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Grape Index
Chapter 8. Soils
Soil is that part of the earth?s surface in which plant roots grow. In sustaining plant life, its major functions are to supply mineral nutrients and water and serve as a medium for root anchorage. It is difficult to grow grapes without soil (hydroponics) either with nutrient solutions only or with sand or gravel.

The four major components of soil mineral materials, organic matter, water, and air are intimately mixed, but are not independent in nature. If one system is changed, there is a change in all. An average silt loam soil may be composed of about 50 percent pore space (air and water); the solid space is made up of about 45 percent mineral matter and 5 percent organic matter.

Soil Parent Material

Parent materials of soils consist of rock and unconsolidated materials from which the mineral components of the soil have arisen. The physical and chemical properties of soil are determined largely by climate and parent material. While some materials are residual (not moved from the place they were formed), others have been transported by water, wind, gravity, or ice. Alluvial soils, for example, have been deposited in the bottomlands along rivers. Sometimes landslides carry unconsolidated materials down slopes that later become colluvial soil parent materials.

Mineral Constituents of Soil

The mineral or inorganic part of the soil is comprised of small rock fragments and minerals of different sizes, and may be original (e.g., sand derived from quartz) and relatively unchanged form the parent materials, or secondary (e.g, clay) and formed by weathering of less resistant minerals.

Soil texture refers to the size of the individual mineral particles (Buckman and Brady, 1960). Textural designations generally used to describe soils are sand, silt, or clay and the textural triangle shows the relative percentages of these components (Fig. 8-1). For example, a typical loam soil, might have 40 percent sand, 40 percent silt, and 20 percent clay. A ?heavy? soil refers to one that is high in clay and other fine particles, and a ?light? soil is one composed of much sand and coarse particles but little clay. Silts are intermediate in size and have properties midway between sand and clay, the finest of the mineral fractions.

Organic Soil Matter

Organic soil matter consists of partially decayed and partially resynthesized plant and animal residues. While it may comprise only about 1 to 5 percent of a mineral topsoil, it has a great influence on soil properties. Many of the valley grape soils such as Hanford. Delhi, Tujunga, and Hesperia contain 1 percent or less organic matter, which is continuously being attacked and decayed by soil micro organisms and is thus a transitory soil constituent that must be replenished by the addition of plant or animal residues. Humus is an organic, colloidal fraction consisting of resistant products that result form decay of original tissues such as leaves and plant tissues.

Organic matter has a high water holding capacity that acts like a sponge in the soil. It is also a source of minerals made available during its decomposition. Small particles of organic matter cover mineral particles and keep them from sticking together, thus helping to maintain a good soil structure. A muck soil contains 10 to 40 percent organic matter and a peat soil about 40 to 100 percent. When such soils are properly managed, they can be extremely productive.

Colloidal Soil Particles

Clay and organic matter are the finer parts of the soil and exist in the colloidal state. Such particles are characterized by a large surface area per unit of weight, and have surface charges that attract ions and water. Chemical reactions and nutrient exchanges occur at the surfaces. The capacity of a soil to retain and exchange cations such as H+, Ca++, and K+ is termed base exchange. The attraction of ions to negatively charged colloidal surfaces prevents essential, positively charged nutrients form leaching from the soil and releases them slowly for plant use. The best soils for grape growing contain a good balance of clay and humus.

Water and Air in Soil

Water in the soil and some dissolved salts comprise the soil solution that is important in supplying plant nutrients. Air is the other component of the pore spaces in the soil. As one increases the other decreases, and there must be an optimum balance between the two for optimum plant growth. Soil air may often consist of isolated, unconnected pore space. The carbon dioxide content of the soil is greater than that in the atmosphere above it because of the decomposition of organic matter in the soil. Oxygen in the soil is less than in the air above it because it is used in respiration by roots and micro organisms.

If too much water is added to the soil, the air is forced out of it and plant roots may be deprived or oxygen. Flooding of grapes during the dormant season does not appear to be harmful, although prolonged flooding in the growing season can be injurious.

Living Organisms in Soil

The soil contains many living organisms ranging from bacteria to worms, insects, rodents, and roots of plants. Excluding roots of higher plants, the weights of soil organisms can be as high as 6000 lb per acre (6725 kg per hectare) (Janick et al., 1974). Insects and earthworms can physically disintegrate plant residues, and smaller organisms such as bacteria and fungi can then completely decompose these residues. Nutrient elements such as phosphorus and nitrogen are released in the decomposition process, although micro organisms also require such elements for their growth and can convert them into organic compounds not available to higher plants.

Soils for Grapes

Grapevines are well adapted for many types of soils. In various parts of the world grapes are grown commercially in almost all types of soils from shallow to deep, gravely sands to clay loams, and in soils high to low in fertility. However, heavy clays, very shallow, poorly drained soils, and soils containing relatively high concentrations of alkali salts, boron, or other toxic materials should be avoided.

