Review site fertility for in-ground nursery stock production

Field grown nursery stock requires fertile sites that supply moisture, air, and nutrients. If there are no problems with winter drainage, or subsurface compaction, and there is supplemental water during the summer months, plants raised on these sites should grow vigorously and develop large healthy root systems. Healthy root systems in turn enable good uptake of soil nutrients. Plants that exhibit good health are better able to ward off insects and diseases, and withstand pressure from weed competition. In short, site fertility forms one of the most important building blocks for plant growth and development (1).

Goals of nutrient management
On well-drained sites sandy loam soil, with high levels of organic matter, there may be sufficient essential plant nutrients to ensure adequate growth and development for the life of the rotation of the stock being raised. Under these conditions, managers may even take soil fertility for granted. The goal of a nutrient management plan has to start with an understanding of the available site in terms of soil structure, texture, and pH. If the native site can not supply adequate nutrients, plants can be stunted, and loose their inherent quality. At the same time, excessive site fertility can result in excessive foliage growth that can detract from standard form that customers have come to expect. In addition, when plants receive more nutrients than they can absorb, the grower will have increased the costs of production unnecessarily, and may have harmed the surrounding environment.

Soil tests
In order to determine which amendments or fertilizers are needed to successfully raise an outdoor nursery crop, managers should collect a soil sample prior to planting. While soil sampling can be during done any month, the fall is good time for testing as soils are dry and accessible. Adopting a policy of collecting samples at the same time each year is just one more factor in consistent, efficient crop production. Sampling prior to crop establishment in the spring will enable growers time to incorporate amendments such as pre-plant fertilizer and lime. Once lime and other soil amendments have been added, further soil sampling is rarely required during the span of the rotation for trees and shrubs grown under field conditions.

Immobile elements such as phosphorus and limestone need to be worked into the plow zone prior to planting as otherwise they will remain near the soil surface and thus not be available to the stock during it life span. As such soil analysis refers only to plant nutrition. It will not address soil physical properties such as texture, or drainage. For perennial nursery crops a soil deficiency for a nutrient rarely has the deleterious effects of soil pathogens or standing water which either separately or together can easily kill plants. Soil testing is especially important in areas west of the Cascade Mountains where acidic soils predominate. Agricultural lime will be required to raise the soil ph (2). It will take time for lime for to raise soil pH. Fall applications are preferred so as to allow the winter rains to move the lime through the soil profile.

Soil testing irregularities
Each soil sample sent in for testing should represent a snapshot of one homogenous section of ground. Each sample to be tested should consist of a defined area of land where the crop will be established. Sites that have different degrees of slope, tillage history, cropping history, and native vegetation should all be treated separately. Avoid collecting samples from small areas where the soils very quite differently, or where there have been burn piles, saturated soils, or compost or manure piles. Combining unlike soil areas into one sample only reduces the chances of obtaining an accurate assessment of inherent soil fertility (3).

Sub-samples from a particular field should all be collected from the same depth as site nutrition can vary considerably over the surface to 24" depth. Each sample should consist of 15-20 sub-samples collected at random from the sampling area. Sub-samples should be collected from the top 6"-9" of the soil and should not include grass of other vegetation. Only use steel shovels or stainless steel tube core samplers as galvanized, brass or bronze tools can interfere with accurate micro-nutrient sampling. Carefully mix the sub-samples in clean plastic pail (not galvanized). Each soil sample typically consists of only a pint or so of soil.

If the farm has had different rates of fertilizer applied to various sections, sample accordingly. If fields have been fertilized recently or applied fertilizer has not had to time to dissolve soil testing should be delayed. Soil containing un-decomposed organic matter should not be sent for sampling. The cost of soil sampling is nominal ($30-$40) in relationship to the total cost of producing field nursery stock. Trying to save money by minimizing the expense of soil sampling can potentially shortchange the grower in terms of plant growth and development at harvest time.

Soil testing labs
Nursery crop producers in the Pacific Northwest should utilize the services of commercial soil testing lab that makes recommendations based on Northwest's native soils and climate. In order to ensure consistency between samples over time, growers are urged to explore the services of 2-3 labs and then settle on upon one lab. Oregon State University has compiled an on-line extension publication entitled A List of Analytical Laboratories serving Oregon (4). Many of the firms listed in this publication also cover Washington as well.

Essential nutrients
There are 16 different chemical elements that are considered essential for plant growth (5). Three of the elements, carbon, hydrogen, and oxygen are viewed as non-mineral elements. All 3 of these are obtained from the atmosphere and from water. Together they comprise more than 90% of the plant's dry weight. Carbon (C) is taken up in the form of carbon dioxide (CO2) from the air through the plants leaves. When combined with water (H2O) and oxygen (O2), under conditions of sunlight, plants will produce sugars and starches in the important process known as photosynthesis. As outdoor conditions have all of the essential factors for photosynthesis, C, O2 shortages are never considered limiting factors (2). In ground nursery stock requires various levers of supplemental H2O during the dry summer months.

