Assessing Infiltration in Orchards

Depositional crust. The source of particles is from irrigation water runoff.

Something I’ve noticed in some orchards, even those with microirrigation, is poor water infiltration. The symptoms are varied and often only affect localized areas, but can range from runoff, standing water, soils that stay saturated for long periods of time, water stressed trees, and diseased or dead trees. It’s an important issue to address as poor infiltration, as well as some causes of poor infiltration, can negatively affect your orchard’s yield long before tree death.

Before we go into what causes infiltration reductions, it’s important to understand the process under normal conditions. Infiltration is defined as the entry of water into the soil. The initial rate is relatively rapid, yet as clays in the soil begin to swell, the rate will decrease over the course of several hours until it steadies out into the final infiltration rate. The initial and final infiltration rates are determined to a large degree by soil texture and soil structure. This is because the two determine soil porosity. In general, coarser textured and well aggregated soils have greater rates of infiltration due to presence of large pores that water can move through.

Your irrigation method and management practices should reflect your soil’s infiltration rate. Soils with a coarser texture can handle systems that deliver high volumes of water, but those systems would result in runoff or standing water in finer textured soils. Thus, some symptoms of poor infiltration could be due to a mismatch in soil type and application rate. Overirrigation can also show some of the same symptoms, so it is important to take water quantity into account when evaluating your soil’s infiltration rate. In orchards with fine textured soils and naturally low infiltration rates, the best management strategy is ensuring the soil profile is filled before the growing season.

In this article I will focus on issues that limit the initial movement of water into the soil. Therefore, I won’t be discussing hardpan or stratified soil layers, which occur deeper in the soil and usually reduce infiltration later in an irrigation/rainfall cycle. So, what causes a reduction in water infiltration? Soil sodicity, crusts, and shallow compaction.

But first, some soil science: the soil is composed of mineral and organic particles, and pores that can be filled with either water or air. Their size determines their role in soils. Smaller pores are responsible for holding plant available water, and larger ones transport water into and through the soil. Soil pores are formed by many processes, including soil particle arrangement, plant roots, earthworms, among others. One critical process is the formation of soil aggregates, which can result in larger pores between aggregates. There are multiple agents that help bind aggregates together, such as organic matter, fungi, and carbonates, to mention a few. On the individual particle level, however, cations, or positively charged molecules, are a critical component to the stability of aggregates, as some help hold groups of clay particles, also known as clay floccules, together. You are already familiar with some of the common players in the cation world: calcium, magnesium, potassium, and sodium.

Cations are not equally helpful in flocculating clay particles. Some, like calcium, have a strong charge, while others, like sodium and potassium, have weaker charges. In soils that have appreciable amounts of sodium, clay particles aren’t held together as strongly or as closely, will deflocculate, and pore space will decrease and as a result water infiltration rate can slow. It can also make soils more vulnerable to crusting, which I will cover below. Sodium is introduced to orchard soils through irrigation water.

A general rule of thumb is that infiltration issues start when ESP, or the exchangeable sodium percentage, is five, though the actual value is dependent on soil texture and mineralogy. This number is a measure of how much of the CEC, or cation exchange capacity, is taken up by sodium, and it can be obtained from commercial laboratories through a soil test. The cation exchange capacity is a measure of how many cations a soil can hold, and is determined by the amount of clay and organic matter in your soil. If you think your soil is sodium affected, you can get a test done at one of the many soil testing laboratories found throughout the state.

In order to reclaim sodium affected soil, you need to have three things: calcium to replace the sodium ions, water of a high enough quality so that it will not increase soil salinity, and drainage to allow the unwanted ions to leave the soil profile. If you only have two of the three, you won’t be successful. Calcium sources can be gypsum, agricultural lime, or calcium carbonates already present in the soil, so knowing your soil is an important part to soil reclamation. In depth discussion of reclamation methods and materials is another article in itself, so I will only offer generalized pointers. In neutral soils that are slightly affected by sodium, one ton of soil-applied gypsum per acre, which will remove approximately one meq/100g soil of sodium, and one acre-foot of water may be sufficient. These assume pure gypsum and high quality water, as well as the soil profile being saturated before leaching begins. For soils with free lime, you can apply sulfur producing amendments; approximately 400 lbs of soil sulfur is equivalent to one ton of gypsum. In soils that have appreciable amounts of sodium, you will need to do reclamation leaching, which can take years. There are many UC resources that cover reclaiming sodium affected soils in depth.

Magnesium can also interfere with soil water infiltration, but only in specific circumstances: when there is far more magnesium than calcium, and in soils with certain types of clay minerals, such as montmorillonite clays. However, caution is warranted – magnesium doesn’t negatively affect soils as strongly as sodium, and magnesium is an essential plant nutrient and should not be eliminated from the soil. If you have a soil high in magnesium but do not suffer from poor infiltration or potassium deficiency, save the money you’d spend applying gypsum and use it somewhere else. In California, soils that contain serpentine clay minerals may have infiltration issues. If in doubt, contact your local farm advisor, crop consultant, or laboratory agronomist.

