Soil Fertility and Nutrient Use: Fertilizer Recommendations
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811: Agronomy Guide > Soil
Fertility and Nutrient Use > Fertilizer Recommendations
The pH scale ranging from 0 to 14 is used to indicate acidity and alkalinity. A pH value of 7.0 is neutral. Values below 7.0 are acidic, and those above 7.0 are alkaline. Most field crops grow well in a soil pH range from 6.0 to 8.0.
To correct soil acidity, broadcast ground limestone and work it into the soil at rates determined by a soil test. Table 9-2, Soil pH at Which Lime Is Recommended for Ontario Crops, shows the pH values below which liming is recommended and the target soil pH to which soils should be limed for different crops. In Ontario, most crops grow quite well at pH values higher than the target pH to which liming is recommended.
The Buffer pH
The soil pH measures the amount of acidity in the soil solution and indicates whether liming is necessary for crop production. It does not measure the amount of reserve acidity held on the clay and organic matter particles in the soil, which will dictate how much lime is needed. Different amounts of reserve acidity will mean that two soils at the same pH value will need different amounts of lime to raise the pH to the desired level. The reserve acidity is measured in a separate test: the buffer pH. The greater the amount of reserve acidity, the lower the buffer pH and the more lime is required to raise the pH. For soils needing lime, Table 9-3, Lime Requirements to Correct Soil Acidity Based on Soil pH and Soil Buffer pH, may be used to determine the amount of lime required to reach different "target" soil pH values required.
1 Where a crop is grown in rotation with other crops requiring a higher pH (for example, corn in rotation with wheat or alfalfa), lime the soil to the higher pH.
Calcitic limestone consists largely of calcium carbonate, and dolomitic limestone is a mixture of both calcium and magnesium carbonates. Dolomitic limestone should be used on soils with a magnesium soil test of 100 or less, as it is an excellent and inexpensive source of magnesium for acidic soils. On soils with magnesium tests greater than 100, calcitic or dolomitic limestone may be used.
The two main factors that affect the value of limestone for soil application are the neutralizing value and particle size. Neutralizing value is the amount of acid a given quantity of limestone will neutralize when it is totally dissolved. It is expressed as a percentage of the neutralizing value of pure calcium carbonate. A limestone that will neutralize 90% is said to have a neutralizing value of 90. In general, the higher the calcium and magnesium content of a limestone, the higher the neutralizing value.
pH in Ontario is measured using the Shoemaker, MacLean and Pratt
(SMP) buffer. Other jurisdictions may use different buffers, which
will give similar but not identical results.
The second factor that affects the value of limestone as a neutralizer of acidity is the particle size. Limestone rock has much less surface area to react with acid soil than finely powdered limestone and, hence, it neutralizes acidity much slower - so much slower that it is of little value. The calculation of a fineness rating for ground limestone is illustrated in Table 9-4, Example Calculation of the Fineness Rating of a Limestone.
1 A #10
Tyler sieve has wires spaced 2.0 mm apart.
The Agricultural Index
This index has been developed in Ontario as a means of combining the neutralizing value and the fineness rating to compare various limestones that are available.
The Agricultural Index = (neutralizing value x fineness rating) ÷ 100
The Agricultural Index can be used to compare the relative value of different limestones for neutralization of soil acidity. Lime with a high Agricultural Index is worth proportionately more than lime with a low index because it may be applied at a lower rate.
For example, if two ground limestones, A and B, have Agricultural Indices of 50 and 80 respectively, the rate of application of limestone A required for a particular soil will be 80/50 x the rate required for limestone B. Limestone A spread on your farm is worth 50/80 x the price of limestone B per tonne.
Recommendations from the OMAFRA soil test service are based on limestone with an Agricultural Index of 75. If the Agricultural Index is known, a rate of application specifically for limestone of that quality can be calculated. This can be done using the following equation:
Limestone application rate from soil text x (75 ÷ Agricultural Index of Limestone) = Rate of Application of Limestone
For example, if there is a limestone requirement by soil test of 9 t/ha, and the most suitable source of limestone from a quality and price standpoint has an Agricultural Index of 90, then apply 7.5 t/ha (75/90 x 9).
The Agricultural Index does not provide information about magnesium content. Dolomitic limestone should be used on soils low in magnesium.
Lime recommendations presented here should raise the pH of the top 15 cm (6 in.) of a soil to the listed target pH. If the soil is plowed to a lesser or greater depth than 15 cm, proportionately more or less lime is required to reach the same target pH. Where reduced tillage depths are used, reduce rates of application proportionately. More frequent liming will be needed.
