Tomato fertilizer requirements can be calculated if you know your soil type and have a good idea of the yield potential of your land and climate. These are basic norms and your specific microclimate and variety can have an influence on the total amount applied. These norms do not take top dressings into account, only the total amount of N, P and K applied during the growth season. The fertilizer can be applied either through the irrigation system or as granular fertilizer. Consult your supplier on the effectiveness of these fertilizers on the various soils to make adjustments to these amounts. Use a drip system when fertigating as the salts will burn the leaves in hot sunny weather leaving necrotic spots. Never use granular fertilizers through the drip irrigation system as it is not 100% water soluble. It will block the drippers eventually and it is very difficult and expensive to rectify.
Three soils are used in these tables:
- S/SL – Sandy to Sandy Loam
- SL/S – Sandy Loam to Sand
- SCL/C – Sandy Clay Loam to Clay
Nitrogen (N) application
Tomato fertilizer requirements ito nitrogen is easy. You don’t need a soil analysis just a good idea of your yield potential in the area. Be careful not to overestimate your potential yields as too much nitrogen will also have a negative effect on total yield and the quality of the fruit. High nitrogen content makes the plant more susceptible to insect damage and diseases. Too much nitrogen on will promote more leaf growth instead of fruit. Too little nitrogen and the leaf development suffers with lower yields. Less leaves also result in a lot of fruit getting sunburn which are not marketable.
|Yield target t/ha||S/SL||SL/S||SCL/C|
Phosphorus (P) – (Bray 1)
Phosphorus fertilizer requirements are based on Bray 1 lab analysis. If another method is used the values must be adjusted. The minimum amount applied is always about 20 kg P per hectare as P is not that mobile and a certain level of P must always be maintained in the soil.
|P analysis (ppm)||Yield target <80 t/ha||Yield target >80 t/ha|
Potassium requirements are based on a soil analysis and yield estimates. Potassium is extremely important for flower formation and water regulation. Over fertilization can increase the salt content of the soil to such an extent that water uptake is reduced. Note that low nitrogen soil content will impair the translocation of potassium to other parts of the plant. A potassium deficiency in the plant will not be noticed as quickly as nitrogen or iron. Deficiency symptoms occur first in the older leaves as potassium is transferred to younger leaves first.
|Soil type||Soil analysis||Yield t/ha|
Below is the well known soil texture triangle on which the recommendations are based. Most commercial agricultural soils are found in the left bottom corner as they are the most productive and provide the plant with best yield potentials.
|↑1||https://commons.wikimedia.org/wiki/File:USDA_Soil_Texture.svg by Christopher Aragón|
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