

gph/100 ft

gpm/100 ft


Sand

Loamy
Sand

Sandy
Loam

Clay
Loam

Silt
Loam

12

0.2

5.0

8.0

11.5

15.5

17.5

18

0.3

3.5

5.0

7.5

10.5

11.5

24

0.4

2.5

4.0

5.5

8.0

8.5

30

0.5

2.0

3.0

4.5

6.5

7.0

36

0.6

1.5

2.5

4.0

5.0

6.0

42

0.7

1.5

2.0

3.0

4.5

5.0

48

0.8

1.5

2.0

3.0

4.0

4.5

Choosing the right emitter spacing for the crop that you are growing is very important. Melons grown on wide spacings are quite different than crops, such as peppers, grown on double rows with tight spacings. The emitter needs to be close enough to planting holes so that young transplants can survive without having to wet the bed so much that you cause leaching. In very sandy soils, this is even more important because the zone of wetting is much narrower.

As to how much to irrigate, during the initial growth phase after seedling establishment and up to the time that plants cover the beds, one inch or less of water per week is generally sufficient based on a mulched acre. As plants reach their full size, this will need to be increased based on the crop. Some crops will need over two inches per week at peak in mid-summer. Irrigation will need to be run multiple days per week to achieve this and on very sandy soils, this will require more management attention so as not to water out of the root zone. Irrigation can be twice daily, daily, every 2 days, or every 3 days depending on the soil type during peak growth.

Estimating crop needs based on evapotranspiration data will be necessary to determine the amount of time to run the drip system under plastic mulch, especially at peak usage. This is complicated by trying to assess the impact of rainfall and water infiltrating under the plastic from row middles. To better manage irrigation under plastic, it is recommended to install or use portable moisture sensors to determine moisture status of the plant root zone under mulch. The standard has been the tensiometer but there are several other moisture sensors that will work equally well or better. Irrigate when available soil moisture is depleted 30-50% based on sensor readings.

Irrigating Lima Beans

Thursday, July 24th, 2008

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

There is considerable controversy about when and how to irrigate lima beans for best yields. Past research has shown that irrigated lima beans significantly out-yield dry land lima beans and top yields generally come from irrigated fields. However, when and how much to irrigate is still a question as is the need to irrigate late in the season for fall harvested lima beans.

Research has suggested that irrigating when soil moisture drops to 50% of field capacity gives better yields than letting the soil dry out further for lima beans. It is also better to irrigate throughout the season rather than just from full flowering onward.

To complicate matters, over-irrigation, especially during pod development when there are very dense vines, increases the potential for pod diseases such as Sclerotinia white mold, lima bean pod blight (Phytophthora capsici), Pythium blight, and downy mildew and can lead to significant yield losses. A balance must be maintained between providing needed water and allowing the crop to dry out to reduce disease pressure later in crop development.

It is also important to consider temperature. The following is from Ed Kee during a heat wave in 2006: “The tremendous heat we’re experiencing makes irrigation of lima beans more critical. Blossom drop occurs when the plant is stressed, and even with adequate irrigation blossoms will abort with high temperatures. However, maintaining soil moisture is important to keep the plant cool, which alleviates stress and reduces blossom and pod drop. Lima beans will use 0.25 inches of water per day through evapotranspiration when temperatures are in the nineties. Reducing plant stress as much as possible will help in the retention of “pins” (small pods) and larger pods.”

In lima beans, first flowering generally occurs at 35 days from planting, and peak flowering at 60 days. Providing adequate water with irrigation during the entire flowering period and through pod set is critical for developing and maintaining yield.

Overhead Irrigation of Vegetables

Friday, May 2nd, 2008

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

Irrigation is a critical management tool for producing high yielding and high quality vegetable crops. Scheduling irrigation for different vegetables grown under center pivot, travelling gun, or solid set overhead systems involves knowledge of the soil water holding capacity, the effective rooting depth of the crop (how deep water can be drawn by the crop), how efficiently water is being delivered (water losses to evaporation before it reaches the crop and how much water is lost to runoff), how much water is being used by the crop (transpiration) and how much water is being lost from the soil and wetted surfaces directly (evaporation). The combination of transpiration and evaporation losses is termed evapotranspiration.

To schedule irrigation, the goal is to replace water lost through evapotranspiration without excessive runoff or excessive loss through percolation out of the root zone. Another factor to consider is the permissible water depletion; how much will you allow the soil to dry down between irrigations. For most crops we set this at 50% of the water holding capacity of the soil. However, for some shallow rooted crops you may want to keep that value lower (only allow for 40% depletion between irrigations). By knowing how much water is being lost and how much is left in the soil, you can determine when to irrigate and how much to irrigate.

In classic work done by the University of Delaware Agriculture Engineering Department in the 1970s and 1980s, water use estimates were developed for a number of vegetable crops. These values remain useful guides for irrigating these crops. A summary follows:

Sweet Corn: Water use 40 days after planting was 0.10 inches per day, water use 60 days after planting was 0.23 inches per day and water use at peak (75 days) was 0.26 inches per day.

Potatoes: Water use 40 days after planting was 0.15 inches per day, water use 60 days after planting was 0.27 inches per day and water use at peak (80 days) was 0.37 inches per day.

Peas: Water use 40 days after planting was 0.16 inches per day and water use 60 days after planting was 0.33 inches per day (peak).

Lima Beans: Water use 20 days after planting was 0.13 inches per day, water use 40 days after planting was 0.25 inches per day, water use 60 days after planting (peak) was 0.33 inches per day and water use 80 days after planting was 0.23 inches per day.

Cucumbers: Water use 20 days after planting was 0.13 inches per day, water use 40 days after planting was 0.27 inches per day, and water use at peak (50 days) was 0.30 inches per day.

Watermelons: Water use 20 days after planting was 0.10 inches per day, water use 40 days after planting was 0.23 inches per day, water use 60 days after planting (peak) was 0.30 inches per day, water use 80 days after planting was 0.28 inches per day and water use 100 days after planting was 0.22 inches per day.

Tomatoes: Water use 20 days after planting was 0.15 inches per day, water use 40 days after planting was 0.27 inches per day, water use 60 days after planting (peak) was 0.33 inches per day, water use 80 days after planting was 0.28 inches per day and water use 100 days after planting was 0.25 inches per day.

In future articles information on irrigation scheduling aids (such as computer scheduling programs), soil moisture sensors, and irrigation scheduling under plastic mulch will be presented.