Ontario Fertility Guide for Vegetable Crops

Macro Nutrients

Nitrogen

Nitrogen is an essential element for the growth and development of all crop plants. As a component of chlorophyll, it plays a vital role in photosynthesis. Nitrogen is also one of the building blocks for the formation of amino acids, protein and enzymes.

Nitrogen is naturally present in all soils. As soil microbes feed on crop residues and soil organic matter, they release nitrogen into the soil. As soil organic matter levels increase, so do the levels of naturally available nitrogen. As a result, good soil management practices enhance a soil’s natural fertility levels. Legumes, such as alfalfa, build soil fertility by capturing atmospheric nitrogen and releasing it into the soil as they decompose.

Nitrogen deficiencies usually first appear on older leaves. These leaves will turn light green or yellow as nitrogen is relocated from older, less productive leaves to the newest growth. Cool growing conditions in early spring often cause plants to develop a temporary nitrogen deficiency. This is usually due to poor growing conditions, and not necessarily a lack of nitrogen in the soil.

Many nitrogen fertilizer materials contain high amounts of salts. If a germinating seedling or young transplant comes into contact with a concentrated fertilizer band, the tender roots may become seriously damaged. Affected plants may become stunted or they may wilt and die. This damage is easily confused with damping off or soil insect feeding.

Ensure that starter or transplant fertilizers contain only as much nitrogen as necessary to get the crop started. Fertilizers containing more than half as much nitrogen as phosphate frequently contain urea and may cause crop damage.

The Pre Side-dress Nitrate Test (PSNT) is a tool that can help assess the level of plant available nitrate-nitrogen present in the soil during the growing season. Developed for use in field corn production, this test can aid in determining additional fertilizer requirements.

The PSNT results are expressed in part per million (ppm) of nitrate-nitrogen. To convert this number into lbs/ac, multiply by 4. For example a nitrate-nitrogen test reading 15 ppm represents 60 lbs/ac.

Keep in mind that the PSNT was developed for use in field corn production, and has not been thoroughly researched for use with horticultural crops. The PSNT is a tool. It can give growers an indication of the nitrate-nitrogen content in the soil at one given point in time. Where PSNT values are high, growers may be able to reduce the side-dress nitrogen fertilizer rate. However, nitrate-nitrogen sampling should not entirely replace current OMAFRA fertility recommendations or grower experience.

Phosphorus

Like nitrogen, phosphorus is an important component of photosynthesis and the development of enzymes and protein. It also plays an important role in cell division and the creation and transport of sugars and starches.

Soil phosphorus levels across Ontario can be quite varied. However, many coarse sandy-loam soils often contain high levels of phosphorus. Former tobacco land and soils with a history of regular manure applications often have phosphorus soil test ratings of “Low Response” (LR) to “No Response” (NR). These soils are capable of producing high-yielding vegetable crops with little or no additional phosphorus fertilizer.

Phosphorus deficiency symptoms usually develop on the older leaf tissue first. It causes these leaves to develop a purplish-red colour. This may be more noticeable on the underside of the leaves. Severe deficiencies may also cause the leaf tips to die back.

Research has shown that phosphorus availability and root growth are reduced during cool soil conditions. As a result, many early-season vegetable crops benefit greatly from having small amounts of phosphorus applied to the rooting zone at seeding or transplanting. For crops planted later in the season, under more favourable soil conditions, there is little benefit to adding extra phosphorus at the time of planting.

Excessive phosphorus applications may negatively impact the crop’s ability to remove zinc and iron from the soil. Where phosphorus levels are high, zinc and iron deficiencies may occur.

Potassium

Potassium is an important component of plant cells. It also influences the uptake of water by the roots and plays a role in both respiration and photosynthesis. The sugar and starch content of crops like potatoes and tomatoes may be affected by potassium levels. Most crops require equal amounts of potassium and nitrogen.

