Last month, a grower I have had the privilege of knowing for a long time reminded me of something he heard me say at a no-till meeting over 20 years ago: What we remove from the field defines our present, what we leave behind defines our future.
This simplistic assertion relates to everything from crop residue and compaction to nutrient budgets. If the nutrients removed exceed the nutrients applied, the deficit left behind points to a future that must eventually involve repaying the temporary loan or experiencing the initially insidious erosion of yield potential and other crop properties. And this would occur during an age when recent and continuing advances in technology promise significant improvement in these same properties.
Let’s establish some context. Three underlying factors that encompass many of the major issues humankind will be facing for the next several decades are human nutrition, carbon, and land (Figure 1). Carbon issues include climate change, cheap energy, and bioenergy. Land issues include land use, soil quality, water use and quality, and waste disposal. The intersection of carbon and land represents soil organic matter and the issues associated with carbon sequestration (and nitrogen, phosphate, and sulfur sequestration). Human nutrition brings into the picture the issues of food quantity, quality, and cost. Of critical importance in the discussion of nutrient management is that a significant component of the common ground of all three of these huge factors is soil fertility and how the management of plant nutrients affects our food supply, our land, and the carbon cycle.
The Human Factor
The human nutrition factor is particularly challenging. Some have estimated that the world will need twice as much food within 30 years. That is equivalent to maintaining a proportional annual rate of increase of over 2.4% over that 30-year period. Others predict a 50% increase in food demand by 2030, which translates into a 1.8% annual increase. Others predict more modest demand increases, but all reflect more rapid increases than we have experienced over the last half century. The magnitude of the challenge is better appreciated when these proportional rates of increase are compared to historical cereal yield trends, which have been linear for nearly half a century with slopes equal to only 1.2% to 1.3% of 2007 yields (Figure 2 & Figure 3). Please remember that the time period represented includes the advent of the Green Revolution and tremendous advances in genetics, pest management, and soil and nutrient management.
This analysis tells us that we must accomplish yield increases in the future at a faster rate than we did during the era when all these impressive past advances occurred. Sustainably accomplishing this feat is a huge challenge and will require close cooperation and understanding among disciplines, across geographies, and between public and private sectors. It will also require every crop plant in the field to have access to the nutrients it needs throughout its life cycle, but not more than it needs — 4R nutrient stewardship at its best.
Crop plants and the fields they occupy offer signs of their nutritional status. Our crop advisers and field reps need to help growers refocus attention on the early warning signs of declining nutrient status. Many growers have likely spent most of their careers without needing to be concerned over such signs, but that may now have changed. These early warning signs of declining phosphorus (P) and potassium (K) status for corn include:
Early Season: Stunted growth in part or most of the field, delayed plant development in part or most of the field, reddish-purple discoloration of leaves, a shallow root system with little spread.
Near Pollination: Unusually high variability in plant growth and development, delay in pollination or poor pollination, unusually high incidence of nitrogen (N) deficiency symptoms suggesting poor N efficiency.
At Maturity: Slow grain dry down, reduced kernel weight or test weight, reduced number of kernels per ear, poor kernel fill and small twisted ears, high frequency of weak spindly stalks, often with barren ears.
Early Season: Delayed plant development in part or most of the field.
Near Pollination: Unusually high variability in plant growth and development, delay in pollination or silking, unusually high incidence of foliar or root diseases, marginal browning of lower leaves in part or most of the field, unusually high incidence of N deficiency symptoms suggesting poor N efficiency.
At Maturity: Cut-away of stalks shows dark brown discoloration at the nodes, slow grain dry down, reduced kernel weight or test weight, ears with poorly filled tips and loose, chaffy kernels, stalk lodging in part or most of the field.
Identifying these signs is often difficult without having a reference — a strong argument for helping growers establish perennial P or K non-limiting strips in their fields to provide that reference. The weather in 2009 presented some amazing yields, but many fields undoubtedly also exhibited many of these signs. It would not hurt to simply ask if these signs were present. If nothing else, it will get the wheels turning concerning the consequences of past practices.
As we look to 2010-11, the time is right for the fertilizer industry and customers to refocus on the grand challenge of crop production — sustainably producing the food, feed, fiber, and fuel needs of an ever more demanding world. The first step is determining what we left behind in 2009.