A nutrição das plantas deve ser uma questão muito fácil de lidar. As plantas precisam de certos elementos essenciais, nas proporções certas, nas quantidades corretas e no momento correto. Uma abordagem sistemática levará em consideração os fatores importantes: água para irrigação, meio de cultivo, a safra, o ambiente e os objetivos de produção. Isso geralmente levará a um bom programa geral de fertilizantes - um que seja simples de implementar, que atenda à maioria das necessidades nutricionais da cultura; mas que também emprega uma estratégia ímpar que pode não satisfazer todas as culturas que você produz. Normalmente, você terá que fornecer a certas culturas mais exigentes algum tratamento especial para manter a qualidade.

Situações frequentes que podem dar errado na seleção de fertilizantes incluem: escolha de fertilizantes errados, adicionar muito fertilizante, adicionar pouco fertilizante e problemas de pH. Ao desenvolver seu programa de fertilizantes, os dados analíticos podem ser muito úteis para ajudá-lo a evitar problemas de colheita que afetam a qualidade e o valor da colheita:

• Um teste abrangente de água pode ser responsável pelo tamponamento da água (alcalinidade ou nível de bicarbonato), nutrientes fornecidos com a água, como cálcio e magnésio, e quaisquer íons problemáticos como sódio ou cloretos.
• Testes internos de CE (condutividade elétrica ou sais solúveis) podem validar se a receita do fertilizante está correta e se o injetor está funcionando corretamente.
• A realização de testes de pH e EC na mídia pode fornecer uma visão geral do estado nutricional da zona radicular.
• Usar um laboratório nutricional de serviço completo pode fornecer a você um instantâneo mais específico do estado nutricional da zona raiz.

Invariavelmente, as coisas darão errado, mesmo no programa de fertilizantes mais bem projetado. Parte disso se deve à grande diversidade de tipos de safras com genética e estágios de cultivo diversos que os produtores costumam cultivar simultaneamente. Parte disso se deve a variações de ambiente que estão fora de nosso controle. Parte disso é devido à variabilidade nos sistemas de cultivo (por exemplo, injetores, sistemas de irrigação e meios de cultivo) que afetarão a eficácia dos fertilizantes. Lembre-se de que a maioria dos nutrientes absorvidos pela planta estão na forma iônica na solução do solo e a eficácia do fertilizante é diretamente afetada por fatores como baixa disponibilidade de água ou evapotranspiração, pH incorreto da zona da raiz, má saúde da raiz, excesso de lixiviação, temperaturas extremas.

Plant nutrition should be a very easy issue to deal with. Plants need certain essential elements, at the right ratios, the correct amounts, and at the correct times.  A systematic approach will consider the important factors: irrigation water, growing media, crops types, environment, and production objectives.  This will lead generally to a good general fertilizer program – one that is simple to implement, fits most crop nutritional needs; but also employs a one size fits all strategy that just might not satisfy every crop you grow.  Typically you will have to provide certain picky crops some special treatment to maintain quality.

Typical things that can go wrong in fertilizer selection include: selection of the wrong fertilizers, adding too much fertilizer, adding too little fertilizer and pH issues. When developing your fertilizer program, analytical data can be very useful to help you prevent crop issues that impact quality and crop value: 

• A comprehensive water test can account for the buffering of the water (Alkalinity or bicarbonate level), nutrient provided with the water such as calcium & magnesium and any problematic ions like sodium or chlorides.
• In-house testing of EC (electrical conductivity or soluble salts) can validate that the fertilizer recipe is correct and that the injector is functioning correctly.
• Conducting pH & EC tests on media can give you a general snapshot of the nutritional status of the root zone.
• Using a full service nutritional lab will can give you a more specific snapshot of the nutritional status of the root zone.

Invariably things will go awry even in the best designed fertilizer program. Part of this is due to the large diversity in crops types with diverse genetics and crop stages that growers typically grow concurrently.  Part of this is due to environment variations that are out of our control.  Part of this is due to variability in growing systems (e.g. injectors, irrigation systems, and growing media) that will impact the efficacy of fertilizers.  Remember that most of the nutrients taken up by the plant are in ionic form in the soil solution and fertilizer efficacy is directly impacted by factors such as poor water availability or evapotranspiration, incorrect root zone pH’s,  poor root health, over leaching, extreme temperatures.

When things go wrong, crops are the ones to alert us of problems in our nutritional systems by exhibiting symptoms of nutrient deficiencies/ toxicities. Unfortunately at this point it may too late to change the fertilizer program, reverse the condition or make the crop salable.  Plants cannot talk; by this I mean they clearly can’t shout out: “Hey Grower, calcium levels are down this week and my growing points are killing me, give me some Peters® Professional 15-0-15…” 

However if you “listen” to your plants through careful scouting and observation; you will find that plants are continuously providing feedback. Subtle crop appearance changes in foliar color, turgidity, wilting, and growth rates of both shoots & roots can help you address nutrient shortcomings before they become big problems.  Most symptoms develop slowly and gradually and it is best to tackle these issues early on.  Look for patterns of symptoms over time between benches/ houses; indicator crops or cultivars, crop turns to identify/ eliminate possible causes of the problem.

When trying to diagnose the cause of deficiency symptoms and come up with an effective mitigation program, it is best to think holistically and consider the entire production system. It is easy to fall into traps based on assumptions and truisms:

• Example: This past year I have seen numerous cases in nurseries with extremely low root zone pH’s (3.5 – 4.0) where crop exhibit chlorotic newer leaves.
• Diagnosis: We are trained to think that low media pH’s cause micronutrients like iron (Fe) & manganese (Mn) to become more soluble and excessive to plants. However in this case after media & tissue testing, the cause of the symptoms was actually determined to be iron deficiency. How could this be? Low pHs were resulting in very high Mn levels in the soil solution. The source of the Mn was likely from the bark used in the mix. This was flooding the system and preventing adequate iron uptake by the plant. By adding a Liquid Stem treatment, symptoms were reversed.

Finally here are a few comments on diagnosis:

• In a problem situation media and tissue testing can help identify/ eliminate possible causes of a problem; but interpretation can be confusing and improper sampling can muddy the picture.
• Just because you have analytical data, doesn’t mean the problem is nutritional in nature.
• Confer with your ICL Specialty Fertilizer Territory Manager to help assess data and work up possible solutions.
• Anything that will impact root or shoot health can impact fertilizer efficacy (e.g. Pythium, viruses, PGR’s, viruses, heat stress, and planting depth).
• Listen to your plants. They know a lot about nutrition.

Dr. Fred Hulme, Ph.D., Technical Services, ICL Specialty Fertilizer