Potassium deficiency usually appears on the older leaves first. It can cause a yellowing or burning of the leaf margins. In tomatoes, low potassium levels have been associated with yellow shoulders on ripe fruit. In other fruiting vegetables, severe potassium deficiencies may result in misshapen fruit and increased levels of fruit abortion. Insufficient levels of potassium may reduce the shelf-life of many vegetable crops.

Excessive potash applications may negatively impact the crop’s ability to remove calcium and magnesium from the soil. Where potash levels are high, calcium and magnesium deficiencies may occur.

Calcium

Calcium is a vital component of cell walls and is involved in the metabolism and formation of the cell nucleus. Calcium pectate in the cell walls provides a physical barrier to disease entry. Calcium does not move readily within the plant.

Calcium deficiencies may result in the death of the plant’s growing point. It may also cause the blossoms and buds to drop prematurely.

Calcium-related disorders may occur in some crops (e.g., blossom-end rot in tomatoes, blackheart in celery or tipburn in lettuce and cabbage). Calcium enters the plant through the transpiration stream. If the water supply is limited, calcium tends to accumulate in the large leaves where the water requirement is greatest. Under these conditions, the dissolved calcium bypasses the fruit and developing leaves, which have lower transpiration rates.

Several cultural management practices will reduce the occurrence of calcium-related disorders. Efficient nitrogen use will help prevent excessive vegetative growth. Good soil management practices ensure good root growth, promoting both water and nutrient uptake. Timely irrigation will help to maintain a steady supply of calcium throughout the plant.

Magnesium

Magnesium is an essential part of chlorophyll. It also aids in the formation of sugars, oils and fats.

Magnesium deficiency usually appears on the older leaves first. The leaf tissue between the veins turns yellow, while the veins remain green. Severe deficiencies will cause the leaf margins to curl.

A basic soil test may be used to identify or confirm potential magnesium deficiencies. Soils testing less than 20 ppm Mg are considered to be deficient.

Excessive potassium applications may induce a magnesium deficiency. Avoid using high rates of potash on soils with a low magnesium rating.

Sulfur

Sulfur is an important component of chlorophyll. It also aids in seed production and is involved in nitrogen fixation in legumes. Sulfur adds flavour, colour and distinctive odours to several crops including; brassica crops, onions, garlic and horseradish.

Sulfur deficiency is very similar to nitrogen deficiency. Affected plants are often stunted and have pale foliage. Sulfur deficiency may delay maturity. Nitrogen deficiency usually appears on the older leaves first, while sulfur deficiency will affect the entire plant.

Deficiencies occasionally occur on coarse, sandy-loam, low pH soils. Improving soil organic matter will help increase soil sulfur levels. Many livestock manures also contain significant amounts of sulfur.

Historically, sulfur was a component of many synthetic fertilizers. Modern production methods have removed much of the sulfur found in fertilizer. Never-the-less many Southwestern Ontario farms still receive considerable amounts of sulfur in the form of acid rain and dry deposition. Each year, 8- 13 kg/ha (7- 12 lbs/ac) of sulfur are deposited in rainfall.

Interpreting the Soil Test Reports

In a soil test report, each nutrient is given a numerical value, a letter rating and a suggested fertilizer application rate.  The suggested fertilizer rate is usually listed in lb/acre or kg/ha. The soil test nutrient ratings reflect the likelihood of a yield response to applied fertilizer in the year of application. The yield benefits to an application should exceed the value of the fertilizer and the associated application costs.

The rating system is explained in Table XX, Soil Test Nutrient Ratings.

Table XX. Soil Test Nutrient Ratings

Response Category

(From the Soil Test Report)

Level of Soil Fertility

Probability of Profitable Crop Response to Applied Nutrients

High Response (HR) Low High (most of the cases)
Medium Response (MR) Medium Medium (about half the cases)
Low Response (LR) High Low (few of the cases)
Rare Response (RR) Very High Rare (very few of the cases)
No or Negative Response (NR) Excessive Not profitable to apply nutrients