The deeper and more fertile soils usually produce the heaviest crops and are therefore preferred for raisins, common wine grapes, and table grapes such as Thompson Seedless and Tokay. Soils of limited depth are preferable for varieties such as Emperor and Malaga. Wine grapes often produce high quality fruit on infertile or rocky soil; thus good grape crops can often be grown commercially where other crops are unsuccessful.

Drainage

The grapevine cannot grow well in a wet soil; the vine does not like ?wet feet?. The water table should not be too close to the surface of the soil; otherwise the soil must be tile drained.

Characteristics of Good Soil

To a grape grower soil is a reservoir to supply the needs of his vines. Water, air, nutrients, and growing space are soil requirements for grapevines, and sufficient growing space is needed for roots to explore the soil for these ingredients.

The best soils for grapes are loams to loamy in texture, well structured, moderately deep to deep and well drained, uniform, and free from harmful salt accumulations, damaging soil pests, and soil pathogens. Relatively flat land is preferred.

A loam soil is a mixture of sand, silt, and clay particles containing about 35-45 percent sand, 35-40 percent silt and 10-25 percent clay. Loams and fine sandy loams are often the best soil textures for agricultural soils. Sands cannot retain much water or supply many nutrients, but good aeration may allow roots to penetrate deeply. Clays have good water holding capacity and nutrient supplying ability, but root penetration may be limited by poor aeration. Silts are similar to clays in many respects.

Soil Texture

A person looking for land to grow grapes should obtain a soil map that shows soil texture in the various areas from the local soil conservation service or agriculture extension office. Another method of determining soil texture is to feel the moist soil : sand particles feel gritty, silt particles are smooth and slick, and clay particles are sticky and plastic. Commercial laboratories can accurately analyze the percentage of sand, silt, and clay, and a textural triangle can then be used to determine the textural type (Fig. 8-1). Thus a soil containing 3 percent clay, 42 percent silt, and 50 percent sand, found at point A on the textural triangle, would be considered a loam soil.

Loam and fine sandy loams are considered excellent texture, while sand, silt, and clay are often poor (Neja and Wildman, 1973). Coarse-textured sandy soils cannot hold as much water or supply it as rapidly to the vine as can the finer textured loams. As soil depth increases, however, the preference of loams over fine sands decreases. For example, a 6-ft (1.83 m) deep, fine sand can be equal or even superior to a 6-ft (1.83 m) deep loam soil.

Soil Structure

Soil structure refers to the arrangement of soil particles into aggregates. Most crop soils have a compound structure in which the aggregates stick together. Good agricultural soil has a crumbly nature dependent upon the soil texture and presence of humus. Very wet or dry soils must not be cultivated, lest the structured be damaged. When wet soils are worked, dry clods form that are difficult to work into the soil. When soils are worked too dry, a powder is formed that causes crusting and sealing against water intake. Granular structure is poorer with increasing proportions of sand and decreasing organic matter.

Fine textured silts and clayey soils can sometimes limit the depth of rooting of irrigated grapes. Since shallow soils, say 30 inches (76.2 cm) deep, require frequent irrigation, a good irrigation setup is required.

Soil texture is only one of several aspects of the soil environment, and soil structure, depth, uniformity, and compaction must also be considered.

Soil Depth

Two major factors that usually limit or prevent deep rooting are lack of pore space for roots to grow into and lack of air caused by extremely wet soil. A tight layer of soil can also block root penetration. For example, if a subsoil layer has an increase in clay content, as in a claypan soil, water may accumulate above this layer and roots may be injured because of poor aeration. This condition is known as waterlogging. A man made compact layer occurring in the upper two feet (60.9 cm) of soil can sharply decrease the rate of water penetration (Fig. 8-2, 8-3).

Another type of barrier to root growth is an abrupt change from a fine or moderately fine topsoil to a coarse textured subsoil. Water will not move from the topsoil to the subsoil until several inches of soil above the interface are saturated. This lingering saturated zone remains because particle to particle flow of water is slow from the upper to the lower layer. Good drainage is essential to obtain root depth, and perched water tables block root growth.

Sometimes very dense, unfractured rocklike layers called hardpan occur. Such cemented hardpans are impervious to both water and roots. During the winter, rainfall can accumulate above the hardpan but can not pass through it. Unless the hardpan is mechanically shattered so that drainage is improved, vines may grow poorly (Widlman and Gowans, 1974).

Methods of detecting topsoil depth limitations include a direct study of the soil profile by appearance and feel, a laboratory analysis of texture and bulk density (weight per unit volume of a core of oven dry soil), and a study of the rate of water infiltration at different soil depths (Fig. 8-4).

Soil Uniformity

The uniformity of the soil reservoir is based on evenness of expected root depth and of topsoil across the field. Depth of rooting can be estimated by mapping the differences in soil structure and texture, with depth, over the field. Generally depth uniformity is considered good if deepest soils are no more than 11/2 times the shallowest depth, and is considered unacceptable if deepest soils are greater than twice the depth of the shallowest soils.