Soil tests are used to measure the nutrients that are expected to become available to plants (6). As such soil tests are not used to measure the total amount of nutrients in the soil. Only a very small percentage of nutrients are actually available to plants, thus limiting the usefulness of total nutrient concentration testing.

Nitrogen
Soil scientists describe the process whereby nitrogen is either used by plants, stays in the soil, or is lost due to leaching as the nitrogen cycle. Nearly 98% of the total soil nitrogen is in the organic form that is largely unavailable to plants. The remaining 2% of soil nitrogen is available to plants in the form of mineral or inorganic nitrogen. In a process known as mineralization a small amount of organic nitrogen is converted to the inorganic form each growing season. The inorganic forms of available soil nitrogen include ammonium (NH4+), nitrate (NO3-). Under conditions that are conducive to plant growth the ammonium ion is quickly converted to the nitrate ion within a week (7) in a process called nitrification. Plants then take up the nitrate ion in order to grow and expand.

In the reverse of this process, the inorganic form of nitrogen is converted back to the organic form in a process known as immobilization. Both mineralization and immobilization occur simultaneously in the soil. However these two reactions are dependent upon microbial activity which is largely influenced by soil moisture, temperature, pH, and soil aeration. Because of these factors soil sampling for perennial crops does not include total nitrogen. Nursery stock growers typically fertilize their stock with urea, ammonium nitrate, or nitrate nitrogen. Nitrate nitrogen is very mobile in the soil and can be easily leached. As the various forms of available nitrogen are in a constant flux in the soil, a soil test for available nitrogen is not commonly conducted in areas west of the Cascades. While soil testing labs can be asked to perform NH4+ and NO3- concentrations at the time of sampling, the values do not reflect the future availability of accessible nitrogen to plants.

Phosphorus
Phosphorus is contained in various forms in the soil, such as iron phosphate, calcium phosphate, and aluminum phosphate. Even though total soil phosphorus may be high, plants can still suffer from phosphorus shortages. Soil phosphorus is available to plants in the form of either the ortho (HPO4-) or di-hydrogen (H2PO4=) ion phosphate. Under conditions of soil pH lower than 5-6, soil phosphorus is tied up on soil colloids in the form of insoluble aluminum phosphates. Under field conditions nursery stock can suffer from phosphorus deficiency if the soil pH is not amended prior to establishing the crop. Conversely, above a soil pH of 8 soil phosphorus is tied up in the form of calcium phosphate.

Soils should be sampled for the availability of phosphorus prior to establishing the crop. To supply adequate phosphorus to plants this element has to be incorporated into the eventual root zone of the crop. Once this practice has been accomplished, there should be adequate levels of available soil phosphorus to sustain the nursery stock over the rotation of the crop. After the stock has been lifted soils should be sampled again for the next cropping cycle. There no established criteria for phosphorus that can be applied to all the different types of nursery crops. Oregon State University (OSU) has released a soil interpretive guide that describes the level of available phosphorus based on different soil test values (Table 1). In general of 40-60 ppm is considered desirable for most crops (3). Managers should avoid sampling portions of the field where phosphorus had previously been applied in bands as levels of this nutrient can build up over time. For commercial Christmas trees, OSU has released a commercial fertilizer guide (9) that addresses soil phosphorus testing as well as suggested pre-plant fertilizer recommendations.

Excess phosphorus
Phosphorus has been implicated by U.S. Environmental Protection Agency as a prime contributor to pollution problems in our nation's streams and lakes (8). Conventional surface applications to level and gently sloping agricultural fields are usually not of concern as phosphorus fertilizer is immobilized by attaching itself to soil colloids and thus does not leach. However, if more phosphorus fertilizer is applied than can be absorbed by either the soil colloids or the crop itself, there is a very strong chance of phosphorus being carried away by either winter rains or supplemental irrigation. Once it moves into surrounding streams, rivers, and lakes it can lead to the excessive growth of algae in the water which can harm the water quality in a process referred to as eutrophication. Shallow slow moving bodies of water are most prone to suffering from the effects of eutrophication. Once they have become choked with weeds and algae it is extremely difficult to return them to clear cool bodies of water that can be enjoyed for swimming and other recreational activities. Nursery crop producers should adopt the use of vegetation buffers surrounding their field stock in order to lower the risk of harming aquatic areas (9).

Once inside the plant phosphorus is considered a mobile element. New growth would receive the phosphorus absorbed by the roots, while older growth would suffer from a shortage.

Potassium
Potassium joins nitrogen and phosphorus as one of the three most heavily used nutrients for sustained plants growth. In the soil 90-98% of the total potassium is tied up in the form of insoluble compounds tied to soil rock particles (3). In this condition potassium is considered relatively unavailable to plants. The slowly available forms of soil potassium (1-10% of the total potassium) are found on the organic matter and clay particles. This form of potassium is slowly released by natural weathering. Readily available soil potassium is derived from potassium fertilizer application as well as from the soil solution as the soil colloids shrink and swell. Soil test values for potassium are listed in Table 2.

