Guidelines for Drip Irrigation and Fertigation of Pines and Hardwoods

Ilan Bar, Netafim, Altomonte Springs, FL

Introduction

Drip irrigation technology is relatively new to the forestry industry. As with any new technology, the objective of drip irrigation is to maximize profits through optimizing tree growth, resulting in higher yields and better quality. For that purpose, drip irrigation is considered as a system to grow short rotation woody crops rather than just a method of irrigation. In order to use this technology to its fullest potential, the basic principles that constitute a drip irrigation system need to be understood.

The concept of drip irrigation is to create a continuous wetted strip along the tree line. This wetted strip should be homogeneous and uniform thus providing even distribution of water and nutrients to the trees. The even supply of water and nutrients directly to the root zone creates an optimal environment for the roots to efficiently absorb the soil solution in order to maximize growth.

Drip irrigation delivers precise amounts of water in a very uniform fashion directly to the root zone without runoff, wind drift, leaching below the root zone or wetting the canopy. Furthermore, the dripperline applies water only to a portion of the surface thus maintaining high moisture within the root zone without water logging due to dry surroundings. These facts permit the use of “marginal water” such as wastewater, mill effluent and brackish water. Marginal water can be used through drip without the risk of injuring the canopy, building-up high concentrations of salts or leaching contaminants into the groundwater. However, only the careful selection of a dripperline will bring the full expression of the aforementioned benefits.

Dripperline Selection

The selection of the most effective dripperline is comprised of determining the best type of dripper, dripper discharge (flow rate) and spacing. The continuous wetted strip consists of individual wetted “bulbs” which, to a certain degree, overlap each other. The size and shape of the “bulbs” are determined by the dripper’s discharge, soil type and duration of the application.

• Dripper Discharge – The higher the flow, the better the lateral movement.
• Soil Type – The heavier the soil, the better the lateral movement.
• Duration of Application – The longer the application, the better the lateral movement, to a certain degree.

The distribution of water into the soil profile from a point source is very different compared to overhead irrigation. In overhead irrigation, the water moves through the soil like a piston. The upper layer is saturated before the water reaches the lower layer. As the water progresses downward, it forces the air out of the wetted soil profile.

In drip irrigation, the soil moisture created by a point source includes all of the various states of moisture from saturation to dry soil. By providing this array of different moisture levels within the root zone, we enable the roots to “choose” the optimum combination of water, oxygen, and nutrient absorption.

There is a differentiation in soil moisture dependent on the distance from the point water source in drip irrigation. This has a detrimental affect on the placement of soil moisture metering devices and their ability to represent the true fluctuation of moisture related to consumptive use and application. For example, placement of such devices within the saturated zone will result in relatively high, flat moisture readings.

The following table will help new users to select the appropriate dripper discharge and consequently, the resulting spacing as related to the specific soil type. Please note there is a trade-off between discharge and spacing on a given soil type and between these two, on different soil types.

There is a minimum coverage, expressed as a percentage of the wetted strip from the total available tree’s surface, for the drip system to be operable under real world conditions.

P=Available Wetted Area/Available Tree Area x 100

P greater than or equal to 35 – 40% Relatively Low Rainfall

P greater than or equal to 25 – 30% Humid Area

The minimum coverage under rainy conditions should be 25 – 30%. The coverage will depend on the type of tree, root zone characteristics, soil type, and climate. Certain trees will be less sensitive to changes in root structures than the others. Some trees have wide and superficial root systems as opposed to a deep, tapered root system. Eventually, trees in a lighter soil will benefit from a wider wetted strip. Frequent rains can always compensate for a narrower wetted strip. Other factors that affect the selection of the dripperline include topography, water quality, and agro-economics.

The only way to use drip irrigation economically (or at all) on a rolling terrain is to use a pressure compensated dripperline. Even on flat ground where it is not hydraulically necessary to use a pressure compensated dripperline, selecting such a line will permit longer runs resulting in overall lower costs as well as extremely accurate and reliable performance.

Not all drippers are alike in terms of accuracy, clogging resistance and durability. On some fiber farms, the water source may be surface water (creek, pond) or mill effluent. These water sources contain numerous contaminants and organic slime. Only years of field tested and proven dripperlines which are equipped with a self-flushing mechanism and, if needed, an internal algae control device, should be selected.

Agro-economical factors should also be considered. In some instances, the drip systems may have to retrofit to an existing infrastructure. This obviously limits the flexibility of the system selection and design. Automated systems are normally less expensive because the flows can be reduced on the expense of time.

The most common spacings and flows which are being used in forestry are listed below.

• 40” -42” @ .92 gph
• 36” @ .92 gph
• 24” @ .92 gph
• 36” @ .61 gph
• 42” @ .42 gph

Irrigation Scheduling

There are several methods used to schedule irrigation. We have selected what is commonly referred to as the bookkeeping or budget approach. The peak demand is estimated based on prior experience, available case studies, literature, and weather data from universities and agricultural operations. For example purposes, we will use a stand of 500/trees/acre which is considered average for short rotation woody crops in the Southeast.

Estimating the peak demand of water consumptive use in various stages.

