Before thumbing through seed catalogs and making your trip to a garden supply center, collect a soil sample and have it tested. Most states offer soil testing services through their State Cooperative Extension Service (County Extension Offices), and if not, there are commercial soil testing laboratories that provide such services at a reasonable cost. Select a soil testing laboratory that services your area, therefore their lime and fertilizer recommendations will fit your soil type and climatic conditions. Make contact with the organization or laboratory that will do the assay so that you know the procedures required to collect and submit the soil sample.
Normally, requesting what most identify as a “routine” test will provide all the information you will need. It might be tempting to assay for other soil properties, but in most instances, the obtained test results can be misleading unless your soil fits those conditions associated with the test objectives. Keep the soil test results so that over time, you can plot the change that is occurring in pH and level of test element concentrations. Based on changing trends, adjustments can be made in the liming and fertilization routine so that the soil stays within the desired range that ensures “fertility.”
An “infertile” soil can be easily made into a “fertile” soil by appropriate additions of lime and fertilizer based on a soil test recommendation, and in addition, by adding moderate amounts of organic materials to improve soil structure and tilth if needed. Deep spading and uniform mixing of soil additives into a garden soil profile creates uniformity with depth, significantly improving plant growth as roots are able to occupy a larger volume of soil, increasing the potential for water and essential element access. With greater root occupancy within the soil profile, the stress on a plant when growing conditions are less than ideal will be moderated.
Probably the most overlooked, and yet a most important factor that defines the fertility of a soil, is its pH. Most plants grow best when the soil water pH is between 5.6 to 6.5, the optimum lower and upper pH limits being determined by the soil’s physio-chemical properties and the growing, or to be grown, plant species. Some species grow best when the soil pH is on the more acid side, between 5.4 and 5.8.Allowing the soil water pH to drop below 5.4 will adversely affect plant growth and reproduction for most species. Liming to maintain the soil pH within the desired range satisfies several other soil fertility factors, such as optimizing the availability of most essential plant elements. Since all liming materials contain calcium, with some liming materials, such as dolomite, also magnesium, one or both of these essential major plant elements being supplied with each lime application. For most acid soils, dolimite should be the liming material of choice. Liming should be done in the fall or early spring, allowing sufficient time for the lime to react with the soil. When correcting an acid soil condition, the objective should be lime to bring the soil within the desired pH range, and then make supplemental applications (can be yearly depending on what fertilizers are being applied and plants grown) in order to maintain the desired established pH range. Long periods between liming cycles the soil pH which results in periods of good (when the pH is within the desired range) and poor (pH out of the desired range) plant growth.
Adding too much, or even the wrong type of fertilizer, are common errors when fertilizing a garden soil, adding more plant nutrient elements than needed, or not sufficient for that needed by the plants. Of the three fertilizer elements, nitrogen, phosphorus and potassium, over fertilizing with phosphorus is most common since many fertilizer formulations contain relatively high quantities of this element, and with repeated applications, the soil phosphorus level slowly increases. High soil phosphorus content poses two potential problems, one environmental, contributing to ground and surface water pollution, and reducing the availability of several essential elements, for example the micronutrients iron and zinc. A soil test will measure the phosphorus level and advise on whether phosphorus fertilization is required. A phosphorus requirement is primarily based on that amount in the soil, and not necessarily based on a plant requirement.
Nitrogen is the fertilizer element that can be either under or over applied, depending on what grade of fertilizer (content and form) is used and rate of application. Plant growth and development are impaired when nitrogen is insufficient, while excessive nitrogen makes plants more susceptible to stress and will reduce flowering and fruit set for some vegetables. There is no soil test for nitrogen; the amount required being based on the plant requirement.
The other fertilizer element is potassium and its requirement is determined by a soil test. The only danger from over fertilization is possible reduction in the availability of both calcium and magnesium. Potassium is that element which affects flower and fruit quality, reducing both when insufficient.
Matching a fertilizer grade with a soil test and/or plant requirement recommendation can be a challenge. It is best to avoid selecting fertilizer grades based on plant use, for example, selecting a fertilizer that is recommended for certain garden vegetables or flowers that may, or may not, fit what is being recommended based on a soil test result. Depending on the soil test level and plant requirement, it could be that only nitrogen fertilizer is needed when there is sufficient soil phosphorus and potassium. Therefore be cautious and do not apply what is not called for in the soil test recommendation, or specified by the plant requirement.
Some fertilizer formulations contain other essential elements, sulfur and some of the micronutrients. Unless there is a specified need, these formulations will not benefit plant growth, their additional cost not justified. If there is a specific need for such other elements, they should be applied at rates, using fertilizer sources designated for that particular element.
Organic substances, such as manures, composts, etc, if indiscriminately applied, poise a potential hazard, making the soil more difficult to manage and creating a situation that could make the soil “infertile,” therefore, requiring significant treatment to restore fertility. Most organic substances will increase the soil water-holding capacity and improve both soil structure and tilth, all desirable effects. But large and repeated applications can make the soil unsuitable for tilling when wet, and cool in temperature, particularly when wet, thereby slowing plant growth. Composted manures and sewage sludge may contain significant quantities of elements (essential and non-essential) that may not be needed, raising an element level to excessive or toxic levels, particularly with repeated use. Before adding such materials, have the elemental content determined by a laboratory-conducted assay. If herbicides are being used to control lawn weeds, collected lawn clippings should not be added to a garden soil.
Vegetable garden soil fertility management has different requirements than that for a lawn, ornamental and flower beds, and trees. Most garden vegetables have fairly high essential element requirements, therefore responsive to fertilization. Most trees and ornamental plants have relatively low essential element requirements, and grow best in acid soils (pH 5.4 to 5.6) with minimum fertilization requirements. Trees within and around a well maintained lawn may be adversely affected by the lime and fertilizers being applied to the lawn, stimulating growth, subjecting the tree to possible disease and insect infestation, and environmental stress. Trees surrounded by flower beds that are frequently cultivated may begin to slow in growth, and may eventually die. Tree feeder roots grow up, not down, so frequent cultivation around the tree will injure these roots, therefore impairing their normal function of supplying water and essential elements to the tree.
The need to apply fertilizer during the growing season should be done only under special soil-plant situations and should not be a standard routine. If the soil had been adequately limed and fertilized, plants should be sufficiently supplied with all the essential elements required to grow normally during their seasonal growth cycle. Many of the advertised supplemental fertilizers would suggest that significant improvement in growth, flower production, etc. can be obtained by applying a foliar fertilizer formulation. Many package their supplemental fertilizers so that they can be easily applied, some through an attached garden watering hose system. Following the directions, no harm will occur – the question being, “was the cost and time justified in terms of better growth and flower production?”
Element absorption through plant leaves is minimal at best, the amount absorbed being determined by leaf structure, and the time of day and weather conditions when applied, form of the element, and the formulation characteristics of the spray material. Most of the major essential elements are not easily adsorbed through plant leaves. A few of the micronutrients are readily absorbed if they are in the right form and the conditions are favorable for adsorption.
J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydropopnics: A Practical Guide for the Soilless Grower and Tomato Plant Culture. It is available at crcpress.com. Dr. Jones has written extensively on hydroponic growing and has been outdoor vegetable gardening employing sub-irrigation hydroponic growing systems (see: hydrogrosystems.com), and using domestic water for making his nutrient solution.