When all things are in place, Nitrogen is the variable which we use to regulate growth and fruiting to suit the variety, type of tomato and conditions. Too many farmers like to calculate their fertiliser programme in advance and include nitrogen. The problem is that nitrogen is so fickle that the standard soil analysis doesn’t even include this element. In some cases it can be leached out of the soil by heavy rain. To schedule a side-dressing programme of nitrogen in advance is pretty risky unless one has experience and plants in a season when it won’t rain.
We also start with a variable as generally we are not sure how much nitrogen there is in the soil before planting and preparation. In most cases, there is a gradual release of nitrogen from crop residues and humus being decomposed by the soil microorganisms. soil with a high humus content, more nitrogen will be released into the soil solution than in one which is low in humus. Furthermore, soil with more humus can prevent the nitrogen in nitrate form from leaching out in heavy rain. Mechanical cultivation of the soil during preparation stimulates microorganism activity, which results in a spike of nitrogen released later on.
Newly transplanted tomatoes therefore have more than adequate amounts of nitrogen available, as the plant biomass at this stage is low and little is required. The root system is limited on these transplants, so in some circumstances the nitrogen can leach out of reach of the plants when overirrigated, especially with drippers in hot weather. We need to look out for this and rectify if necessary. Very little nitrogen applied near the plant or through the drippers will rectify this until the root system enables the plant to forage. We should frequently inspect the plants for adequate vigour and uniformity. When levels of nitrogen are getting low in the early stages, it usually reflects in patches showing this up first. Look out for smaller plants with a lighter colour. Lack of uniformity affects management right through the crop.
Sometimes, it doesn’t start in patches, but there may be a variation in individual plants throughout the land. slightly more vigorous plants would have sufficient energy to get more roots out after transplanting and have more access to soil nutrients. he plants with marginally lower nitrogen and leaching after post-transplant irrigation wouldn’t have the energy to produce roots and therefore remain stunted. Farmers who don’t know the causes tend to blame the seedling nursery for supplying root-bound seedlings. – Bill Kerr ([email protected] or (016) 366 0616)
