As with tillage crops, fertiliser advice for vegetables is based on soil analysis results for P, K, Mg and micronutrients. The results of the individual elements are expressed in milligrams per litre (mg/l) and to keep the nutrient advice as simple as possible they are classified into one of four separate Soil Index groups. Since there is no soil N test capable of predicting crop response, N advice is based on experience of the crops to be grown and the previous cropping and manurial history of the field.
The nutrient advice for vegetables is, for the most part, derived from the results of field experiments on major and trace element nutrition in this country and abroad, as well as on the considerable amount of field experience gained in the use of fertilizers commercially on a wide variety of soil and climatic situations in vegetable crop production in Ireland. For each area, the advice may be varied on the basis of other factors such as soil type, previous cropping history and local experience. For example, on P fixing soils, such as the Clonroche soil series in parts of Wexford and Louth, heavier than the recommended dressings of P fertilizer are needed to ensure an adequate supply. Thus, optimum fertilizer application for vegetables may be developed from this tabulated advice by modifying it according to the local circumstances which apply in the area of the country in which the vegetables are to be grown
Nutrients for Vegetable crops
Nitrogen advice for mineral soils is based on an arable/ley rotation. Under conditions of intensive tillage and depletion of soil organic matter, heavier rates than those recommended should be used. Likewise, if considerable quantities of FYM are used in spring or if the crop is grown after peas or following the ploughing down of a ley rich in clover, smaller dressings than those listed here should be used. Nitrogen fertiliser use should be restricted where rainfall is heavy and increased on light sandy soils low in organic matter.
In general, P fertilizer dressings should be increased:
A. On soil of low total P i.e. soils derived from shales, sandstones, boulder clays or outwash gravels
B. On poorly drained soils - poor drainage restricts root development and therefore, the uptake of P
C. On very acid soils and because P may be rendered unavailable under such conditions
In general, K fertilizer dressings should be increased on:
A. Soils low in total K i.e. newly reclaimed peat and limestone soils
B. Limestone soils - in the Dublin, Meath, Kildare, Offaly and Carlow areas because K fixation is greater in these soils
C. Poorly drained soils, where restricted rooting reduces uptake.
Forms of K compound
Potassium chloride has a higher salt index than potassium sulphate. Consequently in soils where intensive cropping is practised and heavy amounts of inorganic fertilizers are applied, it is suggested that potassium sulphate should be used. Where vegetable crops are grown as part of a farm rotation, potassium chloride is satisfactory for most crops. However, research findings have shown that potassium sulphate should be used on carrots and also on crops such as onions, leeks and French beans where extra heavy dressings of K are required.
Although Mg is a major plant nutrient, it behaves in many ways like a micronutrient or trace element. Deficiency can occur on acid, sandy or compacted soils and under conditions of either drought or high rainfall. If soil analysis indicates a deficiency of Mg in the soil, apply Kieserite (17.5% Mg) at 350 - 400 kg/ha. If lime is being applied use dolomitic limestone which will supply the Mg needed. Crops showing deficiency symptoms can be sprayed at 14-day intervals with Epsom salts (10% Mg) at 20 kg/ha in 1,000 litres of water.
Nutritional Deficiencies in Vegetable Crops
|Black heart in celery
|Boron deficiency (Cat's claw)|
|Boron toxicity - celery|
|Excess nitrogen on swede|
|Low pH - swedes around pH 4.9|
Caused by calcium deficiency
|Molybdenum deficiency in cauliflower
|Brownheart in swede - boron deficiency|
|Excessive nitrogen on sprouts causing botrytis and wet rot|
|Induced iron deficiency in modular sprout plants brought on by watering with hard water. The pH in the peat medium was 7.4. Note the youngest leaves are very pale, which is indicative of iron deficiency. The normal plant is second on the right.|
|Magnesium deficiency on broccoli|
|Nitrogen deficiency in parsley. It shows the classic symptoms of paleness on the older leaf with the younger leaves green. This is the opposite to iron deficiency symptoms.|
|Nitrogen deficiency in onions - modular propagated in organic compost|
|Nitrogen deficiency symptoms on broccoli. Note purpling of outer leaves, light green colour on the middle leaves and the greenest leaves are in the centre.|
|Zinc deficiency in maize. Symptoms: distinct yellow strips. Areas near the stalk may develop a general white to yellow discoloration. Stunted plants due to shortened internodes and lower leaves have a reddish or yellowish streak about one third of the way from the leaf margin. Treat with foliar Zn based on soil test results / plant symptoms|