A grower makes the best of a nonuniform soil by using several techniques. Vineyard blocks should be laid out so that there is even texture and depth within each, leaving nonuniformity among blocks. Irrigation systems should be set up to allow irrigation of individual blocks. The uniformity of soil depth can be increased by plowing and ripping. One should use grape varieties and/ or rootstocks that are adapted to grow well on the particular type of soil.

Soil Compaction

Compacted soil results when improper cultivation practices break down the natural soil structure. Sometimes irrigation water may remain on the soil surface for 2 days or a week after normal irrigation because the large soil aggregates have been broken down, destroying the large pore spaces essential for rapid water percolation and good soil aeration. When deep cuts and fills are made in land grading, old soil surfaces and compact layer may be buried deeply and act as hardpans.

The best means of avoiding compaction is to cultivate as little as possible and only when the soil water content is intermediate, not too wet or too dry. Keep all traffic off the freshly loosened soil. The size of water conducting pores can be increased by planting permanent cover crops; deep rooted grasses that do not require tillage are best and can increase water intake rates5 to 10 times over a 5 to 20 year period. In some cases, however, a sod cover in vineyards may have deleterious effects on vine development and fruit production.

Soil compaction result in a loss of good structure, and often shows up as a slight layering or increasing density just below the usual depth of cultivation (Fig. 8-3). Man made compaction often extends to a depth of 2 ft (60.9 cm) (Neja and Wildman, 1974).

Root Growth

Root growth can be restricted by lack of pore space (tight soils) for roots to grow into or by lack of air in soil that is too wet. Water movement through compacted soil can be very slow. In coastal soils, perched water tables that can cause root pruning often determine root depth (Neja and Wildman, 1973).

Abrupt changes in structure and / or texture can cause a barrier to root growth. For example, if a fine or moderately fine topsoil is under laid with a coarse sand subsoil, roots will not grow across the abrupt boundary. Irrigation water will not pass from the upper layer into the sand until several inches of water above the interface is saturated because of the higher capillarity of upper finer soil compared to that of lower sandy soil.

Temporary saturation above the interface can limit aeration in the area just above the juncture of the two textures and thus restrict root growth. Abrupt boundaries in stratified soils break up the normal downward percolation of water, resulting in poorly aerated zones above the boundaries. A uniformly mixed soil is a much better root medium than a stratified soil.

Improving Physical Soil Factors Before Planting

Frequently the soil reservoir available to grape roots can be increased, but success depends upon the texture and structure of the soil, the location and extent of the soil restriction and the type of deep tillage equipment available for use. It is generally harder to eliminate or alleviate soil restrictions that have a high clay content.

Deep tillage refers to loosening, breaking up, or mixing restricting subsoil layers located below the depth of normal or ordinary cultivation. Its purpose is to break up man made compact layers in the top 2 feet of soil, break through natural subsoil claypans, hardpans and dense layers, mix stratified soils, and eliminate abrupt boundaries between unlike soil textures (Wildman et al., 1974). Types of equipment usually used in deep tillage include rippers, chisels, subsoilers, slip plows, disc plows and moldboard plows. The rippers and subsoilers, names which are often used interchangeably, operate at a depth of 2 to 7 ft (0.61-2.13 m) or more, where they break up hard layers by cracking and shattering; little mixing or dislocation of layers occurs. They are most effective in moderately dry, brittle soils and least efficient in moist sands and clays. Very dry soils require tremendous power to rip and may produce large clods that are difficult to manage.

Slip plowing is usually done to a depth of 3 to 6 ft. (0.91-1.83 m). A slip plow consists fo a verticle ripping shank with a 12-15 in. (30.5-38.1 cm) wide inclined beam extending to the rear from the ripping point at an angle up to the soil surface. Chunks of subsoil are torn loose by the point, slide up the beam, and are permanently moved from their original position. Surface soils fall into the channel produced, resulting in some permanent mixing and thus improving water and root penetration (Wildman et al., 1974).

Disc plows are valuable for relieving soil compaction and for mixing shallow clays in the upper 2 feet (60.9 cm) of soil. Moldboard plows developed in the 1950s could plow 4 to 6 ft (1.2-1.8 m) deep and effectively loosen and mix soil. Today, only a few 3 and 4 ft(0.91 and 1.21 m) plows are still in operation.

The success of the ripping operation can best be checked by backhoe appraisal. Often slip or plowing ripping along a future vine row may be beneficial, unless you plan to disc plow to 18 to 24 in. (45.7-60.9 cm) after deep tillage. It is important that young vine roots penetrate quickly into loosened and well aerated soil.

Backhoes are probably the most universally available, and easily transportable type of equipment listed for soil modification, and are adaptable mainly to small acreages.

Wheel trenches are usually used to dig trenches for water and drain lines, and have been used to a limited extent to loosen and mix soil layers before planting a vineyard.

Continue to Chapter 9