As a positively charged cation potassium does not readily leach from the rooting depth of nursery stock. The movement of potassium within the soil is largely influenced by the amount of soil moisture. During drought conditions there is less available potassium for plants to absorb. Growing roots have to into contact with the potassium ions in order to be absorbed. In order to minimize the deleterious effects of drought there must be adequate potassium in the root zone as provided by supplement fertilization applications. On mineral and loamy soils potassium deficiency is relatively uncommon on field grown nursery stock (11).

Calcium
Calcium is considered a secondary plant nutrient for field nursery stock as it is not used in quantities as extensive as nitrogen, phosphorus, and potassium. Calcium (Ca++) exists in the soil as a positively cation which leads to it being attracted to the negatively charged soil colloids. On loamy soils west of the Cascades calcium is the predominate cation found on the soil colloids (12). Under conditions of low soil pH or low cation exchange capacity (CEC) calcium deficiencies can occur. Soil test values for calcium are presented in Table 3.

Field crop producers can easily correct calcium shortages by supplying agricultural lime (calcium carbonate, CaCO3), gypsum (calcium sulphate, CaSO4), or calcium nitrate prior to planting and tilling it into the plow zone. While the calcium ion itself will not alter the soil's pH, the carbonate ion will. If the soil pH is adequate, calcium itself can be provided by incorporating gypsum prior to planting.

Soil moisture plays a key role in the movement of calcium uptake in plants. When plants suffer from drought, calcium will not be readily taken up and thus not transported to the growing tips of leaves, fruits, and stems. Examples of adverse growth due to shortages of calcium include bitter pit in apples, and blossom end rot in tomatoes. Outdoor field grown nursery stock rarely exhibit shortages of calcium.

Magnesium
In the soil magnesium occurs as the Mg++ ion and is thus firmly held to the soil colloids as is the case for calcium. The natural level of magnesium in Northwest soils depends upon the parent material of the soil type, the cation exchange capacity (CEC), and levels of calcium and potassium. If the soil colloids are filled with high rates of calcium or potassium as a result recent lime or potassium fertilization, localized shortages of magnesium can occur. If soil tests show that the level of exchangeable potassium (as expressed in ppm) exceeds that of exchangeable magnesium by a ratio of 3:1 induced magnesium deficiency is possible. Suggested soil test levels are listed in Table 4.

In order to correct magnesium shortages, growers are advised to incorporate dolomitic lime as opposed to tradition agricultural lime when they prepare beds for planting. Once within the plant the magnesium ion is mobile. Under conditions of magnesium shortages, older leaves may show intervienal chlorosis or even leaf drop. Leaf analysis should be used if magnesium deficiency is suspected. Magnesium deficiency is associated with low pH (5).

Sulfur
Sulfur exists in the soil as suphate, a negatively charged ion (SO4-2), and is thus susceptible to leaching as is the case for nitrogen. Nursery stock can not absorb elemental suphur (frequently sold as gypsum), which is the preferred amendment, especially when growers wish to lower the soil pH. Under warm moist conditions in the spring elemental suphur is converted to sulphate which can then be absorbed and utilized by growing nursery stock. In plants lack of sulphate will first be noted in the growing tips of developing foliage (12). The relationship between the level of elemental sulfur and field nursery stock growth has yet to be established.

References:
1. Building Soils for Better Crops. 2000. Fred Magdoff and Harold van Es. 2nd Edition Sustainable Agriculture Network, Handbook #4. Available from: Sustainable Agriculture Publications, Hills Building, Room 10, University of Vermont, Burlington, VT 05405-0082, phone: 802/ 656-0484.

2. Soil sampling for home gardens and small acreages. 2003. Oregon State University Extension Service. EC 628.

3. Agronomy Handbook. A & L Agricultural Laboratories, edited by Don Ackerman, and Richard Large.

4. A List of Analytical Laboratories serving Oregon. Revised January 2002. John Harte, Extension soil scientist, Oregon State University. EM 8677.

5. Basic principles of soil fertility I: Plant nutrients. 2000. All Bandel. College of Agriculture and Natural Resources, University of Maryland. FS-639.

6. Soil test interpretive guide. E.S. Marx, J.Hart, and R.G. Stevens. Oregon State University Extension Service. EC 1478, reprinted August 1999.

7. The big three. Pay attention to the elements nitrogen, phosphorus, and potassium. James Altlund, Oregon State University Cooperative Extension. In: The Digger, February, 2003.

8. The phosphorus cycle. United States Environmental Protection Agency, Ag 101.

9. Christmas Tree Nutrient Management Guide for Western Oregon and Washington. February 2004. John Hart, Extension soil specialist, Oregon State University. EM 8856-E.

10. Nursery run-off management and regulations. Tom Bilderback, nursery crops extension specialist, North Carolina State University Cooperative Extension.

11. Soil and nutrition management for field-grown plants. Harold Davidson, Curtis Peterson, and Roy Mecklenburg, In: Nursery Management Administration and Culture, 3rd Edition, Prentice Hall, Englewood Cliffs, New Jersey.

12. Secondary, but still important. Calcium, magnesium and sulfur: the lesser macronutrients. James Altlund, Oregon State University. In: The Digger, March 2003, pages 52-54.

First posted: December, 2004.