Lowering Soil pH
On soils with pH values below 6.5, it is possible to lower the pH (make the soil more acidic) by adding sulphur or ammonium sulphate. This may be desirable for some crops, for example, potatoes for scab control, but usually will not be suitable for rotation crops. Soil pH cannot be adjusted from a low pH to a more moderate pH year to year. If the soil pH is above 6.5, it is not advisable and also usually quite impractical to lower the soil pH because of the very large amounts of sulphur or ammonium sulphate required.
Changes in Crop Management
Fertilizer requirements on the OMAFRA soil test report are specific to selected crop and management. Adjustments in fertilizer requirements may be needed where manure is applied or legumes will be incorporated. Changing the crop from the original soil test recommendation will require a new fertilizer recommendation. This can be obtained by looking up the appropriate table under the specific commodity chapter in this publication.
Nitrogen fertilizer recommendations for field crops are based on estimated crop requirements. Recommended rates are presented in tables found in the Fertility Management section for each crop chapter. Rates are adjusted downward if manure is applied, or if the previous crop contains perennial legumes such as alfalfa.
To protect crop quality and avoid movement of nitrogen into the groundwater, the combined application of fertilizer, manure, biosolids and residuals, and other sources of nitrogen should not supply plant-available nitrogen in excess of the crop's requirement.
Phosphate and potash recommendations are based on OMAFRA-accredited soil tests. The requirements of these nutrients are presented in the Fertility Management section for each crop chapter. Only use these tables with OMAFRA-accredited soil tests. Non-accredited tests may use extractants that pull out different amounts of nutrient so they will not give correct values if used with the published OMAFRA tables.
Where a soil test is not available, a rough estimate of requirements can be obtained from these tables using the following guidelines:
The risk of surface water contamination by phosphorus may be increased at higher soil test phosphorus levels. Since phosphorus binds tightly to soil particles, the movement of soil off the field through erosion is a major factor in determining the risk of surface water contamination. It is for this reason that the risk of surface water contamination by phosphorus cannot be based solely on a soil test phosphorus level. Wherever soil tests show a very low or no probability of response to added phosphorus, application of any source of phosphorus should be guided by a Phosphorus Index (see the NMAN Software, the Nutrient Management Workbook or the OMAFRA Factsheet, Determining the Phosphorus Index for a Field, Order No. 05-067). A Phosphorus Index ranks the relative risk of surface water contamination for applications made to a particular area of land. It will also determine maximum application rates, if manure is applied, and suggested setbacks from watercourses.
Some clay and clay loam soils are naturally high in potassium and do not require any potash fertilizers. Only a soil test can adequately determine potash requirements.
Magnesium is a nutrient that is naturally plentiful in many Ontario soils. Although rare, when magnesium soil tests are below 20, according to an OMAFRA soil test, magnesium application will be required for production of most crops. If the soil pH is below 6.0, the most effective and inexpensive means of supplying magnesium is by application of dolomitic lime. Use dolomitic lime to correct soil pH if the magnesium soil test is below 100. On soils with a higher magnesium test, use either dolomitic or calcitic lime to correct soil pH. If the pH is above 6.0 and the soil test result is 20 or below, supply magnesium by either magnesium sulfate or sulfate of potash magnesia (a mixture of sulfate of potash and magnesium sulfate). Apply 30 kg/ha (27 lb/acre) of actual soluble magnesium. These latter sources of magnesium are usually quite expensive compared to supplying magnesium from dolomitic limestone.
Potassium competes with magnesium for uptake by crops, and potash applications can therefore induce or increase magnesium deficiency. For this reason, it is important to monitor soil potassium levels and carefully control potash fertilizer applications on low magnesium soils.
Crops grown on a number of Ontario soils are low in magnesium to the extent that livestock health is affected, although the crops themselves do not suffer from magnesium deficiency. In these situations, it is usually much more economical to add magnesium to the animal's diet than to add it to the soil. Soil potassium should be closely monitored and potash applications restricted to requirements as measured by soil test.
Calcium deficiency has not been a problem in Ontario soils with soil pH adequate for field crop growth. Adequate soil pH is a reliable indicator of adequate calcium in the soil.
Sulphur is generally provided in adequate amounts for crop growth in acid precipitation, livestock manure and organic matter breakdown. There have been reports of sulphur deficiencies from a few fields in Northwestern Ontario, mainly on sandy soils with low organic matter. Where sulphur is required, apply it as either ammonium sulphate or potassium sulphate. Elemental sulphur can be used, but it must be oxidized to the sulphate form before the plants can absorb it. It therefore needs to be applied 12-18 months before a crop requiring sulphur is planted.
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