• Peak demand for a mature forest in the Southeast is 1.25”/week.
• Peak demand at first year is ~ 0.5”/week.
• Assuming 500 trees/acre=approximately 3.5 gallons/tree/day.

In the Southeast, most soils under short rotation woody crops (SRWC) are sandy loam soils. The total water retention capacity of a typical sandy loam soil is 1.3”/ft. Therefore, assuming a root zone diameter of 5’ and 12” depth, the total water holding capacity in the first year is approximately 16 gallons. The maximum depletion of the available water is established at 50% to prevent any stress to the trees. That equates to eight gallons of allowable depletion. Consequently, the irrigation interval would be every other day at eight gallons/tree/application. The duration of the application will depend on the characteristics of the drip system. A dripperline spacing of 40” at 0.92 gph on a tree spacing of 12’ x 7’ will result in a flow of approximately 2 gph/tree or four hours of irrigation every other day.

Based on all of the above, the following table depicts a tentative irrigation schedule for SRWC first year in the Southeast. Some species of fast growing hardwoods like cottonwood may require a slightly higher water application.

Fertigation

Fertigation is the essence of drip irrigation. Drip irrigation should actually be viewed as a method of growing crops and not simply as a method of irrigation. Many times people tend to compare drip irrigation to overhead irrigation (pivots, sprinklers, mini-jets) or flood irrigation. This is not an accurate comparison because the latter methods are viable mainly for irrigation while in drip irrigation, fertigation is a very integral part of the system. Fertigation is a must in order to realize the full potential and benefits of the system. Drip irrigation can be used solely for irrigation and would still be the most efficient method, but the foremost benefits are lost.

In many cases, depending on the year and location, the drip system is used predominantly as a fertigation system. In seasons or climates with abundant rainfall, there is many times no need to irrigate, but there is an obvious need to fertilize due to significant leaching conditions. Applying the seasonal amount of fertilizers in small doses at high frequency (spoon-feeding) will ensure a continuous and stable supply of nutrients. Moreover, this method meets the tree’s growth requirements without leaching the fertilizers below the root zone.

The advantages of fertigation are:

• Less labor, equipment and energy needed for receiving, storing and fertilizer application.
• Reduced soil compaction.
• Prevents damage to crop during delivery.
• No restrictions or limitation on application timing.
• Accurate and uniform distribution for superior efficiency.
• Application restricted to most active root zone which reduces waste.
• Adaptability of nutrients supply to the growth curve resulting in better crop response.
• Split applications for better control of run-off and leaching into groundwater.
• Extremely efficient method of accurately delivering uniform, minute quantities of minor elements.
• Complete adaptability to automation.
• Can be used for other purposes, i.e. pestigation, soil amendments, maintenance.
• Can overcome negative effects of saline/waste water.

Fiber farms are already using systemic insecticides through the drip system to control the cottonwood leaf beetle. Chlorine and acid can also be injected to maintain the cleanliness of the dripperline. Furthermore, certain nutrients can be injected into wastewater, that may contain toxic elements, to counteract their negative effect on tree growth. In many cases, the water contains relevant nutrients such as calcium and magnesium. These elements should be considered as a part of the Fertigation program, in addition to their clogging potential.

In an attempt to determine a formula for Fertigation of SRWC, we collected data from several studies that concentrated on the removal of nutrients by pines and hardwoods. There is not much information in this field due to the fact that, until recently, fertilization of stands was not a common practice. One of the studies suggests the following formula which was put together by Dr. Claus Steinbeck.

Another study suggests the following ratio:

Based on the latter formula, we recommend the use of 8:2:8 as a complete liquid fertilizer that might be enhanced by adding some minor elements like B and S. In some cases along the East Coast and Southeast, phosphorus levels are notably high and therefore the phosphorus can be omitted and calcium and/or magnesium may be added if needed. It is always preferable to apply calcium and magnesium pre-plant as part of the liming or, if pH adjustments are not required (pines), to utilize land plaster and K-Mag.

The underlying concept of Fertigation is to build an adequate level of P, K, Ca, Mg using pre-plant applications which will be always significantly less expensive compared to liquid fertilizer. And then maintain these levels using Fertigation. To summarize Fertigation:

1. Pre-plant applications might include P, K, Ca, Mg, S, Lime and minor elements – all based on soil analysis.
2. It is preferable to use a complete liquid fertilizer containing N, P, K and minor elements especially in very sandy soil. In medium to heavy soils, only N and K might be used on a continuing basis.
3. Sources of N may include: A.N., urea, A.S., Urea + A.N.
4. Sources of P may include: H3PO4, A.P.P, M.A.P., M.K.P.
5. Sources of K may include: KCl, KNO3, M.K.P.
6. Minors should be chelated.

Conclusion

The success of drip irrigation in forestry will depend on the capacity and the ability of the system to optimize distribution of water and nutrients thus resulting in high yields and better quality.

The prerequisites necessary to ensure optimum performances are:

• High quality products with years of field proven results.
• Appropriate selection of spacing and flows based on soil, water and tree types.
• Complete fertigation program based upon soil and water analysis.
• Irrigation scheduling based on crop demand, soil characteristics and system features.
• A fundamental maintenance program.

File posted on March 17, 1998; Date Modified: February 21, 1999