Irrigation water doesn’t just contribute salts to soil, its chemical properties will also affect how quickly water will move into the soil. The salinity of the water, as well as the sodium content, will affect the stability of surface aggregates. Soil water and irrigation water will equilibrate during irrigation, and any issues the irrigation water has will affect infiltration. Irrigation water with a high sodium adsorption ratio (SAR) will deflocculate clay, reducing pore space and reducing the infiltration rate. However, the total salinity of the water comes into play; if the irrigation water has a high salt content, the dissolved ions help aggregates keep their shape. In case you didn’t think this was complicated enough, the pH and bicarbonate levels of your irrigation water come into play as well. Calcium and magnesium will bind with bicarbonates and precipitate out, which will increase the effect of the sodium in your irrigation water. This is accounted for in the adjusted SAR, which agricultural laboratories can calculate for you.

SAR < 5 * EC

Equation 1: the desired SAR of the irrigation water should be less than five times the EC of your irrigation water to avoid infiltration problems. EC is in dS/m.

Equation 2: Equation to calculate the sodium adsorption ratio. Sodium, calcium, and magnesium are all in meq/L.

Table 1: Likelihood of infiltration problems from irrigation water quality. Source: Adapted from Ayers and Westcot (1985).

Extremely pure water can also reduce infiltration. This water, with an EC less than 0.3 dS/m, will leach out ions from the soil solution, including calcium, and move it deeper into the soil. The resulting effect is similar to high sodium levels, though the removal of calcium rather than the addition of sodium is the cause of deflocculation.

Infiltration issues due to water quality are usually limited to the soil surface, and can be remedied through applying gypsum to irrigation water, or the soil. This will decrease the SAR and increase the EC of the irrigation water. It is important to remember that if you have irrigation water with a high salt load and SAR, adding the gypsum may decrease SAR but will increase the EC to a small extent. Adding approximately 250 pounds of solution grade gypsum to one acre-foot of water will add one meq/l of calcium and increase the total salinity by 0.15 dS/m. However, if your irrigation water has high carbonates and high pH, the added gypsum may cause problems with precipitation.

Physical properties of soils can also reduce infiltration rates. A common form of physical restriction is surface crusting or surface sealing, which is the presence of a thin, dense layer of soil at the soil surface with low porosity. Crusts are formed by two methods. The first is structural: drops of water will break the bonds holding surface aggregates together, the surface particles will rearrange themselves and form a layer with low porosity and high density, and some loosened fine soil particles will be washed into larger pores underlying this dense layer and block them. These drops of water can either be from rain or from sprinkler irrigation systems. The second type of crust is depositional: eroded soil particles will settle onto the soil surface, and can form a crust. The source of eroded particles can either be deposits from flooded rivers, or erosion from a higher part of the field to the lower end, or even from flood or furrow irrigation, where soil particles are picked up at the head end and deposited closer to the tail end.

Soil chemical composition does not cause crusts, but can aid in crusting. As mentioned previously, cations like sodium and magnesium, particularly in soils with certain clays, can weaken clay floccules and aggregates so that less force is needed to destroy surface aggregates. The chemical composition of the crust also influences movement of water through the crust and into the underlying bulk soil; high sodium and low calcium results in a less permeable crust.

There are many methods to prevent crusting, which is dependent on the type of crust present. Soils that are prone to structural crusting should be managed to prevent sodium buildup in the soil, and may need a switch to drip irrigation, which eliminates in-season water drop impact. In fields that are prone to depositional crusts from surface irrigation, soil conditioners such as polymers can help hold soil particles in place however they are not helpful in all soil types, and switching to a microirrigation system could be a more sustainable long-term option. Tillage will break up either type of crust, but crusts will reform if the underlying cause of crusting is not addressed.

One extremely effective method that is usually overlooked in the San Joaquin Valley is to use cover crops or resident vegetation during the winter, especially in fields that are prone to erosion and have elevation changes. The presence of plant matter on the soil surface reduces the impact velocity of water droplets, physically protecting the soil. Cover crops can also create pores in the soil through holes left by decaying roots, and increase soil organic matter, which often will result in an improvement of soil structure and stronger aggregates.

Soil compaction can also reduce infiltration rates. This occurs when a heavy weight physically forces soil particles closer together. The depth of soil affected by compaction depends on the weight of the vehicle, tire surface area, and how often this stress is applied. Water can aid in compaction; it will lubricate soil particles and help them slide closer together.

One tillage or harvester pass won’t destroy a soil’s structure, but many passes will, particularly when the soil is near field capacity. While tree crops’ permanent, deep roots are favorable to preventing soil compaction, orchards are not immune from it. As soil structure can take hundreds of years to form, the best way to deal with soil compaction is to prevent it. Tillage can loosen up soil, but without a change in the behavior that originally caused the compaction, the effect will be temporary. Use good soil management practices such as reducing vehicle passes, avoiding driving through fields when they are wet, and maintaining cover crops or resident winter vegetation that will stabilize soils and help deplete excess moisture during the early spring months. Deep rooted annuals and grasses with fibrous root systems may help reduce minor soil compaction.

Managing water infiltration, especially in soils that are prone to crusting or compaction, or in orchards that are irrigated by sodic ground water, is a process that takes time and resources, but is well worth the effort.